string.c

linux内核源码

/*#************************************************************************#*/
/*#-------------------------------------------------------------------------*/
/*#                                                                         */
/*# FUNCTION NAME: memcpy()                                                 */
/*#                                                                         */
/*# PARAMETERS:  void* dst;   Destination address.                          */
/*#              void* src;   Source address.                               */
/*#              int   len;   Number of bytes to copy.                      */
/*#                                                                         */
/*# RETURNS:     dst.                                                       */
/*#                                                                         */
/*# DESCRIPTION: Copies len bytes of memory from src to dst.  No guarantees */
/*#              about copying of overlapping memory areas. This routine is */
/*#              very sensitive to compiler changes in register allocation. */
/*#              Should really be rewritten to avoid this problem.          */
/*#                                                                         */
/*#-------------------------------------------------------------------------*/
/*#                                                                         */
/*# HISTORY                                                                 */
/*#                                                                         */
/*# DATE      NAME            CHANGES                                       */
/*# ----      ----            -------                                       */
/*# 941007    Kenny R         Creation                                      */
/*# 941011    Kenny R         Lots of optimizations and inlining.           */
/*# 941129    Ulf A           Adapted for use in libc.                      */
/*# 950216    HP              N==0 forgotten if non-aligned src/dst.        */
/*#                           Added some optimizations.                     */
/*# 001025    HP              Make src and dst char *.  Align dst to	    */
/*#			      dword, not just word-if-both-src-and-dst-	    */
/*#			      are-misaligned.				    */
/*#                                                                         */
/*#-------------------------------------------------------------------------*/

#include <linux/types.h>

void *memcpy(void *pdst,
             const void *psrc,
             size_t pn)
{
  /* Ok.  Now we want the parameters put in special registers.
     Make sure the compiler is able to make something useful of this.
      As it is now: r10 -> r13; r11 -> r11 (nop); r12 -> r12 (nop).

     If gcc was alright, it really would need no temporaries, and no
     stack space to save stuff on. */

  register void *return_dst __asm__ ("r10") = pdst;
  register char *dst __asm__ ("r13") = pdst;
  register const char *src __asm__ ("r11") = psrc;
  register int n __asm__ ("r12") = pn;


  /* When src is aligned but not dst, this makes a few extra needless
     cycles.  I believe it would take as many to check that the
     re-alignment was unnecessary.  */
  if (((unsigned long) dst & 3) != 0
      /* Don't align if we wouldn't copy more than a few bytes; so we
	 don't have to check further for overflows.  */
      && n >= 3)
  {
    if ((unsigned long) dst & 1)
    {
      n--;
      *(char*)dst = *(char*)src;
      src++;
      dst++;
    }

    if ((unsigned long) dst & 2)
    {
      n -= 2;
      *(short*)dst = *(short*)src;
      src += 2;
      dst += 2;
    }
  }

  /* Decide which copying method to use.  Movem is dirt cheap, so the
     overheap is low enough to always use the minimum block size as the
     threshold.  */
  if (n >= 44)
  {
    /* For large copies we use 'movem' */

  /* It is not optimal to tell the compiler about clobbering any
     registers; that will move the saving/restoring of those registers
     to the function prologue/epilogue, and make non-movem sizes
     suboptimal.  */
    __asm__ volatile ("							\n\
        ;; Check that the register asm declaration got right.		\n\
        ;; The GCC manual explicitly says TRT will happen.		\n\
	.ifnc %0-%1-%2,$r13-$r11-$r12					\n\
	.err								\n\
	.endif								\n\
									\n\
	;; Save the registers we'll use in the movem process		\n\
									\n\
	;; on the stack.						\n\
	subq 	11*4,$sp						\n\
	movem	$r10,[$sp]						\n\
									\n\
        ;; Now we've got this:						\n\
	;; r11 - src							\n\
	;; r13 - dst							\n\
	;; r12 - n							\n\
									\n\
        ;; Update n for the first loop					\n\
        subq    44,$r12							\n\
0:									\n\
	movem	[$r11+],$r10						\n\
        subq   44,$r12							\n\
        bge     0b							\n\
	movem	$r10,[$r13+]						\n\
									\n\
        addq   44,$r12  ;; compensate for last loop underflowing n	\n\
									\n\
	;; Restore registers from stack					\n\
        movem [$sp+],$r10"

     /* Outputs */ : "=r" (dst), "=r" (src), "=r" (n)
     /* Inputs */ : "0" (dst), "1" (src), "2" (n));

  }

  /* Either we directly starts copying, using dword copying
     in a loop, or we copy as much as possible with 'movem'
     and then the last block (<44 bytes) is copied here.
     This will work since 'movem' will have updated src,dst,n. */

  while ( n >= 16 )
  {
    *((long*)dst)++ = *((long*)src)++;
    *((long*)dst)++ = *((long*)src)++;
    *((long*)dst)++ = *((long*)src)++;
    *((long*)dst)++ = *((long*)src)++;
    n -= 16;
  }

  /* A switch() is definitely the fastest although it takes a LOT of code.
   * Particularly if you inline code this.
   */
  switch (n)
  {
    case 0:
      break;
    case 1:
      *(char*)dst = *(char*)src;
      break;
    case 2:
      *(short*)dst = *(short*)src;
      break;
    case 3:
      *((short*)dst)++ = *((short*)src)++;
      *(char*)dst = *(char*)src;
      break;
    case 4:
      *((long*)dst)++ = *((long*)src)++;
      break;
    case 5:
      *((long*)dst)++ = *((long*)src)++;
      *(char*)dst = *(char*)src;
      break;
    case 6:
      *((long*)dst)++ = *((long*)src)++;
      *(short*)dst = *(short*)src;
      break;
    case 7:
      *((long*)dst)++ = *((long*)src)++;
      *((short*)dst)++ = *((short*)src)++;
      *(char*)dst = *(char*)src;
      break;
    case 8:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      break;
    case 9:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *(char*)dst = *(char*)src;
      break;
    case 10:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *(short*)dst = *(short*)src;
      break;
    case 11:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *((short*)dst)++ = *((short*)src)++;
      *(char*)dst = *(char*)src;
      break;
    case 12:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      break;
    case 13:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *(char*)dst = *(char*)src;
      break;
    case 14:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *(short*)dst = *(short*)src;
      break;
    case 15:
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *((long*)dst)++ = *((long*)src)++;
      *((short*)dst)++ = *((short*)src)++;
      *(char*)dst = *(char*)src;
      break;
  }

  return return_dst; /* destination pointer. */
} /* memcpy() */