최적화 관련 질문입니다..

찾아보니 glibc 2.3.1 - i686 에 최적화 되면 strcmp 및 memcmp는
다음의 코드를 가지게 되는군요.

굉장히 철학적이고도 작고 빠르고! 딱 제가 추구하는 코드네요.

결국 memcmp가 약간의 속도 우월성을 보이는 것은 당연하고요.

참고로 memcmp는 i386이후 별로 최적화할 건더기가 없군요.

/* Highly optimized version for ix86, x>=6.
   Copyright (C) 1999, 2000, 2002 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1999.

   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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <sysdep.h>
#include "asm-syntax.h"
#include "bp-sym.h"
#include "bp-asm.h"

#define PARMS	LINKAGE		/* no space for saved regs */
#define STR1	PARMS
#define STR2	STR1+PTR_SIZE

        .text
ENTRY (BP_SYM (strcmp))
	ENTER

	movl	STR1(%esp), %ecx
	movl	STR2(%esp), %edx
	CHECK_BOUNDS_LOW (%ecx, STR1(%esp))
	CHECK_BOUNDS_LOW (%edx, STR2(%esp))

L(oop):	movb	(%ecx), %al
	cmpb	(%edx), %al
	jne	L(neq)
	incl	%ecx
	incl	%edx
	testb	%al, %al
	jnz	L(oop)

	xorl	%eax, %eax
	/* when strings are equal, pointers rest one beyond
	   the end of the NUL terminators.  */
	CHECK_BOUNDS_HIGH (%ecx, STR1(%esp), jbe)
	CHECK_BOUNDS_HIGH (%edx, STR2(%esp), jbe)
	LEAVE
	ret

#ifndef __BOUNDED_POINTERS__
L(neq):	movl	$1, %eax
	movl	$-1, %ecx
	cmovbl	%ecx, %eax
#else
L(neq):	movl	$1, %eax
	ja	L(chk)
	negl	%eax
	/* When strings differ, pointers rest on
	   the unequal characters.  */
L(chk):	CHECK_BOUNDS_HIGH (%ecx, STR1(%esp), jb)
	CHECK_BOUNDS_HIGH (%edx, STR2(%esp), jb)
#endif

	LEAVE
	ret
END (BP_SYM (strcmp))

/* Compare two memory blocks for differences in the first COUNT bytes.
   Copyright (C) 1995, 1996, 1997, 2000 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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <sysdep.h>
#include "asm-syntax.h"
#include "bp-sym.h"
#include "bp-asm.h"

#define PARMS	LINKAGE+4	/* space for 1 saved reg */
#define BLK1	PARMS
#define BLK2	BLK1+PTR_SIZE
#define LEN	BLK2+PTR_SIZE

	.text
ENTRY (BP_SYM (memcmp))
	ENTER

	pushl %esi		/* Save callee-safe registers.  */
	movl %edi, %edx		/* Note that %edx is not used and can
				   so be used to save %edi.  It's faster.  */

	movl BLK1(%esp), %esi
	movl BLK2(%esp), %edi
	movl LEN(%esp), %ecx
	CHECK_BOUNDS_LOW (%esi, BLK1(%esp))
	CHECK_BOUNDS_LOW (%edi, BLK2(%esp))

	cld			/* Set direction of comparison.  */

	xorl %eax, %eax		/* Default result.  */

	repe			/* Compare at most %ecx bytes.  */
	cmpsb
	jz L(1)			/* If even last byte was equal we return 0.  */

	/* The memory blocks are not equal.  So result of the last
	   subtraction is present in the carry flag.  It is set when
	   the byte in block #2 is bigger.  In this case we have to
	   return -1 (=0xffffffff), else 1.  */
	sbbl %eax, %eax		/* This is tricky.  %eax == 0 and carry is set
				   or not depending on last subtraction.  */

	/* At this point %eax == 0, if the byte of block #1 was bigger, and
	   0xffffffff if the last byte of block #2 was bigger.  The latter
	   case is already correct but the former needs a little adjustment.
	   Note that the following operation does not change 0xffffffff.  */
	orb $1, %al		/* Change 0 to 1.  */

L(1):	CHECK_BOUNDS_HIGH (%esi, BLK1(%esp), jbe)
	CHECK_BOUNDS_HIGH (%edi, BLK2(%esp), jbe)
	popl %esi		/* Restore registers.  */
	movl %edx, %edi

	LEAVE
	ret
END (BP_SYM (memcmp))

#undef bcmp
weak_alias (BP_SYM (memcmp), BP_SYM (bcmp))