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/* strlen(str) -- determine the length of the string STR.
Optimized for Intel 80x86, x>=4.
Copyright (C) 1991, 92, 93, 94, 95, 96 Free Software Foundation, Inc.
Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>.
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 Library General Public License as
published by the Free Software Foundation; either version 2 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
Library General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB. 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"
/*
INPUT PARAMETERS:
str (sp + 4)
*/
.text
ENTRY (strlen)
movl 4(%esp), %ecx /* get string pointer */
movl %ecx, %eax /* duplicate it */
andl $3, %ecx /* mask alignment bits */
jz L1 /* aligned => start loop */
cmpb %ch, (%eax) /* is byte NUL? */
je L2 /* yes => return */
incl %eax /* increment pointer */
xorl $3, %ecx /* was alignment = 3? */
jz L1 /* yes => now it is aligned and start loop */
cmpb %ch, (%eax) /* is byte NUL? */
je L2 /* yes => return */
addl $1, %eax /* increment pointer */
subl $1, %ecx /* was alignment = 2? */
jz L1 /* yes => now it is aligned and start loop */
cmpb %ch, (%eax) /* is byte NUL? */
je L2 /* yes => return */
/* Don't change the above `addl $1,%eax' and `subl $1, %ecx' into `incl %eax'
and `decl %ecx' resp. The additional two byte per instruction make the
label 4 to be aligned on a 16 byte boundary with nops.
The following `sub $15, %eax' is part of this trick, too. Together with
the next instruction (`addl $16, %eax') it is in fact a `incl %eax', just
as expected from the algorithm. But doing so has the advantage that
no jump to label 1 is necessary and so the pipeline is not flushed. */
subl $15, %eax /* effectively +1 */
L4: addl $16, %eax /* adjust pointer for full loop */
L1: movl (%eax), %ecx /* get word (= 4 bytes) in question */
movl $0xfefefeff, %edx /* magic value */
addl %ecx, %edx /* add the magic value to the word. We get
carry bits reported for each byte which
is *not* 0 */
jnc L3 /* highest byte is NUL => return pointer */
xorl %ecx, %edx /* (word+magic)^word */
orl $0xfefefeff, %edx /* set all non-carry bits */
incl %edx /* add 1: if one carry bit was *not* set
the addition will not result in 0. */
jnz L3 /* found NUL => return pointer */
movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
movl $0xfefefeff, %edx /* magic value */
addl %ecx, %edx /* add the magic value to the word. We get
carry bits reported for each byte which
is *not* 0 */
jnc L5 /* highest byte is NUL => return pointer */
xorl %ecx, %edx /* (word+magic)^word */
orl $0xfefefeff, %edx /* set all non-carry bits */
incl %edx /* add 1: if one carry bit was *not* set
the addition will not result in 0. */
jnz L5 /* found NUL => return pointer */
movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
movl $0xfefefeff, %edx /* magic value */
addl %ecx, %edx /* add the magic value to the word. We get
carry bits reported for each byte which
is *not* 0 */
jnc L6 /* highest byte is NUL => return pointer */
xorl %ecx, %edx /* (word+magic)^word */
orl $0xfefefeff, %edx /* set all non-carry bits */
incl %edx /* add 1: if one carry bit was *not* set
the addition will not result in 0. */
jnz L6 /* found NUL => return pointer */
movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
movl $0xfefefeff, %edx /* magic value */
addl %ecx, %edx /* add the magic value to the word. We get
carry bits reported for each byte which
is *not* 0 */
jnc L7 /* highest byte is NUL => return pointer */
xorl %ecx, %edx /* (word+magic)^word */
orl $0xfefefeff, %edx /* set all non-carry bits */
incl %edx /* add 1: if one carry bit was *not* set
the addition will not result in 0. */
jz L4 /* no NUL found => continue loop */
L7: addl $4, %eax /* adjust pointer */
L6: addl $4, %eax
L5: addl $4, %eax
L3: testb %cl, %cl /* is first byte NUL? */
jz L2 /* yes => return */
incl %eax /* increment pointer */
testb %ch, %ch /* is second byte NUL? */
jz L2 /* yes => return */
incl %eax /* increment pointer */
testl $0xff0000, %ecx /* is third byte NUL? */
jz L2 /* yes => return pointer */
incl %eax /* increment pointer */
L2: subl 4(%esp), %eax /* compute difference to string start */
ret
END (strlen)
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