浮点优化选项 -ffast-math：极大地提高浮点运算速度
关于浮点优化选项：（摘自C in a nutshell）
C99 浮点环境支持科学和数学级别的应用，这些应用必须有相当高的精度，但是某些应用却不是如此，注重速度高于精度。对于这些以速度为重的应用， -ffast-math 选项定义了预处理器宏 __FAST_MATH__, 指示编译不必遵循 IEEE 和 ISO 的浮点运算标准。-ffast-math标记是一个群组选项，可以分别启用下面六个优化选项：
-fno-math-errno
Disables the use of the global variable errno for math functions that represent a single floating-point instruction.
-funsafe-math-optimizations
The "unsafe math optimizations" are those that might violate floating-point math standards, or that do away with verification of arguments and results. Using such optimizations may involve linking code that modifies the floating-point processor's control flags.
-fno-trapping-math
Generates "nonstop" code, on the assumption that no math exceptions will be raised that can be handled by the user program.
-ffinite-math-only
Generates executable code that disregards infinities and NaN ("not a number") values in arguments and results.
-fno-rounding-math
This option indicates that your program does not depend on a certain rounding behavior, and does not attempt to change the floating-point environment's default rounding mode. This setting is currently the default, and its opposite, -frounding-math, is still experimental.
-fno-signaling-nans
This option permits optimizations that limit the number of floating-point exceptions that may be raised by signaling NaNs. This setting is currently the default, and its opposite, -fsignaling-nans, is still experimental.
测试
测试方法
分别使用marvell（gcc 4.1.1 ） 以及 maemo 编译器（codesourcery 2005q3-2，gcc3.4.4）并使用不同的编译选项进行测试对比。 程序运行在 PXA310 624M CPU 下。
marvell 编译器 + -ffast-math
arm-iwmmxt-linux-gnueabi-gcc float.c -o float1
arm-iwmmxt-linux-gnueabi-strip float1
arm-iwmmxt-linux-gnueabi-gcc -ffast-math float.c -o float2
arm-iwmmxt-linux-gnueabi-strip float2
测试结果
/mnt/share # ./float1
Start time is: 0.0
DIV End time is: 4.4294564932
MUL End time is: 5.4294622841
ADD End time is: 6.4294699686
SUB End time is: 7.4294810851
/mnt/share # ./float2
Start time is: 0.0
DIV End time is: 1.66616
MUL End time is: 2.122699
ADD End time is: 3.253242
SUB End time is: 4.363897
Meamo 编译器 + -ffast-math
gcc -mfloat-abi=soft -march=iwmmxt float.c -o cs_float1
strip cs_float1
gcc -mfloat-abi=soft -march=iwmmxt -ffast-math float.c -o cs_float2
strip cs_float2
测试结果
/mnt/share # ./cs_float1
Start time is: 0.0
DIV End time is: 4.4294865258
MUL End time is: 5.258137
ADD End time is: 7.4294683333
SUB End time is: 8.169521
/mnt/share # ./cs_float2
Start time is: 0.0
DIV End time is: 1.330878
MUL End time is: 3.4294658034
ADD End time is: 4.110022
SUB End time is: 6.4294530884
以下运行在 meamo文件系统下：
Nokia-N810-42-19:/mnt/share# ./cs_float1
Start time is: 0.0
DIV End time is: 4.4294844089
MUL End time is: 5.229302
ADD End time is: 7.4294646888
SUB End time is: 8.127102
Nokia-N810-42-19:/mnt/share# ./cs_float2
Start time is: 0.0
DIV End time is: 1.320814
MUL End time is: 2.673556
ADD End time is: 4.84794
SUB End time is: 5.530572