gcc源代码分析，语法树
< call_expr 840f0
type <integer_type 824d0 int permanent SI
size <integer_cst 8254c literal permanent 4
align 32 size_unit 8 sep_unit 32 symtab 0
sep <integer_cst 8251c literal permanent -2147483648 precision 32 min <integer_cst 8251c -2147483648>
max <integer_cst 82534 literal permanent 2147483647
pointer_to_this <pointer_type 88b4c> chain <integer_type 825bc char>
volatile
arg 0 <addr_expr 84090
type <pointer_type 98f48 type <function_type 956e8>
permanent unsigned SI size <integer_cst 8254c 4>
align 32 size_unit 8 sep_unit 32 symtab 0
arg 0 <function_decl 95740 printf type <function_type 956e8>
external public permanent used QI file /usr/include/stdio.h line 214
align 1 size_unit 1 offset 0 chain <function_decl 954d8 ungetc>
arg 1 <tree_list 83270
value <nop_expr 840d8 type <pointer_type 9117c>
literal
arg 0 <nop_expr 840c0 type <pointer_type 88a44>
literal
arg 0 <addr_expr 840a8 type <pointer_type 94f58>
literal
arg 0 <string_cst 84014 type <array_type 94ef4>
static literal "Hello, world!
"(nil)
printf在stdio.h里面有函数声明，并且在函数rest_of_decl_compilation()里面生成了rtx
rest_of_decl_compilation
<function_decl 95740 printf
type <function_type 956e8
type <integer_type 824d0 int permanent SI
size <integer_cst 8254c literal permanent 4
align 32 size_unit 8 sep_unit 32 symtab 0
sep <integer_cst 8251c literal permanent -2147483648 precision 32 min <integer_cst 8251c -2147483648>
max <integer_cst 82534 literal permanent 2147483647
pointer_to_this <pointer_type 88b4c> chain <integer_type 825bc char>
permanent EP
size <integer_cst 82dd8 literal permanent 8
align 32 size_unit 8 sep_unit 0 symtab 0
arg-types <tree_list 95698 permanent value <pointer_type 9117c>
external public permanent QI file /usr/include/stdio.h line 214
align 1 size_unit 1 offset 0 chain <function_decl 954d8 ungetc>
(symbol_ref:SI ("printf"))
(mem:QI (symbol_ref:SI ("printf")))