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133 lines
4.9 KiB
ArmAsm
133 lines
4.9 KiB
ArmAsm
/* strlen(str) -- determine the length of the string STR.
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Optimized for Intel 80x86, x>=4.
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Copyright (C) 1991-2017 Free Software Foundation, Inc.
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Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <sysdep.h>
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#include "asm-syntax.h"
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#define PARMS 4 /* no space for saved regs */
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#define STR PARMS
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.text
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ENTRY (strlen)
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movl STR(%esp), %ecx
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movl %ecx, %eax /* duplicate it */
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andl $3, %ecx /* mask alignment bits */
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jz L(1) /* aligned => start loop */
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cmpb %ch, (%eax) /* is byte NUL? */
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je L(2) /* yes => return */
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incl %eax /* increment pointer */
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xorl $3, %ecx /* was alignment = 3? */
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jz L(1) /* yes => now it is aligned and start loop */
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cmpb %ch, (%eax) /* is byte NUL? */
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je L(2) /* yes => return */
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addl $1, %eax /* increment pointer */
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subl $1, %ecx /* was alignment = 2? */
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jz L(1) /* yes => now it is aligned and start loop */
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cmpb %ch, (%eax) /* is byte NUL? */
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je L(2) /* yes => return */
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/* Don't change the above `addl $1,%eax' and `subl $1, %ecx' into `incl %eax'
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and `decl %ecx' resp. The additional two byte per instruction make the
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label 4 to be aligned on a 16 byte boundary with nops.
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The following `sub $15, %eax' is part of this trick, too. Together with
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the next instruction (`addl $16, %eax') it is in fact a `incl %eax', just
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as expected from the algorithm. But doing so has the advantage that
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no jump to label 1 is necessary and so the pipeline is not flushed. */
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subl $15, %eax /* effectively +1 */
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L(4): addl $16, %eax /* adjust pointer for full loop */
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L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edx /* magic value */
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addl %ecx, %edx /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(3) /* highest byte is NUL => return pointer */
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xorl %ecx, %edx /* (word+magic)^word */
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orl $0xfefefeff, %edx /* set all non-carry bits */
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incl %edx /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jnz L(3) /* found NUL => return pointer */
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movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edx /* magic value */
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addl %ecx, %edx /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(5) /* highest byte is NUL => return pointer */
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xorl %ecx, %edx /* (word+magic)^word */
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orl $0xfefefeff, %edx /* set all non-carry bits */
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incl %edx /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jnz L(5) /* found NUL => return pointer */
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movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edx /* magic value */
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addl %ecx, %edx /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(6) /* highest byte is NUL => return pointer */
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xorl %ecx, %edx /* (word+magic)^word */
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orl $0xfefefeff, %edx /* set all non-carry bits */
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incl %edx /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jnz L(6) /* found NUL => return pointer */
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movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edx /* magic value */
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addl %ecx, %edx /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(7) /* highest byte is NUL => return pointer */
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xorl %ecx, %edx /* (word+magic)^word */
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orl $0xfefefeff, %edx /* set all non-carry bits */
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incl %edx /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jz L(4) /* no NUL found => continue loop */
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L(7): addl $4, %eax /* adjust pointer */
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L(6): addl $4, %eax
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L(5): addl $4, %eax
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L(3): testb %cl, %cl /* is first byte NUL? */
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jz L(2) /* yes => return */
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incl %eax /* increment pointer */
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testb %ch, %ch /* is second byte NUL? */
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jz L(2) /* yes => return */
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incl %eax /* increment pointer */
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testl $0xff0000, %ecx /* is third byte NUL? */
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jz L(2) /* yes => return pointer */
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incl %eax /* increment pointer */
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L(2): subl STR(%esp), %eax /* compute difference to string start */
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ret
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END (strlen)
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libc_hidden_builtin_def (strlen)
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