看LIBC源码中关于strncpy实现的问题

/* Copyright (C) 1991, 1997, 2003 Free Software Foundation, Inc.

   This file is part of the GNU C Library.



   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.



   The GNU C Library is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   Lesser General Public License for more details.



   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */



#include <string.h>

#include <memcopy.h>



#undef strncpy



#ifndef STRNCPY

#define STRNCPY strncpy

#endif



char *

STRNCPY (char *s1, const char *s2, size_t n)

{

  char c;

  char *s = s1;



  --s1;



  if (n >= 4)

    {

      size_t n4 = n >> 2;



      for (;;)

	{

	  c = *s2++;

	  *++s1 = c;

	  if (c == '\0')

	    break;

	  c = *s2++;

	  *++s1 = c;

	  if (c == '\0')

	    break;

	  c = *s2++;

	  *++s1 = c;

	  if (c == '\0')

	    break;

	  c = *s2++;

	  *++s1 = c;

	  if (c == '\0')

	    break;

	  if (--n4 == 0)

	    goto last_chars;

	}

      n = n - (s1 - s) - 1;

      if (n == 0)

	return s;

      goto zero_fill;

    }



 last_chars:

  n &= 3;

  if (n == 0)

    return s;



  do

    {

      c = *s2++;

      *++s1 = c;

      if (--n == 0)

	return s;

    }

  while (c != '\0');



 zero_fill:

  do

    *++s1 = '\0';

  while (--n > 0);



  return s;

}

libc_hidden_builtin_def (strncpy)

这不明摆着吗，现在的机器字长多半是32位，4字节，n>>2就是把char划成32个位一起处理，到了汇编层次，这个代码看起来就像这样：

loop:

    inc      %esi

    inc      %edi

    movb     src(, %esi), %al

    movb     %al, dst(, %edi)

    inc      %esi

    inc      %edi

    movb     src(, %esi), %al

    movb     %al, dst(, %edi)

    inc      %esi

    inc      %edi

    movb     src(, %esi), %al

    movb     %al, dst(, %edi)

    inc      %esi

    inc      %edi

    movb     src(, %esi), %al

    movb     %al, dst(, %edi)

    jmp      loop

然后编译器只要稍微聪明一些，就能够将它们拼凑到一起：

loop:

    inc      %esi

    inc      %edi

    movl     src(, %esi, 4), %eax

    movl     %eax, dst(, %edi, 4)

    jmp      loop