Glibc中strlen的汇编实现

#include <string.h>

size_t
strlen (const char *str)
{
  int cnt;

  asm("cld\n"			/* Search forward.  */
      /* Some old versions of gas need `repne' instead of `repnz'.  */
      "repnz\n"			/* Look for a zero byte.  */
      "scasb" /* %0, %1, %3 */ :
      "=c" (cnt) : "D" (str), "0" (-1), "a" (0));

  return -2 - cnt;
}

Disassembly of section .text:

00000000 <strlen>:
   0:   57                      push   %edi
   1:   b9 ff ff ff ff          mov    $0xffffffff,%ecx
   6:   b8 00 00 00 00          mov    $0x0,%eax
   b:   8b 7c 24 08             mov    0x8(%esp),%edi
   f:   fc                      cld
  10:   f2 ae                   repnz scas %es:(%edi),%al
  12:   b8 fe ff ff ff          mov    $0xfffffffe,%eax
  17:   29 c8                   sub    %ecx,%eax
  19:   5f                      pop    %edi
  1a:   c3                      ret