NetBSD-5.0.2/lib/libc/arch/i386/string/swab.S

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/*
 * Written by J.T. Conklin <jtc@NetBSD.org>.
 * Public domain.
 */

#include <machine/asm.h>

#if defined(LIBC_SCCS)
	RCSID("$NetBSD: swab.S,v 1.13 2007/11/12 18:42:00 ad Exp $")
#endif

/*
 * On the i486, this code is negligibly faster than the code generated
 * by gcc at about half the size.  If my i386 databook is correct, it
 * should be considerably faster than the gcc code on a i386.
 */

ENTRY(swab)
	pushl	%esi
	pushl	%edi
	movl	12(%esp),%esi
	movl	16(%esp),%edi
	movl	20(%esp),%ecx

	shrl	$1,%ecx
	testl	$7,%ecx			# copy first group of 1 to 7 words
	jz	L2			# while swapping alternate bytes.
	_ALIGN_TEXT,0x90
L1:	lodsw
	rorw	$8,%ax
	stosw
	decl	%ecx
	testl	$7,%ecx
	jnz	L1

L2:	shrl	$3,%ecx			# copy remainder 8 words at a time
	jz	L4			# while swapping alternate bytes.
	_ALIGN_TEXT,0x90
L3:	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	lodsw
	rorw	$8,%ax
	stosw
	decl	%ecx
	jnz	L3

L4:	popl	%edi
	popl	%esi
	ret