GCC Manual
GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c│-S│-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options
as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options"
allow you to stop this process at an intermediate stage. For example, the -c option says not to run the
linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the
compiler itself. Yet other options control the assembler and linker; most of these are not documented here,
since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option is only
useful with another language (usually C++), the explanation says so explicitly. If the description for a par-
ticular option does not mention a source language, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore
multiple single-letter options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order does mat-
ter when you use several options of the same kind; for example, if you specify -L more than once, the directo-
ries are searched in the order specified.
Many options have long names starting with -f or with -W---for example, -fstrength-reduce, -Wformat and so on.
Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual
documents only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -combine -pipe -pass-exit-codes -x language -v -### --help --target-help --ver-
sion
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp
-fallow-single-precision -fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -fun-
signed-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space -ffriend-injection -fno-const-strings
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti -fstats
-ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-depre-
cated -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link -gen-decls -Was-
sign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once│every-line] -fdiagnostics-show-options
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return -Wno-attributes
-Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declara-
tions -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror-implicit-function-dec-
laration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral
-Wformat-security -Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport
-Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch
-Wlarger-than-len -Wunsafe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-ini-
tializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull
-Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type
-Wsequence-point -Wshadow -Wsign-compare -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas
-Wno-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter
-Wunused-value -Wunused-variable -Wvariadic-macros -Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n]
-fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias
-fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect
-fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-salias -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vec-
torizer-verbose=n -fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -feliminate-unused-debug-types -felimi-
nate-unused-debug-symbols -fmem-report -fprofile-arcs -frandom-seed=string -fsched-verbose=n -ftest-cover-
age -ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff
-gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -save-temps
-time
Optimization Options
-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fmudflap -fmudflapth -fmud-
flapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-tar-
get-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks -fcx-limited-range -fdata-sections -fdelayed-branch -fdelete-null-pointer-checks
-fearly-inlining -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -ffunction-sections
-fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload -floop-optimize -fcrossjumping -fif-conver-
sion -fif-conversion2 -finline-functions -finline-functions-called-once -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmodulo-sched
-fno-branch-count-reg -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants
-fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peep-
hole2 -funsafe-math-optimizations -funsafe-loop-optimizations -ffinite-math-only -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls
-fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove -frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -frerun-loop-opt -frounding-math
-fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns[=n] -fsched-stalled-insns-dep[=n]
-fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans -fsin-
gle-precision-constant -fstack-protector -fstack-protector-all -fstrength-reduce -fstrict-aliasing
-ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -fsplit-ivs-in-unroller
-funswitch-loops -fvariable-expansion-in-unroller -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize
-ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse
-ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize
-ftree-vect-loop-version -ftree-salias -fweb -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop
-ftree-vrp -funit-at-a-time -fwhole-program --param name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir
-include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -isysroot
dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro
-Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static
-static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-
data-section
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian
-mbig-endian -mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb
-marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump
-mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly
-mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mid-shared-library
-mno-id-shared-library -mshared-library-id=n -mlong-calls -mno-long-calls
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4
-metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit
-m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CRX Options -mmac -mpush-args
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_names-
pace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -image_base -init
-install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size
-prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols
-whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot
-segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -what-
sloaded -F -gused -gfull -mmacosx-version-min=version -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-confor-
mant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic -minline-plt
-mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc
-mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indi-
rect-calls -mgas -mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay -mlinker-opt
-mlong-calls -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-indexing -mno-fast-indi-
rect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float
-mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -msched-
ule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static -threads
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-double -mpre-
ferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -mssse3 -msse4a -msse5 -m3dnow -mpopcnt -mabm
-mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num -msseregparm -momit-leaf-frame-pointer
-mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num
-mfused-madd -mno-fused-madd
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregis-
ter-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -min-
line-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -min-
line-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=regis-
ter-range -mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name
-mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881
-mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int
-mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library
-mno-id-shared-library
M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls
-mshort -msoft-reg-count=count
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates
-mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-call-
graph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64
-mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls -mxgot -mno-xgot -mgp32 -mgp64 -mfp32
-mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mdsp -mpaired-single -mips3d -mlong64
-mlong32 -msym32 -mno-sym32 -Gnum -membedded-data -mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-r4000
-mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mfix-sb1
-mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend
-mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mreturn-pointer-on-d0
-mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32
-mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm -mdec-asm
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2
-mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpow-
erpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd -mmfpgpr
-mno-mfpgpr -mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc
-m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float
-mmultiple -mno-multiple -mstring -mno-string -mupdate -mno-update -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocat-
able-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdy-
namic-no-pic -maltivec -mswdiv -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=abi-
type -msecure-plt -mbss-plt -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mvr-
save -mno-vrsave -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G
num -pthread
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mlong-double-64
-mlong-double-128 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mtpf-trace
-mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-sin-
gle-only -m4a-single -m4a -m4al -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact
-m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mno-
macsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy -mdi-
vsi3_libfunc=name -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mimpure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -threads -pthreads
-pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti
-mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpre-
serve-float
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e1 -mv850e
-mv850 -mbig-switch
VAX Options -mg -mgnu -munix
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-section-literals -mno-text-sec-
tion-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -funwind-tables -fasyn-
chronous-unwind-tables -finhibit-size-directive -finstrument-functions -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -freg-struct-return -fshared-data
-fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fargument-alias -fargument-noalias
-fargument-noalias-global -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisi-
bility -fopenmp
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in
that order. GCC is capable of preprocessing and compiling several files either into several assembler input
files, or into one assembler input file; then each assembler input file produces an object file, and linking
combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program
work.
file.mi
Objective-C source code which should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ pro-
gram work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the last two letters must both be literally
x. Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the traditional preprocessor).
file.ads
Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or
generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or sub-
program renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are also
called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated this
way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a
default based on the file name suffix). This option applies to all following input files until the next -x
option. Possible values for language are:
c c-header c-cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f95 f95-cpp-input
java
treelang
-x none
Turn off any specification of a language, so that subsequent files are handled according to their file name
suffixes (as they are if -x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success
return code. If you specify -pass-exit-codes, the gcc program will instead return with numerically highest
error produced by any phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to
start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for
example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate
output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with
.o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler
code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with
.s.
Input files that don’t require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of prepro-
cessed source code, which is sent to the standard output.
Input files which don’t require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced, whether
it be an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suf-
fix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all
preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the
version number of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and all command arguments are quoted. This is useful for
shell scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This
fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
-combine
If you are compiling multiple source files, this option tells the driver to pass all the source files to
the compiler at once (for those languages for which the compiler can handle this). This will allow inter-
module analysis (IMA) to be performed by the compiler. Currently the only language for which this is sup-
ported is C. If you pass source files for multiple languages to the driver, using this option, the driver
will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appro-
priate for it. For those languages that do not support IMA this option will be ignored, and the compiler
will be invoked once for each source file in that language. If you use this option in conjunction with
-save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only
one (combined) .o or .s file.
--help
Print (on the standard output) a description of the command line options understood by gcc. If the -v
option is also specified then --help will also be passed on to the various processes invoked by gcc, so
that they can display the command line options they accept. If the -Wextra option is also specified then
command line options which have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target specific command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header
files often use .hh or .H; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these
names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs
(usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands the C++ lan-
guage---and under some circumstances, you might want to compile programs or header files from standard input,
or otherwise without a suffix that flags them as C++ programs. You might also like to precompile a C header
file with a .h extension to be used in C++ compilations. g++ is a program that calls GCC with the default lan-
guage set to C++, and automatically specifies linking against the C++ library. On many systems, g++ is also
installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling
programs in any language; or command-line options meaningful for C and related languages; or options that are
meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and
Objective-C++) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of
standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such
as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as
well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in
header files that might be included in compilations done with -ansi. Alternate predefined macros such as
"__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -pedantic is
required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may notice this
macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn’t
call for; this is to avoid interfering with any programs that might use these names for other things.
Functions which would normally be built in but do not have semantics defined by ISO C (such as "alloca" and
"ffs") are not built-in functions with -ansi is used.
-std=
Determine the language standard. This option is currently only supported when compiling C or C++. A value
for this option must be provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see tus.html> for more information. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default.
The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
Even when this option is not specified, you can still use some of the features of newer standards in so far
as they do not conflict with previous C standards. For example, you may use "__restrict__" even when
-std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as -ansi, except that features that
were not in ISO C90 but are in the specified version (for example, // comments and the "inline" keyword in
ISO C99) are not disabled.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99
mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline func-
tions.
This option is accepted by GCC versions 4.1.3 and up. In GCC versions prior to 4.3, C99 inline semantics
are not supported, and thus this option is effectively assumed to be present regardless of whether or not
it is specified; the only effect of specifying it explicitly is to disable warnings about using inline
functions in C99 mode. Likewise, the option -fno-gnu89-inline is not supported in versions of GCC before
4.3. It will be supported only in C99 or gnu99 mode, not in C89 or gnu89 mode.
The preprocesor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which seman-
tics are in effect for "inline" functions.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a trans-
lation unit, including those in header files. This option is silently ignored in any language other than
C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and
line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respec-
tively, in the first character after the line number and the colon), and whether it came from a declaration
or a definition (C or F, respectively, in the following character). In the case of function definitions, a
K&R-style list of arguments followed by their declarations is also provided, inside comments, after the
declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers.
You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords. You
may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or
-std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword
in ISO C99.
-fno-builtin
-fno-builtin-function
Don’t recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance,
calls to "alloca" may become single instructions that adjust the stack directly, and calls to "memcpy" may
become inline copy loops. The resulting code is often both smaller and faster, but since the function
calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library. In addition, when a function is recognized as a
built-in function, GCC may use information about that function to warn about problems with calls to that
function, or to generate more efficient code, even if the resulting code still contains calls to that func-
tion. For example, warnings are given with -Wformat for bad calls to "printf", when "printf" is built in,
and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not
begin with __builtin_. If a function is named this is not built-in in this version of GCC, this option is
ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted environment
is one in which the entire standard library is available, and in which "main" has a return type of "int".
Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin. A free-
standing environment is one in which the standard library may not exist, and program startup may not neces-
sarily be at "main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only accepted with this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -tri-
graphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied
"cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an
additional preprocessing step after normal preprocessing but before compiling. The default is to use the
integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only sup-
ported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP
manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such
an expression is void. This option is not supported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default
or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the
signedness of an object. But many programs have been written to use plain "char" and expect it to be
signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its
inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its
behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Like-
wise, the option -fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use either
"signed" or "unsigned". By default, such a bit-field is signed, because this is consistent: the basic
integer types such as "int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you can also use
most of the GNU compiler options regardless of what language your program is in. For example, you might com-
pile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any
language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the ver-
sion that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0
will change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control
code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allo-
cated. This check is normally unnecessary because the C++ standard specifies that "operator new" will only
return 0 if it is declared throw(), in which case the compiler will always check the return value even
without this option. In all other cases, when "operator new" has a non-empty exception specification, mem-
ory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves
space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag
and your program mysteriously crashes after "main()" has completed, you may have an object that is being
destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting variables into
BSS without making them common.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the
class in which they are declared. Friend functions were documented to work this way in the old Annotated
C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend
function which is not declared in an enclosing scope can only be found using argument dependent lookup.
This option causes friends to be injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future release of G++.
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By default, G++ uses type "const char
*" as required by the standard. Even if you use -fno-const-strings, you cannot actually modify the value
of a string constant.
This option might be removed in a future release of G++. For maximum portability, you should structure
your code so that it works with string constants that have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is only used to initialize
another object of the same type. Specifying this option disables that optimization, and forces G++ to call
the copy constructor in all cases.
-fno-enforce-eh-specs
Don’t generate code to check for violation of exception specifications at runtime. This option violates
the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.
This does not give user code permission to throw exceptions in violation of the exception specifications;
the compiler will still optimize based on the specifications, so throwing an unexpected exception will
result in undefined behavior.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for
loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables
declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions
of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for old-style
code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the
keyword "__typeof__" instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code
for explicit instantiations.
-fno-implicit-inline-templates
Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines
differently so that compiles with and without optimization will need the same set of explicit instantia-
tions.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation.
This will cause linker errors if these functions are not inlined everywhere they are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to mem-
ber function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs",
"alloca", "_exit", "index", "bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as syn-
onyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such
diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will
allow some nonconforming code to compile.
-frepo
Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ runtime type
identification features (dynamic_cast and typeid). If you don’t use those parts of the language, you can
save some space by using this flag. Note that exception handling uses the same information, but it will
generate it as needed.
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is generally
only useful to the G++ development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth
is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming pro-
grams must not rely on a maximum depth greater than 17.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of
local statics. You can use this option to reduce code size slightly in code that doesn’t need to be
thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than
the "atexit" function. This option is required for fully standards-compliant handling of static destruc-
tors, but will only work if your C library supports "__cxa_atexit".
-fvisibility-inlines-hidden
Causes all inlined methods to be marked with "__attribute__ ((visibility ("hidden")))" so that they do not
appear in the export table of a DSO and do not require a PLT indirection when used within the DSO.
Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the
size of the dynamic export table when the library makes heavy use of templates.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method.
For example, if you do want to compare pointers to a particular inline method, or the method has local
static data, you might mark it as having default visibility.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak sym-
bols if they are available. This option exists only for testing, and should not be used by end-users; it
will result in inferior code and has no benefits. This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search the other
standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won’t be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an
effort has been made to warn about all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code. There may also be cases where warnings are emitted even
though the code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code gener-
ated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as
a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can
avoid this problem by explicitly padding "A" so that its size is a multiple of the byte size on your
platform; that will cause G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out
virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will. You can
avoid this problem by explicitly padding "A" so that its size is a multiple of its alignment (ignoring
virtual base classes); that will cause G++ and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when
the bit-fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the number of bits
in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++
mistakenly believes that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or template template parameters can be man-
gled incorrectly.
template
void f(typename Q::X) {}
template class Q>
void f(typename Q::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private, and
it has neither friends nor public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-
virtual one. This warning is enabled by -Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not match the order in which they must be
executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of the mem-
bers, emitting a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ book:
* Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated
memory.
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don’t try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers’ More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
* Item 7: Never overload "&&", "││", or ",".
When selecting this option, be aware that the standard library headers do not obey all of these guidelines;
use grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid
sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant not a null pointer, it
is guaranteed to of the same size as a pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the advent of
explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e.,
friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary,
nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids
could be interpreted as a particular specialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check
existing code for potential trouble spots and is on by default. This new compiler behavior can be turned
off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warn-
ing.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts
(dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and
much easier to search for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a
conversion to an unsigned type of the same size. Previous versions of G++ would try to preserve unsigned-
ness, but the standard mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-
defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ pro-
grams, but you can also use most of the language-independent GNU compiler options. For example, you might com-
pile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use
the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options
specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific
options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the syntax
"@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and "NSCon-
stantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also
present, will override the -fconstant-string-class setting and cause "@"..."" literals to be laid out as
constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most
types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including
Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]") in this translation unit
ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to be
used. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and
later.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial
default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method that will run
non-trivial default constructors on any such instance variables, in order, and then return "self". Simi-
larly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize
a special "- (void) .cxx_destruct" method that will run all such default destructors, in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly generated will only operate on
instance variables declared in the current Objective-C class, and not those inherited from superclasses.
It is the responsibility of the Objective-C runtime to invoke all such methods in an object’s inheritance
hierarchy. The "- (id) .cxx_construct" methods will be invoked by the runtime immediately after a new
object instance is allocated; the "- (void) .cxx_destruct" methods will be invoked immediately before the
runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "- (id)
.cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by
C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X
10.3 and later.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a
@catch block, the @throw may appear without an argument (as shown above), in which case the object caught
by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an
object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type,
analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may
also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if
any).
The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ...
@catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown
inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with "NS_HANDLER"-style idioms provided by the
"NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems,
due to additional functionality needed in the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling types other than Objective-C objects.
Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with
C++ exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it
in C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been
placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait
until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will
place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the
lock (thereby making "guard" available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing
exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked prop-
erly.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow
dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging
mode, where the object file in question may be recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is
only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")" (when
the name of the class is known at compile time) with static class references that get initialized at load
time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and
causes calls to "objc_getClass("...")" to be retained. This is useful in Zero-Link debugging mode, since
it allows for individual class implementations to be modified during program execution.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued for every method in the protocol that
is not implemented by the class. The default behavior is to issue a warning for every method not explic-
itly implemented in the class, even if a method implementation is inherited from the superclass. If you
use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector are found during compilation. The check
is performed on the list of methods in the final stage of compilation. Additionally, a check is performed
for each selector appearing in a "@selector(...)" expression, and a corresponding method for that selector
has been found during compilation. Because these checks scan the method table only at the end of compila-
tion, these warnings are not produced if the final stage of compilation is not reached, for example because
an error is found during compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match
Warn if multiple methods with differing argument and/or return types are found for a given selector when
attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is
off (which is the default behavior), the compiler will omit such warnings if any differences found are con-
fined to types which share the same size and alignment.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is consid-
ered undeclared if no method with that name has been declared before the "@selector(...)" expression,
either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section.
This option always performs its checks as soon as a "@selector(...)" expression is found, while -Wselector
only performs its checks in the final stage of compilation. This also enforces the coding style convention
that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device’s aspect (e.g. its
width, ...). The options described below can be used to control the diagnostic messages formatting algorithm,
e.g. how many characters per line, how often source location information should be reported. Right now, only
the C++ front end can honor these options. However it is expected, in the near future, that the remaining
front ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The default is 72 characters
for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be
done; each error message will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source
location information; that is, in case the message is too long to fit on a single physical line and has to
be wrapped, the source location won’t be emitted (as prefix) again, over and over, in subsequent continua-
tion lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source
location information (as prefix) for physical lines that result from the process of breaking a message
which is too long to fit on a single line.
-fdiagnostics-show-options
This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates
which command line option directly controls that diagnostic, when such an option is known to the diagnostic
machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are
risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on
implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn
off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the
default.
The following options control the amount and kinds of warnings produced by GCC; for further, language-specific
options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-fsyntax-only
Check the code for syntax errors, but don’t do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden exten-
sions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the
ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few
will require -ansi or a -std option specifying the required version of ISO C). However, without this
option, certain GNU extensions and traditional C and C++ features are supported as well. With this option,
they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with
__. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only
system header files should use these escape routes; application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find that it
does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C
requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require
considerable additional work and would be quite different from -pedantic. We don’t have plans to support
such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or gnu99,
there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.
Warnings from -pedantic are given where they are required by the base standard. (It would not make sense
for such warnings to be given only for features not in the specified GNU C dialect, since by definition the
GNU dialects of C include all features the compiler supports with the given option, and there would be
nothing to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warnings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget
that this type is signed on some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears
in a // comment. This warning is enabled by -Wall.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to keep
going and printing further error messages.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate
to the format string specified, and that the conversions specified in the format string make sense. This
includes standard functions, and others specified by format attributes, in the "printf", "scanf", "strf-
time" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific fami-
lies). Which functions are checked without format attributes having been specified depends on the standard
version selected, and such checks of functions without the attribute specified are disabled by -ffreestand-
ing or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all
ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these features; GCC does not support warning
about features that go beyond a particular library’s limitations. However, if -pedantic is used with
-Wformat, warnings will be given about format features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnon-
null.
-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wfor-
mat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and
-Wformat=2 are available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The
C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifica-
tions, normally warnings are still given, since the implementation could not know what type to pass to
"va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this option will suppress
the warning if the unused arguments are all pointers, since the Single Unix Specification says that such
unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length
formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked,
unless the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security prob-
lems. At present, this warns about calls to "printf" and "scanf" functions where the format string is not
a string literal and there are no format arguments, as in "printf (foo);". This may be a security hole if
the format string came from untrusted input and contains %n. (This is currently a subset of what -Wfor-
mat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included
in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat -Wfor-
mat-nonliteral -Wformat-security -Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull" func-
tion attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with themselves. Note this option can only be
used with the -Wuninitialized option, which in turn only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snippet only when -Winit-self has
been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared. The form
-Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning,
not an error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage, returning int,
taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by
-Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initial-
izer for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where
a truth value is expected, or when operators are nested whose precedence people often get confused about.
Only the warning for an assignment used as a truth value is supported when compiling C++; the other warn-
ings are only supported when compiling C.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a dif-
ferent interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which "if" statement an "else" branch
belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if
(b)". This is often not what the programmer expected, as illustrated in the above example by indentation
the programmer chose. When there is the potential for this confusion, GCC will issue a warning when this
flag is specified. To eliminate the warning, add explicit braces around the innermost "if" statement so
there is no way the "else" could belong to the enclosing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in the C
standard.
The C standard defines the order in which expressions in a C program are evaluated in terms of sequence
points, which represent a partial ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur after the evaluation of a full expres-
sion (one which is not part of a larger expression), after the evaluation of the first operand of a "&&",
"││", "? :" or "," (comma) operator, before a function is called (but after the evaluation of its arguments
and the expression denoting the called function), and in certain other places. Other than as expressed by
the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order, since, for example, if two functions
are called within one expression with no sequence point between them, the order in which the functions are
called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Pro-
grams whose behavior depends on this have undefined behavior; the C standard specifies that "Between the
previous and next sequence point an object shall have its stored value modified at most once by the evalua-
tion of an expression. Furthermore, the prior value shall be read only to determine the value to be
stored.". If a program breaks these rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more com-
plicated cases are not diagnosed by this option, and it may give an occasional false positive result, but
in general it has been found fairly effective at detecting this sort of problem in programs.
The present implementation of this option only works for C programs. A future implementation may also work
for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise meaning of the
sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal defi-
nitions, may be found on the GCC readings page, at.
This warning is enabled by -Wall.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int". Also warn about any
"return" statement with no return-value in a function whose return-type is not "void".
For C, also warn if the return type of a function has a type qualifier such as "const". Such a type quali-
fier has no effect, since the value returned by a function is not an lvalue. ISO C prohibits qualified
"void" return types on function definitions, so such return types always receive a warning even without
this option.
For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is
specified. The only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case"
labels outside the enumeration range also provoke warnings when this option is used. This warning is
enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings
when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within com-
ments are not warned about). This warning is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused.
This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This
warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. This warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused
(note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by
a "setjmp" call.
These warnings are possible only in optimizing compilation, because they require data flow information that
is computed only when optimizing. If you don’t specify -O, you simply won’t get these warnings.
If you want to warn about code which uses the uninitialized value of the variable in its own initializer,
use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array vari-
ables as well as for variables which are uninitialized or clobbered as a whole. They do not occur for
variables or elements declared "volatile". Because these warnings depend on optimization, the exact vari-
ables or elements for which there are warnings will depend on the precise optimization options and version
of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is
never used, because such computations may be deleted by data flow analysis before the warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why the code might
be correct despite appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn’t know this. Here
is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp".
These warnings as well are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a
signal handler could call it at any point in the code. As a result, you may get a warning even when there
is in fact no problem because "longjmp" cannot in fact be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as "nore-
turn".
This warning is enabled by -Wall.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option
is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if
the warnings were only enabled by the -Wall command line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between
pragmas. See also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code which might break the
strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases,
but does attempt to catch the more common pitfalls. It is included in -Wall.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It warns about code which might break the
strict aliasing rules that the compiler is using for optimization. This warning catches more cases than
-Wstrict-aliasing, but it will also give a warning for some ambiguous cases that are safe.
-Wall
All of the above -W options combined. This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction
with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objec-
tive-C and Objective-C++ Dialect Options.
The following -W... options are not implied by -Wall. Some of them warn about constructions that users gener-
ally do not consider questionable, but which occasionally you might wish to check for; others warn about con-
structions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to
suppress the warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the newer name is more descrip-
tive.) Print extra warning messages for these events:
* A function can return either with or without a value. (Falling off the end of the function body is
considered returning without a value.) For example, this function would evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma expression contains no side effects. To sup-
press the warning, cast the unused expression to void. For example, an expression such as x[i,j] will
cause a warning, but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things in a declaration. According to the C
Standard, this usage is obsolescent.
* If -Wall or -Wunused is also specified, warn about unused arguments.
* A comparison between signed and unsigned values could produce an incorrect result when the signed value
is converted to unsigned. (But don’t warn if -Wno-sign-compare is also specified.)
* An aggregate has an initializer which does not initialize all members. This warning can be indepen-
dently controlled by -Wmissing-field-initializers.
* A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or >=.
* A variable might be changed by longjmp or vfork.
* Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of
precision, etc.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a class without constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared register.
*<(C++ only)>
A base class is not initialized in a derived class’ copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by zero is not warned
about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are nor-
mally suppressed, on the assumption that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command line option tells GCC to emit warnings from system
headers as if they occurred in user code. However, note that using -Wall in conjunction with this option
will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point
values as approximations to infinitely precise real numbers. If you are doing this, then you need to com-
pute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation
introduces, and allow for it when performing comparisons (and when producing output, but that’s a different
problem). In particular, instead of testing for equality, you would check to see whether the two values
have ranges that overlap; and this is done with the relational operators, so equality comparisons are prob-
ably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and/or problematic constructs which should be avoided.
* Macro parameters that appear within string literals in the macro body. In traditional C macro replace-
ment takes place within string literals, but does not in ISO C.
* In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only
consider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional
warns about directives that traditional C understands but would ignore because the # does not appear as
the first character on the line. It also suggests you hide directives like #pragma not understood by
traditional C by indenting them. Some traditional implementations would not recognize #elif, so it
suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional C does
support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the sys-
tem headers of most modern systems, e.g. the _MIN/_MAX macros in "". Use of these macros in
user code might normally lead to spurious warnings, however GCC’s integrated preprocessor has enough
context to avoid warning in these cases.
* A function declared external in one block and then used after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct is not accepted by some
traditional C compilers.
* The ISO type of an integer constant has a different width or signedness from its traditional type.
This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values,
which typically represent bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
* Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the
assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid miss-
ing initializer warnings and relies on default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values and vice versa. The absence of these
prototypes when compiling with traditional C would cause serious problems. This is a subset of the
possible conversion warnings, for the full set use -Wconversion.
* Use of ISO C style function definitions. This warning intentionally is not issued for prototype decla-
rations or variadic functions because these ISO C features will appear in your code when using libib-
erty’s traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for
nested functions because that feature is already a GCC extension and thus not relevant to traditional C
compatibility.
-Wdeclaration-after-statement (C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was intro-
duced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported
by GCC versions before GCC 3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a
built-in function is shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the
loop indices. With -funsafe-loop-optimizations warn if the compiler made such assumptions.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types
a size of 1, for convenience in calculations with "void *" pointers and pointers to functions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast
to "anything *".
-Wc++-compat
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for
implicit conversion from "void *" to a pointer to non-"void" type.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn
if a "const char *" is cast to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example,
warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-
byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of one
into a non-"const" "char *" pointer will get a warning; when compiling C++, warn about the deprecated con-
version from string constants to "char *". These warnings will help you find at compile time code that can
try to write into a string constant, but only if you have been very careful about using "const" in declara-
tions and prototypes. Otherwise, it will just be a nuisance; this is why we did not make -Wall request
these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from what would happen to the same argument
in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed point argument except when the same as the default
promotion.
Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For
example, warn about the assignment "x = -1" if "x" is unsigned. But do not warn about explicit casts like
"(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed
value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of
-Wextra without this warning, use -Wextra -Wno-sign-compare.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where you can
return an array, this also elicits a warning.)
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes
applied to variables, etc. This will not stop errors for incorrect use of supported attributes.
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style function
definition is permitted without a warning if preceded by a declaration which specifies the argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous proto-
type.
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype declaration. This warning is issued even
if the definition itself provides a prototype. The aim is to detect global functions that fail to be
declared in header files.
-Wmissing-declarations (C only)
Warn if a global function is defined without a previous declaration. Do so even if the definition itself
provides a prototype. Use this option to detect global functions that are not declared in header files.
-Wmissing-field-initializers
Warn if a structure’s initializer has some fields missing. For example, the following code would cause
such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification would not trigger a
warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn". Note these are only possible can-
didates, not absolute ones. Care should be taken to manually verify functions actually do not ever return
before adding the "noreturn" attribute, otherwise subtle code generation bugs could be introduced. You
will not get a warning for "main" in hosted C environments.
-Wmissing-format-attribute
Warn about function pointers which might be candidates for "format" attributes. Note these are only possi-
ble candidates, not absolute ones. GCC will guess that function pointers with "format" attributes that are
used in assignment, initialization, parameter passing or return statements should have a corresponding
"format" attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the
type of the parameter variable, or the return type of the containing function respectively should also have
a "format" attribute to avoid the warning.
GCC will also warn about function definitions which might be candidates for "format" attributes. Again,
these are only possible candidates. GCC will guess that "format" attributes might be appropriate for any
function that calls a function like "vprintf" or "vscanf", but this might not always be the case, and some
functions for which "format" attributes are appropriate may not be detected.
-Wno-multichar
Do not warn if a multicharacter constant (’FOOF’) is used. Usually they indicate a typo in the user’s
code, as they have implementation-defined values, and should not be used in portable code.
-Wnormalized=
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. How-
ever, sometimes when characters outside the basic ASCII character set are used, you can have two different
character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normal-
ization rules which when applied ensure that two sequences that look the same are turned into the same
sequence. GCC can warn you if you are using identifiers which have not been normalized; this option con-
trols that warning.
There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any
identifier which is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most
uses.
Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into
NFC aren’t allowable as identifiers. That is, there’s no way to use these symbols in portable ISO C or C++
and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct this, which is why this option is not the
default.
You can switch the warning off for all characters by writing -Wnormalized=none. You would only want to do
this if you were using some other normalization scheme (like "D"), because otherwise you can easily create
bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodolo-
gies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER
N", will display just like a regular "n" which has been placed in a superscript. ISO 10646 defines the
NFKC normalisation scheme to convert all these into a standard form as well, and GCC will warn if your code
is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier
that contains the letter O because it might be confused with the digit 0, and so is not the default, but
may be useful as a local coding convention if the programming environment is unable to be fixed to display
these characters distinctly.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated"
attribute. (@pxref{Function Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or
size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code,
the variable "f.x" in "struct bar" will be misaligned even though "struct bar" does not itself have the
packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align the
whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is
valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole line of source code will
never be executed, because some condition is never satisfied or because it is after a procedure that never
returns.
It is possible for this option to produce a warning even though there are circumstances under which part of
the affected line can be executed, so care should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only one
inlined copy of the function.
This option is not made part of -Wall because in a debugging version of a program there is often substan-
tial code which checks correct functioning of the program and is, hopefully, unreachable because the pro-
gram does work. Another common use of unreachable code is to provide behavior which is selectable at com-
pile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler
will not warn about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example,
the compiler takes into account the size of the function being inlined and the amount of inlining that has
already been done in the current function. Therefore, seemingly insignificant changes in the source pro-
gram can cause the warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ stan-
dard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof
typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple
struct that fails to be a POD type only by virtue of having a constructor.) This flag is for users who are
aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about
it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
-Wno-int-to-pointer-cast (C only)
Suppress warnings from casts to pointer type of an integer of a different size.
-Wno-pointer-to-int-cast (C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can’t be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the warning messages, use -Wno-long-long.
Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO
C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations
that may eliminate reads and/or writes to register variables.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is
anything wrong with your code; it merely indicates that GCC’s optimizers were unable to handle the code
effectively. Often, the problem is that your code is too big or too complex; GCC will refuse to optimize
programs when the optimization itself is likely to take inordinate amounts of time.
-Wpointer-sign
Warn for pointer argument passing or assignment with different signedness. This option is only supported
for C and Objective-C. It is implied by -Wall and by -pedantic, which can be disabled with
-Wno-pointer-sign.
-Werror
Make all warnings into errors.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that will not be
protected against stack smashing.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
-g Produce debugging information in the operating system’s native format (stabs, COFF, XCOFF, or DWARF 2).
GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use;
this extra information makes debugging work better in GDB but will probably make other debuggers crash or
refuse to read the program. If you want to control for certain whether to generate the extra information,
use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surpris-
ing results: some variables you declared may not exist at all; flow of control may briefly move where you
did not expect it; some statements may not be executed because they compute constant results or their val-
ues were already at hand; some statements may execute in different places because they were moved out of
loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer
for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one debugging for-
mat.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available
(DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at
all possible.
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the
format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces
stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option
requires the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only symbols that are actually
used.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions understood only
by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse
to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported). This is the format used by SDB on
most System V systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX
debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only
by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse
to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by
DBX on IRIX 6. With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is
upward compatible with version 2, but may still cause problems for older debuggers.
-gvms
Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG
on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don’t
plan to debug. This includes descriptions of functions and external variables, but no information about
local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debug-
gers support macro expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that
meant to generate debug information in version 1 of the DWARF format (which is very different from version
2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be
changed now. Instead use an additional -glevel option to change the debug level for DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option
only makes sense when generating DWARF2 debugging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the analysis program prof. You must use this
option when compiling the source files you want data about, and you must also use it when linking.
-pg Generate extra code to write profile information suitable for the analysis program gprof. You must use
this option when compiling the source files you want data about, and you must also use it when linking.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about each
pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many times
each branch and call is executed and how many times it is taken or returns. When the compiled program
exits it saves this data to a file called auxname.gcda for each source file. The data may be used for pro-
file-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage).
Each object file’s auxname is generated from the name of the output file, if explicitly specified and it is
not the final executable, otherwise it is the basename of the source file. In both cases any suffix is
removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o
dir/foo.o).
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a synonym
for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for
those options for more details.
@bullet
Compile the source files with -fprofile-arcs plus optimization and code generation options. For test
coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source
file in a program.
@cvmmfu
Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
@dwnngv
Run the program on a representative workload to generate the arc profile information. This may be
repeated any number of times. You can run concurrent instances of your program, and provided that the
file system supports locking, the data files will be correctly updated. Also "fork" calls are detected
and correctly handled (double counting will not happen).
@exoohw
For profile-directed optimizations, compile the source files again with the same optimization and code
generation options plus -fbranch-probabilities.
@fyppix
For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda
files. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a span-
ning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler
adds code to count the number of times that these arcs are executed. When an arc is the only exit or only
entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must
be created to hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source
file’s note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of
auxname and instructions on how to generate test coverage data. Coverage data will match the source files
more closely, if you do not optimize.
-dletters
-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified by letters. This is used for debug-
ging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a
pass number and a word to the dumpname. dumpname is generated from the name of the output file, if explic-
itly specified and it is not an executable, otherwise it is the basename of the source file.
Most debug dumps can be enabled either passing a letter to the -d option, or with a long -fdump-rtl switch;
here are the possible letters for use in letters and pass, and their meanings:
-dA Annotate the assembler output with miscellaneous debugging information.
-db
-fdump-rtl-bp
Dump after computing branch probabilities, to file.09.bp.
-dB
-fdump-rtl-bbro
Dump after block reordering, to file.30.bbro.
-dc
-fdump-rtl-combine
Dump after instruction combination, to the file file.17.combine.
-dC
-fdump-rtl-ce1
-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if conversion, to the file file.11.ce1. -dC and
-fdump-rtl-ce2 enable dumping after the second if conversion, to the file file.18.ce2.
-dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load optimization, to file.31.btl. -dd and
-fdump-rtl-dbr enable dumping after delayed branch scheduling, to file.36.dbr.
-dD Dump all macro definitions, at the end of preprocessing, in addition to normal output.
-dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.28.ce3.
-df
-fdump-rtl-cfg
-fdump-rtl-life
-df and -fdump-rtl-cfg enable dumping after control and data flow analysis, to file.08.cfg. -df and
-fdump-rtl-cfg enable dumping dump after life analysis, to file.16.life.
-dg
-fdump-rtl-greg
Dump after global register allocation, to file.23.greg.
-dG
-fdump-rtl-gcse
-fdump-rtl-bypass
-dG and -fdump-rtl-gcse enable dumping after GCSE, to file.05.gcse. -dG and -fdump-rtl-bypass enable
dumping after jump bypassing and control flow optimizations, to file.07.bypass.
-dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.
-di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.01.sibling.
-dj
-fdump-rtl-jump
Dump after the first jump optimization, to file.03.jump.
-dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to file.33.stack.
-dl
-fdump-rtl-lreg
Dump after local register allocation, to file.22.lreg.
-dL
-fdump-rtl-loop
-fdump-rtl-loop2
-dL and -fdump-rtl-loop enable dumping after the first loop optimization pass, to file.06.loop. -dL
and -fdump-rtl-loop2 enable dumping after the second pass, to file.13.loop2.
-dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.20.sms.
-dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, to file.35.mach.
-dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.29.rnreg.
-dN
-fdump-rtl-regmove
Dump after the register move pass, to file.19.regmove.
-do
-fdump-rtl-postreload
Dump after post-reload optimizations, to file.24.postreload.
-dr
-fdump-rtl-expand
Dump after RTL generation, to file.00.expand.
-dR
-fdump-rtl-sched2
Dump after the second scheduling pass, to file.32.sched2.
-ds
-fdump-rtl-cse
Dump after CSE (including the jump optimization that sometimes follows CSE), to file.04.cse.
-dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.
-dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to
file.15.cse2.
-dT
-fdump-rtl-tracer
Dump after running tracer, to file.12.tracer.
-dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile transformations, to file.10.vpt. -dV and
-fdump-rtl-vartrack enable dumping after variable tracking, to file.34.vartrack.
-dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.26.flow2.
-dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.27.peephole2.
-dZ
-fdump-rtl-web
Dump after live range splitting, to file.14.web.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dH Produce a core dump whenever an error occurs.
-dm Print statistics on memory usage, at the end of the run, to standard error.
-dp Annotate the assembler output with a comment indicating which pattern and alternative was used. The
length of each instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annota-
tion.
-dv For each of the other indicated dump files (either with -d or -fdump-rtl-pass), dump a representation
of the control flow graph suitable for viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it. Usually used with r (-fdump-rtl-expand).
-dy Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output.
This makes it more feasible to use diff on debugging dumps for compiler invocations with different options,
in particular with and without -g.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file name is
made by appending .tu to the source file name. If the -options form is used, options controls the details
of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class’s hierarchy and virtual function table layout to a file. The file name
is made by appending .class to the source file name. If the -options form is used, options controls the
details of the dump as described for the -fdump-tree options.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name
is generated by appending a switch specific suffix to the source file name. The following dumps are possi-
ble:
all Enables all inter-procedural analysis dumps; currently the only produced dump is the cgraph dump.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
-fdump-tree-switch
-fdump-tree-switch-options
Control the dumping at various stages of processing the intermediate language tree to a file. The file
name is generated by appending a switch specific suffix to the source file name. If the -options form is
used, options is a list of - separated options that control the details of the dump. Not all options are
applicable to all dumps, those which are not meaningful will be ignored. The following options are avail-
able
address
Print the address of each node. Usually this is not meaningful as it changes according to the environ-
ment and source file. Its primary use is for tying up a dump file with a debug environment.
slim
Inhibit dumping of members of a scope or body of a function merely because that scope has been reached.
Only dump such items when they are directly reachable by some other path. When dumping pretty-printed
trees, this option inhibits dumping the bodies of control structures.
raw Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representa-
tion.
details
Enable more detailed dumps (not honored by every dump option).
stats
Enable dumping various statistics about the pass (not honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
all Turn on all options, except raw, slim and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
gimple
Dump each function before and after the gimplification pass to a file. The file name is made by
appending .gimple to the source file name.
cfg Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the
source file name.
vcg Dump the control flow graph of each function to a file in VCG format. The file name is made by append-
ing .vcg to the source file name. Note that if the file contains more than one function, the generated
file cannot be used directly by VCG. You will need to cut and paste each function’s graph into its own
separate file first.
ch Dump each function after copying loop headers. The file name is made by appending .ch to the source
file name.
ssa Dump SSA related information to a file. The file name is made by appending .ssa to the source file
name.
salias
Dump structure aliasing variable information to a file. This file name is made by appending .salias to
the source file name.
alias
Dump aliasing information for each function. The file name is made by appending .alias to the source
file name.
ccp Dump each function after CCP. The file name is made by appending .ccp to the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file
name.
pre Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source
file name.
fre Dump trees after full redundancy elimination. The file name is made by appending .fre to the source
file name.
copyprop
Dump trees after copy propagation. The file name is made by appending .copyprop to the source file
name.
store_copyprop
Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the
source file name.
dce Dump each function after dead code elimination. The file name is made by appending .dce to the source
file name.
mudflap
Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap
to the source file name.
sra Dump each function after performing scalar replacement of aggregates. The file name is made by append-
ing .sra to the source file name.
sink
Dump each function after performing code sinking. The file name is made by appending .sink to the
source file name.
dom Dump each function after applying dominator tree optimizations. The file name is made by appending
.dom to the source file name.
dse Dump each function after applying dead store elimination. The file name is made by appending .dse to
the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline code. The file name is made by append-
ing .phiopt to the source file name.
forwprop
Dump each function after forward propagating single use variables. The file name is made by appending
.forwprop to the source file name.
copyrename
Dump each function after applying the copy rename optimization. The file name is made by appending
.copyrename to the source file name.
nrv Dump each function after applying the named return value optimization on generic trees. The file name
is made by appending .nrv to the source file name.
vect
Dump each function after applying vectorization of loops. The file name is made by appending .vect to
the source file name.
vrp Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the
source file name.
all Enable all the available tree dumps with the flags provided in this option.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer prints. This information is written to
standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the
usual dump listing file, .vect.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to gener-
ate certain symbol names that have to be different in every compiled file. It is also used to place unique
stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option
to produce reproducibly identical object files.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the sched-
uler prints. This information is written to standard error, unless -dS or -dR is specified, in which case
it is output to the usual dump listing file, .sched or .sched2 respectively. However for n greater than
nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater than one, it
also output basic block probabilities, detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-ver-
bose also includes dependence info.
-save-temps
Store the usual "temporary" intermediate files permanently; place them in the current directory and name
them based on the source file. Thus, compiling foo.c with -c -save-temps would produce files foo.i and
foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now nor-
mally uses an integrated preprocessor.
When used in combination with the -x command line option, -save-temps is sensible enough to avoid over
writing an input source file with the same extension as an intermediate file. The corresponding intermedi-
ate file may be obtained by renaming the source file before using -save-temps.
-time
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if linking is done). The output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself. The
second number is "system time", time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better
debugging information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g)
and the debug info format supports it.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don’t do
anything else. With this option, GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the com-
mand line. This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory name
is separated from the switches by ;, and each switch starts with an @} instead of the @samp{-, without
spaces between multiple switches. This is supposed to ease shell-processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS
libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS
libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if
OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do
gcc -nostdlib... ‘gcc -print-libgcc-file-name‘
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc
will search---and don’t do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or
directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects
to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed
them. Don’t forget the trailing /.
-dumpmachine
Print the compiler’s target machine (for example, i686-pc-linux-gnu)---and don’t do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don’t do anything else.
-dumpspecs
Print the compiler’s built-in specs---and don’t do anything else. (This is used when GCC itself is being
built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a
compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes
this is useful, such as if, in the debugger, you want to cast a value to a type that is not actually used
in your program (but is declared). More often, however, this results in a significant amount of wasted
space. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in
the source file being compiled.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging
produce the expected results. Statements are independent: if you stop the program with a breakpoint between
statements, you can then assign a new value to any variable or change the program counter to any other state-
ment in the function and get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the
expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Optimization levels -O and
above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from
later functions in the file when compiling a function. Compiling multiple files at once to a single output
file in unit-at-a-time mode allows the compiler to use information gained from all of the files when compiling
each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations
that take a great deal of compilation time.
-O turns on the following optimization flags: -fdefer-pop -fdelayed-branch -fguess-branch-probability
-fcprop-registers -floop-optimize -fif-conversion -fif-conversion2 -ftree-ccp -ftree-dce -ftree-domina-
tor-opts -ftree-dse -ftree-ter -ftree-lrs -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch
-funit-at-a-time -fmerge-constants
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
-O doesn’t turn on -ftree-sra for the Ada compiler. This option must be explicitly specified on the com-
mand line to be enabled for the Ada compiler.
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed
tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As com-
pared to -O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:
-fthread-jumps -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks -fgcse
-fgcse-lm -fexpensive-optimizations -fstrength-reduce -frerun-cse-after-loop -frerun-loop-opt
-fcaller-saves -fpeephole2 -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fregmove
-fstrict-aliasing -fdelete-null-pointer-checks -freorder-blocks -freorder-functions -falign-functions
-falign-jumps -falign-loops -falign-labels -ftree-vrp -ftree-pre
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-func-
tions, -funswitch-loops and -fgcse-after-reload options.
-O0 Do not optimize. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also
performs further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays
-ftree-vect-loop-version
If you use multiple -O options, with or without level numbers, the last such option is the one that is
effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative
forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed---the
one you typically will use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related
to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be
performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the class scope (C++
only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by
default; i.e., you don’t need to add inline in front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines which must
pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for sev-
eral function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Force memory operands to be copied into registers before doing arithmetic on them. This produces better
code by making all memory references potential common subexpressions. When they are not common subexpres-
sions, instruction combination should eliminate the separate register-load. This option is now a nop and
will be removed in 4.2.
-fforce-addr
Force memory address constants to be copied into registers before doing arithmetic on them.
-fomit-frame-pointer
Don’t keep the frame pointer in a register for functions that don’t need one. This avoids the instructions
to save, set up and restore frame pointers; it also makes an extra register available in many functions.
It also makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automati-
cally handles the frame pointer and nothing is saved by pretending it doesn’t exist. The machine-descrip-
tion macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don’t pay attention to the "inline" keyword. Normally this option is used to keep the compiler from
expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically decides which functions are
simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function
is normally not output as assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not marked
"inline". If a call to a given function is integrated, then the function is not output as assembler code
in its own right.
Enabled if -funit-at-a-time is enabled.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function call
overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes pro-
filing significantly cheaper and usually inlining faster on programs having large chains of nested wrapper
functions.
Enabled by default.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows the control of this
limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined
within the class definition in c++). n is the size of functions that can be inlined in number of pseudo
instructions (not counting parameter handling). The default value of n is 600. Increasing this value can
result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which presumably means slower programs). This
option is particularly useful for programs that use inlining heavily such as those based on recursive tem-
plates with C++.
Inlining is actually controlled by a number of parameters, which may be specified individually by using
--param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to I/2.
max-inline-insns-auto
is set to I/2.
min-inline-insns
is set to 130 or I/4, whichever is smaller.
max-inline-insns-rtl
is set to I.
See below for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function’s
size. In no way does it represent a count of assembly instructions and as such its exact meaning might
change from one release to an another.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function has
been inlined into all of its callers. This switch does not affect functions using the "extern inline"
extension in GNU C. In C++, emit any and all inline functions into the object file.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn’t turned on, even if the variables aren’t ref-
erenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was refer-
enced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating point constants) across compilation
units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant
initialized arrays or initialized constant variables with integral or floating point types. Languages like
C or C++ require each non-automatic variable to have distinct location, so using this option will result in
non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost
loops and reorders their instructions by overlapping different iterations.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of
instructions that decrement a register, compare it against zero, then branch based upon the result. This
option is only meaningful on architectures that support such instructions, which include x86, PowerPC,
IA-64 and S/390.
The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function contain
the function’s address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be
confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS.
This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the data
section. E.g., so that the resulting executable can find the beginning of that section and/or make assump-
tions based on that.
The default is -fzero-initialized-in-bss.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations,
some standard library string/heap functions, and some other associated constructs with range/validity
tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other
classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (libmud-
flap), which will be linked into a program if -fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env MUD-
FLAP_OPTIONS=-help a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmud-
flapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This pro-
duces less instrumentation (and therefore faster execution) and still provides some protection against out-
right memory corrupting writes, but allows erroneously read data to propagate within a program.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another comparison sub-
sumed by the first is found. If so, the first branch is redirected to either the destination of the second
branch or a point immediately following it, depending on whether the condition is known to be true or
false.
Enabled at levels -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions when the target of the jump is not
reached by any other path. For example, when CSE encounters an "if" statement with an "else" clause, CSE
will follow the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which conditionally skip over blocks.
When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow the
jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and copy
propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime perfor-
mance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command
line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads which are
only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed
to a load outside the loop, and a copy/store within the loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This
pass will attempt to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a
load/store sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads
that come after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The pur-
pose of this pass is to cleanup redundant spilling.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and
optionally do strength-reduction as well.
Enabled at levels -O, -O2, -O3, -Os.
-floop-optimize2
Perform loop optimizations using the new loop optimizer. The optimizations (loop unrolling, peeling and
unswitching, loop invariant motion) are enabled by separate flags.
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with non-
trivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop
optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the
compiler will warn you if it finds this kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional
moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use
of conditional execution on chips where it is available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler
assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it
has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple instructions in
order to maximize the amount of register tying. This is especially helpful on machines with two-operand
instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available
after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to
required data being unavailable. This helps machines that have slow floating point or memory load instruc-
tions by allowing other instructions to be issued until the result of the load or floating point instruc-
tion is required.
Enabled at levels -O2, -O3, -Os.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allo-
cation has been done. This is especially useful on machines with a relatively small number of registers
and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don’t schedule instructions across basic blocks. This is normally enabled by default when scheduling
before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don’t allow speculative motion of non-load instructions. This is normally enabled by default when schedul-
ing before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready
list, during the second scheduling pass. -fno-fsched-stalled-insns and -fsched-stalled-insns=0 are equiva-
lent and mean that no insns will be moved prematurely. If n is unspecified then there is no limit on how
many queued insns can be moved prematurely.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate
for premature removal from the queue of stalled insns. This has an effect only during the second schedul-
ing pass, and only if -fsched-stalled-insns is used and its value is not zero.
+-fno-sched-stalled-insns-dep is equivalent to +-fsched-stalled-insns-dep=0. +-fsched-stalled-insns-dep
without a value is equivalent to +-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, do use superblock scheduling algorithm. Superblock scheduling
allows motion across basic block boundaries resulting on faster schedules. This option is experimental, as
not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the
algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or
higher.
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform
code duplication in order to increase the size of superblocks using tracer pass. See -ftracer for details
on trace formation.
This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the
traces constructed may not match the reality and hurt the performance. This only makes sense when schedul-
ing after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want
to prevent the later scheduling passes from changing its schedule, we use this option to control that.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra
instructions to save and restore the registers around such calls. Such allocation is done only when it
seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no call-preserved
registers to use instead.
Enabled at levels -O2, -O3, -Os.
-ftree-pre
Perform Partial Redundancy Elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
-ftree-fre
Perform Full Redundancy Elimination (FRE) on trees. The difference between FRE and PRE is that FRE only
considers expressions that are computed on all paths leading to the redundant computation. This analysis
faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled
by default at -O and higher.
-ftree-store-copy-prop
Perform copy propagation of memory loads and stores. This pass eliminates unnecessary copy operations in
memory references (structures, global variables, arrays, etc). This flag is enabled by default at -O2 and
higher.
-ftree-salias
Perform structural alias analysis on trees. This flag is enabled by default at -O and higher.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O and higher.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar
variables and is enabled by default at -O and higher.
-ftree-store-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass operates on both local scalar
variables and memory stores and loads (global variables, structures, arrays, etc). This flag is enabled by
default at -O2 and higher.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range prop-
agation and expression simplification) based on a dominator tree traversal. This also performs jump
threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion
optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not
enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
-ftree-loop-linear
Perform linear loop transformations on tree. This flag can improve cache performance and allow further
loop optimizations to take place.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that would be hard to handle at
RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops
it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in the loop for that determining number of iterations
requires complicated analysis. Later optimizations then may determine the number easily. Useful espe-
cially in connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and induction
variable elimination) on trees.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent
committing structures to memory too early. This flag is enabled by default at -O and higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at
copy locations, usually resulting in variable names which more closely resemble the original variables.
This flag is enabled by default at -O and higher.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries
are replaced at their use location with their defining expression. This results in non-GIMPLE code, but
gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled
by default at -O and higher.
-ftree-lrs
Perform live range splitting during the SSA->normal phase. Distinct live ranges of a variable are split
into unique variables, allowing for better optimization later. This is enabled by default at -O and
higher.
-ftree-vectorize
Perform loop vectorization on trees.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable
except that data alignment or data dependence cannot be determined at compile time then vectorized and non-
vectorized versions of the loop are generated along with runtime checks for alignment or dependence to con-
trol which version is executed. This option is enabled by default except at level -Os where it is dis-
abled.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of
values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks
like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null
pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of
the function allowing other optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -fun-
roll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option makes code larger, and
may or may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually
makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops,
-fsplit-ivs-in-unroller
Enables expressing of values of induction variables in later iterations of the unrolled loop using the
value in the first iteration. This breaks long dependency chains, thus improving efficiency of the
scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body
is more complicated than a single basic block, this is not reliable. It also does not work at all on some
of the architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some local variables when unrolling a loop
which can result in superior code.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of
loops that access large arrays.
These options may generate better or worse code; results are highly dependent on the structure of loops
within the source code.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peep-
hole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use
both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are
specified by __builtin_expect, then the heuristics will be used to guess branch probabilities for the rest
of the control flow graph, taking the __builtin_expect info into account. The interactions between the
heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuris-
tics so that the effects of __builtin_expect are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code
locality.
Enabled at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken
branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to
improve paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling, for linkonce sections,
for functions with a user-defined section attribute and on any architecture that does not support named
sections.
-freorder-functions
Reorder functions in the object file in order to improve code locality. This is implemented by using spe-
cial subsections ".text.hot" for most frequently executed functions and ".text.unlikely" for unlikely exe-
cuted functions. Reordering is done by the linker so object file format must support named sections and
linker must place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For
C (and C++), this activates optimizations based on the type of expressions. In particular, an object of
one type is assumed never to reside at the same address as an object of a different type, unless the types
are almost the same. For example, an "unsigned int" can alias an "int", but not a "void*" or a "double".
A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called
"type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is
accessed through the union type. So, the code above will work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias analysis should define a function that com-
putes, given an "tree" node, an alias set for the node. Nodes in different alias sets are not allowed to
alias. For an example, see the C front-end function "c_get_alias_set".
Enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For
instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24
would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This
option can easily make code slower, because it must insert dummy operations for when the branch target is
reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are
used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no
alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that
the loop will be executed many times, which will make up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached
by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be exe-
cuted.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to
take place but consumes more memory (in general). There are some compatibility issues with unit-at-at-time
mode:
* enabling unit-at-a-time mode may change the order in which functions, variables, and top-level "asm"
statements are emitted, and will likely break code relying on some particular ordering. The majority
of such top-level "asm" statements, though, can be replaced by "section" attributes.
* unit-at-a-time mode removes unreferenced static variables and functions. This may result in undefined
references when an "asm" statement refers directly to variables or functions that are otherwise unused.
In that case either the variable/function shall be listed as an operand of the "asm" statement operand
or, in the case of top-level "asm" statements the attribute "used" shall be used on the declaration.
* Static functions now can use non-standard passing conventions that may break "asm" statements calling
functions directly. Again, attribute "used" will prevent this behavior.
As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future
releases of GCC.
Enabled at levels -O, -O2, -O3, -Os.
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo
register. This allows the register allocation pass to operate on pseudos directly, but also strengthens
several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, how-
ever, make debugging impossible, since variables will no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents whole program being compiled. All public functions and
variables with the exception of "main" and those merged by attribute "externally_visible" become static
functions and in a affect gets more aggressively optimized by interprocedural optimizers. While this
option is equivalent to proper use of "static" keyword for programs consisting of single file, in combina-
tion with option --combine this flag can be used to compile most of smaller scale C programs since the
functions and variables become local for the whole combined compilation unit, not for the single source
file itself.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, we perform a copy-propagation
pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-generate
Enable options usually used for instrumenting application to produce profile useful for later recompilation
with profile feedback based optimization. You must use -fprofile-generate both when compiling and when
linking your program.
The following options are enabled: "-fprofile-arcs", "-fprofile-values", "-fvpt".
-fprofile-use
Enable profile feedback directed optimizations, and optimizations generally profitable only with profile
feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops",
"-ftracer", "-fno-loop-optimize".
The following options control compiler behavior regarding floating point arithmetic. These options trade off
between speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating point variables in registers, and inhibit other options that might change whether a
floating point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating regis-
ters (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the x86 archi-
tecture. For most programs, the excess precision does only good, but a few programs rely on the precise
definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all
pertinent intermediate computations into variables.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only, -fno-round-
ing-math, -fno-signaling-nans and fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt. A
program that relies on IEEE exceptions for math error handling may want to use this flag for speed while
maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to
consider the possibility that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and
(b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files
that change the default FPU control word or other similar optimizations.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or
+-Infs.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps
include division by zero, overflow, underflow, inexact result and invalid operation. This option implies
-fno-signaling-nans. Setting this option may allow faster code if one relies on "non-stop" IEEE arith-
metic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default floating point rounding behavior. This is
round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic
truncations. This option should be specified for programs that change the FP rounding mode dynamically, or
that may be executed with a non-default rounding mode. This option disables constant folding of floating
point expressions at compile-time (which may be affected by rounding mode) and arithmetic transformations
that are unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are
affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99’s
"FENV_ACCESS" pragma. This command line option will be used to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point opera-
tions. Setting this option disables optimizations that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect
signaling NaN behavior.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of implicitly converting it to double
precision constant.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex divi-
sion. The default is -fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the
option applies to all languages.
The following options control optimizations that may improve performance, but are not enabled by any -O
options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-prob-
abilities, to improve optimizations based on the number of times each branch was taken. When the program
compiled with -fprofile-arcs exits it saves arc execution counts to a file called sourcename.gcda for each
source file The information in this data file is very dependent on the structure of the generated code, so
you must use the same source code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be
used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing
which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is
taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program
is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and adds
REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about values
of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based
on them. Currently the optimizations include specialization of division operation using the knowledge
about the value of the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register
allocation. This optimization will most benefit processors with lots of registers. Depending on the debug
information format adopted by the target, however, it can make debugging impossible, since variables will
no longer stay in a "home register".
Enabled by default with -funroll-loops.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of
the function allowing other optimizations to do better job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -fun-
roll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually
makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
-fpeel-loops
Peels the loops for that there is enough information that they do not roll much (from profile feedback).
It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of itera-
tions).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the new loop optimizer. Enabled at level -O1
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches
(modified according to result of the condition).
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of
loops that access large arrays.
Disabled at level -Os.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports arbitrary
sections. The name of the function or the name of the data item determines the section’s name in the out-
put file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in
the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2
have linkers with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so. When you specify these options,
the assembler and linker will create larger object and executable files and will also be slower. You will
not be able to use "gprof" on all systems if you specify this option and you may have problems with debug-
ging if you specify both this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of target
registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-
block scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don’t reuse branch target registers in within any
basic block.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a
guard variable to functions with vulnerable objects. This includes functions that call alloca, and func-
tions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then
checked when the function exits. If a guard check fails, an error message is printed and the program
exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For exam-
ple, GCC will not inline functions that contain more that a certain number of instructions. You can con-
trol some of these constants on the command-line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler,
and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are given in the following table:
salias-max-implicit-fields
The maximum number of fields in a variable without direct structure accesses for which structure alias-
ing will consider trying to track each field. The default is 5
sra-max-structure-size
The maximum structure size, in bytes, at which the scalar replacement of aggregates (SRA) optimization
will perform block copies. The default value, 0, implies that GCC will select the most appropriate
size itself.
sra-field-structure-ratio
The threshold ratio (as a percentage) between instantiated fields and the complete structure size. We
say that if the ratio of the number of bytes in instantiated fields to the number of bytes in the com-
plete structure exceeds this parameter, then block copies are not used. The default is 75.
max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjump-
ing is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive
optimization, making the compile time increase with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions which must be matched at the end of two blocks before crossjumping
will be performed on them. This value is ignored in the case where all instructions in the block being
crossjumped from are matched. The default value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is
relative to a jump instruction. The default value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid
O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and
unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than
max-goto-duplication-insns are unfactored. The default value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a delay slot.
If more than this arbitrary number of instructions is searched, the time savings from filling the delay
slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making
the compile time increase with probably small improvement in executable run time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when searching for a
block with valid live register information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compile time. This parameter should be removed when the delay
slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated in order to perform the global common
subexpression elimination optimization. If more memory than specified is required, the optimization
will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run. The default is 1.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow before flushing the current state and
starting over. Large functions with few branches or calls can create excessively large lists which
needlessly consume memory and resources.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This number sets the maximum number of
instructions (counted in GCC’s internal representation) in a single function that the tree inliner will
consider for inlining. This only affects functions declared inline and methods implemented in a class
declaration (C++). The default value is 450.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be con-
sidered for inlining by the compiler will be investigated. To those functions, a different (more
restrictive) limit compared to functions declared inline can be applied. The default value is 90.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after inlining
inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid
extreme compilation time caused by non-linear algorithms used by the backend. This parameter is
ignored when -funit-at-a-time is not used. The default value is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored
when -funit-at-a-time is not used. The default value is 100 which limits large function growth to 2.0
times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units larger than this limit
is limited by --param inline-unit-growth. For small units this might be too tight (consider unit con-
sisting of function A that is inline and B that just calls A three time. If B is small relative to A,
the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of
small inlininable functions however the overall unit growth limit is needed to avoid exponential explo-
sion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before
aplying --param inline-unit-growth. The default is 10000
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored
when -funit-at-a-time is not used. The default value is 50 which limits unit growth to 1.5 times the
original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow
into by performing recursive inlining.
For functions declared inline --param max-inline-insns-recursive is taken into account. For function
not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-insns-recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive inlining.
For functions declared inline --param max-inline-recursive-depth is taken into account. For function
not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-recursive-depth-auto is used. The default value is 450.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt for
function having little recursion depth by increasing the prologue size or complexity of function body
to other optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed
from probability that function will recurse via given call expression. This parameter limits inlining
only to call expression whose probability exceeds given threshold (in percents). The default value is
10.
inline-call-cost
Specify cost of call instruction relative to simple arithmetics operations (having cost of 1).
Increasing this cost disqualifies inlining of non-leaf functions and at the same time increases size of
leaf function that is believed to reduce function size by being inlined. In effect it increases amount
of inlining for code having large abstraction penalty (many functions that just pass the arguments to
other functions) and decrease inlining for code with low abstraction penalty. The default value is 16.
max-unrolled-insns
The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is
unrolled, it determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop should have
if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is
unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is
peeled, it determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables below that all candidates are considered for each
use in induction variable optimizations. Only the most relevant candidates are considered if there are
more candidates, to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs
from the set during its optimization when a new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the ana-
lyzer.
vect-max-version-checks
The maximum number of runtime checks that can be performed when doing loop versioning in the vector-
izer. See option ftree-vect-loop-version for more information.
max-iterations-to-track
The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of
the loop tries to evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in program given basic block needs
to have to be considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic block in function given basic block
needs to have to be considered hot
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where function
contain single loop with known bound and other loop with unknown. We predict the known number of iter-
ations correctly, while the unknown number of iterations average to roughly 10. This means that the
loop without bounds would appear artificially cold relative to the other one.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is
covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real pro-
files (as opposed to statically estimated ones) are much less balanced allowing the threshold to be
larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as
most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher val-
ues than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback
and one for compilation without. The value for compilation with profile feedback needs to be more con-
servative (higher) in order to make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers. The default is 10.
max-cse-insns
The maximum instructions CSE process before flushing. The default is 1000.
global-var-threshold
Counts the number of function calls (n) and the number of call-clobbered variables (v). If nxv is
larger than this limit, a single artificial variable will be created to represent all the call-clob-
bered variables at function call sites. This artificial variable will then be made to alias every
call-clobbered variable. (done as "int * size_t" on the host machine; beware overflow).
max-aliased-vops
Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping
heuristic. Alias grouping reduces compile times and memory consumption needed for aliasing at the
expense of precision loss in alias information.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum
percentage by which the garbage collector’s heap should be allowed to expand between collections. Tun-
ing this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is
available, the notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC
is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this
parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is
extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector’s heap before it begins bothering to collect garbage. The first
collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning
this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to ensure that RLIMIT_DATA or
RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound
is used. Setting this parameter very large effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register. Increasing val-
ues mean more aggressive optimization, making the compile time increase with probably slightly better
performance. The default value is 100.
max-cselib-memory-location
The maximum number of memory locations cselib should take into account. Increasing values mean more
aggressive optimization, making the compile time increase with probably slightly better performance.
The default value is 500.
max-flow-memory-location
Similar as max-cselib-memory-location but for dataflow liveness. The default value is 100.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code
on its destination. Code is duplicated when its estimated size is smaller than this value multiplied
by the estimated size of unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to
higher values than reorder-block-duplicate since information about the hot spots is more accurate.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider at any given time
during the first scheduling pass. Increasing values mean more thorough searches, making the compila-
tion time increase with probably little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling. The default value
is 10.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling. The default value
is 100.
min-sched-prob
The minimum probability of reaching a source block for interblock speculative scheduling. The default
value is 40.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a
pseudo register as last known value of that register. The default is 10000.
integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler’s memory usage and
increasing its speed. This sets the maximum value of a shared integer constant’s. The default value
is 256.
min-virtual-mappings
Specifies the minimum number of virtual mappings in the incremental SSA updater that should be regis-
tered to trigger the virtual mappings heuristic defined by virtual-mappings-ratio. The default value
is 100.
virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols
to be updated, then the incremental SSA updater switches to a full update for those symbols. The
default ratio is 3.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when -fstack-pro-
tection is used.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure we will treat in a field sensitive manner during pointer analy-
sis.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together
with -E because they cause the preprocessor output to be unsuitable for actual compilation.
You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor.
If option contains commas, it is split into multiple options at the commas. However, many options are mod-
ified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp
forcibly bypasses this phase. The preprocessor’s direct interface is undocumented and subject to change,
so whenever possible you should avoid using -Wp and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor
options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the
option and once for the argument.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase three
in a #define directive. In particular, the definition will be truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell’s
quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding
parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh, -D’name(args...)=definition’ works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and
-include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain
defined.
-I dir
Add the directory dir to the list of directories to be searched for header files. Directories named by -I
are searched before the standard system include directories. If the directory dir is a standard system
include directory, the option is ignored to ensure that the default search order for system directories and
the special treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc
has a different interpretation of a second non-option argument, so you must use -o to specify the output
file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtri-
graphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions.
Note that many of the preprocessor’s warnings are on by default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears
in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form
an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends. There-
fore, only trigraphs that would form escaped newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs
-Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and problematic constructs which should be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined.
Such identifiers are replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested
for existence at least once. The preprocessor will also warn if the macro has not been used at the time it
is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned
about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as
unused. To avoid the warning in such a case, you might improve the scope of the macro’s definition by, for
example, moving it into the first skipped block. Alternatively, you could provide a dummy use with some-
thing like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This warning is on
by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code,
therefore suppressed. If you are responsible for the system library, you may want to see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since
they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes manda-
tory diagnostics that GCC issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependen-
cies of the main source file. The preprocessor outputs one make rule containing the object file name for
that source file, a colon, and the names of all the included files, including those coming from -include or
-imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of the source
file with any suffix replaced with object file suffix. If there are many included files then the rule is
split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor’s debug output, such as -dM. To avoid mixing such debug
output with the dependency rules you should explicitly specify the dependency output file with -MF, or use
an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output
stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header files that
are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself
determine whether that header will appear in -MM dependency output. This is a slight change in semantics
from GCC versions 3.0 and earlier.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files
are generated files and adds them to the dependency list without raising an error. The dependency filename
is taken directly from the "#include" directive without prepending any path. -MG also suppresses prepro-
cessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each
to depend on nothing. These dummy rules work around errors make gives if you remove header files without
updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main
input file, including any path, deletes any file suffix such as .c, and appends the platform’s usual object
suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you
can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT ’$(objpfx)foo.o’ might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ ’$(objpfx)foo.o’ gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise
it take the basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file
(but @pxref{dashMF,,-MF}), but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the com-
pilation process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to also list the files
from the precompiled header’s dependencies. If not specified only the precompiled header would be listed
and not the files that were used to create it because those files are not consulted when a precompiled
header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma
GCC pch_preprocess """ in the output to mark the place where the precompiled header was found,
and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input
to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file
is available in a different location. The filename may be absolute or it may be relative to GCC’s current
directory.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards con-
formance or extensions; it merely selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will
treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards
conformance level. This option has been removed, because it conflicts with the -l option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; oth-
ers may be added in the future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I- Split the include path. Any directories specified with -I options before -I- are searched only for headers
requested with "#include "file""; they are not searched for "#include". If additional directories
are specified with -I options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search direc-
tory for "#include "file"". This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the directories you have specified
with -I options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the other
standard directories. (This option is used when building the C++ library.)
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the
first directory searched for file is the preprocessor’s working directory instead of the directory contain-
ing the main source file. If not found there, it is searched for in the remainder of the "#include "...""
search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the command
line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines
remain defined. This allows you to acquire all the macros from a header without also processing its decla-
rations.
All files specified by -imacros are processed before all files specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard system
directories have been exhausted. dir is treated as a system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory,
you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include
search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter
would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files. See the --sysroot option for
more information.
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system directo-
ries. Mark it as a system directory, so that it gets the same special treatment as is applied to the stan-
dard system directories.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not searched for
"#include", before all directories specified by -I and before the standard system directories.
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is experimental; in a future version of GCC,
it will be enabled by default for C99 and C++.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things
like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to
the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer
for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings
or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is
ignored. The default is 8.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8. charset
can be any encoding supported by the system’s "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is UTF-32
or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any
encoding supported by the system’s "iconv" library routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the source
character set used by GCC. If the locale does not specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by either the locale or this command line option.
Currently the command line option takes precedence if there’s a conflict. charset can be any encoding sup-
ported by the system’s "iconv" library routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the compiler know the current
working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it’s present in the preprocessed input, as the directory emit-
ted as the current working directory in some debugging information formats. This option is implicitly
enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-work-
ing-directory. If the -P flag is present in the command line, this option has no effect, since no "#line"
directives are emitted whatsoever.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a
program that does not understand the column numbers, such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form
-A predicate(answer), which is still supported, because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other
characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are
silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define directives for all the macros defined during
the execution of the preprocessor, including predefined macros. This gives you a way of finding out
what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
D Like M except in two respects: it does not include the predefined macros, and it outputs both the
#define directives and the result of preprocessing. Both kinds of output go to the standard output
file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running
the preprocessor on something that is not C code, and will be sent to a program which might be confused by
the linemarkers.
-C Do not discard comments. All comments are passed through to the output file, except for comments in pro-
cessed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as
tokens in their own right. For example, comments appearing at the start of what would be a directive line
have the effect of turning that line into an ordinary source line, since the first token on the line is no
longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments contained
within macros are also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a
macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently
commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting with ??, that are defined by
ISO C to stand for single characters. For example, ??/ stands for \, so ’??/n’ is a character constant for
a newline. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the
-std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??’ ??! ??-
Replacement: [ ] { } # \ ^ │ ~
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP’s version number at the beginning of execution, and report the final form
of the include path.
-H Print the name of each header file used, in addition to other normal activities. Each name is indented to
show how deep in the #include stack it is. Precompiled header files are also printed, even if they are
found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
-version
--version
Print out GNU CPP’s version number. With one dash, proceed to preprocess as normal. With two dashes, exit
immediately.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple options at
the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler options
which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option
and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are
meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or
library. (Object files are distinguished from libraries by the linker according to the file contents.) If
linking is done, these object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used as
arguments.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a separate
argument is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes
libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named libli-
brary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with -L.
Normally the files found this way are library files---archive files whose members are object files. The
linker handles an archive file by scanning through it for members which define symbols that have so far
been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in
the usual fashion. The only difference between using an -l option and specifying a file name is that -l
surrounds library with lib and .a and searches several directories.
-lobjc
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used nor-
mally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be passed to
the linker. The standard startup files are used normally, unless -nostartfiles is used. The compiler may
generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by
entries in libc. These entry points should be supplied through some other mechanism when this option is
specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only the
libraries you specify will be passed to the linker. The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should
be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal
subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some lan-
guages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words,
when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that
you have no unresolved references to internal GCC library subroutines. (For example, __main, used to
ensure C++ constructors will be called.)
-pie
Produce a position independent executable on targets which support it. For predictable results, you must
also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions)
when you specify this option.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to
add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of
"dlopen" or to allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On other sys-
tems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all systems
support this option. For predictable results, you must also specify the same set of options that were used
to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the shared or
static version respectively. If no shared version of libgcc was built when the compiler was configured,
these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static
version. The most common of these is when the application wishes to throw and catch exceptions across dif-
ferent shared libraries. In that case, each of the libraries as well as the application itself should use
the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or
a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to
do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be
linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or
a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into
shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking
with the shared version of libgcc, linking with the static version of libgcc by default. This allows
exceptions to propagate through such shared libraries, without incurring relocation costs at library load
time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using
the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared libgcc.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved references
(unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this
option.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options which
GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the option and
once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker
definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire
string as a single argument, which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options at
the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u
multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This can be
used to override a system header file, substituting your own version, since these directories are searched
before the system header file directories. However, you should not use this option to add directories that
contain vendor-supplied system header files (use -isystem for that). If you use more than one -I option,
the directories are scanned in left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I,
the -I option will be ignored. The directory will still be searched but as a system directory at its nor-
mal position in the system include chain. This is to ensure that GCC’s procedure to fix buggy system head-
ers and the ordering for the include_next directive are not inadvertently changed. If you really need to
change the search order for system directories, use the -nostdinc and/or -isystem options.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header files only for the
case of #include "file"; they are not searched for #include, otherwise just like -I.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the com-
piler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a
prefix for each program it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not
found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and
/usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name
is searched for using the directories specified in your PATH environment variable.
The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it
will add a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the
compiler translates these options into -L options for the linker. They also apply to includes files in the
preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In
this case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not
found there, the two standard prefixes above are tried, and that is all. The file is left out of the link
if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9,
then it will be replaced by [dir/]include. This is to help with boot-strapping the compiler.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults that
the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than
one -specs=file can be specified on the command line, and they are processed in order, from left to right.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler would nor-
mally search for headers in /usr/include and libraries in /usr/lib, it will instead search dir/usr/include
and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option will apply to libraries,
but the -isysroot option will apply to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker
does not support this option, the header file aspect of --sysroot will still work, but the library aspect
will not.
-I- This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove
the -I-. Any directories you specify with -I options before the -I- option are searched only for the case
of #include "file"; they are not searched for #include.
If additional directories are specified with -I options after the -I-, these directories are searched for
all #include directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input file came
from) as the first search directory for #include "file". There is no way to override this effect of -I-.
With -I. you can specify searching the directory which was current when the compiler was invoked. That is
not exactly the same as what the preprocessor does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc
are independent.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or-gcc when cross-compiling, or
-gcc- to run a version other than the one that was installed last. Sometimes this is incon-
venient, so GCC provides options that will switch to another cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type when configuring GCC as a
cross-compiler. For example, if a cross-compiler was configured with configure arm-elf, meaning to compile
for an arm processor with elf binaries, then you would specify -b arm-elf to run that cross compiler.
Because there are other options beginning with -b, the configuration must contain a hyphen.
-V version
The argument version specifies which version of GCC to run. This is useful when multiple versions are
installed. For example, version might be 4.0, meaning to run GCC version 4.0.
The -V and -b options work by running the-gcc- executable, so there’s no real reason to use
them if you can just run that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed compilers for completely
different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with -m, to choose
among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A
single installed version of the compiler can compile for any model or configuration, according to the options
specified.
Some configurations of the compiler also support additional special options, usually for compatibility with
other compilers on the same platform.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
-mmangle-cpu
Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC
variants with different instruction and register set characteristics. This flag prevents code compiled for
one cpu to be linked with code compiled for another. No facility exists for handling variants that are
"almost identical". This is an all or nothing option.
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants
support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-section, data-section, and readonly-data-section respec-
tively by default. This can be overridden with the "section" attribute.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwm-
mxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if
this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with
this option will cause the stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code which supports calling between the ARM and Thumb instruction sets. Without this option the
two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork,
since slightly larger code is generated when -mthumb-interwork is specified.
-mno-sched-prolog
Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the
instructions in the function’s body. This means that all functions will start with a recognizable set of
instructions (or in fact one of a choice from a small set of different function prologues), and this infor-
mation can be used to locate the start if functions inside an executable piece of code. The default is
-msched-prolog.
-mhard-float
Generate output containing floating point instructions. This is the default.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all ARM targets. Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you need to compile libgcc.a, the library that comes
with GCC, with -msoft-float in order for this to work.
-mfloat-abi=name
Specifies which ABI to use for floating point values. Permissible values are: soft, softfp and hard.
soft and hard are equivalent to -msoft-float and -mhard-float respectively. softfp allows the generation
of floating point instructions, but still uses the soft-float calling conventions.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard configu-
rations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a little-
endian processor.
-mwords-little-endian
This option only applies when generating code for big-endian processors. Generate code for a little-endian
word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option
should only be used if you require compatibility with code for big-endian ARM processors generated by ver-
sions of the compiler prior to 2.8.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instruc-
tions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710,
arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm, strongarm110, strongarm1100,
arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore,
mpcorenovfp, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312.
-mtune=name
This option is very similar to the -mcpu= option, except that instead of specifying the actual target pro-
cessor type, and hence restricting which instructions can be used, it specifies that GCC should tune the
performance of the code as if the target were of the type specified in this option, but still choosing the
instructions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementa-
tions better performance can be obtained by using this option.
-march=name
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. This option can be used in conjunction with or
instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5,
armv5t, armv5te, armv6, armv6j, iwmmxt, ep9312.
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating point hardware (or hardware emulation) is available on the target. Permissi-
ble names are: fpa, fpe2, fpe3, maverick, vfp. -mfp and -mfpe are synonyms for -mfpu=fpenumber, for com-
patibility with older versions of GCC.
If -msoft-float is specified this specifies the format of floating point values.
-mstructure-size-boundary=n
The size of all structures and unions will be rounded up to a multiple of the number of bits set by this
option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the
COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI sup-
ports it.
Specifying the larger number can produce faster, more efficient code, but can also increase the size of the
program. Different values are potentially incompatible. Code compiled with one value cannot necessarily
expect to work with code or libraries compiled with another value, if they exchange information using
structures or unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It will be executed if the
function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register
and then performing a subroutine call on this register. This switch is needed if the target function will
lie outside of the 64 megabyte addressing range of the offset based version of subroutine call instruction.
Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic is
that static functions, functions which have the short-call attribute, functions that are inside the scope
of a #pragma no_long_calls directive and functions whose definitions have already been compiled within the
current compilation unit, will not be turned into long calls. The exception to this rule is that weak
function definitions, functions with the long-call attribute or the section attribute, and functions that
are within the scope of a #pragma long_calls directive, will always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as
will placing the function calls within the scope of a #pragma long_calls_off directive. Note these
switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each
function. The run-time system is responsible for initializing this register with an appropriate value
before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled,
when R9 is used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruc-
tion combinations. This option is only valid if the -mcpu=ep9312 option has been used to enable generation
of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by
default, since the problem is only present in older Maverick implementations. The default can be re-
enabled by use of the -mno-cirrus-fix-invalid-insns switch.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The gen-
erated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace
function then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function
name embedded immediately preceding this location and has length "((pc[-3]) & 0xff000000)".
-mthumb
Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions.
(A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A
leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which
switches to Thumb mode before executing the rest of the function. This allows these functions to be called
from non-interworking code.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether
the target code has been compiled for interworking or not. There is a small overhead in the cost of exe-
cuting a function pointer if this option is enabled.
-mtp=name
Specify the access model for the thread local storage pointer. The valid models are soft, which generates
calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the
arm6k architecture), and auto, which uses the best available method for the selected processor. The
default setting is auto.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler pro-
grams (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types:
at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types:
atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8,
atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types:
atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a symbol or numeric value, __stack is the default.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size will be smaller.
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate subroutines. Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes increase code size.
-mtiny-stack
Change only the low 8 bits of the stack pointer.
-mint8
Assume int to be 8 bit integer. This affects the sizes of all types: A char will be 1 byte, an int will be
1 byte, an long will be 2 bytes and long long will be 4 bytes. Please note that this option does not com-
ply to the C standards, but it will provide you with smaller code size.
Blackfin Options
-momit-leaf-frame-pointer
Don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set
up and restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler will ensure that the generated code does not contain speculative loads after
jump instructions. This option is enabled by default.
-mno-specld-anomaly
Don’t generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC instructions
too soon after conditional branches. This option is enabled by default.
-mno-csync-anomaly
Don’t generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a condi-
tional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the
low 64k of memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place
and shared libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn’t assume ID based shared libraries are being used. This is the default.
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0
will generate more compact code, specifying other values will force the allocation of that number to the
current library but is no more space or time efficient than omitting this option.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register
and then performing a subroutine call on this register. This switch is needed if the target function will
lie outside of the 24 bit addressing range of the offset based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior.
Note these switches have no effect on how the compiler generates code to handle function calls via function
pointers.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the
default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The choices for architecture-type are the same as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-melinux-stacksize=n
Only available with the cris-axis-aout target. Arranges for indications in the program to the kernel
loader that the stack of the program should be set to n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is
active by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the
effect to turn off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly
file.
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test instructions
before use of condition codes.
-mno-side-effects
Do not emit instructions with side-effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and con-
stants to be aligned for the maximum single data access size for the chosen CPU model. The default is to
arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable
data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue that sets up the stack-frame are
omitted and no return instructions or return sequences are generated in the code. Use this option only
together with visual inspection of the compiled code: no warnings or errors are generated when call-saved
registers must be saved, or storage for local variable needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don’t generate (do generate) instruction sequences that load addresses for functions
from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The
default is -mgotplt.
-maout
Legacy no-op option only recognized with the cris-axis-aout target.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files
and instruction set for -march=v8.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output func-
tions from a simulator library. Code, initialized data and zero-initialized data are allocated consecu-
tively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at
0x80000000.
CRX Options
These options are defined specifically for the CRX ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mpush-args
Push instructions will be used to pass outgoing arguments when functions are called. Enabled by default.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it will create an object file for the single architecture
that it was built to target. Apple’s GCC on Darwin does create "fat" files if multiple -arch options are used;
it does so by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the
ISA that GCC is targetting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override
this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, will only per-
mit instructions to be used that are valid for the subtype of the file it is generating, so you cannot put
64-bit instructions in an ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail and
print an error if asked to create a shared library with a less restrictive subtype than its input files (for
instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, will qui-
etly give the executable the most restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header files.
These directories are interleaved with those specified by -I options and are scanned in a left-to-right
order.
A framework directory is a directory with frameworks in it. A framework is a directory with a "Headers"
and/or "PrivateHeaders" directory contained directly in it that ends in ".framework". The name of a frame-
work is the name of this directory excluding the ".framework". Headers associated with the framework are
found in one of those two directories, with "Headers" being searched first. A subframework is a framework
directory that is in a framework’s "Frameworks" directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two
subframeworks are siblings if they occur in the same framework. A subframework should not have the same
name as a framework, a warning will be issued if this is violated. Currently a subframework cannot have
subframeworks, in the future, the mechanism may be extended to support this. The standard frameworks can
be found in "/System/Library/Frameworks" and "/Library/Frameworks". An example include looks like
"#include", where Framework denotes the name of the framework and header.h is found in
the "PrivateHeaders" or "Headers" directory.
-gused
Emit debugging information for symbols that are used. For STABS debugging format, this enables -felimi-
nate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical values of version
include 10.1, 10.2, and 10.3.9.
The default for this option is to make choices that seem to be most useful.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for
Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Using this switch may require recompiling all other modules in a program,
including system libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turn around development. Needed to enable gdb to dynamically load ".o"
files into already running programs. -findirect-data and -ffix-and-continue are provided for backwards
compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when
the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will be loading the build output file being linked. See man
ld(1) for more information.
-dynamiclib
When passed this option, GCC will produce a dynamic library instead of an executable when linking, using
the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC’s output file to have the ALL subtype, instead of one controlled by the -mcpu or -march
option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float
is specified, functions in libgcc.a will be used to perform floating-point operations. Unless they are
replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point operations. If you are compiling for an
Alpha without floating-point operations, you must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point reg-
isters.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float.
If the floating-point register set is not used, floating point operands are passed in integer registers as
if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard
calling sequence, so any function with a floating-point argument or return value called by code compiled
with -mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore,
any floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly
compliant with the IEEE floating point standard. However, for full compliance, software assistance is
required. This option generates code fully IEEE compliant code except that the inexact-flag is not main-
tained (see below). If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during com-
pilation. The resulting code is less efficient but is able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this
option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option
causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may
execute significantly slower than the code generated by default. Since there is very little code that
depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this option
-fptm trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot be dis-
abled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like su, but the instructions are marked to be safe for software completion (see Alpha architecture
manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-
mode can be one of:
n Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or
towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture
reference manual) controls the rounding mode in effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round
towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating point traps are imprecise. This means without software assistance it
is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC
can generate code that can assist operating system trap handlers in determining the exact location that
caused a floating point trap. Depending on the requirements of an application, different levels of preci-
sions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which program
caused a floating point exception.
f Function precision. The trap handler can determine the function that caused a floating point excep-
tion.
i Instruction precision. The trap handler can determine the exact instruction that caused a floating
point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you also
specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit
the line .eflag 48 in the function prologue of the generated assembly file. Under DEC Unix, this has the
effect that IEEE-conformant math library routines will be linked in.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants
in two or three instructions. If it cannot, it will output the constant as a literal and generate code to
load it from the data segment at runtime.
Use this option to require GCC to construct all integer constants using code, even if it takes more
instructions (the maximum is six).
You would typically use this option to build a shared library dynamic loader. Itself a shared library, it
must relocate itself in memory before it can find the variables and constants in its own data segment.
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by the GNU
assembler -mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The
default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the
CPU on which GCC was built if none was specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating point arithmetic instead of IEEE single and
double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of
these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new
syntax that allows the compiler to explicitly mark which relocations should apply to which instructions.
This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is
built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When
-msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and
".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of
the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that
require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in
the program’s data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library)
fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can
assume that all local symbols share the same $gp value, and thus reduce the number of instructions required
for a function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify
either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4,
EV5 and EV6 family of processors and will choose the default values for the instruction set from the pro-
cessor you specify. If you do not specify a processor type, GCC will default to the processor on which the
compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not
changed.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the application.
This number is highly dependent on the memory access patterns used by the application and the size of the
external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the
Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that
L3 is only valid for EV5.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.
FRV Options
-mgpr-32
Only use the first 32 general purpose registers.
-mgpr-64
Use all 64 general purpose registers.
-mfpr-32
Use only the first 32 floating point registers.
-mfpr-64
Use all 64 floating point registers
-mhard-float
Use hardware instructions for floating point operations.
-msoft-float
Use library routines for floating point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating point double instructions.
-mno-double
Do not use floating point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, that uses function descriptors to represent pointers to functions. Without any
PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are
within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32
bits.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has
no effect without -mfdpic. It’s enabled by default if optimizing for speed and compiling for shared
libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the
command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections.
It’s enabled by default, except for -fpic or -fpie: even though it may help make the global offset table
smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which
may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so
it’s more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It’s implied by -mlibrary-pic, as well as by -fPIC and
-fpic without -mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This
option is enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the functions
to be placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting nops into the previous packet. This option only has
an effect when VLIW packing is enabled. It doesn’t create new packets; it merely adds nops to existing
ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "││" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "││" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler generated code. It is enabled by
default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500,
fr450, fr405, fr400, fr300 and simple.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and
H8S is to align longs and floats on 4 byte boundaries. -malign-300 causes them to be aligned on 2 byte
boundaries. This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1
for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine
the proper architecture option for your machine. Code compiled for lower numbered architectures will run
on higher numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about
out of range branches within a switch table.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for
the function call to be the target of the conditional jump.
-mdisable-fpregs
Prevent floating point registers from being used in any manner. This is necessary for compiling kernels
which perform lazy context switching of floating point registers. If you use this option and attempt to
perform floating point operations, the compiler will abort.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when com-
piling MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect
calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which performs
faster indirect calls.
This option will not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the reg-
ister allocator can not use. This is useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is
equivalent to the +k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also
triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some
programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all HPPA targets. Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded target hppa1.1-*-pro does provide software floating
point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you need to compile libgcc.a, the library that comes
with GCC, with -msoft-float in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines,
"__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX
and HI-UX.
-mgnu-ld
Use GNU ld specific options. This passes -shared to ld when building a shared library. It is the default
when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not have any
affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called
is determined by the --with-ld configure option, GCC’s program search path, and finally by the user’s PATH.
The linker used by GCC can be printed using which ‘gcc -print-prog-name=ld‘. This option is only available
on the 64 bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use HP ld specific options. This passes -b to ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with
the HP linker. This option does not have any affect on which ld is called, it only changes what parameters
are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC’s pro-
gram search path, and finally by the user’s PATH. The linker used by GCC can be printed using which ‘gcc
-print-prog-name=ld‘. This option is only available on the 64 bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker gen-
erated stubs. The default is to generate long calls only when the distance from the call site to the
beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the
branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the
PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when
using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it will degrade performance. However, it may be useful
in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code
being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or
pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in
pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-
std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later.
98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10
though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for
"XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional pre-
defines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also
affects the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the
variable __xpg4_extended_mask as appropriate. Most GNU software doesn’t provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX
10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn’t an archive ver-
sion of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve
this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static
option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers gen-
erate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver
from adding these link options.
-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both
the preprocessor and linker.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of avail-
able instructions. The choices for cpu-type are:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which
your code will run, then you should use the corresponding -mtune option instead of -mtune=generic.
But, if you do not know exactly what CPU users of your application will have, then you should use this
option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore,
if you upgrade to a newer version of GCC, the code generated option will change to reflect the proces-
sors that were most common when that version of GCC was released.
There is no -march=generic option because -march indicates the instruction set the compiler can use,
and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates
the processor (or, in this case, collection of processors) for which the code is optimized.
i386
Original Intel’s i386 CPU.
i486
Intel’s i486 CPU. (No scheduling is implemented for this chip.)
i586, pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro instruction set will be used, so the
code will run on all i686 familly chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
pentium3, pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Cen-
trino notebooks.
pentium4, pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set
support.
core2
Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2, k6-3
Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
athlon, athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions support.
athlon-4, athlon-xp, athlon-mp
Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE instruction set support.
k8, opteron, athlon64, athlon-fx
AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3dNOW!,
enhanced 3dNOW! and 64-bit instruction set extensions.)
amdfam10
AMD Family 10 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2,
SSE3, SSE4A, SSE5, 3dNOW!, enhanced 3dNOW!, ABM and 64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2
IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW! instruction set support.
c3 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is implemented for this chip.)
c3-2
Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler
will not generate any code that does not run on the i386 without the -march=cpu-type option being used.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune.
Moreover, specifying -march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respec-
tively. These synonyms are deprecated.
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The choices for unit are:
387 Use the standard 387 floating point coprocessor present majority of chips and emulated otherwise. Code
compiled with this option will run almost everywhere. The temporary results are computed in 80bit pre-
cision instead of precision specified by the type resulting in slightly different results compared to
most of other chips. See -ffloat-store for more detailed description.
This is the default choice for i386 compiler.
sse Use scalar floating point instructions present in the SSE instruction set. This instruction set is
supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The
earlier version of SSE instruction set supports only single precision arithmetics, thus the double and
extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and
the future AMD x86-64 chips supports double precision arithmetics too.
For the i386 compiler, you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE exten-
sions and make this option effective. For the x86-64 compiler, these extensions are enabled by
default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical
instability problems of 387 code, but may break some existing code that expects temporaries to be
80bit.
This is the default choice for the x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This effectively double the amount of available reg-
isters and on chips with separate execution units for 387 and SSE the execution resources too. Use
this option with care, as it is still experimental, because the GCC register allocator does not model
separate functional units well resulting in instable performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or att (the default one).
Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case
where the result of a comparison is unordered.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not part
of GCC. Normally the facilities of the machine’s usual C compiler are used, but this can’t be done
directly in cross-compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation.
On machines where a function returns floating point results in the 80387 register stack, some floating
point opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU register,
even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this
option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD.
This option is overridden when -march indicates that the target cpu will always have an FPU and so the
instruction will not need emulation. As of revision 2.6.1, these instructions are not generated unless you
also use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two word boundary or a
one word boundary. Aligning "double" variables on a two word boundary will produce code that runs somewhat
faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types will be aligned dif-
ferently than the published application binary interface specifications for the 386 and will not be binary
compatible with structures in code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The i386 application binary interface specifies the
size to be 96 bits, so -m96bit-long-double is the default in 32 bit mode.
Modern architectures (Pentium and newer) would prefer "long double" to be aligned to an 8 or 16 byte bound-
ary. In arrays or structures conforming to the ABI, this would not be possible. So specifying a
-m128bit-long-double will align "long double" to a 16 byte boundary by padding the "long double" with an
additional 32 bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double"
is to be aligned on 16 byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a
"long double".
Warning: if you override the default value for your target ABI, the structures and arrays containing "long
double" variables will change their size as well as function calling convention for function taking "long
double" will be modified. Hence they will not be binary compatible with arrays or structures in code com-
piled without that switch.
-mmlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold are placed in large data section. This
value must be the same across all object linked into the binary and defaults to 65535.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the "bss" or "data" segments. -msvr3-shlib
places them into "bss". These options are meaningful only on System V Release 3.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "ret" num instruction, which pops their arguments while returning. This saves one instruc-
tion in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function
attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it
if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Nor-
mally, extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to pass
arguments, and at most 3 registers can be used. You can control this behavior for a specific function by
using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value,
including any libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can control
this behavior for a specific function by using the function attribute sseregparm.
Warning: if you use this switch then you must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-bound-
ary is not specified, the default is 4 (16 bytes or 128 bits).
On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8 byte boundary (see
-malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD
Extension (SSE) data type "__m128" may not work properly if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that
required by any value stored on the stack. Further, every function must be generated such that it keeps
the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function
compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that
libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is sen-
sitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the
preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4a
-mno-sse4a
-msse5
-mno-sse5
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2 or 3DNow! extended instruc-
tion sets. These extensions are also available as built-in functions: see X86 Built-in Functions, for
details of the functions enabled and disabled by these switches.
To have SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387 instruc-
tions), see -mfpmath=sse.
These options will enable GCC to use these extended instructions in generated code, even without -mfp-
math=sse. Applications which perform runtime CPU detection must compile separate files for each supported
architecture, using the appropriate flags. In particular, the file containing the CPU detection code
should be compiled without these options.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as
method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve perfor-
mance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments will be computed in the function
prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and
reduced stack usage when preferred stack boundary is not equal to 2. The drawback is a notable increase in
code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception handling must
compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when link-
ing, it links in a special thread helper library -lmingwthrd which cleans up per thread exception handling
data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch reduces code size and improves perfor-
mance in case the destination is already aligned, but GCC doesn’t know about it.
-minline-all-stringops
By default GCC inlines string operations only when destination is known to be aligned at least to 4 byte
boundary. This enables more inlining, increase code size, but may improve performance of code that depends
on fast memcpy, strlen and memset for short lengths.
-momit-leaf-frame-pointer
Don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set
up and restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit,
%fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is legal depends on
the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long
and pointer to 64 bits and generates code for AMD’s x86-64 architecture.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a
128-byte area beyond the location of the stack pointer that will not be modified by signal or interrupt
handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag
-mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the
address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This
model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the lower 2 GB of the address space but sym-
bols can be located anywhere in the address space. Programs can be statically or dynamically linked, but
building of shared libraries are not supported with the medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of sections.
Currently GCC does not implement this model.
-mfused-madd
-mno-fused-madd
Enable automatic generation of fused floating point multiply-add instructions if the ISA supports such
instructions. The -mfused-madd option is on by default.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don’t) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don’t) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent code,
and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don’t) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don’t) in, loc, and out register names for the stacked registers. This may make assembler
output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working around
optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling
firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating point values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating point values using the maximum throughput algorithm.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-dwarf2-asm
-mdwarf2-asm
Don’t (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when
not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop
bit. This can improve instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the reg-
ister allocator can not use. This is useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2,
and mckinley.
-mt
-pthread
Add support for multithreading using the POSIX threads library. This option sets flags for both the pre-
processor and linker. It does not affect the thread safety of object code produced by the compiler or that
of libraries supplied with it. These are HP-UX specific flags.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX spe-
cific flags.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the
M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to be
linked in which supports, for example, file I/O. You must not use this option when generating programs
that will run on real hardware; you must provide your own runtime library for whatever I/O functions are
needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC will use during code generation. These pseudo-
registers will be used like real registers, so there is a tradeoff between GCC’s ability to fit the code
into available registers, and the performance penalty of using memory instead of registers. Note that all
modules in a program must be compiled with the same value for this option. Because of that, you must not
use this option with the default runtime libraries gcc builds.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instruc-
tions to load their addresses), and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instruc-
tions to load their addresses), and assume subroutines may not be reachable with the "bl" instruction (the
compiler will generate the much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into one of .data, bss, or .rodata (unless the
"section" attribute has been specified). This is the default.
The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data
area with the "section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to reference
them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to reference
them.
-G num
Put global and static objects less than or equal to num bytes into the small data or bss sections instead
of the normal data or bss sections. The default value of num is 8. The -msdata option must be set to one
of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or
may not work; if it doesn’t the linker will give an error message---incorrect code will not be generated.
-mdebug
Makes the M32R specific code in the compiler display some statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it is 2,
then the opposite will apply.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and
15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is
_flush_cache, but a function call will only be used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for the 68000 series. The default values for these options depends on which
style of 68000 was selected when the compiler was configured; the defaults for the most common choices are
given below.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307,
68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating point. This is the default for most 68020 sys-
tems unless --nfp was specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040.
Use this option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on
the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333,
68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X "coldfire" family cpu. This is the default when the compiler is configured for
520X-based systems.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code which can run
relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68040.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code which can run
relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68060.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all m68k targets. Normally the facilities of the machine’s usual C compiler are used, but
this can’t be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded targets m68k-*-aout and m68k-*-coff do provide soft-
ware floating point support.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack are
also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
-mnobitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use
a configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "rtd" instruction, which pops their arguments while returning. This saves one instruction
in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you
need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Nor-
mally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not
by the 68000 or 5200.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a
32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit
boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC will align structures containing the above types differ-
ently than most published application binary interface specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At
present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is
not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references will be handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text seg-
ment. This allows for execute in place in an environment without virtual memory management. This option
implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place
and shared libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn’t assume ID based shared libraries are being used. This is the default.
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0
will generate more compact code, specifying other values will force the allocation of that number to the
current library but is no more space or time efficient than omitting this option.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these
options depends on which style of microcontroller was selected when the compiler was configured; the defaults
for the most common choices are given below.
-m6811
-m68hc11
Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based sys-
tems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based sys-
tems.
-m68S12
-m68hcs12
Generate output for a 68HCS12.
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the
"call" instruction to call a function and the "rtc" instruction for returning.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is
32. Using more pseudo-soft register may or may not result in better code depending on the program. The
default is 4 for 68HC11 and 2 for 68HC12.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a four byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little endian target.
-m210
-m340
Generate code for the 210 processor.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
-march=arch
Generate code that will run on arch, which can be the name of a generic MIPS ISA, or the name of a particu-
lar processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, and mips64. The processor
names are: 4kc, 4km, 4kp, 5kc, 5kf, 20kc, 24k, 24kc, 24kf, 24kx, m4k, orion, r2000, r3000, r3900, r4000,
r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300,
vr5000, vr5400 and vr5500. The special value from-abi selects the most compatible architecture for the
selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional,
and vr may be written r.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of
target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized
value of _MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to "r2000" and define the macro
_MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have the full
prefix and will not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architec-
ture (either "mips1" or "mips3"). It names the default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the
perceived cost of arithmetic operations. The list of arch values is the same as for -march.
When this option is not used, GCC will optimize for the processor specified by -march. By using -march and
-mtune together, it is possible to generate code that will run on a family of processors, but optimize the
code for one particular member of that family.
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones
described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or MIPS64 architecture, it will make
use of the MIPS16e ASE.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a
64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see.
-mabicalls
-mno-abicalls
Generate (do not generate) SVR4-style position-independent code. -mabicalls is the default for SVR4-based
systems.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it
will only work if the GOT is smaller than about 64k. Anything larger will cause the linker to report an
error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs,
although it will also be less efficient, since it will take three instructions to fetch the value of a
global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use
-mxgot when a single object file accesses more than 64k’s worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using library
calls instead.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the default.
-mdsp
-mno-dsp
Use (do not use) the MIPS DSP ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option can only be used when generating 64-bit code and requires hardware floating-point support to
be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the
pointer size is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit
"int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers
are the same size as "long"s, or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is
useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster
references to symbolic addresses.
-G num
Put global and static items less than or equal to num bytes into the small data or bss section instead of
the normal data or bss section. This allows the data to be accessed using a single instruction.
All modules should be compiled with the same -G num value.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data section if
possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of
RAM required when executing, and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only meaningful in con-
junction with -membedded-data.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative,
selected by -mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation opera-
tors.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero. The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break instruction.
Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of
the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdi-
vide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the
use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using
--with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which
allows GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but
requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating point multiply-accumulate instructions, when they are available. The
default is -mfused-madd.
When multiply-accumulate instructions are used, the intermediate product is calculated to infinite preci-
sion and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when
assembling them.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting
an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplica-
tion is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a
jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting
an integer division.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions
are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a nop to be inserted between certain pairs of instructions. These errata are
handled by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather than
by GCC, although GCC will avoid using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and "dmac-
chi" instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and
"F2" floating point errata.)
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If called,
the function must take the same arguments as the common "_flush_func()", that is, the address of the memory
range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both
caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or
__cpu_flush.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected architec-
ture. By default, Branch Likely instructions may be generated if they are supported by the selected archi-
tecture. An exception is for the MIPS32 and MIPS64 architectures and processors which implement those
architectures; for those, Branch Likely instructions will not be generated by default because the MIPS32
and MIPS64 architectures specifically deprecate their use.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we schedule FP instructions for some proces-
sors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use
both FP pipes. Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one
is 8-byte aligned. When this option is enabled, GCC will align pairs of instructions that it thinks should
execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the
expense of making it bigger. It is enabled by default at optimization level -O3.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the
size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as
registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instruc-
tions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically
valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX"
assembly directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable
branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically generates a
request (handled by the assembler and the linker) for a constant to be set up in a global register. The
register is used for one or more base address requests within the range 0 to 255 from the value held in the
register. The generally leads to short and fast code, but the number of different data items that can be
addressed is limited. This means that a program that uses lots of static data may require
-mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33
Generate code which uses features specific to the AM33 processor.
-mno-am33
Do not generate code which uses features specific to the AM33 processor. This is the default.
-mreturn-pointer-on-d0
When generating a function which returns a pointer, return the pointer in both "a0" and "d0". Otherwise,
the pointer is returned only in a0, and attempts to call such functions without a prototype would result in
errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and
absolute memory addresses. This option only has an effect when used on the command line for the final link
step.
This option makes symbolic debugging impossible.
MT Options
These -m options are defined for Morpho MT architectures:
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor
type. Possible values for cpu-type are ms1-64-001, ms1-16-002, ms1-16-003 and ms2.
When this option is not used, the default is -march=ms1-16-002.
-mbacc
Use byte loads and stores when generating code.
-mno-bacc
Do not use byte loads and stores when generating code.
-msim
Use simulator runtime
-mno-crt0
Do not link in the C run-time initialization object file crti.o. Other run-time initialization and termi-
nation files such as startup.o and exit.o are still included on the linker command line.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not sup-
ported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-msplit
Generate code for a system with split I&D.
-mno-split
Generate code for a system without split I&D. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than
pdp11-*-bsd.
PowerPC Options
These are listed under
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mfprnd
-mno-fprnd
-mmfpgpr
-mno-mfpgpr
GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction
set are those instructions supported by the rios chip set used in the original RS/6000 systems and the Pow-
erPC instruction set is the architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the
IBM 4xx, 6xx, and follow-on microprocessors.
Neither architecture is a subset of the other. However there is a large common subset of instructions sup-
ported by both. An MQ register is included in processors supporting the POWER architecture.
You use these options to specify which instructions are available on the processor you are using. The
default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides
the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options
listed above.
The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to
use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that
are present in the POWER2 architecture but not the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the
PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional
PowerPC architecture instructions in the General Purpose group, including floating-point square root.
Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture
instructions in the Graphics group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on
the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb
option allows GCC to generate the popcount and double precision FP reciprocal estimate instruction imple-
mented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The
-mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ pro-
cessor and other processors that support the PowerPC V2.03 architecture. The -mmfpgpr option allows GCC to
generate the FP move to/from general purpose register instructions implemented on the POWER6X processor and
other processors that support the extended PowerPC V2.05 architecture.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full
PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of
both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying
both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of
the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the assem-
bler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assembler mnemonics
defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic;
GCC uses that mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type sometimes
overrides the value of these option. Unless you are building a cross-compiler, you should normally not
specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for
machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602,
603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, ec603e, G3, G4, G5,
power, power2, power3, power4, power5, power5+, power6, power6x, common, powerpc, powerpc64, rios, rios1,
rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code generated under this option will run on any
POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures,
and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit
PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic
processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best on that
processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options: -maltivec, -mfprnd, -mhard-float,
-mmfcrf, -mmultiple, -mnew-mnemonics, -mpopcntb, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpow-
erpc-gfxopt, -mstring, -mmfpgpr. The particular options set for any particular CPU will vary between com-
piler versions, depending on what setting seems to produce optimal code for that CPU; it doesn’t necessar-
ily reflect the actual hardware’s capabilities. If you wish to set an individual option to a particular
value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present
because AIX does not have full support for these options. You may still enable or disable them individu-
ally if you’re sure it’ll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type,
register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for
-mtune as for -mcpu. If both are specified, the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by -mtune.
-mswdiv
-mno-swdiv
Generate code to compute division as reciprocal estimate and iterative refinement, creating opportunities
for increased throughput. This feature requires: optional PowerPC Graphics instruction set for single pre-
cision and FRE instruction for double precision, assuming divides cannot generate user-visible traps, and
the domain values not include Infinities, denormals or zero denominator.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions
that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to
adjust the current ABI with AltiVec ABI enhancements.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-exec .plt and
.got sections. This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that
are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe
-mno-isel
This switch enables or disables the generation of SPE simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating point operations on the general purpose regis-
ters for architectures that support it.
The argument yes or single enables the use of single-precision floating point operations.
The argument double enables the use of single and double-precision floating point operations.
The argument no disables floating point operations on the general purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The
32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC vari-
ant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for
PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The
-mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC entry for each
unique non-automatic variable reference in your program. GCC will also place floating-point constants in
the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can
reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc
prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate
code to calculate the sum of an address and a constant at run-time instead of putting that sum into the
TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and
larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc
instead. This option causes GCC to make only one TOC entry for every file. When you specify this option,
GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure
needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32 disables the
64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass
floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addi-
tion to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly
rounded when comparing values and converting to double. Use XL symbol names for long double support rou-
tines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of
calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL com-
pilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is com-
piled without optimization. Because always storing floating-point arguments on the stack is inefficient
and rarely needed, this option is not enabled by default and only is necessary when calling subroutines
compiled by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with
special startup code to enable the application to run. The system must have PE installed in the standard
location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the
appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and
the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The
option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the stan-
dard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating point emulation
is provided if you use the -msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple word
instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC
systems. Do not use -mmultiple on little endian PowerPC systems, since those instructions do not work when
the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions
usage in little endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store string word instructions
to save multiple registers and do small block moves. These instructions are generated by default on POWER
systems, and not generated on PowerPC systems. Do not use -mstring on little endian PowerPC systems, since
those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and
PPC750 which permit the instructions usage in little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register to the
address of the calculated memory location. These instructions are generated by default. If you use
-mno-update, there is a small window between the time that the stack pointer is updated and the address of
the previous frame is stored, which means code that walks the stack frame across interrupts or signals may
get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions. These
instructions are generated by default if hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields
to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 would be
aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align, the structure would be
aligned to a 1 byte boundary and be one byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be han-
dled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a
different address at runtime. If you use -mrelocatable on any module, all objects linked together must be
compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a
different address at runtime. Modules compiled with -mrelocatable-lib can be linked with either modules
compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with the -mrelocatable
options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a
global area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The
-mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external refer-
ences are relocatable. The resulting code is suitable for applications, but not shared libraries.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during the sec-
ond scheduling pass. The argument priority takes the value 0/1/2 to assign no/highest/second-highest pri-
ority to dispatch slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during instruction scheduling.
The argument dependence_type takes one of the following values: no: no dependence is costly, all: all
dependences are costly, true_store_to_load: a true dependence from store to load is costly, store_to_load:
any dependence from store to load is costly, number: any dependence which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be used during the second scheduling pass. The argu-
ment scheme takes one of the following values: no: Don’t insert nops. pad: Pad with nops any dispatch
group which has vacant issue slots, according to the scheduler’s grouping. regroup_exact: Insert nops to
force costly dependent insns into separate groups. Insert exactly as many nops as needed to force an insn
to a new group, according to the estimated processor grouping. number: Insert nops to force costly depen-
dent insns into separate groups. Insert number nops to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March
1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default
unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec,
no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the
SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended precision long double. This is a PowerPC 32-bit SYSV ABI
option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended precision long double. This is a PowerPC 32-bit Linux ABI
option.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are prop-
erly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to
set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were
passed in the floating point registers in case the function takes a variable arguments. With -mprototype,
only calls to prototyped variable argument functions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C
libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim. configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-mwindiss
Specify that you are compiling for the WindISS simulation environment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended
relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface
(eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the
stack is aligned to an 8 byte boundary, a function "__eabi" is called to from "main" to set up the eabi
environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas.
Selecting -mno-eabi means that the stack is aligned to a 16 byte boundary, do not call an initialization
function from "main", and the -msdata option will only use "r13" to point to a single small data area. The
-meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the
.sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and static
data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and
static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which
is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section,
which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable
option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, oth-
erwise compile code the same as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section. Put
small uninitialized global and static data in the .sbss section. Do not use register "r13" to address
small data however. This is the default behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data section, and all
uninitialized data in the .bss section.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small
data or bss sections instead of the normal data or bss section. By default, num is 8. The -G num switch
is also passed to the linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output
using symbolic forms.
-mlongcall
-mno-longcall
Default to making all function calls indirectly, using a register, so that functions which reside further
than 32 megabytes (33,554,432 bytes) from the current location can be called. This setting can be overrid-
den by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these
systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do
this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit
PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" will generate "jbsr callee, L42", plus a "branch island" (glue
code). The two target addresses represent the callee and the "branch island". The Darwin/PPC linker will
prefer the first address and generate a "bl callee" if the PPC "bl" instruction will reach the callee
directly; otherwise, the linker will generate "bl L42" to call the "branch island". The "branch island" is
appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps
to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and
the Darwin linker decides whether to use or discard it.
In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate
glue.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preproces-
sor and linker.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point operations.
When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations.
When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the
default.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64bit makes the "long double" type equiva-
lent to the "double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller’s frame as backchain pointer into the callee’s stack frame.
A backchain may be needed to allow debugging using tools that do not understand DWARF-2 call frame informa-
tion. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack
frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte
register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; how-
ever, use of the backchain for debugging purposes usually requires that the whole binary is built with
-mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.
In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all
fields of the 96/160 byte register save area only for their default purpose; unused fields still take up
stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the
register save area; unused space is reused for other purposes, allowing for more efficient use of the
available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register is always saved two words below the
backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible
with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or
zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes.
Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination
of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed 64k. The default is to use the "basr" instruction
instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified,
generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit
instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is
specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with
-m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generat-
ing code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle
is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug
information.
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor
type. Possible values for cpu-type are g5, g6, z900, and z990. When generating code using the instruc-
tions available on z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of avail-
able instructions. The list of cpu-type values is the same as for -march. The default is the value used
for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating sys-
tem. This option is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile time check
it doesn’t need to be a real problem when the program runs. It is intended to identify functions which
most probably cause a stack overflow. It is useful to be used in an environment with limited stack size
e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls alloca or uses dynamically sized arrays. This is generally a bad idea
with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
These arguments always have to be used in conjunction. If they are present the s390 back end emits addi-
tional instructions in the function prologue which trigger a trap if the stack size is stack-guard bytes
above the stack-size (remember that the stack on s390 grows downward). These options are intended to be
used to help debugging stack overflow problems. The additionally emitted code causes only little overhead
and hence can also be used in production like systems without greater performance degradation. The given
values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding
64k. In order to be efficient the extra code makes the assumption that the stack starts at an address
aligned to the value given by stack-size.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn’t generate any DSP
instructions at the moment.
-mb Compile code for the processor in big endian mode.
-ml Compile code for the processor in little endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some func-
tions from the standard C library will not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by Renesas.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available. This
option is the default for all targets of the SH toolchain except for sh-symbianelf.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code. At the moment, this is equivalent to
-fno-finite-math-only. When generating 16 bit SH opcodes, getting IEEE-conforming results for comparisons
of NANs / infinities incurs extra overhead in every floating point comparison, therefore the default is set
to -ffinite-math-only.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH
ABI.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls using the Global Offset Table instead of the
Procedure Linkage Table.
-musermode
Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline.
This library function call doesn’t assume it can write to the whole memory address space. This is the
default when the target is "sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv, inv:min-
lat, inv20u, inv20l, inv:call, inv:call2, inv:fp . "fp" performs the operation in floating point. This
has a very high latency, but needs only a few instructions, so it might be a good choice if your code has
enough easily exploitable ILP to allow the compiler to schedule the floating point instructions together
with other instructions. Division by zero causes a floating point exception. "inv" uses integer opera-
tions to calculate the inverse of the divisor, and then multiplies the dividend with the inverse. This
strategy allows cse and hoisting of the inverse calculation. Division by zero calculates an unspecified
result, but does not trap. "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
have been found, or if the entire operation has been hoisted to the same place, the last stages of the
inverse calculation are intertwined with the final multiply to reduce the overall latency, at the expense
of using a few more instructions, and thus offering fewer scheduling opportunities with other code. "call"
calls a library function that usually implements the inv:minlat strategy. This gives high code density for
m5-*media-nofpu compilations. "call2" uses a different entry point of the same library function, where it
assumes that a pointer to a lookup table has already been set up, which exposes the pointer load to cse /
code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
code generation, but if the code stays unoptimized, revert to the "call", "call2", or "fp" strategies,
respectively. Note that the potentially-trapping side effect of division by zero is carried by a separate
instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the
side effect stays where it is. A recombination to fp operations or a call is not possible in that case.
"inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case that the inverse calculation
was nor separated from the multiply, they speed up division where the dividend fits into 20 bits (plus sign
where applicable), by inserting a test to skip a number of operations in this case; this test slows down
the case of larger dividends. inv20u assumes the case of a such a small dividend to be unlikely, and
inv20l assumes it to be likely.
-mdivsi3_libfunc=name
Set the name of the library function used for 32 bit signed division to name. This only affect the name
used in the call and inv:call division strategies, and the compiler will still expect the same sets of
input/output/clobbered registers as if this option was not present.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This option only has an effect if the gcc code
base supports the TARGET_ADJUST_UNROLL_MAX target hook.
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware
and/or OS implement 32 bit wrap-around semantics for the indexed addressing mode. The architecture allows
the implementation of processors with 64 bit MMU, which the OS could use to get 32 bit addressing, but
since no current hardware implementation supports this or any other way to make the indexed addressing mode
safe to use in the 32 bit ABI, the default is -mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the gettr instruction to number. The default is 2 if -mpt-fixed is in effect, 100
otherwise.
-mpt-fixed
Assume pt* instructions won’t trap. This will generally generate better scheduled code, but is unsafe on
current hardware. The current architecture definition says that ptabs and ptrel trap when the target anded
with 3 is 3. This has the unintentional effect of making it unsafe to schedule ptabs / ptrel before a
branch, or hoist it out of a loop. For example, __do_global_ctors, a part of libgcc that runs constructors
at program startup, calls functions in a list which is delimited by -1. With the -mpt-fixed option, the
ptabs will be done before testing against -1. That means that all the constructors will be run a bit
quicker, but when the loop comes to the end of the list, the program crashes because ptabs loads -1 into a
target register. Since this option is unsafe for any hardware implementing the current architecture speci-
fication, the default is -mno-pt-fixed. Unless the user specifies a specific cost with -mgettrcost,
-mno-pt-fixed also implies -mgettrcost=100; this deters register allocation using target registers for
storing ordinary integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the compiler will always be valid
to load with movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it is possible
to generate symbols that will cause ptabs / ptrel to trap. This option is only meaningful when
-mno-pt-fixed is in effect. It will then prevent cross-basic-block cse, hoisting and most scheduling of
symbol loads. The default is -mno-invalid-symbols.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI
reserves for applications. This is the default.
To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs. You should
compile libraries and system software with this option.
-mfpu
-mhard-float
Generate output containing floating point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all SPARC targets. Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide
software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you need to compile libgcc.a, the library that comes
with GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating point instructions. The
functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word float-
ing point instructions. They all invoke a trap handler for one of these instructions, and then the trap
handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower
than calling the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8 byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained in
another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment.
Specifying this option avoids some rare compatibility problems with code generated by other compilers. It
is not the default because it results in a performance loss, especially for floating point code.
-mno-faster-structs
-mfaster-structs
With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment. This enables the
use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many
"ld" and "st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus,
it’s intended only for use on targets where the developer acknowledges that their resulting code will not
be directly in line with the rules of the ABI.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when link-
ing a shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker
error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not
actually shared across processes. Instead of using -mimpure-text, you should compile all source code with
-fpic or -fPIC.
This option is only available on SunOS and Solaris.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are v7, cypress, v8, supersparc, sparclite, f930, f934, hypersparc, spar-
clite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, and niagara.
Default instruction scheduling parameters are used for values that select an architecture and not an imple-
mentation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3, niagara
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture.
With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from
V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in
SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the Super-
SPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds
the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in
SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is
the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the
Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the
integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in
SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC
SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer
and floating-point move instructions, 3 additional floating-point condition code registers and conditional
move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC
I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those
that select a particular cpu implementation. Those are cypress, supersparc, hypersparc, f930, f934, spar-
clite86x, tsc701, ultrasparc, ultrasparc3, and niagara.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global
and out registers are considered 64-bit wide. This is enabled by default on Solaris in 32-bit mode for all
SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions.
The default is -mno-vis.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-mlittle-endian
Generate code for a processor running in little-endian mode. It is only available for a few configurations
and most notably not on Solaris and Linux.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=medlow
Generate code for the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits
of memory. Programs can be statically or dynamically linked.
-mcmodel=medmid
Generate code for the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44
bits of memory, the text and data segments must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.
-mcmodel=medany
Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in mem-
ory, the text and data segments must be less than 2GB in size and the data segment must be located within
2GB of the text segment.
-mcmodel=embmedany
Generate code for the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data
segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The
global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not
supported.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047
which must be added back when making stack frame references. This is the default in 64-bit mode. Other-
wise, assume no such offset is present.
These switches are supported in addition to the above on Solaris:
-threads
Add support for multithreading using the Solaris threads library. This option sets flags for both the pre-
processor and linker. This option does not affect the thread safety of object code produced by the com-
piler or that of libraries supplied with it.
-pthreads
Add support for multithreading using the POSIX threads library. This option sets flags for both the pre-
processor and linker. This option does not affect the thread safety of object code produced by the com-
piler or that of libraries supplied with it.
-pthread
This is a synonym for -pthreads.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those
systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the
TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The small memory model assumed that all data fits into
one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing
the .bss and .data program sections. The big memory model is the default and requires reloading of the DP
register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the block count register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled
by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration
count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note that GCC
will try to reverse a loop so that it can utilize the decrement and branch instruction, but will give up if
there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can
generate slightly more efficient code, in cases where the RPTB instruction cannot be utilized.
-mdp-isr-reload
-mparanoid
Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the
data section, and restored on exit from the ISR. This should not be required unless someone has violated
the small memory model by modifying the DP register, say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee
32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed
using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are
performed inline instead of a library call.
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest inte-
ger less than or equal to the floating point value rather than to the nearest integer. Thus if the float-
ing point number is negative, the result will be incorrectly truncated an additional code is necessary to
detect and correct this case. This option can be used to disable generation of the additional code
required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping.
The RPTB construct is only used for innermost loops that do not call functions or jump across the loop
boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and
restore the RC, RS, and RE registers. This is enabled by default with -O2.
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a
single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a
RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop
count cannot be determined at compile time. Note that the repeated instruction following RPTS does not
have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands. However, since
interrupts are blocked by this instruction, it is disabled by default.
-mloop-unsigned
-mno-loop-unsigned
The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these instruc-
tions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned
there is a possibility than the 2^{31 + 1} maximum iteration count may be exceeded. This switch allows an
unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compati-
bility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures
rather than in floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are
passed in registers where possible rather than by pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by default with -O2.
-mparallel-mpy
-mno-parallel-mpy
Allow the generation of MPY││ADD and MPY││SUB parallel instructions, provided -mparallel-insns is also
specified. These instructions have tight register constraints which can pessimize the code generation of
large functions.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always
load the functions address up into a register, and call indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer
into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default
if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a
function. The external functions are slower, but use less code space if more than one function saves the
same number of registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function
options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register "ep"
points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register "gp"
points to. The small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about
out of range branches within a switch table.
-mapp-regs
This option will cause r2 and r5 to be used in the code generated by the compiler. This setting is the
default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ will
be defined if this option is used.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant __v850e__ will be defined if
this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target processor will be chosen and
the relevant __v850*__ preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is
the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the v850
architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot
handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-format.
x86-64 Options
These are listed under
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in
place of the standard "L32R" instructions. The use of "CONST16" is enabled by default only if the "L32R"
instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point
option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add
and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and
add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions do not round the intermediate result, thereby pro-
ducing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program output is not sensitive to the compiler’s ability
to combine multiply and add/subtract operations.
-mtext-section-literals
-mno-text-section-literals
Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals
in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and
it also allows the linker to combine literal pools from separate object files to remove redundant literals
and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in
order to keep them as close as possible to their references. This may be necessary for large assembly
files.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce
branch penalties at the expense of some code density. The assembler attempts to widen density instructions
to align branch targets and the instructions following call instructions. If there are not enough preced-
ing safe density instructions to align a target, no widening will be performed. The default is -mtar-
get-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which the
assembler will always align, either by widening density instructions or by inserting no-op instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless
it can determine that the target of a direct call is in the range allowed by the call instruction. This
translation typically occurs for calls to functions in other source files. Specifically, the assembler
translates a direct "CALL" instruction into an "L32R" followed by a "CALLX" instruction. The default is
-mno-longcalls. This option should be used in programs where the call target can potentially be out of
range. This option is implemented in the assembler, not the compiler, so the assembly code generated by
GCC will still show direct call instructions---look at the disassembled object code to see the actual
instructions. Note that the assembler will use an indirect call for every cross-file call, not just those
that really will be out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table
below, only one of the forms is listed---the one which is not the default. You can figure out the other form
by either removing no- or adding it.
-fbounds-check
For front-ends that support it, generate additional code to check that indices used to access arrays are
within the declared range. This is currently only supported by the Java and Fortran front-ends, where this
option defaults to true and false respectively.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement representation. This flag enables some optimizations and
disables others. This option is enabled by default for the Java front-end, as required by the Java lan-
guage specification.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this
implies GCC will generate frame unwind information for all functions, which can produce significant data
size overhead, although it does not affect execution. If you do not specify this option, GCC will enable
it by default for languages like C++ which normally require exception handling, and disable it for lan-
guages like C that do not normally require it. However, you may need to enable this option when compiling
C code that needs to interoperate properly with exception handlers written in C++. You may also wish to
disable this option if you are compiling older C++ programs that don’t use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-spe-
cific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to
throw exceptions, i.e. memory references or floating point instructions. It does not allow exceptions to
be thrown from arbitrary signal handlers such as "SIGALRM".
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the
generated code in any other way. You will normally not enable this option; instead, a language processor
that needs this handling would enable it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each
instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or
garbage collector).
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This con-
vention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files
and files compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with
the -freg-struct-return switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small structures
than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except
on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose
the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with
the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values.
Specifically, the "enum" type will be equivalent to the smallest integer type which has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a non-default application binary interface.
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int instead of the default for the target.
This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a non-default application binary interface.
-fshared-data
Requests that the data and non-"const" variables of this compilation be shared data rather than private
data. The distinction makes sense only on certain operating systems, where shared data is shared between
processes running the same program, while private data exists in one copy per process.
-fno-common
In C, allocate even uninitialized global variables in the data section of the object file, rather than gen-
erating them as common blocks. This has the effect that if the same variable is declared (without
"extern") in two different compilations, you will get an error when you link them. The only reason this
might be useful is if you wish to verify that the program will work on other systems which always work this
way.
-fno-ident
Ignore the #ident directive.
-finhibit-size-directive
Don’t output a ".size" assembler directive, or anything else that would cause trouble if the function is
split in the middle, and the two halves are placed at locations far apart in memory. This option is used
when compiling crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This option is
generally only of use to those who actually need to read the generated assembly code (perhaps while debug-
ging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two
assembler files.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target
machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic
loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile
with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no
such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the
386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and
avoiding any limit on the size of the global offset table. This option makes a difference on the m68k,
PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position independent code can be only linked
into executables. Usually these options are used when -pie GCC option will be used during linking.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code genera-
tion strategies. This option is of use in conjunction with -fpic or -fPIC for building code which forms
part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do
not require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps
as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in
the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allo-
cated for temporaries or variables that do not live across a call. Functions compiled this way will not
save and restore the register reg.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other reg-
isters that have fixed pervasive roles in the machine’s execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even for
temporaries or variables that live across a call. Functions compiled this way will save and restore the
register reg if they use it.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other reg-
isters that have fixed pervasive roles in the machine’s execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values
may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is specified
(which must be a small power of two), pack structure members according to this value, representing the max-
imum alignment (that is, objects with default alignment requirements larger than this will be output poten-
tially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-
default application binary interface.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just before
function exit, the following profiling functions will be called with the address of the current function
and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current func-
tion, so the call site information may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in
the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls
will indicate where, conceptually, the inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a function are expanded inline, this may
mean an additional expansion of code size. If you use extern inline in your C code, an addressable version
of such functions must be provided. (This is normally the case anyways, but if you get lucky and the opti-
mizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation will not
be done. This can be used, for example, for the profiling functions listed above, high-priority interrupt
routines, and any functions from which the profiling functions cannot safely be called (perhaps signal han-
dlers, if the profiling routines generate output or allocate memory).
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag
if you are running in an environment with multiple threads, but only rarely need to specify it in a single-
threaded environment since stack overflow is automatically detected on nearly all systems if there is only
one stack.
Note that this switch does not actually cause checking to be done; the operating system must do that. The
switch causes generation of code to ensure that the operating system sees the stack being extended.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register
or the address of a symbol. If the stack would grow beyond the value, a signal is raised. For most tar-
gets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal
without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias each other and may alias global stor-
age.-fargument-noalias specifies that arguments do not alias each other, but may alias global storage.-far-
gument-noalias-global specifies that arguments do not alias each other and do not alias global storage.
Each language will automatically use whatever option is required by the language standard. You should not
need to use these options yourself.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented
in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary interface. Not
all targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of "global-dynamic",
"local-dynamic", "initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".
-fvisibility=default│internal│hidden│protected
Set the default ELF image symbol visibility to the specified option---all symbols will be marked with this
unless overridden within the code. Using this feature can very substantially improve linking and load
times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent
symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.
Despite the nomenclature, "default" always means public ie; available to be linked against from outside the
shared object. "protected" and "internal" are pretty useless in real-world usage so the only other com-
monly used option will be "hidden". The default if -fvisibility isn’t specified is "default", i.e., make
every symbol public---this causes the same behavior as previous versions of GCC.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by
"How To Write Shared Libraries" by Ulrich Drepper (which can be found at per/>)---however a superior solution made possible by this option to marking things hidden when the default
is public is to make the default hidden and mark things public. This is the norm with DLL’s on Windows and
with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)"
you get almost identical semantics with identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This
works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visi-
bility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed
as part of the API interface contract and thus all new code should always specify visibility when it is not
the default ie; declarations only for use within the local DSO should always be marked explicitly as hidden
as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-doc-
umentation of the code. Note that due to ISO C++ specification requirements, operator new and operator
delete must always be of default visibility.
extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibil-
ity=hidden with no modifications. However, this means that calls to extern functions with no explicit vis-
ibility will use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC vis-
ibility to tell the compiler which extern declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception
class that will be thrown between DSOs must be explicitly marked with default visibility so that the
type_info nodes will be unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at ity>.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is spec-
ified, the compiler generates parallel code according to the OpenMP Application Program Interface v2.5
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by speci-
fying directories or prefixes to use when searching for various kinds of files. Some are used to specify other
aspects of the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take precedence
over places specified using environment variables, which in turn take precedence over those specified by the
configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information that allow GCC to work
with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has
been configured to do so. These locale categories can be set to any value supported by your installation.
A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the charac-
ter boundaries in a string; this is needed for some multibyte encodings that contain quote and escape char-
acters that would otherwise be interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise,
LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables
are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold
the output of one stage of compilation which is to be used as input to the next stage: for example, the
output of the preprocessor, which is the input to the compiler proper.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the
compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can spec-
ify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the
pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the
subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the value of "prefix" when you ran
the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header files.
For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely,
with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce
an alternate directory name. Thus, with -Bfoo/, GCC will search foo/bar where it would normally search
/usr/local/lib/bar. These alternate directories are searched first; the standard directories come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the direc-
tories thus specified when searching for subprograms, if it can’t find the subprograms using GCC_EXEC_PRE-
FIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a
native compiler, GCC tries the directories thus specified when searching for special linker files, if it
can’t find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for
ordinary libraries for the -l option (but directories specified with -L come first).
LANG
This variable is used to pass locale information to the compiler. One way in which this information is
used is to determine the character set to be used when character literals, string literals and comments are
parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values
for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as
defined by the default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable’s value is a list of directories separated by a special character, much like PATH, in which
to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at
GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it
is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given
with -I options on the command line. This environment variable is used regardless of which language is
being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if specified with -isystem, but after any paths given
with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory.
Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system
header files processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that
file, guessing the target name from the source file name. Or the value can have the form file target, in
which case the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an
optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not
ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.
BUGS
For instructions on reporting bugs, see.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-
libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply
the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is
innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info
entries for gcc, cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or, for contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documen-
tation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant
Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see
below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7)
man page.
(a) The FSF’s Front-Cover Text is:
A GNU Manual
(b) The FSF’s Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.1.2 2010-10-06 GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c│-S│-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options
as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options"
allow you to stop this process at an intermediate stage. For example, the -c option says not to run the
linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the
compiler itself. Yet other options control the assembler and linker; most of these are not documented here,
since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option is only
useful with another language (usually C++), the explanation says so explicitly. If the description for a par-
ticular option does not mention a source language, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore
multiple single-letter options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order does mat-
ter when you use several options of the same kind; for example, if you specify -L more than once, the directo-
ries are searched in the order specified.
Many options have long names starting with -f or with -W---for example, -fstrength-reduce, -Wformat and so on.
Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual
documents only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -combine -pipe -pass-exit-codes -x language -v -### --help --target-help --ver-
sion
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp
-fallow-single-precision -fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -fun-
signed-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space -ffriend-injection -fno-const-strings
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti -fstats
-ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-depre-
cated -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link -gen-decls -Was-
sign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once│every-line] -fdiagnostics-show-options
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return -Wno-attributes
-Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declara-
tions -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror-implicit-function-dec-
laration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral
-Wformat-security -Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport
-Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch
-Wlarger-than-len -Wunsafe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-ini-
tializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull
-Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type
-Wsequence-point -Wshadow -Wsign-compare -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas
-Wno-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter
-Wunused-value -Wunused-variable -Wvariadic-macros -Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n]
-fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias
-fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect
-fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-salias -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vec-
torizer-verbose=n -fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -feliminate-unused-debug-types -felimi-
nate-unused-debug-symbols -fmem-report -fprofile-arcs -frandom-seed=string -fsched-verbose=n -ftest-cover-
age -ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff
-gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -save-temps
-time
Optimization Options
-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fmudflap -fmudflapth -fmud-
flapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-tar-
get-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks -fcx-limited-range -fdata-sections -fdelayed-branch -fdelete-null-pointer-checks
-fearly-inlining -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -ffunction-sections
-fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload -floop-optimize -fcrossjumping -fif-conver-
sion -fif-conversion2 -finline-functions -finline-functions-called-once -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmodulo-sched
-fno-branch-count-reg -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants
-fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peep-
hole2 -funsafe-math-optimizations -funsafe-loop-optimizations -ffinite-math-only -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls
-fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove -frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -frerun-loop-opt -frounding-math
-fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns[=n] -fsched-stalled-insns-dep[=n]
-fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans -fsin-
gle-precision-constant -fstack-protector -fstack-protector-all -fstrength-reduce -fstrict-aliasing
-ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -fsplit-ivs-in-unroller
-funswitch-loops -fvariable-expansion-in-unroller -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize
-ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse
-ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize
-ftree-vect-loop-version -ftree-salias -fweb -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop
-ftree-vrp -funit-at-a-time -fwhole-program --param name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir
-include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -isysroot
dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro
-Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static
-static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-
data-section
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian
-mbig-endian -mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb
-marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump
-mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly
-mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mid-shared-library
-mno-id-shared-library -mshared-library-id=n -mlong-calls -mno-long-calls
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4
-metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit
-m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CRX Options -mmac -mpush-args
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_names-
pace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -image_base -init
-install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size
-prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols
-whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot
-segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -what-
sloaded -F -gused -gfull -mmacosx-version-min=version -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-confor-
mant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic -minline-plt
-mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc
-mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indi-
rect-calls -mgas -mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay -mlinker-opt
-mlong-calls -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-indexing -mno-fast-indi-
rect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float
-mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -msched-
ule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static -threads
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-double -mpre-
ferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -mssse3 -msse4a -msse5 -m3dnow -mpopcnt -mabm
-mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num -msseregparm -momit-leaf-frame-pointer
-mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num
-mfused-madd -mno-fused-madd
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregis-
ter-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -min-
line-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -min-
line-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=regis-
ter-range -mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name
-mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881
-mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int
-mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library
-mno-id-shared-library
M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls
-mshort -msoft-reg-count=count
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates
-mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-call-
graph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64
-mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls -mxgot -mno-xgot -mgp32 -mgp64 -mfp32
-mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mdsp -mpaired-single -mips3d -mlong64
-mlong32 -msym32 -mno-sym32 -Gnum -membedded-data -mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-r4000
-mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mfix-sb1
-mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend
-mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mreturn-pointer-on-d0
-mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32
-mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm -mdec-asm
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2
-mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpow-
erpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd -mmfpgpr
-mno-mfpgpr -mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc
-m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float
-mmultiple -mno-multiple -mstring -mno-string -mupdate -mno-update -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocat-
able-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdy-
namic-no-pic -maltivec -mswdiv -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=abi-
type -msecure-plt -mbss-plt -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mvr-
save -mno-vrsave -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G
num -pthread
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mlong-double-64
-mlong-double-128 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mtpf-trace
-mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-sin-
gle-only -m4a-single -m4a -m4al -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact
-m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mno-
macsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy -mdi-
vsi3_libfunc=name -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mimpure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -threads -pthreads
-pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti
-mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpre-
serve-float
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e1 -mv850e
-mv850 -mbig-switch
VAX Options -mg -mgnu -munix
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-section-literals -mno-text-sec-
tion-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -funwind-tables -fasyn-
chronous-unwind-tables -finhibit-size-directive -finstrument-functions -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -freg-struct-return -fshared-data
-fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fargument-alias -fargument-noalias
-fargument-noalias-global -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisi-
bility -fopenmp
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in
that order. GCC is capable of preprocessing and compiling several files either into several assembler input
files, or into one assembler input file; then each assembler input file produces an object file, and linking
combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program
work.
file.mi
Objective-C source code which should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ pro-
gram work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the last two letters must both be literally
x. Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the traditional preprocessor).
file.ads
Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or
generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or sub-
program renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are also
called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated this
way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a
default based on the file name suffix). This option applies to all following input files until the next -x
option. Possible values for language are:
c c-header c-cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f95 f95-cpp-input
java
treelang
-x none
Turn off any specification of a language, so that subsequent files are handled according to their file name
suffixes (as they are if -x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success
return code. If you specify -pass-exit-codes, the gcc program will instead return with numerically highest
error produced by any phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to
start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for
example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate
output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with
.o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler
code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with
.s.
Input files that don’t require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of prepro-
cessed source code, which is sent to the standard output.
Input files which don’t require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced, whether
it be an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suf-
fix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all
preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the
version number of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and all command arguments are quoted. This is useful for
shell scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This
fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
-combine
If you are compiling multiple source files, this option tells the driver to pass all the source files to
the compiler at once (for those languages for which the compiler can handle this). This will allow inter-
module analysis (IMA) to be performed by the compiler. Currently the only language for which this is sup-
ported is C. If you pass source files for multiple languages to the driver, using this option, the driver
will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appro-
priate for it. For those languages that do not support IMA this option will be ignored, and the compiler
will be invoked once for each source file in that language. If you use this option in conjunction with
-save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only
one (combined) .o or .s file.
--help
Print (on the standard output) a description of the command line options understood by gcc. If the -v
option is also specified then --help will also be passed on to the various processes invoked by gcc, so
that they can display the command line options they accept. If the -Wextra option is also specified then
command line options which have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target specific command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header
files often use .hh or .H; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these
names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs
(usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands the C++ lan-
guage---and under some circumstances, you might want to compile programs or header files from standard input,
or otherwise without a suffix that flags them as C++ programs. You might also like to precompile a C header
file with a .h extension to be used in C++ compilations. g++ is a program that calls GCC with the default lan-
guage set to C++, and automatically specifies linking against the C++ library. On many systems, g++ is also
installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling
programs in any language; or command-line options meaningful for C and related languages; or options that are
meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and
Objective-C++) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of
standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such
as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as
well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in
header files that might be included in compilations done with -ansi. Alternate predefined macros such as
"__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -pedantic is
required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may notice this
macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn’t
call for; this is to avoid interfering with any programs that might use these names for other things.
Functions which would normally be built in but do not have semantics defined by ISO C (such as "alloca" and
"ffs") are not built-in functions with -ansi is used.
-std=
Determine the language standard. This option is currently only supported when compiling C or C++. A value
for this option must be provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default.
The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
Even when this option is not specified, you can still use some of the features of newer standards in so far
as they do not conflict with previous C standards. For example, you may use "__restrict__" even when
-std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as -ansi, except that features that
were not in ISO C90 but are in the specified version (for example, // comments and the "inline" keyword in
ISO C99) are not disabled.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99
mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline func-
tions.
This option is accepted by GCC versions 4.1.3 and up. In GCC versions prior to 4.3, C99 inline semantics
are not supported, and thus this option is effectively assumed to be present regardless of whether or not
it is specified; the only effect of specifying it explicitly is to disable warnings about using inline
functions in C99 mode. Likewise, the option -fno-gnu89-inline is not supported in versions of GCC before
4.3. It will be supported only in C99 or gnu99 mode, not in C89 or gnu89 mode.
The preprocesor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which seman-
tics are in effect for "inline" functions.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a trans-
lation unit, including those in header files. This option is silently ignored in any language other than
C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and
line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respec-
tively, in the first character after the line number and the colon), and whether it came from a declaration
or a definition (C or F, respectively, in the following character). In the case of function definitions, a
K&R-style list of arguments followed by their declarations is also provided, inside comments, after the
declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers.
You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords. You
may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or
-std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword
in ISO C99.
-fno-builtin
-fno-builtin-function
Don’t recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance,
calls to "alloca" may become single instructions that adjust the stack directly, and calls to "memcpy" may
become inline copy loops. The resulting code is often both smaller and faster, but since the function
calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library. In addition, when a function is recognized as a
built-in function, GCC may use information about that function to warn about problems with calls to that
function, or to generate more efficient code, even if the resulting code still contains calls to that func-
tion. For example, warnings are given with -Wformat for bad calls to "printf", when "printf" is built in,
and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not
begin with __builtin_. If a function is named this is not built-in in this version of GCC, this option is
ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted environment
is one in which the entire standard library is available, and in which "main" has a return type of "int".
Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin. A free-
standing environment is one in which the standard library may not exist, and program startup may not neces-
sarily be at "main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only accepted with this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -tri-
graphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied
"cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an
additional preprocessing step after normal preprocessing but before compiling. The default is to use the
integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only sup-
ported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP
manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such
an expression is void. This option is not supported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default
or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the
signedness of an object. But many programs have been written to use plain "char" and expect it to be
signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its
inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its
behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Like-
wise, the option -fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use either
"signed" or "unsigned". By default, such a bit-field is signed, because this is consistent: the basic
integer types such as "int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you can also use
most of the GNU compiler options regardless of what language your program is in. For example, you might com-
pile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any
language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the ver-
sion that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0
will change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control
code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allo-
cated. This check is normally unnecessary because the C++ standard specifies that "operator new" will only
return 0 if it is declared throw(), in which case the compiler will always check the return value even
without this option. In all other cases, when "operator new" has a non-empty exception specification, mem-
ory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves
space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag
and your program mysteriously crashes after "main()" has completed, you may have an object that is being
destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting variables into
BSS without making them common.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the
class in which they are declared. Friend functions were documented to work this way in the old Annotated
C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend
function which is not declared in an enclosing scope can only be found using argument dependent lookup.
This option causes friends to be injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future release of G++.
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By default, G++ uses type "const char
*" as required by the standard. Even if you use -fno-const-strings, you cannot actually modify the value
of a string constant.
This option might be removed in a future release of G++. For maximum portability, you should structure
your code so that it works with string constants that have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is only used to initialize
another object of the same type. Specifying this option disables that optimization, and forces G++ to call
the copy constructor in all cases.
-fno-enforce-eh-specs
Don’t generate code to check for violation of exception specifications at runtime. This option violates
the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.
This does not give user code permission to throw exceptions in violation of the exception specifications;
the compiler will still optimize based on the specifications, so throwing an unexpected exception will
result in undefined behavior.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for
loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables
declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions
of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for old-style
code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the
keyword "__typeof__" instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code
for explicit instantiations.
-fno-implicit-inline-templates
Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines
differently so that compiles with and without optimization will need the same set of explicit instantia-
tions.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation.
This will cause linker errors if these functions are not inlined everywhere they are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to mem-
ber function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs",
"alloca", "_exit", "index", "bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as syn-
onyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such
diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will
allow some nonconforming code to compile.
-frepo
Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ runtime type
identification features (dynamic_cast and typeid). If you don’t use those parts of the language, you can
save some space by using this flag. Note that exception handling uses the same information, but it will
generate it as needed.
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is generally
only useful to the G++ development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth
is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming pro-
grams must not rely on a maximum depth greater than 17.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of
local statics. You can use this option to reduce code size slightly in code that doesn’t need to be
thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than
the "atexit" function. This option is required for fully standards-compliant handling of static destruc-
tors, but will only work if your C library supports "__cxa_atexit".
-fvisibility-inlines-hidden
Causes all inlined methods to be marked with "__attribute__ ((visibility ("hidden")))" so that they do not
appear in the export table of a DSO and do not require a PLT indirection when used within the DSO.
Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the
size of the dynamic export table when the library makes heavy use of templates.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method.
For example, if you do want to compare pointers to a particular inline method, or the method has local
static data, you might mark it as having default visibility.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak sym-
bols if they are available. This option exists only for testing, and should not be used by end-users; it
will result in inferior code and has no benefits. This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search the other
standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won’t be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an
effort has been made to warn about all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code. There may also be cases where warnings are emitted even
though the code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code gener-
ated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as
a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can
avoid this problem by explicitly padding "A" so that its size is a multiple of the byte size on your
platform; that will cause G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out
virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will. You can
avoid this problem by explicitly padding "A" so that its size is a multiple of its alignment (ignoring
virtual base classes); that will cause G++ and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when
the bit-fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the number of bits
in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++
mistakenly believes that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or template template parameters can be man-
gled incorrectly.
template
void f(typename Q::X) {}
template class Q>
void f(typename Q
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private, and
it has neither friends nor public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-
virtual one. This warning is enabled by -Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not match the order in which they must be
executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of the mem-
bers, emitting a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ book:
* Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated
memory.
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don’t try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers’ More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
* Item 7: Never overload "&&", "││", or ",".
When selecting this option, be aware that the standard library headers do not obey all of these guidelines;
use grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid
sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant not a null pointer, it
is guaranteed to of the same size as a pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the advent of
explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e.,
friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary,
nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids
could be interpreted as a particular specialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check
existing code for potential trouble spots and is on by default. This new compiler behavior can be turned
off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warn-
ing.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts
(dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and
much easier to search for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a
conversion to an unsigned type of the same size. Previous versions of G++ would try to preserve unsigned-
ness, but the standard mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-
defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ pro-
grams, but you can also use most of the language-independent GNU compiler options. For example, you might com-
pile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use
the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options
specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific
options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the syntax
"@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and "NSCon-
stantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also
present, will override the -fconstant-string-class setting and cause "@"..."" literals to be laid out as
constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most
types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including
Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]") in this translation unit
ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to be
used. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and
later.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial
default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method that will run
non-trivial default constructors on any such instance variables, in order, and then return "self". Simi-
larly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize
a special "- (void) .cxx_destruct" method that will run all such default destructors, in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly generated will only operate on
instance variables declared in the current Objective-C class, and not those inherited from superclasses.
It is the responsibility of the Objective-C runtime to invoke all such methods in an object’s inheritance
hierarchy. The "- (id) .cxx_construct" methods will be invoked by the runtime immediately after a new
object instance is allocated; the "- (void) .cxx_destruct" methods will be invoked immediately before the
runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "- (id)
.cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by
C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X
10.3 and later.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a
@catch block, the @throw may appear without an argument (as shown above), in which case the object caught
by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an
object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type,
analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may
also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if
any).
The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ...
@catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown
inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with "NS_HANDLER"-style idioms provided by the
"NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems,
due to additional functionality needed in the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling types other than Objective-C objects.
Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with
C++ exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it
in C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been
placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait
until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will
place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the
lock (thereby making "guard" available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing
exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked prop-
erly.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow
dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging
mode, where the object file in question may be recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is
only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")" (when
the name of the class is known at compile time) with static class references that get initialized at load
time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and
causes calls to "objc_getClass("...")" to be retained. This is useful in Zero-Link debugging mode, since
it allows for individual class implementations to be modified during program execution.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued for every method in the protocol that
is not implemented by the class. The default behavior is to issue a warning for every method not explic-
itly implemented in the class, even if a method implementation is inherited from the superclass. If you
use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector are found during compilation. The check
is performed on the list of methods in the final stage of compilation. Additionally, a check is performed
for each selector appearing in a "@selector(...)" expression, and a corresponding method for that selector
has been found during compilation. Because these checks scan the method table only at the end of compila-
tion, these warnings are not produced if the final stage of compilation is not reached, for example because
an error is found during compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match
Warn if multiple methods with differing argument and/or return types are found for a given selector when
attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is
off (which is the default behavior), the compiler will omit such warnings if any differences found are con-
fined to types which share the same size and alignment.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is consid-
ered undeclared if no method with that name has been declared before the "@selector(...)" expression,
either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section.
This option always performs its checks as soon as a "@selector(...)" expression is found, while -Wselector
only performs its checks in the final stage of compilation. This also enforces the coding style convention
that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device’s aspect (e.g. its
width, ...). The options described below can be used to control the diagnostic messages formatting algorithm,
e.g. how many characters per line, how often source location information should be reported. Right now, only
the C++ front end can honor these options. However it is expected, in the near future, that the remaining
front ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The default is 72 characters
for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be
done; each error message will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source
location information; that is, in case the message is too long to fit on a single physical line and has to
be wrapped, the source location won’t be emitted (as prefix) again, over and over, in subsequent continua-
tion lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source
location information (as prefix) for physical lines that result from the process of breaking a message
which is too long to fit on a single line.
-fdiagnostics-show-options
This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates
which command line option directly controls that diagnostic, when such an option is known to the diagnostic
machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are
risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on
implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn
off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the
default.
The following options control the amount and kinds of warnings produced by GCC; for further, language-specific
options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-fsyntax-only
Check the code for syntax errors, but don’t do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden exten-
sions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the
ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few
will require -ansi or a -std option specifying the required version of ISO C). However, without this
option, certain GNU extensions and traditional C and C++ features are supported as well. With this option,
they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with
__. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only
system header files should use these escape routes; application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find that it
does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C
requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require
considerable additional work and would be quite different from -pedantic. We don’t have plans to support
such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or gnu99,
there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.
Warnings from -pedantic are given where they are required by the base standard. (It would not make sense
for such warnings to be given only for features not in the specified GNU C dialect, since by definition the
GNU dialects of C include all features the compiler supports with the given option, and there would be
nothing to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warnings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget
that this type is signed on some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears
in a // comment. This warning is enabled by -Wall.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to keep
going and printing further error messages.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate
to the format string specified, and that the conversions specified in the format string make sense. This
includes standard functions, and others specified by format attributes, in the "printf", "scanf", "strf-
time" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific fami-
lies). Which functions are checked without format attributes having been specified depends on the standard
version selected, and such checks of functions without the attribute specified are disabled by -ffreestand-
ing or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all
ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these features; GCC does not support warning
about features that go beyond a particular library’s limitations. However, if -pedantic is used with
-Wformat, warnings will be given about format features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnon-
null.
-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wfor-
mat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and
-Wformat=2 are available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The
C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifica-
tions, normally warnings are still given, since the implementation could not know what type to pass to
"va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this option will suppress
the warning if the unused arguments are all pointers, since the Single Unix Specification says that such
unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length
formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked,
unless the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security prob-
lems. At present, this warns about calls to "printf" and "scanf" functions where the format string is not
a string literal and there are no format arguments, as in "printf (foo);". This may be a security hole if
the format string came from untrusted input and contains %n. (This is currently a subset of what -Wfor-
mat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included
in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat -Wfor-
mat-nonliteral -Wformat-security -Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull" func-
tion attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with themselves. Note this option can only be
used with the -Wuninitialized option, which in turn only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snippet only when -Winit-self has
been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared. The form
-Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning,
not an error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage, returning int,
taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by
-Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initial-
izer for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where
a truth value is expected, or when operators are nested whose precedence people often get confused about.
Only the warning for an assignment used as a truth value is supported when compiling C++; the other warn-
ings are only supported when compiling C.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a dif-
ferent interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which "if" statement an "else" branch
belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if
(b)". This is often not what the programmer expected, as illustrated in the above example by indentation
the programmer chose. When there is the potential for this confusion, GCC will issue a warning when this
flag is specified. To eliminate the warning, add explicit braces around the innermost "if" statement so
there is no way the "else" could belong to the enclosing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in the C
standard.
The C standard defines the order in which expressions in a C program are evaluated in terms of sequence
points, which represent a partial ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur after the evaluation of a full expres-
sion (one which is not part of a larger expression), after the evaluation of the first operand of a "&&",
"││", "? :" or "," (comma) operator, before a function is called (but after the evaluation of its arguments
and the expression denoting the called function), and in certain other places. Other than as expressed by
the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order, since, for example, if two functions
are called within one expression with no sequence point between them, the order in which the functions are
called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Pro-
grams whose behavior depends on this have undefined behavior; the C standard specifies that "Between the
previous and next sequence point an object shall have its stored value modified at most once by the evalua-
tion of an expression. Furthermore, the prior value shall be read only to determine the value to be
stored.". If a program breaks these rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more com-
plicated cases are not diagnosed by this option, and it may give an occasional false positive result, but
in general it has been found fairly effective at detecting this sort of problem in programs.
The present implementation of this option only works for C programs. A future implementation may also work
for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise meaning of the
sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal defi-
nitions, may be found on the GCC readings page, at
This warning is enabled by -Wall.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int". Also warn about any
"return" statement with no return-value in a function whose return-type is not "void".
For C, also warn if the return type of a function has a type qualifier such as "const". Such a type quali-
fier has no effect, since the value returned by a function is not an lvalue. ISO C prohibits qualified
"void" return types on function definitions, so such return types always receive a warning even without
this option.
For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is
specified. The only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case"
labels outside the enumeration range also provoke warnings when this option is used. This warning is
enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings
when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within com-
ments are not warned about). This warning is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused.
This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This
warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. This warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused
(note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by
a "setjmp" call.
These warnings are possible only in optimizing compilation, because they require data flow information that
is computed only when optimizing. If you don’t specify -O, you simply won’t get these warnings.
If you want to warn about code which uses the uninitialized value of the variable in its own initializer,
use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array vari-
ables as well as for variables which are uninitialized or clobbered as a whole. They do not occur for
variables or elements declared "volatile". Because these warnings depend on optimization, the exact vari-
ables or elements for which there are warnings will depend on the precise optimization options and version
of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is
never used, because such computations may be deleted by data flow analysis before the warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why the code might
be correct despite appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn’t know this. Here
is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp".
These warnings as well are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a
signal handler could call it at any point in the code. As a result, you may get a warning even when there
is in fact no problem because "longjmp" cannot in fact be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as "nore-
turn".
This warning is enabled by -Wall.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option
is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if
the warnings were only enabled by the -Wall command line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between
pragmas. See also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code which might break the
strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases,
but does attempt to catch the more common pitfalls. It is included in -Wall.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It warns about code which might break the
strict aliasing rules that the compiler is using for optimization. This warning catches more cases than
-Wstrict-aliasing, but it will also give a warning for some ambiguous cases that are safe.
-Wall
All of the above -W options combined. This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction
with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objec-
tive-C and Objective-C++ Dialect Options.
The following -W... options are not implied by -Wall. Some of them warn about constructions that users gener-
ally do not consider questionable, but which occasionally you might wish to check for; others warn about con-
structions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to
suppress the warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the newer name is more descrip-
tive.) Print extra warning messages for these events:
* A function can return either with or without a value. (Falling off the end of the function body is
considered returning without a value.) For example, this function would evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma expression contains no side effects. To sup-
press the warning, cast the unused expression to void. For example, an expression such as x[i,j] will
cause a warning, but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things in a declaration. According to the C
Standard, this usage is obsolescent.
* If -Wall or -Wunused is also specified, warn about unused arguments.
* A comparison between signed and unsigned values could produce an incorrect result when the signed value
is converted to unsigned. (But don’t warn if -Wno-sign-compare is also specified.)
* An aggregate has an initializer which does not initialize all members. This warning can be indepen-
dently controlled by -Wmissing-field-initializers.
* A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or >=.
* A variable might be changed by longjmp or vfork.
* Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of
precision, etc.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a class without constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared register.
*<(C++ only)>
A base class is not initialized in a derived class’ copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by zero is not warned
about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are nor-
mally suppressed, on the assumption that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command line option tells GCC to emit warnings from system
headers as if they occurred in user code. However, note that using -Wall in conjunction with this option
will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point
values as approximations to infinitely precise real numbers. If you are doing this, then you need to com-
pute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation
introduces, and allow for it when performing comparisons (and when producing output, but that’s a different
problem). In particular, instead of testing for equality, you would check to see whether the two values
have ranges that overlap; and this is done with the relational operators, so equality comparisons are prob-
ably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and/or problematic constructs which should be avoided.
* Macro parameters that appear within string literals in the macro body. In traditional C macro replace-
ment takes place within string literals, but does not in ISO C.
* In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only
consider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional
warns about directives that traditional C understands but would ignore because the # does not appear as
the first character on the line. It also suggests you hide directives like #pragma not understood by
traditional C by indenting them. Some traditional implementations would not recognize #elif, so it
suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional C does
support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the sys-
tem headers of most modern systems, e.g. the _MIN/_MAX macros in "
user code might normally lead to spurious warnings, however GCC’s integrated preprocessor has enough
context to avoid warning in these cases.
* A function declared external in one block and then used after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct is not accepted by some
traditional C compilers.
* The ISO type of an integer constant has a different width or signedness from its traditional type.
This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values,
which typically represent bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
* Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the
assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid miss-
ing initializer warnings and relies on default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values and vice versa. The absence of these
prototypes when compiling with traditional C would cause serious problems. This is a subset of the
possible conversion warnings, for the full set use -Wconversion.
* Use of ISO C style function definitions. This warning intentionally is not issued for prototype decla-
rations or variadic functions because these ISO C features will appear in your code when using libib-
erty’s traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for
nested functions because that feature is already a GCC extension and thus not relevant to traditional C
compatibility.
-Wdeclaration-after-statement (C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was intro-
duced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported
by GCC versions before GCC 3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a
built-in function is shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the
loop indices. With -funsafe-loop-optimizations warn if the compiler made such assumptions.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types
a size of 1, for convenience in calculations with "void *" pointers and pointers to functions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast
to "anything *".
-Wc++-compat
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for
implicit conversion from "void *" to a pointer to non-"void" type.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn
if a "const char *" is cast to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example,
warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-
byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of one
into a non-"const" "char *" pointer will get a warning; when compiling C++, warn about the deprecated con-
version from string constants to "char *". These warnings will help you find at compile time code that can
try to write into a string constant, but only if you have been very careful about using "const" in declara-
tions and prototypes. Otherwise, it will just be a nuisance; this is why we did not make -Wall request
these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from what would happen to the same argument
in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed point argument except when the same as the default
promotion.
Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For
example, warn about the assignment "x = -1" if "x" is unsigned. But do not warn about explicit casts like
"(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed
value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of
-Wextra without this warning, use -Wextra -Wno-sign-compare.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where you can
return an array, this also elicits a warning.)
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes
applied to variables, etc. This will not stop errors for incorrect use of supported attributes.
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style function
definition is permitted without a warning if preceded by a declaration which specifies the argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous proto-
type.
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype declaration. This warning is issued even
if the definition itself provides a prototype. The aim is to detect global functions that fail to be
declared in header files.
-Wmissing-declarations (C only)
Warn if a global function is defined without a previous declaration. Do so even if the definition itself
provides a prototype. Use this option to detect global functions that are not declared in header files.
-Wmissing-field-initializers
Warn if a structure’s initializer has some fields missing. For example, the following code would cause
such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification would not trigger a
warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn". Note these are only possible can-
didates, not absolute ones. Care should be taken to manually verify functions actually do not ever return
before adding the "noreturn" attribute, otherwise subtle code generation bugs could be introduced. You
will not get a warning for "main" in hosted C environments.
-Wmissing-format-attribute
Warn about function pointers which might be candidates for "format" attributes. Note these are only possi-
ble candidates, not absolute ones. GCC will guess that function pointers with "format" attributes that are
used in assignment, initialization, parameter passing or return statements should have a corresponding
"format" attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the
type of the parameter variable, or the return type of the containing function respectively should also have
a "format" attribute to avoid the warning.
GCC will also warn about function definitions which might be candidates for "format" attributes. Again,
these are only possible candidates. GCC will guess that "format" attributes might be appropriate for any
function that calls a function like "vprintf" or "vscanf", but this might not always be the case, and some
functions for which "format" attributes are appropriate may not be detected.
-Wno-multichar
Do not warn if a multicharacter constant (’FOOF’) is used. Usually they indicate a typo in the user’s
code, as they have implementation-defined values, and should not be used in portable code.
-Wnormalized=
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. How-
ever, sometimes when characters outside the basic ASCII character set are used, you can have two different
character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normal-
ization rules which when applied ensure that two sequences that look the same are turned into the same
sequence. GCC can warn you if you are using identifiers which have not been normalized; this option con-
trols that warning.
There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any
identifier which is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most
uses.
Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into
NFC aren’t allowable as identifiers. That is, there’s no way to use these symbols in portable ISO C or C++
and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct this, which is why this option is not the
default.
You can switch the warning off for all characters by writing -Wnormalized=none. You would only want to do
this if you were using some other normalization scheme (like "D"), because otherwise you can easily create
bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodolo-
gies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER
N", will display just like a regular "n" which has been placed in a superscript. ISO 10646 defines the
NFKC normalisation scheme to convert all these into a standard form as well, and GCC will warn if your code
is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier
that contains the letter O because it might be confused with the digit 0, and so is not the default, but
may be useful as a local coding convention if the programming environment is unable to be fixed to display
these characters distinctly.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated"
attribute. (@pxref{Function Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or
size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code,
the variable "f.x" in "struct bar" will be misaligned even though "struct bar" does not itself have the
packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align the
whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is
valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole line of source code will
never be executed, because some condition is never satisfied or because it is after a procedure that never
returns.
It is possible for this option to produce a warning even though there are circumstances under which part of
the affected line can be executed, so care should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only one
inlined copy of the function.
This option is not made part of -Wall because in a debugging version of a program there is often substan-
tial code which checks correct functioning of the program and is, hopefully, unreachable because the pro-
gram does work. Another common use of unreachable code is to provide behavior which is selectable at com-
pile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler
will not warn about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example,
the compiler takes into account the size of the function being inlined and the amount of inlining that has
already been done in the current function. Therefore, seemingly insignificant changes in the source pro-
gram can cause the warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ stan-
dard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof
typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple
struct that fails to be a POD type only by virtue of having a constructor.) This flag is for users who are
aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about
it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
-Wno-int-to-pointer-cast (C only)
Suppress warnings from casts to pointer type of an integer of a different size.
-Wno-pointer-to-int-cast (C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can’t be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the warning messages, use -Wno-long-long.
Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO
C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations
that may eliminate reads and/or writes to register variables.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is
anything wrong with your code; it merely indicates that GCC’s optimizers were unable to handle the code
effectively. Often, the problem is that your code is too big or too complex; GCC will refuse to optimize
programs when the optimization itself is likely to take inordinate amounts of time.
-Wpointer-sign
Warn for pointer argument passing or assignment with different signedness. This option is only supported
for C and Objective-C. It is implied by -Wall and by -pedantic, which can be disabled with
-Wno-pointer-sign.
-Werror
Make all warnings into errors.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that will not be
protected against stack smashing.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
-g Produce debugging information in the operating system’s native format (stabs, COFF, XCOFF, or DWARF 2).
GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use;
this extra information makes debugging work better in GDB but will probably make other debuggers crash or
refuse to read the program. If you want to control for certain whether to generate the extra information,
use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surpris-
ing results: some variables you declared may not exist at all; flow of control may briefly move where you
did not expect it; some statements may not be executed because they compute constant results or their val-
ues were already at hand; some statements may execute in different places because they were moved out of
loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer
for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one debugging for-
mat.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available
(DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at
all possible.
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the
format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces
stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option
requires the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only symbols that are actually
used.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions understood only
by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse
to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported). This is the format used by SDB on
most System V systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX
debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only
by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse
to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by
DBX on IRIX 6. With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is
upward compatible with version 2, but may still cause problems for older debuggers.
-gvms
Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG
on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don’t
plan to debug. This includes descriptions of functions and external variables, but no information about
local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debug-
gers support macro expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that
meant to generate debug information in version 1 of the DWARF format (which is very different from version
2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be
changed now. Instead use an additional -glevel option to change the debug level for DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option
only makes sense when generating DWARF2 debugging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the analysis program prof. You must use this
option when compiling the source files you want data about, and you must also use it when linking.
-pg Generate extra code to write profile information suitable for the analysis program gprof. You must use
this option when compiling the source files you want data about, and you must also use it when linking.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about each
pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many times
each branch and call is executed and how many times it is taken or returns. When the compiled program
exits it saves this data to a file called auxname.gcda for each source file. The data may be used for pro-
file-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage).
Each object file’s auxname is generated from the name of the output file, if explicitly specified and it is
not the final executable, otherwise it is the basename of the source file. In both cases any suffix is
removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o
dir/foo.o).
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a synonym
for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for
those options for more details.
@bullet
Compile the source files with -fprofile-arcs plus optimization and code generation options. For test
coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source
file in a program.
@cvmmfu
Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
@dwnngv
Run the program on a representative workload to generate the arc profile information. This may be
repeated any number of times. You can run concurrent instances of your program, and provided that the
file system supports locking, the data files will be correctly updated. Also "fork" calls are detected
and correctly handled (double counting will not happen).
@exoohw
For profile-directed optimizations, compile the source files again with the same optimization and code
generation options plus -fbranch-probabilities.
@fyppix
For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda
files. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a span-
ning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler
adds code to count the number of times that these arcs are executed. When an arc is the only exit or only
entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must
be created to hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source
file’s note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of
auxname and instructions on how to generate test coverage data. Coverage data will match the source files
more closely, if you do not optimize.
-dletters
-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified by letters. This is used for debug-
ging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a
pass number and a word to the dumpname. dumpname is generated from the name of the output file, if explic-
itly specified and it is not an executable, otherwise it is the basename of the source file.
Most debug dumps can be enabled either passing a letter to the -d option, or with a long -fdump-rtl switch;
here are the possible letters for use in letters and pass, and their meanings:
-dA Annotate the assembler output with miscellaneous debugging information.
-db
-fdump-rtl-bp
Dump after computing branch probabilities, to file.09.bp.
-dB
-fdump-rtl-bbro
Dump after block reordering, to file.30.bbro.
-dc
-fdump-rtl-combine
Dump after instruction combination, to the file file.17.combine.
-dC
-fdump-rtl-ce1
-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if conversion, to the file file.11.ce1. -dC and
-fdump-rtl-ce2 enable dumping after the second if conversion, to the file file.18.ce2.
-dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load optimization, to file.31.btl. -dd and
-fdump-rtl-dbr enable dumping after delayed branch scheduling, to file.36.dbr.
-dD Dump all macro definitions, at the end of preprocessing, in addition to normal output.
-dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.28.ce3.
-df
-fdump-rtl-cfg
-fdump-rtl-life
-df and -fdump-rtl-cfg enable dumping after control and data flow analysis, to file.08.cfg. -df and
-fdump-rtl-cfg enable dumping dump after life analysis, to file.16.life.
-dg
-fdump-rtl-greg
Dump after global register allocation, to file.23.greg.
-dG
-fdump-rtl-gcse
-fdump-rtl-bypass
-dG and -fdump-rtl-gcse enable dumping after GCSE, to file.05.gcse. -dG and -fdump-rtl-bypass enable
dumping after jump bypassing and control flow optimizations, to file.07.bypass.
-dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.
-di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.01.sibling.
-dj
-fdump-rtl-jump
Dump after the first jump optimization, to file.03.jump.
-dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to file.33.stack.
-dl
-fdump-rtl-lreg
Dump after local register allocation, to file.22.lreg.
-dL
-fdump-rtl-loop
-fdump-rtl-loop2
-dL and -fdump-rtl-loop enable dumping after the first loop optimization pass, to file.06.loop. -dL
and -fdump-rtl-loop2 enable dumping after the second pass, to file.13.loop2.
-dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.20.sms.
-dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, to file.35.mach.
-dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.29.rnreg.
-dN
-fdump-rtl-regmove
Dump after the register move pass, to file.19.regmove.
-do
-fdump-rtl-postreload
Dump after post-reload optimizations, to file.24.postreload.
-dr
-fdump-rtl-expand
Dump after RTL generation, to file.00.expand.
-dR
-fdump-rtl-sched2
Dump after the second scheduling pass, to file.32.sched2.
-ds
-fdump-rtl-cse
Dump after CSE (including the jump optimization that sometimes follows CSE), to file.04.cse.
-dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.
-dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to
file.15.cse2.
-dT
-fdump-rtl-tracer
Dump after running tracer, to file.12.tracer.
-dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile transformations, to file.10.vpt. -dV and
-fdump-rtl-vartrack enable dumping after variable tracking, to file.34.vartrack.
-dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.26.flow2.
-dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.27.peephole2.
-dZ
-fdump-rtl-web
Dump after live range splitting, to file.14.web.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dH Produce a core dump whenever an error occurs.
-dm Print statistics on memory usage, at the end of the run, to standard error.
-dp Annotate the assembler output with a comment indicating which pattern and alternative was used. The
length of each instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annota-
tion.
-dv For each of the other indicated dump files (either with -d or -fdump-rtl-pass), dump a representation
of the control flow graph suitable for viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it. Usually used with r (-fdump-rtl-expand).
-dy Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output.
This makes it more feasible to use diff on debugging dumps for compiler invocations with different options,
in particular with and without -g.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file name is
made by appending .tu to the source file name. If the -options form is used, options controls the details
of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class’s hierarchy and virtual function table layout to a file. The file name
is made by appending .class to the source file name. If the -options form is used, options controls the
details of the dump as described for the -fdump-tree options.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name
is generated by appending a switch specific suffix to the source file name. The following dumps are possi-
ble:
all Enables all inter-procedural analysis dumps; currently the only produced dump is the cgraph dump.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
-fdump-tree-switch
-fdump-tree-switch-options
Control the dumping at various stages of processing the intermediate language tree to a file. The file
name is generated by appending a switch specific suffix to the source file name. If the -options form is
used, options is a list of - separated options that control the details of the dump. Not all options are
applicable to all dumps, those which are not meaningful will be ignored. The following options are avail-
able
address
Print the address of each node. Usually this is not meaningful as it changes according to the environ-
ment and source file. Its primary use is for tying up a dump file with a debug environment.
slim
Inhibit dumping of members of a scope or body of a function merely because that scope has been reached.
Only dump such items when they are directly reachable by some other path. When dumping pretty-printed
trees, this option inhibits dumping the bodies of control structures.
raw Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representa-
tion.
details
Enable more detailed dumps (not honored by every dump option).
stats
Enable dumping various statistics about the pass (not honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
all Turn on all options, except raw, slim and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
gimple
Dump each function before and after the gimplification pass to a file. The file name is made by
appending .gimple to the source file name.
cfg Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the
source file name.
vcg Dump the control flow graph of each function to a file in VCG format. The file name is made by append-
ing .vcg to the source file name. Note that if the file contains more than one function, the generated
file cannot be used directly by VCG. You will need to cut and paste each function’s graph into its own
separate file first.
ch Dump each function after copying loop headers. The file name is made by appending .ch to the source
file name.
ssa Dump SSA related information to a file. The file name is made by appending .ssa to the source file
name.
salias
Dump structure aliasing variable information to a file. This file name is made by appending .salias to
the source file name.
alias
Dump aliasing information for each function. The file name is made by appending .alias to the source
file name.
ccp Dump each function after CCP. The file name is made by appending .ccp to the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file
name.
pre Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source
file name.
fre Dump trees after full redundancy elimination. The file name is made by appending .fre to the source
file name.
copyprop
Dump trees after copy propagation. The file name is made by appending .copyprop to the source file
name.
store_copyprop
Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the
source file name.
dce Dump each function after dead code elimination. The file name is made by appending .dce to the source
file name.
mudflap
Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap
to the source file name.
sra Dump each function after performing scalar replacement of aggregates. The file name is made by append-
ing .sra to the source file name.
sink
Dump each function after performing code sinking. The file name is made by appending .sink to the
source file name.
dom Dump each function after applying dominator tree optimizations. The file name is made by appending
.dom to the source file name.
dse Dump each function after applying dead store elimination. The file name is made by appending .dse to
the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline code. The file name is made by append-
ing .phiopt to the source file name.
forwprop
Dump each function after forward propagating single use variables. The file name is made by appending
.forwprop to the source file name.
copyrename
Dump each function after applying the copy rename optimization. The file name is made by appending
.copyrename to the source file name.
nrv Dump each function after applying the named return value optimization on generic trees. The file name
is made by appending .nrv to the source file name.
vect
Dump each function after applying vectorization of loops. The file name is made by appending .vect to
the source file name.
vrp Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the
source file name.
all Enable all the available tree dumps with the flags provided in this option.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer prints. This information is written to
standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the
usual dump listing file, .vect.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to gener-
ate certain symbol names that have to be different in every compiled file. It is also used to place unique
stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option
to produce reproducibly identical object files.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the sched-
uler prints. This information is written to standard error, unless -dS or -dR is specified, in which case
it is output to the usual dump listing file, .sched or .sched2 respectively. However for n greater than
nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater than one, it
also output basic block probabilities, detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-ver-
bose also includes dependence info.
-save-temps
Store the usual "temporary" intermediate files permanently; place them in the current directory and name
them based on the source file. Thus, compiling foo.c with -c -save-temps would produce files foo.i and
foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now nor-
mally uses an integrated preprocessor.
When used in combination with the -x command line option, -save-temps is sensible enough to avoid over
writing an input source file with the same extension as an intermediate file. The corresponding intermedi-
ate file may be obtained by renaming the source file before using -save-temps.
-time
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if linking is done). The output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself. The
second number is "system time", time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better
debugging information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g)
and the debug info format supports it.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don’t do
anything else. With this option, GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the com-
mand line. This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory name
is separated from the switches by ;, and each switch starts with an @} instead of the @samp{-, without
spaces between multiple switches. This is supposed to ease shell-processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS
libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS
libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if
OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do
gcc -nostdlib
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc
will search---and don’t do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or
directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects
to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed
them. Don’t forget the trailing /.
-dumpmachine
Print the compiler’s target machine (for example, i686-pc-linux-gnu)---and don’t do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don’t do anything else.
-dumpspecs
Print the compiler’s built-in specs---and don’t do anything else. (This is used when GCC itself is being
built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a
compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes
this is useful, such as if, in the debugger, you want to cast a value to a type that is not actually used
in your program (but is declared). More often, however, this results in a significant amount of wasted
space. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in
the source file being compiled.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging
produce the expected results. Statements are independent: if you stop the program with a breakpoint between
statements, you can then assign a new value to any variable or change the program counter to any other state-
ment in the function and get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the
expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Optimization levels -O and
above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from
later functions in the file when compiling a function. Compiling multiple files at once to a single output
file in unit-at-a-time mode allows the compiler to use information gained from all of the files when compiling
each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations
that take a great deal of compilation time.
-O turns on the following optimization flags: -fdefer-pop -fdelayed-branch -fguess-branch-probability
-fcprop-registers -floop-optimize -fif-conversion -fif-conversion2 -ftree-ccp -ftree-dce -ftree-domina-
tor-opts -ftree-dse -ftree-ter -ftree-lrs -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch
-funit-at-a-time -fmerge-constants
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
-O doesn’t turn on -ftree-sra for the Ada compiler. This option must be explicitly specified on the com-
mand line to be enabled for the Ada compiler.
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed
tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As com-
pared to -O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:
-fthread-jumps -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks -fgcse
-fgcse-lm -fexpensive-optimizations -fstrength-reduce -frerun-cse-after-loop -frerun-loop-opt
-fcaller-saves -fpeephole2 -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fregmove
-fstrict-aliasing -fdelete-null-pointer-checks -freorder-blocks -freorder-functions -falign-functions
-falign-jumps -falign-loops -falign-labels -ftree-vrp -ftree-pre
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-func-
tions, -funswitch-loops and -fgcse-after-reload options.
-O0 Do not optimize. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also
performs further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays
-ftree-vect-loop-version
If you use multiple -O options, with or without level numbers, the last such option is the one that is
effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative
forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed---the
one you typically will use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related
to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be
performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the class scope (C++
only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by
default; i.e., you don’t need to add inline in front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines which must
pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for sev-
eral function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Force memory operands to be copied into registers before doing arithmetic on them. This produces better
code by making all memory references potential common subexpressions. When they are not common subexpres-
sions, instruction combination should eliminate the separate register-load. This option is now a nop and
will be removed in 4.2.
-fforce-addr
Force memory address constants to be copied into registers before doing arithmetic on them.
-fomit-frame-pointer
Don’t keep the frame pointer in a register for functions that don’t need one. This avoids the instructions
to save, set up and restore frame pointers; it also makes an extra register available in many functions.
It also makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automati-
cally handles the frame pointer and nothing is saved by pretending it doesn’t exist. The machine-descrip-
tion macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don’t pay attention to the "inline" keyword. Normally this option is used to keep the compiler from
expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically decides which functions are
simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function
is normally not output as assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not marked
"inline". If a call to a given function is integrated, then the function is not output as assembler code
in its own right.
Enabled if -funit-at-a-time is enabled.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function call
overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes pro-
filing significantly cheaper and usually inlining faster on programs having large chains of nested wrapper
functions.
Enabled by default.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows the control of this
limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined
within the class definition in c++). n is the size of functions that can be inlined in number of pseudo
instructions (not counting parameter handling). The default value of n is 600. Increasing this value can
result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which presumably means slower programs). This
option is particularly useful for programs that use inlining heavily such as those based on recursive tem-
plates with C++.
Inlining is actually controlled by a number of parameters, which may be specified individually by using
--param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to I
max-inline-insns-auto
is set to I
min-inline-insns
is set to 130 or I
max-inline-insns-rtl
is set to I
See below for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function’s
size. In no way does it represent a count of assembly instructions and as such its exact meaning might
change from one release to an another.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function has
been inlined into all of its callers. This switch does not affect functions using the "extern inline"
extension in GNU C. In C++, emit any and all inline functions into the object file.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn’t turned on, even if the variables aren’t ref-
erenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was refer-
enced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating point constants) across compilation
units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant
initialized arrays or initialized constant variables with integral or floating point types. Languages like
C or C++ require each non-automatic variable to have distinct location, so using this option will result in
non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost
loops and reorders their instructions by overlapping different iterations.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of
instructions that decrement a register, compare it against zero, then branch based upon the result. This
option is only meaningful on architectures that support such instructions, which include x86, PowerPC,
IA-64 and S/390.
The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function contain
the function’s address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be
confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS.
This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the data
section. E.g., so that the resulting executable can find the beginning of that section and/or make assump-
tions based on that.
The default is -fzero-initialized-in-bss.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations,
some standard library string/heap functions, and some other associated constructs with range/validity
tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other
classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (libmud-
flap), which will be linked into a program if -fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env MUD-
FLAP_OPTIONS=-help a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmud-
flapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This pro-
duces less instrumentation (and therefore faster execution) and still provides some protection against out-
right memory corrupting writes, but allows erroneously read data to propagate within a program.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another comparison sub-
sumed by the first is found. If so, the first branch is redirected to either the destination of the second
branch or a point immediately following it, depending on whether the condition is known to be true or
false.
Enabled at levels -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions when the target of the jump is not
reached by any other path. For example, when CSE encounters an "if" statement with an "else" clause, CSE
will follow the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which conditionally skip over blocks.
When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow the
jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and copy
propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime perfor-
mance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command
line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads which are
only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed
to a load outside the loop, and a copy/store within the loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This
pass will attempt to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a
load/store sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads
that come after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The pur-
pose of this pass is to cleanup redundant spilling.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and
optionally do strength-reduction as well.
Enabled at levels -O, -O2, -O3, -Os.
-floop-optimize2
Perform loop optimizations using the new loop optimizer. The optimizations (loop unrolling, peeling and
unswitching, loop invariant motion) are enabled by separate flags.
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with non-
trivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop
optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the
compiler will warn you if it finds this kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional
moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use
of conditional execution on chips where it is available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler
assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it
has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple instructions in
order to maximize the amount of register tying. This is especially helpful on machines with two-operand
instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available
after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to
required data being unavailable. This helps machines that have slow floating point or memory load instruc-
tions by allowing other instructions to be issued until the result of the load or floating point instruc-
tion is required.
Enabled at levels -O2, -O3, -Os.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allo-
cation has been done. This is especially useful on machines with a relatively small number of registers
and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don’t schedule instructions across basic blocks. This is normally enabled by default when scheduling
before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don’t allow speculative motion of non-load instructions. This is normally enabled by default when schedul-
ing before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready
list, during the second scheduling pass. -fno-fsched-stalled-insns and -fsched-stalled-insns=0 are equiva-
lent and mean that no insns will be moved prematurely. If n is unspecified then there is no limit on how
many queued insns can be moved prematurely.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate
for premature removal from the queue of stalled insns. This has an effect only during the second schedul-
ing pass, and only if -fsched-stalled-insns is used and its value is not zero.
+-fno-sched-stalled-insns-dep is equivalent to +-fsched-stalled-insns-dep=0. +-fsched-stalled-insns-dep
without a value is equivalent to +-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, do use superblock scheduling algorithm. Superblock scheduling
allows motion across basic block boundaries resulting on faster schedules. This option is experimental, as
not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the
algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or
higher.
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform
code duplication in order to increase the size of superblocks using tracer pass. See -ftracer for details
on trace formation.
This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the
traces constructed may not match the reality and hurt the performance. This only makes sense when schedul-
ing after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want
to prevent the later scheduling passes from changing its schedule, we use this option to control that.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra
instructions to save and restore the registers around such calls. Such allocation is done only when it
seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no call-preserved
registers to use instead.
Enabled at levels -O2, -O3, -Os.
-ftree-pre
Perform Partial Redundancy Elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
-ftree-fre
Perform Full Redundancy Elimination (FRE) on trees. The difference between FRE and PRE is that FRE only
considers expressions that are computed on all paths leading to the redundant computation. This analysis
faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled
by default at -O and higher.
-ftree-store-copy-prop
Perform copy propagation of memory loads and stores. This pass eliminates unnecessary copy operations in
memory references (structures, global variables, arrays, etc). This flag is enabled by default at -O2 and
higher.
-ftree-salias
Perform structural alias analysis on trees. This flag is enabled by default at -O and higher.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O and higher.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar
variables and is enabled by default at -O and higher.
-ftree-store-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass operates on both local scalar
variables and memory stores and loads (global variables, structures, arrays, etc). This flag is enabled by
default at -O2 and higher.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range prop-
agation and expression simplification) based on a dominator tree traversal. This also performs jump
threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion
optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not
enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
-ftree-loop-linear
Perform linear loop transformations on tree. This flag can improve cache performance and allow further
loop optimizations to take place.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that would be hard to handle at
RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops
it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in the loop for that determining number of iterations
requires complicated analysis. Later optimizations then may determine the number easily. Useful espe-
cially in connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and induction
variable elimination) on trees.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent
committing structures to memory too early. This flag is enabled by default at -O and higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at
copy locations, usually resulting in variable names which more closely resemble the original variables.
This flag is enabled by default at -O and higher.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries
are replaced at their use location with their defining expression. This results in non-GIMPLE code, but
gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled
by default at -O and higher.
-ftree-lrs
Perform live range splitting during the SSA->normal phase. Distinct live ranges of a variable are split
into unique variables, allowing for better optimization later. This is enabled by default at -O and
higher.
-ftree-vectorize
Perform loop vectorization on trees.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable
except that data alignment or data dependence cannot be determined at compile time then vectorized and non-
vectorized versions of the loop are generated along with runtime checks for alignment or dependence to con-
trol which version is executed. This option is enabled by default except at level -Os where it is dis-
abled.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of
values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks
like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null
pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of
the function allowing other optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -fun-
roll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option makes code larger, and
may or may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually
makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops,
-fsplit-ivs-in-unroller
Enables expressing of values of induction variables in later iterations of the unrolled loop using the
value in the first iteration. This breaks long dependency chains, thus improving efficiency of the
scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body
is more complicated than a single basic block, this is not reliable. It also does not work at all on some
of the architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some local variables when unrolling a loop
which can result in superior code.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of
loops that access large arrays.
These options may generate better or worse code; results are highly dependent on the structure of loops
within the source code.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peep-
hole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use
both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are
specified by __builtin_expect, then the heuristics will be used to guess branch probabilities for the rest
of the control flow graph, taking the __builtin_expect info into account. The interactions between the
heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuris-
tics so that the effects of __builtin_expect are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code
locality.
Enabled at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken
branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to
improve paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling, for linkonce sections,
for functions with a user-defined section attribute and on any architecture that does not support named
sections.
-freorder-functions
Reorder functions in the object file in order to improve code locality. This is implemented by using spe-
cial subsections ".text.hot" for most frequently executed functions and ".text.unlikely" for unlikely exe-
cuted functions. Reordering is done by the linker so object file format must support named sections and
linker must place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For
C (and C++), this activates optimizations based on the type of expressions. In particular, an object of
one type is assumed never to reside at the same address as an object of a different type, unless the types
are almost the same. For example, an "unsigned int" can alias an "int", but not a "void*" or a "double".
A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called
"type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is
accessed through the union type. So, the code above will work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias analysis should define a function that com-
putes, given an "tree" node, an alias set for the node. Nodes in different alias sets are not allowed to
alias. For an example, see the C front-end function "c_get_alias_set".
Enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For
instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24
would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This
option can easily make code slower, because it must insert dummy operations for when the branch target is
reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are
used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no
alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that
the loop will be executed many times, which will make up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached
by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be exe-
cuted.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to
take place but consumes more memory (in general). There are some compatibility issues with unit-at-at-time
mode:
* enabling unit-at-a-time mode may change the order in which functions, variables, and top-level "asm"
statements are emitted, and will likely break code relying on some particular ordering. The majority
of such top-level "asm" statements, though, can be replaced by "section" attributes.
* unit-at-a-time mode removes unreferenced static variables and functions. This may result in undefined
references when an "asm" statement refers directly to variables or functions that are otherwise unused.
In that case either the variable/function shall be listed as an operand of the "asm" statement operand
or, in the case of top-level "asm" statements the attribute "used" shall be used on the declaration.
* Static functions now can use non-standard passing conventions that may break "asm" statements calling
functions directly. Again, attribute "used" will prevent this behavior.
As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future
releases of GCC.
Enabled at levels -O, -O2, -O3, -Os.
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo
register. This allows the register allocation pass to operate on pseudos directly, but also strengthens
several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, how-
ever, make debugging impossible, since variables will no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents whole program being compiled. All public functions and
variables with the exception of "main" and those merged by attribute "externally_visible" become static
functions and in a affect gets more aggressively optimized by interprocedural optimizers. While this
option is equivalent to proper use of "static" keyword for programs consisting of single file, in combina-
tion with option --combine this flag can be used to compile most of smaller scale C programs since the
functions and variables become local for the whole combined compilation unit, not for the single source
file itself.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, we perform a copy-propagation
pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-generate
Enable options usually used for instrumenting application to produce profile useful for later recompilation
with profile feedback based optimization. You must use -fprofile-generate both when compiling and when
linking your program.
The following options are enabled: "-fprofile-arcs", "-fprofile-values", "-fvpt".
-fprofile-use
Enable profile feedback directed optimizations, and optimizations generally profitable only with profile
feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops",
"-ftracer", "-fno-loop-optimize".
The following options control compiler behavior regarding floating point arithmetic. These options trade off
between speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating point variables in registers, and inhibit other options that might change whether a
floating point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating regis-
ters (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the x86 archi-
tecture. For most programs, the excess precision does only good, but a few programs rely on the precise
definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all
pertinent intermediate computations into variables.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only, -fno-round-
ing-math, -fno-signaling-nans and fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt. A
program that relies on IEEE exceptions for math error handling may want to use this flag for speed while
maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to
consider the possibility that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and
(b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files
that change the default FPU control word or other similar optimizations.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or
+-Infs.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps
include division by zero, overflow, underflow, inexact result and invalid operation. This option implies
-fno-signaling-nans. Setting this option may allow faster code if one relies on "non-stop" IEEE arith-
metic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default floating point rounding behavior. This is
round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic
truncations. This option should be specified for programs that change the FP rounding mode dynamically, or
that may be executed with a non-default rounding mode. This option disables constant folding of floating
point expressions at compile-time (which may be affected by rounding mode) and arithmetic transformations
that are unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are
affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99’s
"FENV_ACCESS" pragma. This command line option will be used to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point opera-
tions. Setting this option disables optimizations that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect
signaling NaN behavior.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of implicitly converting it to double
precision constant.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex divi-
sion. The default is -fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the
option applies to all languages.
The following options control optimizations that may improve performance, but are not enabled by any -O
options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-prob-
abilities, to improve optimizations based on the number of times each branch was taken. When the program
compiled with -fprofile-arcs exits it saves arc execution counts to a file called sourcename.gcda for each
source file The information in this data file is very dependent on the structure of the generated code, so
you must use the same source code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be
used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing
which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is
taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program
is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and adds
REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about values
of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based
on them. Currently the optimizations include specialization of division operation using the knowledge
about the value of the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register
allocation. This optimization will most benefit processors with lots of registers. Depending on the debug
information format adopted by the target, however, it can make debugging impossible, since variables will
no longer stay in a "home register".
Enabled by default with -funroll-loops.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of
the function allowing other optimizations to do better job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -fun-
roll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually
makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
-fpeel-loops
Peels the loops for that there is enough information that they do not roll much (from profile feedback).
It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of itera-
tions).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the new loop optimizer. Enabled at level -O1
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches
(modified according to result of the condition).
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of
loops that access large arrays.
Disabled at level -Os.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports arbitrary
sections. The name of the function or the name of the data item determines the section’s name in the out-
put file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in
the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2
have linkers with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so. When you specify these options,
the assembler and linker will create larger object and executable files and will also be slower. You will
not be able to use "gprof" on all systems if you specify this option and you may have problems with debug-
ging if you specify both this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of target
registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-
block scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don’t reuse branch target registers in within any
basic block.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a
guard variable to functions with vulnerable objects. This includes functions that call alloca, and func-
tions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then
checked when the function exits. If a guard check fails, an error message is printed and the program
exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For exam-
ple, GCC will not inline functions that contain more that a certain number of instructions. You can con-
trol some of these constants on the command-line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler,
and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are given in the following table:
salias-max-implicit-fields
The maximum number of fields in a variable without direct structure accesses for which structure alias-
ing will consider trying to track each field. The default is 5
sra-max-structure-size
The maximum structure size, in bytes, at which the scalar replacement of aggregates (SRA) optimization
will perform block copies. The default value, 0, implies that GCC will select the most appropriate
size itself.
sra-field-structure-ratio
The threshold ratio (as a percentage) between instantiated fields and the complete structure size. We
say that if the ratio of the number of bytes in instantiated fields to the number of bytes in the com-
plete structure exceeds this parameter, then block copies are not used. The default is 75.
max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjump-
ing is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive
optimization, making the compile time increase with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions which must be matched at the end of two blocks before crossjumping
will be performed on them. This value is ignored in the case where all instructions in the block being
crossjumped from are matched. The default value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is
relative to a jump instruction. The default value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid
O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and
unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than
max-goto-duplication-insns are unfactored. The default value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a delay slot.
If more than this arbitrary number of instructions is searched, the time savings from filling the delay
slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making
the compile time increase with probably small improvement in executable run time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when searching for a
block with valid live register information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compile time. This parameter should be removed when the delay
slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated in order to perform the global common
subexpression elimination optimization. If more memory than specified is required, the optimization
will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run. The default is 1.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow before flushing the current state and
starting over. Large functions with few branches or calls can create excessively large lists which
needlessly consume memory and resources.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This number sets the maximum number of
instructions (counted in GCC’s internal representation) in a single function that the tree inliner will
consider for inlining. This only affects functions declared inline and methods implemented in a class
declaration (C++). The default value is 450.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be con-
sidered for inlining by the compiler will be investigated. To those functions, a different (more
restrictive) limit compared to functions declared inline can be applied. The default value is 90.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after inlining
inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid
extreme compilation time caused by non-linear algorithms used by the backend. This parameter is
ignored when -funit-at-a-time is not used. The default value is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored
when -funit-at-a-time is not used. The default value is 100 which limits large function growth to 2.0
times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units larger than this limit
is limited by --param inline-unit-growth. For small units this might be too tight (consider unit con-
sisting of function A that is inline and B that just calls A three time. If B is small relative to A,
the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of
small inlininable functions however the overall unit growth limit is needed to avoid exponential explo-
sion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before
aplying --param inline-unit-growth. The default is 10000
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored
when -funit-at-a-time is not used. The default value is 50 which limits unit growth to 1.5 times the
original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow
into by performing recursive inlining.
For functions declared inline --param max-inline-insns-recursive is taken into account. For function
not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-insns-recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive inlining.
For functions declared inline --param max-inline-recursive-depth is taken into account. For function
not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-recursive-depth-auto is used. The default value is 450.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt for
function having little recursion depth by increasing the prologue size or complexity of function body
to other optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed
from probability that function will recurse via given call expression. This parameter limits inlining
only to call expression whose probability exceeds given threshold (in percents). The default value is
10.
inline-call-cost
Specify cost of call instruction relative to simple arithmetics operations (having cost of 1).
Increasing this cost disqualifies inlining of non-leaf functions and at the same time increases size of
leaf function that is believed to reduce function size by being inlined. In effect it increases amount
of inlining for code having large abstraction penalty (many functions that just pass the arguments to
other functions) and decrease inlining for code with low abstraction penalty. The default value is 16.
max-unrolled-insns
The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is
unrolled, it determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop should have
if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is
unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is
peeled, it determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables below that all candidates are considered for each
use in induction variable optimizations. Only the most relevant candidates are considered if there are
more candidates, to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs
from the set during its optimization when a new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the ana-
lyzer.
vect-max-version-checks
The maximum number of runtime checks that can be performed when doing loop versioning in the vector-
izer. See option ftree-vect-loop-version for more information.
max-iterations-to-track
The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of
the loop tries to evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in program given basic block needs
to have to be considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic block in function given basic block
needs to have to be considered hot
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where function
contain single loop with known bound and other loop with unknown. We predict the known number of iter-
ations correctly, while the unknown number of iterations average to roughly 10. This means that the
loop without bounds would appear artificially cold relative to the other one.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is
covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real pro-
files (as opposed to statically estimated ones) are much less balanced allowing the threshold to be
larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as
most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher val-
ues than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback
and one for compilation without. The value for compilation with profile feedback needs to be more con-
servative (higher) in order to make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers. The default is 10.
max-cse-insns
The maximum instructions CSE process before flushing. The default is 1000.
global-var-threshold
Counts the number of function calls (n) and the number of call-clobbered variables (v). If nxv is
larger than this limit, a single artificial variable will be created to represent all the call-clob-
bered variables at function call sites. This artificial variable will then be made to alias every
call-clobbered variable. (done as "int * size_t" on the host machine; beware overflow).
max-aliased-vops
Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping
heuristic. Alias grouping reduces compile times and memory consumption needed for aliasing at the
expense of precision loss in alias information.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum
percentage by which the garbage collector’s heap should be allowed to expand between collections. Tun-
ing this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is
available, the notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC
is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this
parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is
extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector’s heap before it begins bothering to collect garbage. The first
collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning
this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to ensure that RLIMIT_DATA or
RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound
is used. Setting this parameter very large effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register. Increasing val-
ues mean more aggressive optimization, making the compile time increase with probably slightly better
performance. The default value is 100.
max-cselib-memory-location
The maximum number of memory locations cselib should take into account. Increasing values mean more
aggressive optimization, making the compile time increase with probably slightly better performance.
The default value is 500.
max-flow-memory-location
Similar as max-cselib-memory-location but for dataflow liveness. The default value is 100.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code
on its destination. Code is duplicated when its estimated size is smaller than this value multiplied
by the estimated size of unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to
higher values than reorder-block-duplicate since information about the hot spots is more accurate.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider at any given time
during the first scheduling pass. Increasing values mean more thorough searches, making the compila-
tion time increase with probably little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling. The default value
is 10.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling. The default value
is 100.
min-sched-prob
The minimum probability of reaching a source block for interblock speculative scheduling. The default
value is 40.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a
pseudo register as last known value of that register. The default is 10000.
integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler’s memory usage and
increasing its speed. This sets the maximum value of a shared integer constant’s. The default value
is 256.
min-virtual-mappings
Specifies the minimum number of virtual mappings in the incremental SSA updater that should be regis-
tered to trigger the virtual mappings heuristic defined by virtual-mappings-ratio. The default value
is 100.
virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols
to be updated, then the incremental SSA updater switches to a full update for those symbols. The
default ratio is 3.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when -fstack-pro-
tection is used.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure we will treat in a field sensitive manner during pointer analy-
sis.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together
with -E because they cause the preprocessor output to be unsuitable for actual compilation.
You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor.
If option contains commas, it is split into multiple options at the commas. However, many options are mod-
ified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp
forcibly bypasses this phase. The preprocessor’s direct interface is undocumented and subject to change,
so whenever possible you should avoid using -Wp and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor
options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the
option and once for the argument.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase three
in a #define directive. In particular, the definition will be truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell’s
quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding
parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh, -D’name(args...)=definition’ works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and
-include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain
defined.
-I dir
Add the directory dir to the list of directories to be searched for header files. Directories named by -I
are searched before the standard system include directories. If the directory dir is a standard system
include directory, the option is ignored to ensure that the default search order for system directories and
the special treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc
has a different interpretation of a second non-option argument, so you must use -o to specify the output
file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtri-
graphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions.
Note that many of the preprocessor’s warnings are on by default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears
in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form
an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends. There-
fore, only trigraphs that would form escaped newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs
-Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and problematic constructs which should be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined.
Such identifiers are replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested
for existence at least once. The preprocessor will also warn if the macro has not been used at the time it
is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned
about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as
unused. To avoid the warning in such a case, you might improve the scope of the macro’s definition by, for
example, moving it into the first skipped block. Alternatively, you could provide a dummy use with some-
thing like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This warning is on
by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code,
therefore suppressed. If you are responsible for the system library, you may want to see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since
they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes manda-
tory diagnostics that GCC issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependen-
cies of the main source file. The preprocessor outputs one make rule containing the object file name for
that source file, a colon, and the names of all the included files, including those coming from -include or
-imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of the source
file with any suffix replaced with object file suffix. If there are many included files then the rule is
split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor’s debug output, such as -dM. To avoid mixing such debug
output with the dependency rules you should explicitly specify the dependency output file with -MF, or use
an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output
stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header files that
are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself
determine whether that header will appear in -MM dependency output. This is a slight change in semantics
from GCC versions 3.0 and earlier.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files
are generated files and adds them to the dependency list without raising an error. The dependency filename
is taken directly from the "#include" directive without prepending any path. -MG also suppresses prepro-
cessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each
to depend on nothing. These dummy rules work around errors make gives if you remove header files without
updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main
input file, including any path, deletes any file suffix such as .c, and appends the platform’s usual object
suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you
can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT ’$(objpfx)foo.o’ might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ ’$(objpfx)foo.o’ gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise
it take the basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file
(but @pxref{dashMF,,-MF}), but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the com-
pilation process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to also list the files
from the precompiled header’s dependencies. If not specified only the precompiled header would be listed
and not the files that were used to create it because those files are not consulted when a precompiled
header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma
GCC pch_preprocess "
and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input
to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file
is available in a different location. The filename may be absolute or it may be relative to GCC’s current
directory.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards con-
formance or extensions; it merely selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will
treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards
conformance level. This option has been removed, because it conflicts with the -l option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; oth-
ers may be added in the future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I- Split the include path. Any directories specified with -I options before -I- are searched only for headers
requested with "#include "file""; they are not searched for "#include
are specified with -I options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search direc-
tory for "#include "file"". This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the directories you have specified
with -I options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the other
standard directories. (This option is used when building the C++ library.)
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the
first directory searched for file is the preprocessor’s working directory instead of the directory contain-
ing the main source file. If not found there, it is searched for in the remainder of the "#include "...""
search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the command
line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines
remain defined. This allows you to acquire all the macros from a header without also processing its decla-
rations.
All files specified by -imacros are processed before all files specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard system
directories have been exhausted. dir is treated as a system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory,
you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include
search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter
would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files. See the --sysroot option for
more information.
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system directo-
ries. Mark it as a system directory, so that it gets the same special treatment as is applied to the stan-
dard system directories.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not searched for
"#include
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is experimental; in a future version of GCC,
it will be enabled by default for C99 and C++.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things
like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to
the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer
for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings
or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is
ignored. The default is 8.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8. charset
can be any encoding supported by the system’s "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is UTF-32
or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any
encoding supported by the system’s "iconv" library routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the source
character set used by GCC. If the locale does not specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by either the locale or this command line option.
Currently the command line option takes precedence if there’s a conflict. charset can be any encoding sup-
ported by the system’s "iconv" library routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the compiler know the current
working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it’s present in the preprocessed input, as the directory emit-
ted as the current working directory in some debugging information formats. This option is implicitly
enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-work-
ing-directory. If the -P flag is present in the command line, this option has no effect, since no "#line"
directives are emitted whatsoever.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a
program that does not understand the column numbers, such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form
-A predicate(answer), which is still supported, because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other
characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are
silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define directives for all the macros defined during
the execution of the preprocessor, including predefined macros. This gives you a way of finding out
what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
D Like M except in two respects: it does not include the predefined macros, and it outputs both the
#define directives and the result of preprocessing. Both kinds of output go to the standard output
file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running
the preprocessor on something that is not C code, and will be sent to a program which might be confused by
the linemarkers.
-C Do not discard comments. All comments are passed through to the output file, except for comments in pro-
cessed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as
tokens in their own right. For example, comments appearing at the start of what would be a directive line
have the effect of turning that line into an ordinary source line, since the first token on the line is no
longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments contained
within macros are also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a
macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently
commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting with ??, that are defined by
ISO C to stand for single characters. For example, ??/ stands for \, so ’??/n’ is a character constant for
a newline. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the
-std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??’ ??! ??-
Replacement: [ ] { } # \ ^ │ ~
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP’s version number at the beginning of execution, and report the final form
of the include path.
-H Print the name of each header file used, in addition to other normal activities. Each name is indented to
show how deep in the #include stack it is. Precompiled header files are also printed, even if they are
found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
-version
--version
Print out GNU CPP’s version number. With one dash, proceed to preprocess as normal. With two dashes, exit
immediately.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple options at
the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler options
which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option
and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are
meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or
library. (Object files are distinguished from libraries by the linker according to the file contents.) If
linking is done, these object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used as
arguments.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a separate
argument is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes
libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named libli-
brary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with -L.
Normally the files found this way are library files---archive files whose members are object files. The
linker handles an archive file by scanning through it for members which define symbols that have so far
been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in
the usual fashion. The only difference between using an -l option and specifying a file name is that -l
surrounds library with lib and .a and searches several directories.
-lobjc
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used nor-
mally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be passed to
the linker. The standard startup files are used normally, unless -nostartfiles is used. The compiler may
generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by
entries in libc. These entry points should be supplied through some other mechanism when this option is
specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only the
libraries you specify will be passed to the linker. The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should
be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal
subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some lan-
guages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words,
when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that
you have no unresolved references to internal GCC library subroutines. (For example, __main, used to
ensure C++ constructors will be called.)
-pie
Produce a position independent executable on targets which support it. For predictable results, you must
also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions)
when you specify this option.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to
add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of
"dlopen" or to allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On other sys-
tems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all systems
support this option. For predictable results, you must also specify the same set of options that were used
to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the shared or
static version respectively. If no shared version of libgcc was built when the compiler was configured,
these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static
version. The most common of these is when the application wishes to throw and catch exceptions across dif-
ferent shared libraries. In that case, each of the libraries as well as the application itself should use
the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or
a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to
do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be
linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or
a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into
shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking
with the shared version of libgcc, linking with the static version of libgcc by default. This allows
exceptions to propagate through such shared libraries, without incurring relocation costs at library load
time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using
the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared libgcc.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved references
(unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this
option.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options which
GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the option and
once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker
definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire
string as a single argument, which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options at
the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u
multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This can be
used to override a system header file, substituting your own version, since these directories are searched
before the system header file directories. However, you should not use this option to add directories that
contain vendor-supplied system header files (use -isystem for that). If you use more than one -I option,
the directories are scanned in left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I,
the -I option will be ignored. The directory will still be searched but as a system directory at its nor-
mal position in the system include chain. This is to ensure that GCC’s procedure to fix buggy system head-
ers and the ordering for the include_next directive are not inadvertently changed. If you really need to
change the search order for system directories, use the -nostdinc and/or -isystem options.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header files only for the
case of #include "file"; they are not searched for #include
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the com-
piler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a
prefix for each program it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not
found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and
/usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name
is searched for using the directories specified in your PATH environment variable.
The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it
will add a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the
compiler translates these options into -L options for the linker. They also apply to includes files in the
preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In
this case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not
found there, the two standard prefixes above are tried, and that is all. The file is left out of the link
if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9,
then it will be replaced by [dir/]include. This is to help with boot-strapping the compiler.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults that
the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than
one -specs=file can be specified on the command line, and they are processed in order, from left to right.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler would nor-
mally search for headers in /usr/include and libraries in /usr/lib, it will instead search dir/usr/include
and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option will apply to libraries,
but the -isysroot option will apply to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker
does not support this option, the header file aspect of --sysroot will still work, but the library aspect
will not.
-I- This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove
the -I-. Any directories you specify with -I options before the -I- option are searched only for the case
of #include "file"; they are not searched for #include
If additional directories are specified with -I options after the -I-, these directories are searched for
all #include directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input file came
from) as the first search directory for #include "file". There is no way to override this effect of -I-.
With -I. you can specify searching the directory which was current when the compiler was invoked. That is
not exactly the same as what the preprocessor does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc
are independent.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or
venient, so GCC provides options that will switch to another cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type when configuring GCC as a
cross-compiler. For example, if a cross-compiler was configured with configure arm-elf, meaning to compile
for an arm processor with elf binaries, then you would specify -b arm-elf to run that cross compiler.
Because there are other options beginning with -b, the configuration must contain a hyphen.
-V version
The argument version specifies which version of GCC to run. This is useful when multiple versions are
installed. For example, version might be 4.0, meaning to run GCC version 4.0.
The -V and -b options work by running the
them if you can just run that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed compilers for completely
different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with -m, to choose
among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A
single installed version of the compiler can compile for any model or configuration, according to the options
specified.
Some configurations of the compiler also support additional special options, usually for compatibility with
other compilers on the same platform.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
-mmangle-cpu
Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC
variants with different instruction and register set characteristics. This flag prevents code compiled for
one cpu to be linked with code compiled for another. No facility exists for handling variants that are
"almost identical". This is an all or nothing option.
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants
support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-section, data-section, and readonly-data-section respec-
tively by default. This can be overridden with the "section" attribute.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwm-
mxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if
this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with
this option will cause the stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code which supports calling between the ARM and Thumb instruction sets. Without this option the
two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork,
since slightly larger code is generated when -mthumb-interwork is specified.
-mno-sched-prolog
Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the
instructions in the function’s body. This means that all functions will start with a recognizable set of
instructions (or in fact one of a choice from a small set of different function prologues), and this infor-
mation can be used to locate the start if functions inside an executable piece of code. The default is
-msched-prolog.
-mhard-float
Generate output containing floating point instructions. This is the default.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all ARM targets. Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you need to compile libgcc.a, the library that comes
with GCC, with -msoft-float in order for this to work.
-mfloat-abi=name
Specifies which ABI to use for floating point values. Permissible values are: soft, softfp and hard.
soft and hard are equivalent to -msoft-float and -mhard-float respectively. softfp allows the generation
of floating point instructions, but still uses the soft-float calling conventions.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard configu-
rations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a little-
endian processor.
-mwords-little-endian
This option only applies when generating code for big-endian processors. Generate code for a little-endian
word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option
should only be used if you require compatibility with code for big-endian ARM processors generated by ver-
sions of the compiler prior to 2.8.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instruc-
tions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710,
arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm, strongarm110, strongarm1100,
arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore,
mpcorenovfp, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312.
-mtune=name
This option is very similar to the -mcpu= option, except that instead of specifying the actual target pro-
cessor type, and hence restricting which instructions can be used, it specifies that GCC should tune the
performance of the code as if the target were of the type specified in this option, but still choosing the
instructions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementa-
tions better performance can be obtained by using this option.
-march=name
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. This option can be used in conjunction with or
instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5,
armv5t, armv5te, armv6, armv6j, iwmmxt, ep9312.
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating point hardware (or hardware emulation) is available on the target. Permissi-
ble names are: fpa, fpe2, fpe3, maverick, vfp. -mfp and -mfpe are synonyms for -mfpu=fpenumber, for com-
patibility with older versions of GCC.
If -msoft-float is specified this specifies the format of floating point values.
-mstructure-size-boundary=n
The size of all structures and unions will be rounded up to a multiple of the number of bits set by this
option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the
COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI sup-
ports it.
Specifying the larger number can produce faster, more efficient code, but can also increase the size of the
program. Different values are potentially incompatible. Code compiled with one value cannot necessarily
expect to work with code or libraries compiled with another value, if they exchange information using
structures or unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It will be executed if the
function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register
and then performing a subroutine call on this register. This switch is needed if the target function will
lie outside of the 64 megabyte addressing range of the offset based version of subroutine call instruction.
Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic is
that static functions, functions which have the short-call attribute, functions that are inside the scope
of a #pragma no_long_calls directive and functions whose definitions have already been compiled within the
current compilation unit, will not be turned into long calls. The exception to this rule is that weak
function definitions, functions with the long-call attribute or the section attribute, and functions that
are within the scope of a #pragma long_calls directive, will always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as
will placing the function calls within the scope of a #pragma long_calls_off directive. Note these
switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each
function. The run-time system is responsible for initializing this register with an appropriate value
before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled,
when R9 is used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruc-
tion combinations. This option is only valid if the -mcpu=ep9312 option has been used to enable generation
of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by
default, since the problem is only present in older Maverick implementations. The default can be re-
enabled by use of the -mno-cirrus-fix-invalid-insns switch.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The gen-
erated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace
function then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function
name embedded immediately preceding this location and has length "((pc[-3]) & 0xff000000)".
-mthumb
Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions.
(A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A
leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which
switches to Thumb mode before executing the rest of the function. This allows these functions to be called
from non-interworking code.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether
the target code has been compiled for interworking or not. There is a small overhead in the cost of exe-
cuting a function pointer if this option is enabled.
-mtp=name
Specify the access model for the thread local storage pointer. The valid models are soft, which generates
calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the
arm6k architecture), and auto, which uses the best available method for the selected processor. The
default setting is auto.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler pro-
grams (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types:
at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types:
atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8,
atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types:
atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a symbol or numeric value, __stack is the default.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size will be smaller.
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate subroutines. Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes increase code size.
-mtiny-stack
Change only the low 8 bits of the stack pointer.
-mint8
Assume int to be 8 bit integer. This affects the sizes of all types: A char will be 1 byte, an int will be
1 byte, an long will be 2 bytes and long long will be 4 bytes. Please note that this option does not com-
ply to the C standards, but it will provide you with smaller code size.
Blackfin Options
-momit-leaf-frame-pointer
Don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set
up and restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler will ensure that the generated code does not contain speculative loads after
jump instructions. This option is enabled by default.
-mno-specld-anomaly
Don’t generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC instructions
too soon after conditional branches. This option is enabled by default.
-mno-csync-anomaly
Don’t generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a condi-
tional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the
low 64k of memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place
and shared libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn’t assume ID based shared libraries are being used. This is the default.
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0
will generate more compact code, specifying other values will force the allocation of that number to the
current library but is no more space or time efficient than omitting this option.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register
and then performing a subroutine call on this register. This switch is needed if the target function will
lie outside of the 24 bit addressing range of the offset based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior.
Note these switches have no effect on how the compiler generates code to handle function calls via function
pointers.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the
default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The choices for architecture-type are the same as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-melinux-stacksize=n
Only available with the cris-axis-aout target. Arranges for indications in the program to the kernel
loader that the stack of the program should be set to n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is
active by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the
effect to turn off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly
file.
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test instructions
before use of condition codes.
-mno-side-effects
Do not emit instructions with side-effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and con-
stants to be aligned for the maximum single data access size for the chosen CPU model. The default is to
arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable
data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue that sets up the stack-frame are
omitted and no return instructions or return sequences are generated in the code. Use this option only
together with visual inspection of the compiled code: no warnings or errors are generated when call-saved
registers must be saved, or storage for local variable needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don’t generate (do generate) instruction sequences that load addresses for functions
from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The
default is -mgotplt.
-maout
Legacy no-op option only recognized with the cris-axis-aout target.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files
and instruction set for -march=v8.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output func-
tions from a simulator library. Code, initialized data and zero-initialized data are allocated consecu-
tively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at
0x80000000.
CRX Options
These options are defined specifically for the CRX ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mpush-args
Push instructions will be used to pass outgoing arguments when functions are called. Enabled by default.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it will create an object file for the single architecture
that it was built to target. Apple’s GCC on Darwin does create "fat" files if multiple -arch options are used;
it does so by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the
ISA that GCC is targetting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override
this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, will only per-
mit instructions to be used that are valid for the subtype of the file it is generating, so you cannot put
64-bit instructions in an ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail and
print an error if asked to create a shared library with a less restrictive subtype than its input files (for
instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, will qui-
etly give the executable the most restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header files.
These directories are interleaved with those specified by -I options and are scanned in a left-to-right
order.
A framework directory is a directory with frameworks in it. A framework is a directory with a "Headers"
and/or "PrivateHeaders" directory contained directly in it that ends in ".framework". The name of a frame-
work is the name of this directory excluding the ".framework". Headers associated with the framework are
found in one of those two directories, with "Headers" being searched first. A subframework is a framework
directory that is in a framework’s "Frameworks" directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two
subframeworks are siblings if they occur in the same framework. A subframework should not have the same
name as a framework, a warning will be issued if this is violated. Currently a subframework cannot have
subframeworks, in the future, the mechanism may be extended to support this. The standard frameworks can
be found in "/System/Library/Frameworks" and "/Library/Frameworks". An example include looks like
"#include
the "PrivateHeaders" or "Headers" directory.
-gused
Emit debugging information for symbols that are used. For STABS debugging format, this enables -felimi-
nate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical values of version
include 10.1, 10.2, and 10.3.9.
The default for this option is to make choices that seem to be most useful.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for
Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Using this switch may require recompiling all other modules in a program,
including system libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turn around development. Needed to enable gdb to dynamically load ".o"
files into already running programs. -findirect-data and -ffix-and-continue are provided for backwards
compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when
the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will be loading the build output file being linked. See man
ld(1) for more information.
-dynamiclib
When passed this option, GCC will produce a dynamic library instead of an executable when linking, using
the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC’s output file to have the ALL subtype, instead of one controlled by the -mcpu or -march
option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float
is specified, functions in libgcc.a will be used to perform floating-point operations. Unless they are
replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point operations. If you are compiling for an
Alpha without floating-point operations, you must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point reg-
isters.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float.
If the floating-point register set is not used, floating point operands are passed in integer registers as
if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard
calling sequence, so any function with a floating-point argument or return value called by code compiled
with -mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore,
any floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly
compliant with the IEEE floating point standard. However, for full compliance, software assistance is
required. This option generates code fully IEEE compliant code except that the inexact-flag is not main-
tained (see below). If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during com-
pilation. The resulting code is less efficient but is able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this
option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option
causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may
execute significantly slower than the code generated by default. Since there is very little code that
depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this option
-fptm trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot be dis-
abled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like su, but the instructions are marked to be safe for software completion (see Alpha architecture
manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-
mode can be one of:
n Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or
towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture
reference manual) controls the rounding mode in effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round
towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating point traps are imprecise. This means without software assistance it
is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC
can generate code that can assist operating system trap handlers in determining the exact location that
caused a floating point trap. Depending on the requirements of an application, different levels of preci-
sions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which program
caused a floating point exception.
f Function precision. The trap handler can determine the function that caused a floating point excep-
tion.
i Instruction precision. The trap handler can determine the exact instruction that caused a floating
point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you also
specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit
the line .eflag 48 in the function prologue of the generated assembly file. Under DEC Unix, this has the
effect that IEEE-conformant math library routines will be linked in.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants
in two or three instructions. If it cannot, it will output the constant as a literal and generate code to
load it from the data segment at runtime.
Use this option to require GCC to construct all integer constants using code, even if it takes more
instructions (the maximum is six).
You would typically use this option to build a shared library dynamic loader. Itself a shared library, it
must relocate itself in memory before it can find the variables and constants in its own data segment.
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by the GNU
assembler -mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The
default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the
CPU on which GCC was built if none was specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating point arithmetic instead of IEEE single and
double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of
these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new
syntax that allows the compiler to explicitly mark which relocations should apply to which instructions.
This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is
built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When
-msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and
".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of
the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that
require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in
the program’s data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library)
fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can
assume that all local symbols share the same $gp value, and thus reduce the number of instructions required
for a function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify
either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4,
EV5 and EV6 family of processors and will choose the default values for the instruction set from the pro-
cessor you specify. If you do not specify a processor type, GCC will default to the processor on which the
compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not
changed.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the application.
This number is highly dependent on the memory access patterns used by the application and the size of the
external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the
Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that
L3 is only valid for EV5.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.
FRV Options
-mgpr-32
Only use the first 32 general purpose registers.
-mgpr-64
Use all 64 general purpose registers.
-mfpr-32
Use only the first 32 floating point registers.
-mfpr-64
Use all 64 floating point registers
-mhard-float
Use hardware instructions for floating point operations.
-msoft-float
Use library routines for floating point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating point double instructions.
-mno-double
Do not use floating point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, that uses function descriptors to represent pointers to functions. Without any
PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are
within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32
bits.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has
no effect without -mfdpic. It’s enabled by default if optimizing for speed and compiling for shared
libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the
command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections.
It’s enabled by default, except for -fpic or -fpie: even though it may help make the global offset table
smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which
may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so
it’s more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It’s implied by -mlibrary-pic, as well as by -fPIC and
-fpic without -mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This
option is enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the functions
to be placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting nops into the previous packet. This option only has
an effect when VLIW packing is enabled. It doesn’t create new packets; it merely adds nops to existing
ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "││" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "││" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler generated code. It is enabled by
default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500,
fr450, fr405, fr400, fr300 and simple.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and
H8S is to align longs and floats on 4 byte boundaries. -malign-300 causes them to be aligned on 2 byte
boundaries. This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1
for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine
the proper architecture option for your machine. Code compiled for lower numbered architectures will run
on higher numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about
out of range branches within a switch table.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for
the function call to be the target of the conditional jump.
-mdisable-fpregs
Prevent floating point registers from being used in any manner. This is necessary for compiling kernels
which perform lazy context switching of floating point registers. If you use this option and attempt to
perform floating point operations, the compiler will abort.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when com-
piling MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect
calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which performs
faster indirect calls.
This option will not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the reg-
ister allocator can not use. This is useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is
equivalent to the +k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also
triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some
programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all HPPA targets. Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded target hppa1.1-*-pro does provide software floating
point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you need to compile libgcc.a, the library that comes
with GCC, with -msoft-float in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines,
"__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX
and HI-UX.
-mgnu-ld
Use GNU ld specific options. This passes -shared to ld when building a shared library. It is the default
when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not have any
affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called
is determined by the --with-ld configure option, GCC’s program search path, and finally by the user’s PATH.
The linker used by GCC can be printed using which ‘gcc -print-prog-name=ld‘. This option is only available
on the 64 bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use HP ld specific options. This passes -b to ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with
the HP linker. This option does not have any affect on which ld is called, it only changes what parameters
are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC’s pro-
gram search path, and finally by the user’s PATH. The linker used by GCC can be printed using which ‘gcc
-print-prog-name=ld‘. This option is only available on the 64 bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker gen-
erated stubs. The default is to generate long calls only when the distance from the call site to the
beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the
branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the
PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when
using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it will degrade performance. However, it may be useful
in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code
being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or
pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in
pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-
std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later.
98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10
though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for
"XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional pre-
defines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also
affects the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the
variable __xpg4_extended_mask as appropriate. Most GNU software doesn’t provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX
10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn’t an archive ver-
sion of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve
this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static
option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers gen-
erate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver
from adding these link options.
-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both
the preprocessor and linker.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of avail-
able instructions. The choices for cpu-type are:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which
your code will run, then you should use the corresponding -mtune option instead of -mtune=generic.
But, if you do not know exactly what CPU users of your application will have, then you should use this
option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore,
if you upgrade to a newer version of GCC, the code generated option will change to reflect the proces-
sors that were most common when that version of GCC was released.
There is no -march=generic option because -march indicates the instruction set the compiler can use,
and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates
the processor (or, in this case, collection of processors) for which the code is optimized.
i386
Original Intel’s i386 CPU.
i486
Intel’s i486 CPU. (No scheduling is implemented for this chip.)
i586, pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro instruction set will be used, so the
code will run on all i686 familly chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
pentium3, pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Cen-
trino notebooks.
pentium4, pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set
support.
core2
Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2, k6-3
Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
athlon, athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions support.
athlon-4, athlon-xp, athlon-mp
Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE instruction set support.
k8, opteron, athlon64, athlon-fx
AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3dNOW!,
enhanced 3dNOW! and 64-bit instruction set extensions.)
amdfam10
AMD Family 10 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2,
SSE3, SSE4A, SSE5, 3dNOW!, enhanced 3dNOW!, ABM and 64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2
IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW! instruction set support.
c3 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is implemented for this chip.)
c3-2
Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler
will not generate any code that does not run on the i386 without the -march=cpu-type option being used.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune.
Moreover, specifying -march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respec-
tively. These synonyms are deprecated.
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The choices for unit are:
387 Use the standard 387 floating point coprocessor present majority of chips and emulated otherwise. Code
compiled with this option will run almost everywhere. The temporary results are computed in 80bit pre-
cision instead of precision specified by the type resulting in slightly different results compared to
most of other chips. See -ffloat-store for more detailed description.
This is the default choice for i386 compiler.
sse Use scalar floating point instructions present in the SSE instruction set. This instruction set is
supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The
earlier version of SSE instruction set supports only single precision arithmetics, thus the double and
extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and
the future AMD x86-64 chips supports double precision arithmetics too.
For the i386 compiler, you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE exten-
sions and make this option effective. For the x86-64 compiler, these extensions are enabled by
default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical
instability problems of 387 code, but may break some existing code that expects temporaries to be
80bit.
This is the default choice for the x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This effectively double the amount of available reg-
isters and on chips with separate execution units for 387 and SSE the execution resources too. Use
this option with care, as it is still experimental, because the GCC register allocator does not model
separate functional units well resulting in instable performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or att (the default one).
Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case
where the result of a comparison is unordered.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not part
of GCC. Normally the facilities of the machine’s usual C compiler are used, but this can’t be done
directly in cross-compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation.
On machines where a function returns floating point results in the 80387 register stack, some floating
point opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU register,
even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this
option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD.
This option is overridden when -march indicates that the target cpu will always have an FPU and so the
instruction will not need emulation. As of revision 2.6.1, these instructions are not generated unless you
also use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two word boundary or a
one word boundary. Aligning "double" variables on a two word boundary will produce code that runs somewhat
faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types will be aligned dif-
ferently than the published application binary interface specifications for the 386 and will not be binary
compatible with structures in code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The i386 application binary interface specifies the
size to be 96 bits, so -m96bit-long-double is the default in 32 bit mode.
Modern architectures (Pentium and newer) would prefer "long double" to be aligned to an 8 or 16 byte bound-
ary. In arrays or structures conforming to the ABI, this would not be possible. So specifying a
-m128bit-long-double will align "long double" to a 16 byte boundary by padding the "long double" with an
additional 32 bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double"
is to be aligned on 16 byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a
"long double".
Warning: if you override the default value for your target ABI, the structures and arrays containing "long
double" variables will change their size as well as function calling convention for function taking "long
double" will be modified. Hence they will not be binary compatible with arrays or structures in code com-
piled without that switch.
-mmlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold are placed in large data section. This
value must be the same across all object linked into the binary and defaults to 65535.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the "bss" or "data" segments. -msvr3-shlib
places them into "bss". These options are meaningful only on System V Release 3.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "ret" num instruction, which pops their arguments while returning. This saves one instruc-
tion in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function
attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it
if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Nor-
mally, extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to pass
arguments, and at most 3 registers can be used. You can control this behavior for a specific function by
using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value,
including any libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can control
this behavior for a specific function by using the function attribute sseregparm.
Warning: if you use this switch then you must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-bound-
ary is not specified, the default is 4 (16 bytes or 128 bits).
On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8 byte boundary (see
-malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD
Extension (SSE) data type "__m128" may not work properly if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that
required by any value stored on the stack. Further, every function must be generated such that it keeps
the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function
compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that
libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is sen-
sitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the
preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4a
-mno-sse4a
-msse5
-mno-sse5
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2 or 3DNow! extended instruc-
tion sets. These extensions are also available as built-in functions: see X86 Built-in Functions, for
details of the functions enabled and disabled by these switches.
To have SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387 instruc-
tions), see -mfpmath=sse.
These options will enable GCC to use these extended instructions in generated code, even without -mfp-
math=sse. Applications which perform runtime CPU detection must compile separate files for each supported
architecture, using the appropriate flags. In particular, the file containing the CPU detection code
should be compiled without these options.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as
method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve perfor-
mance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments will be computed in the function
prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and
reduced stack usage when preferred stack boundary is not equal to 2. The drawback is a notable increase in
code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception handling must
compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when link-
ing, it links in a special thread helper library -lmingwthrd which cleans up per thread exception handling
data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch reduces code size and improves perfor-
mance in case the destination is already aligned, but GCC doesn’t know about it.
-minline-all-stringops
By default GCC inlines string operations only when destination is known to be aligned at least to 4 byte
boundary. This enables more inlining, increase code size, but may improve performance of code that depends
on fast memcpy, strlen and memset for short lengths.
-momit-leaf-frame-pointer
Don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set
up and restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit,
%fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is legal depends on
the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long
and pointer to 64 bits and generates code for AMD’s x86-64 architecture.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a
128-byte area beyond the location of the stack pointer that will not be modified by signal or interrupt
handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag
-mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the
address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This
model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the lower 2 GB of the address space but sym-
bols can be located anywhere in the address space. Programs can be statically or dynamically linked, but
building of shared libraries are not supported with the medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of sections.
Currently GCC does not implement this model.
-mfused-madd
-mno-fused-madd
Enable automatic generation of fused floating point multiply-add instructions if the ISA supports such
instructions. The -mfused-madd option is on by default.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don’t) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don’t) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent code,
and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don’t) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don’t) in, loc, and out register names for the stacked registers. This may make assembler
output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working around
optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling
firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating point values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating point values using the maximum throughput algorithm.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-dwarf2-asm
-mdwarf2-asm
Don’t (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when
not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop
bit. This can improve instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the reg-
ister allocator can not use. This is useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2,
and mckinley.
-mt
-pthread
Add support for multithreading using the POSIX threads library. This option sets flags for both the pre-
processor and linker. It does not affect the thread safety of object code produced by the compiler or that
of libraries supplied with it. These are HP-UX specific flags.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX spe-
cific flags.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the
M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to be
linked in which supports, for example, file I/O. You must not use this option when generating programs
that will run on real hardware; you must provide your own runtime library for whatever I/O functions are
needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC will use during code generation. These pseudo-
registers will be used like real registers, so there is a tradeoff between GCC’s ability to fit the code
into available registers, and the performance penalty of using memory instead of registers. Note that all
modules in a program must be compiled with the same value for this option. Because of that, you must not
use this option with the default runtime libraries gcc builds.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instruc-
tions to load their addresses), and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instruc-
tions to load their addresses), and assume subroutines may not be reachable with the "bl" instruction (the
compiler will generate the much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into one of .data, bss, or .rodata (unless the
"section" attribute has been specified). This is the default.
The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data
area with the "section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to reference
them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to reference
them.
-G num
Put global and static objects less than or equal to num bytes into the small data or bss sections instead
of the normal data or bss sections. The default value of num is 8. The -msdata option must be set to one
of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or
may not work; if it doesn’t the linker will give an error message---incorrect code will not be generated.
-mdebug
Makes the M32R specific code in the compiler display some statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it is 2,
then the opposite will apply.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and
15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is
_flush_cache, but a function call will only be used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for the 68000 series. The default values for these options depends on which
style of 68000 was selected when the compiler was configured; the defaults for the most common choices are
given below.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307,
68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating point. This is the default for most 68020 sys-
tems unless --nfp was specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040.
Use this option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on
the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333,
68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X "coldfire" family cpu. This is the default when the compiler is configured for
520X-based systems.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code which can run
relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68040.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code which can run
relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68060.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all m68k targets. Normally the facilities of the machine’s usual C compiler are used, but
this can’t be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded targets m68k-*-aout and m68k-*-coff do provide soft-
ware floating point support.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack are
also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
-mnobitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use
a configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "rtd" instruction, which pops their arguments while returning. This saves one instruction
in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you
need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Nor-
mally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not
by the 68000 or 5200.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a
32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit
boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC will align structures containing the above types differ-
ently than most published application binary interface specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At
present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is
not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references will be handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text seg-
ment. This allows for execute in place in an environment without virtual memory management. This option
implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place
and shared libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn’t assume ID based shared libraries are being used. This is the default.
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0
will generate more compact code, specifying other values will force the allocation of that number to the
current library but is no more space or time efficient than omitting this option.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these
options depends on which style of microcontroller was selected when the compiler was configured; the defaults
for the most common choices are given below.
-m6811
-m68hc11
Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based sys-
tems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based sys-
tems.
-m68S12
-m68hcs12
Generate output for a 68HCS12.
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the
"call" instruction to call a function and the "rtc" instruction for returning.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is
32. Using more pseudo-soft register may or may not result in better code depending on the program. The
default is 4 for 68HC11 and 2 for 68HC12.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a four byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little endian target.
-m210
-m340
Generate code for the 210 processor.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
-march=arch
Generate code that will run on arch, which can be the name of a generic MIPS ISA, or the name of a particu-
lar processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, and mips64. The processor
names are: 4kc, 4km, 4kp, 5kc, 5kf, 20kc, 24k, 24kc, 24kf, 24kx, m4k, orion, r2000, r3000, r3900, r4000,
r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300,
vr5000, vr5400 and vr5500. The special value from-abi selects the most compatible architecture for the
selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional,
and vr may be written r.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of
target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized
value of _MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to "r2000" and define the macro
_MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have the full
prefix and will not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architec-
ture (either "mips1" or "mips3"). It names the default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the
perceived cost of arithmetic operations. The list of arch values is the same as for -march.
When this option is not used, GCC will optimize for the processor specified by -march. By using -march and
-mtune together, it is possible to generate code that will run on a family of processors, but optimize the
code for one particular member of that family.
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones
described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or MIPS64 architecture, it will make
use of the MIPS16e ASE.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a
64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see
-mabicalls
-mno-abicalls
Generate (do not generate) SVR4-style position-independent code. -mabicalls is the default for SVR4-based
systems.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it
will only work if the GOT is smaller than about 64k. Anything larger will cause the linker to report an
error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs,
although it will also be less efficient, since it will take three instructions to fetch the value of a
global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use
-mxgot when a single object file accesses more than 64k’s worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using library
calls instead.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the default.
-mdsp
-mno-dsp
Use (do not use) the MIPS DSP ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option can only be used when generating 64-bit code and requires hardware floating-point support to
be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the
pointer size is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit
"int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers
are the same size as "long"s, or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is
useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster
references to symbolic addresses.
-G num
Put global and static items less than or equal to num bytes into the small data or bss section instead of
the normal data or bss section. This allows the data to be accessed using a single instruction.
All modules should be compiled with the same -G num value.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data section if
possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of
RAM required when executing, and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only meaningful in con-
junction with -membedded-data.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative,
selected by -mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation opera-
tors.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero. The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break instruction.
Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of
the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdi-
vide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the
use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using
--with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which
allows GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but
requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating point multiply-accumulate instructions, when they are available. The
default is -mfused-madd.
When multiply-accumulate instructions are used, the intermediate product is calculated to infinite preci-
sion and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when
assembling them.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting
an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplica-
tion is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a
jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting
an integer division.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions
are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a nop to be inserted between certain pairs of instructions. These errata are
handled by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather than
by GCC, although GCC will avoid using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and "dmac-
chi" instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and
"F2" floating point errata.)
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If called,
the function must take the same arguments as the common "_flush_func()", that is, the address of the memory
range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both
caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or
__cpu_flush.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected architec-
ture. By default, Branch Likely instructions may be generated if they are supported by the selected archi-
tecture. An exception is for the MIPS32 and MIPS64 architectures and processors which implement those
architectures; for those, Branch Likely instructions will not be generated by default because the MIPS32
and MIPS64 architectures specifically deprecate their use.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we schedule FP instructions for some proces-
sors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use
both FP pipes. Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one
is 8-byte aligned. When this option is enabled, GCC will align pairs of instructions that it thinks should
execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the
expense of making it bigger. It is enabled by default at optimization level -O3.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the
size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as
registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instruc-
tions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically
valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX"
assembly directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable
branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically generates a
request (handled by the assembler and the linker) for a constant to be set up in a global register. The
register is used for one or more base address requests within the range 0 to 255 from the value held in the
register. The generally leads to short and fast code, but the number of different data items that can be
addressed is limited. This means that a program that uses lots of static data may require
-mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33
Generate code which uses features specific to the AM33 processor.
-mno-am33
Do not generate code which uses features specific to the AM33 processor. This is the default.
-mreturn-pointer-on-d0
When generating a function which returns a pointer, return the pointer in both "a0" and "d0". Otherwise,
the pointer is returned only in a0, and attempts to call such functions without a prototype would result in
errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and
absolute memory addresses. This option only has an effect when used on the command line for the final link
step.
This option makes symbolic debugging impossible.
MT Options
These -m options are defined for Morpho MT architectures:
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor
type. Possible values for cpu-type are ms1-64-001, ms1-16-002, ms1-16-003 and ms2.
When this option is not used, the default is -march=ms1-16-002.
-mbacc
Use byte loads and stores when generating code.
-mno-bacc
Do not use byte loads and stores when generating code.
-msim
Use simulator runtime
-mno-crt0
Do not link in the C run-time initialization object file crti.o. Other run-time initialization and termi-
nation files such as startup.o and exit.o are still included on the linker command line.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not sup-
ported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-msplit
Generate code for a system with split I&D.
-mno-split
Generate code for a system without split I&D. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than
pdp11-*-bsd.
PowerPC Options
These are listed under
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mfprnd
-mno-fprnd
-mmfpgpr
-mno-mfpgpr
GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction
set are those instructions supported by the rios chip set used in the original RS/6000 systems and the Pow-
erPC instruction set is the architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the
IBM 4xx, 6xx, and follow-on microprocessors.
Neither architecture is a subset of the other. However there is a large common subset of instructions sup-
ported by both. An MQ register is included in processors supporting the POWER architecture.
You use these options to specify which instructions are available on the processor you are using. The
default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides
the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options
listed above.
The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to
use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that
are present in the POWER2 architecture but not the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the
PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional
PowerPC architecture instructions in the General Purpose group, including floating-point square root.
Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture
instructions in the Graphics group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on
the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb
option allows GCC to generate the popcount and double precision FP reciprocal estimate instruction imple-
mented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The
-mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ pro-
cessor and other processors that support the PowerPC V2.03 architecture. The -mmfpgpr option allows GCC to
generate the FP move to/from general purpose register instructions implemented on the POWER6X processor and
other processors that support the extended PowerPC V2.05 architecture.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full
PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of
both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying
both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of
the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the assem-
bler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assembler mnemonics
defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic;
GCC uses that mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type sometimes
overrides the value of these option. Unless you are building a cross-compiler, you should normally not
specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for
machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602,
603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, ec603e, G3, G4, G5,
power, power2, power3, power4, power5, power5+, power6, power6x, common, powerpc, powerpc64, rios, rios1,
rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code generated under this option will run on any
POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures,
and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit
PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic
processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best on that
processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options: -maltivec, -mfprnd, -mhard-float,
-mmfcrf, -mmultiple, -mnew-mnemonics, -mpopcntb, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpow-
erpc-gfxopt, -mstring, -mmfpgpr. The particular options set for any particular CPU will vary between com-
piler versions, depending on what setting seems to produce optimal code for that CPU; it doesn’t necessar-
ily reflect the actual hardware’s capabilities. If you wish to set an individual option to a particular
value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present
because AIX does not have full support for these options. You may still enable or disable them individu-
ally if you’re sure it’ll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type,
register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for
-mtune as for -mcpu. If both are specified, the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by -mtune.
-mswdiv
-mno-swdiv
Generate code to compute division as reciprocal estimate and iterative refinement, creating opportunities
for increased throughput. This feature requires: optional PowerPC Graphics instruction set for single pre-
cision and FRE instruction for double precision, assuming divides cannot generate user-visible traps, and
the domain values not include Infinities, denormals or zero denominator.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions
that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to
adjust the current ABI with AltiVec ABI enhancements.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-exec .plt and
.got sections. This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that
are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe
-mno-isel
This switch enables or disables the generation of SPE simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating point operations on the general purpose regis-
ters for architectures that support it.
The argument yes or single enables the use of single-precision floating point operations.
The argument double enables the use of single and double-precision floating point operations.
The argument no disables floating point operations on the general purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The
32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC vari-
ant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for
PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The
-mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC entry for each
unique non-automatic variable reference in your program. GCC will also place floating-point constants in
the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can
reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc
prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate
code to calculate the sum of an address and a constant at run-time instead of putting that sum into the
TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and
larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc
instead. This option causes GCC to make only one TOC entry for every file. When you specify this option,
GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure
needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32 disables the
64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass
floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addi-
tion to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly
rounded when comparing values and converting to double. Use XL symbol names for long double support rou-
tines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of
calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL com-
pilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is com-
piled without optimization. Because always storing floating-point arguments on the stack is inefficient
and rarely needed, this option is not enabled by default and only is necessary when calling subroutines
compiled by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with
special startup code to enable the application to run. The system must have PE installed in the standard
location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the
appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and
the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The
option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the stan-
dard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating point emulation
is provided if you use the -msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple word
instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC
systems. Do not use -mmultiple on little endian PowerPC systems, since those instructions do not work when
the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions
usage in little endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store string word instructions
to save multiple registers and do small block moves. These instructions are generated by default on POWER
systems, and not generated on PowerPC systems. Do not use -mstring on little endian PowerPC systems, since
those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and
PPC750 which permit the instructions usage in little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register to the
address of the calculated memory location. These instructions are generated by default. If you use
-mno-update, there is a small window between the time that the stack pointer is updated and the address of
the previous frame is stored, which means code that walks the stack frame across interrupts or signals may
get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions. These
instructions are generated by default if hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields
to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 would be
aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align, the structure would be
aligned to a 1 byte boundary and be one byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be han-
dled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a
different address at runtime. If you use -mrelocatable on any module, all objects linked together must be
compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a
different address at runtime. Modules compiled with -mrelocatable-lib can be linked with either modules
compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with the -mrelocatable
options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a
global area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The
-mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external refer-
ences are relocatable. The resulting code is suitable for applications, but not shared libraries.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during the sec-
ond scheduling pass. The argument priority takes the value 0/1/2 to assign no/highest/second-highest pri-
ority to dispatch slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during instruction scheduling.
The argument dependence_type takes one of the following values: no: no dependence is costly, all: all
dependences are costly, true_store_to_load: a true dependence from store to load is costly, store_to_load:
any dependence from store to load is costly, number: any dependence which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be used during the second scheduling pass. The argu-
ment scheme takes one of the following values: no: Don’t insert nops. pad: Pad with nops any dispatch
group which has vacant issue slots, according to the scheduler’s grouping. regroup_exact: Insert nops to
force costly dependent insns into separate groups. Insert exactly as many nops as needed to force an insn
to a new group, according to the estimated processor grouping. number: Insert nops to force costly depen-
dent insns into separate groups. Insert number nops to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March
1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default
unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec,
no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the
SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended precision long double. This is a PowerPC 32-bit SYSV ABI
option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended precision long double. This is a PowerPC 32-bit Linux ABI
option.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are prop-
erly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to
set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were
passed in the floating point registers in case the function takes a variable arguments. With -mprototype,
only calls to prototyped variable argument functions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C
libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim. configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-mwindiss
Specify that you are compiling for the WindISS simulation environment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended
relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface
(eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the
stack is aligned to an 8 byte boundary, a function "__eabi" is called to from "main" to set up the eabi
environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas.
Selecting -mno-eabi means that the stack is aligned to a 16 byte boundary, do not call an initialization
function from "main", and the -msdata option will only use "r13" to point to a single small data area. The
-meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the
.sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and static
data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and
static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which
is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section,
which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable
option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, oth-
erwise compile code the same as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section. Put
small uninitialized global and static data in the .sbss section. Do not use register "r13" to address
small data however. This is the default behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data section, and all
uninitialized data in the .bss section.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small
data or bss sections instead of the normal data or bss section. By default, num is 8. The -G num switch
is also passed to the linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output
using symbolic forms.
-mlongcall
-mno-longcall
Default to making all function calls indirectly, using a register, so that functions which reside further
than 32 megabytes (33,554,432 bytes) from the current location can be called. This setting can be overrid-
den by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these
systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do
this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit
PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" will generate "jbsr callee, L42", plus a "branch island" (glue
code). The two target addresses represent the callee and the "branch island". The Darwin/PPC linker will
prefer the first address and generate a "bl callee" if the PPC "bl" instruction will reach the callee
directly; otherwise, the linker will generate "bl L42" to call the "branch island". The "branch island" is
appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps
to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and
the Darwin linker decides whether to use or discard it.
In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate
glue.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preproces-
sor and linker.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point operations.
When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations.
When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the
default.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64bit makes the "long double" type equiva-
lent to the "double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller’s frame as backchain pointer into the callee’s stack frame.
A backchain may be needed to allow debugging using tools that do not understand DWARF-2 call frame informa-
tion. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack
frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte
register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; how-
ever, use of the backchain for debugging purposes usually requires that the whole binary is built with
-mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.
In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all
fields of the 96/160 byte register save area only for their default purpose; unused fields still take up
stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the
register save area; unused space is reused for other purposes, allowing for more efficient use of the
available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register is always saved two words below the
backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible
with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or
zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes.
Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination
of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed 64k. The default is to use the "basr" instruction
instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified,
generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit
instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is
specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with
-m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generat-
ing code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle
is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug
information.
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor
type. Possible values for cpu-type are g5, g6, z900, and z990. When generating code using the instruc-
tions available on z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of avail-
able instructions. The list of cpu-type values is the same as for -march. The default is the value used
for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating sys-
tem. This option is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile time check
it doesn’t need to be a real problem when the program runs. It is intended to identify functions which
most probably cause a stack overflow. It is useful to be used in an environment with limited stack size
e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls alloca or uses dynamically sized arrays. This is generally a bad idea
with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
These arguments always have to be used in conjunction. If they are present the s390 back end emits addi-
tional instructions in the function prologue which trigger a trap if the stack size is stack-guard bytes
above the stack-size (remember that the stack on s390 grows downward). These options are intended to be
used to help debugging stack overflow problems. The additionally emitted code causes only little overhead
and hence can also be used in production like systems without greater performance degradation. The given
values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding
64k. In order to be efficient the extra code makes the assumption that the stack starts at an address
aligned to the value given by stack-size.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn’t generate any DSP
instructions at the moment.
-mb Compile code for the processor in big endian mode.
-ml Compile code for the processor in little endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some func-
tions from the standard C library will not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by Renesas.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available. This
option is the default for all targets of the SH toolchain except for sh-symbianelf.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code. At the moment, this is equivalent to
-fno-finite-math-only. When generating 16 bit SH opcodes, getting IEEE-conforming results for comparisons
of NANs / infinities incurs extra overhead in every floating point comparison, therefore the default is set
to -ffinite-math-only.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH
ABI.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls using the Global Offset Table instead of the
Procedure Linkage Table.
-musermode
Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline.
This library function call doesn’t assume it can write to the whole memory address space. This is the
default when the target is "sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv, inv:min-
lat, inv20u, inv20l, inv:call, inv:call2, inv:fp . "fp" performs the operation in floating point. This
has a very high latency, but needs only a few instructions, so it might be a good choice if your code has
enough easily exploitable ILP to allow the compiler to schedule the floating point instructions together
with other instructions. Division by zero causes a floating point exception. "inv" uses integer opera-
tions to calculate the inverse of the divisor, and then multiplies the dividend with the inverse. This
strategy allows cse and hoisting of the inverse calculation. Division by zero calculates an unspecified
result, but does not trap. "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
have been found, or if the entire operation has been hoisted to the same place, the last stages of the
inverse calculation are intertwined with the final multiply to reduce the overall latency, at the expense
of using a few more instructions, and thus offering fewer scheduling opportunities with other code. "call"
calls a library function that usually implements the inv:minlat strategy. This gives high code density for
m5-*media-nofpu compilations. "call2" uses a different entry point of the same library function, where it
assumes that a pointer to a lookup table has already been set up, which exposes the pointer load to cse /
code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
code generation, but if the code stays unoptimized, revert to the "call", "call2", or "fp" strategies,
respectively. Note that the potentially-trapping side effect of division by zero is carried by a separate
instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the
side effect stays where it is. A recombination to fp operations or a call is not possible in that case.
"inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case that the inverse calculation
was nor separated from the multiply, they speed up division where the dividend fits into 20 bits (plus sign
where applicable), by inserting a test to skip a number of operations in this case; this test slows down
the case of larger dividends. inv20u assumes the case of a such a small dividend to be unlikely, and
inv20l assumes it to be likely.
-mdivsi3_libfunc=name
Set the name of the library function used for 32 bit signed division to name. This only affect the name
used in the call and inv:call division strategies, and the compiler will still expect the same sets of
input/output/clobbered registers as if this option was not present.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This option only has an effect if the gcc code
base supports the TARGET_ADJUST_UNROLL_MAX target hook.
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware
and/or OS implement 32 bit wrap-around semantics for the indexed addressing mode. The architecture allows
the implementation of processors with 64 bit MMU, which the OS could use to get 32 bit addressing, but
since no current hardware implementation supports this or any other way to make the indexed addressing mode
safe to use in the 32 bit ABI, the default is -mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the gettr instruction to number. The default is 2 if -mpt-fixed is in effect, 100
otherwise.
-mpt-fixed
Assume pt* instructions won’t trap. This will generally generate better scheduled code, but is unsafe on
current hardware. The current architecture definition says that ptabs and ptrel trap when the target anded
with 3 is 3. This has the unintentional effect of making it unsafe to schedule ptabs / ptrel before a
branch, or hoist it out of a loop. For example, __do_global_ctors, a part of libgcc that runs constructors
at program startup, calls functions in a list which is delimited by -1. With the -mpt-fixed option, the
ptabs will be done before testing against -1. That means that all the constructors will be run a bit
quicker, but when the loop comes to the end of the list, the program crashes because ptabs loads -1 into a
target register. Since this option is unsafe for any hardware implementing the current architecture speci-
fication, the default is -mno-pt-fixed. Unless the user specifies a specific cost with -mgettrcost,
-mno-pt-fixed also implies -mgettrcost=100; this deters register allocation using target registers for
storing ordinary integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the compiler will always be valid
to load with movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it is possible
to generate symbols that will cause ptabs / ptrel to trap. This option is only meaningful when
-mno-pt-fixed is in effect. It will then prevent cross-basic-block cse, hoisting and most scheduling of
symbol loads. The default is -mno-invalid-symbols.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI
reserves for applications. This is the default.
To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs. You should
compile libraries and system software with this option.
-mfpu
-mhard-float
Generate output containing floating point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all SPARC targets. Normally the facilities of the machine’s usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide
software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you need to compile libgcc.a, the library that comes
with GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating point instructions. The
functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word float-
ing point instructions. They all invoke a trap handler for one of these instructions, and then the trap
handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower
than calling the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8 byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained in
another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment.
Specifying this option avoids some rare compatibility problems with code generated by other compilers. It
is not the default because it results in a performance loss, especially for floating point code.
-mno-faster-structs
-mfaster-structs
With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment. This enables the
use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many
"ld" and "st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus,
it’s intended only for use on targets where the developer acknowledges that their resulting code will not
be directly in line with the rules of the ABI.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when link-
ing a shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker
error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not
actually shared across processes. Instead of using -mimpure-text, you should compile all source code with
-fpic or -fPIC.
This option is only available on SunOS and Solaris.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are v7, cypress, v8, supersparc, sparclite, f930, f934, hypersparc, spar-
clite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, and niagara.
Default instruction scheduling parameters are used for values that select an architecture and not an imple-
mentation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3, niagara
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture.
With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from
V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in
SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the Super-
SPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds
the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in
SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is
the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the
Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the
integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in
SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC
SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer
and floating-point move instructions, 3 additional floating-point condition code registers and conditional
move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC
I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those
that select a particular cpu implementation. Those are cypress, supersparc, hypersparc, f930, f934, spar-
clite86x, tsc701, ultrasparc, ultrasparc3, and niagara.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global
and out registers are considered 64-bit wide. This is enabled by default on Solaris in 32-bit mode for all
SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions.
The default is -mno-vis.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-mlittle-endian
Generate code for a processor running in little-endian mode. It is only available for a few configurations
and most notably not on Solaris and Linux.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=medlow
Generate code for the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits
of memory. Programs can be statically or dynamically linked.
-mcmodel=medmid
Generate code for the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44
bits of memory, the text and data segments must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.
-mcmodel=medany
Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in mem-
ory, the text and data segments must be less than 2GB in size and the data segment must be located within
2GB of the text segment.
-mcmodel=embmedany
Generate code for the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data
segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The
global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not
supported.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047
which must be added back when making stack frame references. This is the default in 64-bit mode. Other-
wise, assume no such offset is present.
These switches are supported in addition to the above on Solaris:
-threads
Add support for multithreading using the Solaris threads library. This option sets flags for both the pre-
processor and linker. This option does not affect the thread safety of object code produced by the com-
piler or that of libraries supplied with it.
-pthreads
Add support for multithreading using the POSIX threads library. This option sets flags for both the pre-
processor and linker. This option does not affect the thread safety of object code produced by the com-
piler or that of libraries supplied with it.
-pthread
This is a synonym for -pthreads.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those
systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the
TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The small memory model assumed that all data fits into
one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing
the .bss and .data program sections. The big memory model is the default and requires reloading of the DP
register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the block count register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled
by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration
count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note that GCC
will try to reverse a loop so that it can utilize the decrement and branch instruction, but will give up if
there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can
generate slightly more efficient code, in cases where the RPTB instruction cannot be utilized.
-mdp-isr-reload
-mparanoid
Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the
data section, and restored on exit from the ISR. This should not be required unless someone has violated
the small memory model by modifying the DP register, say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee
32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed
using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are
performed inline instead of a library call.
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest inte-
ger less than or equal to the floating point value rather than to the nearest integer. Thus if the float-
ing point number is negative, the result will be incorrectly truncated an additional code is necessary to
detect and correct this case. This option can be used to disable generation of the additional code
required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping.
The RPTB construct is only used for innermost loops that do not call functions or jump across the loop
boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and
restore the RC, RS, and RE registers. This is enabled by default with -O2.
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a
single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a
RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop
count cannot be determined at compile time. Note that the repeated instruction following RPTS does not
have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands. However, since
interrupts are blocked by this instruction, it is disabled by default.
-mloop-unsigned
-mno-loop-unsigned
The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these instruc-
tions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned
there is a possibility than the 2^{31 + 1} maximum iteration count may be exceeded. This switch allows an
unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compati-
bility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures
rather than in floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are
passed in registers where possible rather than by pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by default with -O2.
-mparallel-mpy
-mno-parallel-mpy
Allow the generation of MPY││ADD and MPY││SUB parallel instructions, provided -mparallel-insns is also
specified. These instructions have tight register constraints which can pessimize the code generation of
large functions.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always
load the functions address up into a register, and call indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer
into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default
if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a
function. The external functions are slower, but use less code space if more than one function saves the
same number of registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function
options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register "ep"
points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register "gp"
points to. The small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about
out of range branches within a switch table.
-mapp-regs
This option will cause r2 and r5 to be used in the code generated by the compiler. This setting is the
default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ will
be defined if this option is used.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant __v850e__ will be defined if
this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target processor will be chosen and
the relevant __v850*__ preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is
the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the v850
architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot
handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-format.
x86-64 Options
These are listed under
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in
place of the standard "L32R" instructions. The use of "CONST16" is enabled by default only if the "L32R"
instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point
option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add
and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and
add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions do not round the intermediate result, thereby pro-
ducing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program output is not sensitive to the compiler’s ability
to combine multiply and add/subtract operations.
-mtext-section-literals
-mno-text-section-literals
Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals
in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and
it also allows the linker to combine literal pools from separate object files to remove redundant literals
and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in
order to keep them as close as possible to their references. This may be necessary for large assembly
files.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce
branch penalties at the expense of some code density. The assembler attempts to widen density instructions
to align branch targets and the instructions following call instructions. If there are not enough preced-
ing safe density instructions to align a target, no widening will be performed. The default is -mtar-
get-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which the
assembler will always align, either by widening density instructions or by inserting no-op instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless
it can determine that the target of a direct call is in the range allowed by the call instruction. This
translation typically occurs for calls to functions in other source files. Specifically, the assembler
translates a direct "CALL" instruction into an "L32R" followed by a "CALLX" instruction. The default is
-mno-longcalls. This option should be used in programs where the call target can potentially be out of
range. This option is implemented in the assembler, not the compiler, so the assembly code generated by
GCC will still show direct call instructions---look at the disassembled object code to see the actual
instructions. Note that the assembler will use an indirect call for every cross-file call, not just those
that really will be out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table
below, only one of the forms is listed---the one which is not the default. You can figure out the other form
by either removing no- or adding it.
-fbounds-check
For front-ends that support it, generate additional code to check that indices used to access arrays are
within the declared range. This is currently only supported by the Java and Fortran front-ends, where this
option defaults to true and false respectively.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement representation. This flag enables some optimizations and
disables others. This option is enabled by default for the Java front-end, as required by the Java lan-
guage specification.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this
implies GCC will generate frame unwind information for all functions, which can produce significant data
size overhead, although it does not affect execution. If you do not specify this option, GCC will enable
it by default for languages like C++ which normally require exception handling, and disable it for lan-
guages like C that do not normally require it. However, you may need to enable this option when compiling
C code that needs to interoperate properly with exception handlers written in C++. You may also wish to
disable this option if you are compiling older C++ programs that don’t use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-spe-
cific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to
throw exceptions, i.e. memory references or floating point instructions. It does not allow exceptions to
be thrown from arbitrary signal handlers such as "SIGALRM".
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the
generated code in any other way. You will normally not enable this option; instead, a language processor
that needs this handling would enable it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each
instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or
garbage collector).
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This con-
vention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files
and files compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with
the -freg-struct-return switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small structures
than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except
on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose
the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with
the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values.
Specifically, the "enum" type will be equivalent to the smallest integer type which has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a non-default application binary interface.
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int instead of the default for the target.
This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a non-default application binary interface.
-fshared-data
Requests that the data and non-"const" variables of this compilation be shared data rather than private
data. The distinction makes sense only on certain operating systems, where shared data is shared between
processes running the same program, while private data exists in one copy per process.
-fno-common
In C, allocate even uninitialized global variables in the data section of the object file, rather than gen-
erating them as common blocks. This has the effect that if the same variable is declared (without
"extern") in two different compilations, you will get an error when you link them. The only reason this
might be useful is if you wish to verify that the program will work on other systems which always work this
way.
-fno-ident
Ignore the #ident directive.
-finhibit-size-directive
Don’t output a ".size" assembler directive, or anything else that would cause trouble if the function is
split in the middle, and the two halves are placed at locations far apart in memory. This option is used
when compiling crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This option is
generally only of use to those who actually need to read the generated assembly code (perhaps while debug-
ging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two
assembler files.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target
machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic
loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile
with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no
such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the
386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and
avoiding any limit on the size of the global offset table. This option makes a difference on the m68k,
PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position independent code can be only linked
into executables. Usually these options are used when -pie GCC option will be used during linking.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code genera-
tion strategies. This option is of use in conjunction with -fpic or -fPIC for building code which forms
part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do
not require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps
as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in
the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allo-
cated for temporaries or variables that do not live across a call. Functions compiled this way will not
save and restore the register reg.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other reg-
isters that have fixed pervasive roles in the machine’s execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even for
temporaries or variables that live across a call. Functions compiled this way will save and restore the
register reg if they use it.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other reg-
isters that have fixed pervasive roles in the machine’s execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values
may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is specified
(which must be a small power of two), pack structure members according to this value, representing the max-
imum alignment (that is, objects with default alignment requirements larger than this will be output poten-
tially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code gen-
erated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-
default application binary interface.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just before
function exit, the following profiling functions will be called with the address of the current function
and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current func-
tion, so the call site information may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in
the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls
will indicate where, conceptually, the inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a function are expanded inline, this may
mean an additional expansion of code size. If you use extern inline in your C code, an addressable version
of such functions must be provided. (This is normally the case anyways, but if you get lucky and the opti-
mizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation will not
be done. This can be used, for example, for the profiling functions listed above, high-priority interrupt
routines, and any functions from which the profiling functions cannot safely be called (perhaps signal han-
dlers, if the profiling routines generate output or allocate memory).
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag
if you are running in an environment with multiple threads, but only rarely need to specify it in a single-
threaded environment since stack overflow is automatically detected on nearly all systems if there is only
one stack.
Note that this switch does not actually cause checking to be done; the operating system must do that. The
switch causes generation of code to ensure that the operating system sees the stack being extended.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register
or the address of a symbol. If the stack would grow beyond the value, a signal is raised. For most tar-
gets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal
without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias each other and may alias global stor-
age.-fargument-noalias specifies that arguments do not alias each other, but may alias global storage.-far-
gument-noalias-global specifies that arguments do not alias each other and do not alias global storage.
Each language will automatically use whatever option is required by the language standard. You should not
need to use these options yourself.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented
in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary interface. Not
all targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of "global-dynamic",
"local-dynamic", "initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".
-fvisibility=default│internal│hidden│protected
Set the default ELF image symbol visibility to the specified option---all symbols will be marked with this
unless overridden within the code. Using this feature can very substantially improve linking and load
times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent
symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.
Despite the nomenclature, "default" always means public ie; available to be linked against from outside the
shared object. "protected" and "internal" are pretty useless in real-world usage so the only other com-
monly used option will be "hidden". The default if -fvisibility isn’t specified is "default", i.e., make
every symbol public---this causes the same behavior as previous versions of GCC.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by
"How To Write Shared Libraries" by Ulrich Drepper (which can be found at
is public is to make the default hidden and mark things public. This is the norm with DLL’s on Windows and
with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)"
you get almost identical semantics with identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This
works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visi-
bility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed
as part of the API interface contract and thus all new code should always specify visibility when it is not
the default ie; declarations only for use within the local DSO should always be marked explicitly as hidden
as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-doc-
umentation of the code. Note that due to ISO C++ specification requirements, operator new and operator
delete must always be of default visibility.
extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibil-
ity=hidden with no modifications. However, this means that calls to extern functions with no explicit vis-
ibility will use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC vis-
ibility to tell the compiler which extern declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception
class that will be thrown between DSOs must be explicitly marked with default visibility so that the
type_info nodes will be unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is spec-
ified, the compiler generates parallel code according to the OpenMP Application Program Interface v2.5
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by speci-
fying directories or prefixes to use when searching for various kinds of files. Some are used to specify other
aspects of the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take precedence
over places specified using environment variables, which in turn take precedence over those specified by the
configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information that allow GCC to work
with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has
been configured to do so. These locale categories can be set to any value supported by your installation.
A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the charac-
ter boundaries in a string; this is needed for some multibyte encodings that contain quote and escape char-
acters that would otherwise be interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise,
LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables
are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold
the output of one stage of compilation which is to be used as input to the next stage: for example, the
output of the preprocessor, which is the input to the compiler proper.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the
compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can spec-
ify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the
pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the
subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the value of "prefix" when you ran
the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header files.
For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely,
with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce
an alternate directory name. Thus, with -Bfoo/, GCC will search foo/bar where it would normally search
/usr/local/lib/bar. These alternate directories are searched first; the standard directories come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the direc-
tories thus specified when searching for subprograms, if it can’t find the subprograms using GCC_EXEC_PRE-
FIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a
native compiler, GCC tries the directories thus specified when searching for special linker files, if it
can’t find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for
ordinary libraries for the -l option (but directories specified with -L come first).
LANG
This variable is used to pass locale information to the compiler. One way in which this information is
used is to determine the character set to be used when character literals, string literals and comments are
parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values
for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as
defined by the default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable’s value is a list of directories separated by a special character, much like PATH, in which
to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at
GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it
is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given
with -I options on the command line. This environment variable is used regardless of which language is
being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if specified with -isystem, but after any paths given
with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory.
Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system
header files processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that
file, guessing the target name from the source file name. Or the value can have the form file target, in
which case the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an
optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not
ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.
BUGS
For instructions on reporting bugs, see
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-
libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply
the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is
innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info
entries for gcc, cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documen-
tation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant
Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see
below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7)
man page.
(a) The FSF’s Front-Cover Text is:
A GNU Manual
(b) The FSF’s Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.1.2 2010-10-06 GCC(1)