我对此很好奇并构建了一个测试程序:
#include <string.h>
compare(char* String_1, char* String_2)
{
char TempChar_1,
TempChar_2;
do
{
TempChar_1 = *String_1++;
TempChar_2 = *String_2++;
} while(TempChar_1 && TempChar_1 == TempChar_2);
return TempChar_1 - TempChar_2;
}
int main(){
int i=strcmp("foo","bar");
int j=compare("foo","bar");
return i;
}
我将它编译为汇编程序gcc -S -Os test.c
使用 gcc 4.7.3 生成以下汇编程序:
.file "test.c"
.text
.globl compare
.type compare, @function
compare:
.LFB24:
.cfi_startproc
xorl %edx, %edx
.L2:
movsbl (%rdi,%rdx), %eax
movsbl (%rsi,%rdx), %ecx
incq %rdx
cmpb %cl, %al
jne .L4
testb %al, %al
jne .L2
.L4:
subl %ecx, %eax
ret
.cfi_endproc
.LFE24:
.size compare, .-compare
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "bar"
.LC1:
.string "foo"
.section .text.startup,"ax",@progbits
.globl main
.type main, @function
main:
.LFB25:
.cfi_startproc
movl $.LC0, %esi
movl $.LC1, %edi
call compare
movl $1, %eax
ret
.cfi_endproc
.LFE25:
.size main, .-main
.ident "GCC: (Ubuntu/Linaro 4.7.3-1ubuntu1) 4.7.3"
.section .note.GNU-stack,"",@progbits
我不太擅长 x86 汇编程序,但据我所知,对 strcmp 的调用被删除并简单地替换为常量表达式(movl $1, %eax
)。因此,如果您在测试中使用常量表达式,gcc 可能会将 strcmp 优化为常量。