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/* strlen(str) -- determine the length of the string STR.
Optimized for Intel 80x86, x>=4.
Copyright (C) 1991-2021 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
<https://www.gnu.org/licenses/>. */
#include <sysdep.h>
#include "asm-syntax.h"
#define PARMS 4 /* no space for saved regs */
#define STR PARMS
.text
ENTRY (strlen)
movl STR(%esp), %ecx
movl %ecx, %eax /* duplicate it */
andl $3, %ecx /* mask alignment bits */
jz L(1) /* aligned => start loop */
cmpb %ch, (%eax) /* is byte NUL? */
je L(2) /* yes => return */
incl %eax /* increment pointer */
xorl $3, %ecx /* was alignment = 3? */
jz L(1) /* yes => now it is aligned and start loop */
cmpb %ch, (%eax) /* is byte NUL? */
je L(2) /* yes => return */
addl $1, %eax /* increment pointer */
subl $1, %ecx /* was alignment = 2? */
jz L(1) /* yes => now it is aligned and start loop */
cmpb %ch, (%eax) /* is byte NUL? */
je L(2) /* 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 */
L(4): addl $16, %eax /* adjust pointer for full loop */
L(1): 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 L(3) /* 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 L(3) /* 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 L(5) /* 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 L(5) /* 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 L(6) /* 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 L(6) /* 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 L(7) /* 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 L(4) /* no NUL found => continue loop */
L(7): addl $4, %eax /* adjust pointer */
L(6): addl $4, %eax
L(5): addl $4, %eax
L(3): testb %cl, %cl /* is first byte NUL? */
jz L(2) /* yes => return */
incl %eax /* increment pointer */
testb %ch, %ch /* is second byte NUL? */
jz L(2) /* yes => return */
incl %eax /* increment pointer */
testl $0xff0000, %ecx /* is third byte NUL? */
jz L(2) /* yes => return pointer */
incl %eax /* increment pointer */
L(2): subl STR(%esp), %eax /* compute difference to string start */
ret
END (strlen)
libc_hidden_builtin_def (strlen)
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