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289 lines
11 KiB
ArmAsm
289 lines
11 KiB
ArmAsm
/* strchr (str, ch) -- Return pointer to first occurrence of CH in STR.
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For Intel 80x86, x>=3.
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Copyright (C) 1994-2024 Free Software Foundation, Inc.
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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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<https://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+4 /* space for 1 saved reg */
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#define RTN PARMS
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#define STR RTN
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#define CHR STR+4
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.text
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ENTRY (strchr)
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pushl %edi /* Save callee-safe registers used here. */
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cfi_adjust_cfa_offset (4)
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cfi_rel_offset (edi, 0)
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movl STR(%esp), %eax
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movl CHR(%esp), %edx
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/* At the moment %edx contains C. What we need for the
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algorithm is C in all bytes of the dword. Avoid
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operations on 16 bit words because these require an
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prefix byte (and one more cycle). */
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movb %dl, %dh /* now it is 0|0|c|c */
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movl %edx, %ecx
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shll $16, %edx /* now it is c|c|0|0 */
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movw %cx, %dx /* and finally c|c|c|c */
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/* Before we start with the main loop we process single bytes
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until the source pointer is aligned. This has two reasons:
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1. aligned 32-bit memory access is faster
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and (more important)
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2. we process in the main loop 32 bit in one step although
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we don't know the end of the string. But accessing at
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4-byte alignment guarantees that we never access illegal
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memory if this would not also be done by the trivial
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implementation (this is because all processor inherent
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boundaries are multiples of 4. */
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testb $3, %al /* correctly aligned ? */
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jz L(11) /* yes => begin loop */
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movb (%eax), %cl /* load byte in question (we need it twice) */
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cmpb %cl, %dl /* compare byte */
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je L(6) /* target found => return */
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testb %cl, %cl /* is NUL? */
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jz L(2) /* yes => return NULL */
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incl %eax /* increment pointer */
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testb $3, %al /* correctly aligned ? */
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jz L(11) /* yes => begin loop */
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movb (%eax), %cl /* load byte in question (we need it twice) */
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cmpb %cl, %dl /* compare byte */
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je L(6) /* target found => return */
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testb %cl, %cl /* is NUL? */
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jz L(2) /* yes => return NULL */
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incl %eax /* increment pointer */
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testb $3, %al /* correctly aligned ? */
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jz L(11) /* yes => begin loop */
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movb (%eax), %cl /* load byte in question (we need it twice) */
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cmpb %cl, %dl /* compare byte */
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je L(6) /* target found => return */
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testb %cl, %cl /* is NUL? */
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jz L(2) /* yes => return NULL */
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incl %eax /* increment pointer */
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/* No we have reached alignment. */
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jmp L(11) /* begin loop */
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/* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
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change any of the hole bits of LONGWORD.
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1) Is this safe? Will it catch all the zero bytes?
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Suppose there is a byte with all zeros. Any carry bits
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propagating from its left will fall into the hole at its
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least significant bit and stop. Since there will be no
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carry from its most significant bit, the LSB of the
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byte to the left will be unchanged, and the zero will be
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detected.
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2) Is this worthwhile? Will it ignore everything except
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zero bytes? Suppose every byte of LONGWORD has a bit set
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somewhere. There will be a carry into bit 8. If bit 8
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is set, this will carry into bit 16. If bit 8 is clear,
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one of bits 9-15 must be set, so there will be a carry
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into bit 16. Similarly, there will be a carry into bit
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24. If one of bits 24-31 is set, there will be a carry
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into bit 32 (=carry flag), so all of the hole bits will
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be changed.
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3) But wait! Aren't we looking for C, not zero?
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Good point. So what we do is XOR LONGWORD with a longword,
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each of whose bytes is C. This turns each byte that is C
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into a zero. */
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/* Each round the main loop processes 16 bytes. */
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ALIGN(4)
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L(1): addl $16, %eax /* adjust pointer for whole round */
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L(11): movl (%eax), %ecx /* get word (= 4 bytes) in question */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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* C */
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/* According to the algorithm we had to reverse the effect of the
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XOR first and then test the overflow bits. But because the
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following XOR would destroy the carry flag and it would (in a
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representation with more than 32 bits) not alter then last
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overflow, we can now test this condition. If no carry is signaled
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no overflow must have occurred in the last byte => it was 0. */
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jnc L(7)
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/* We are only interested in carry bits that change due to the
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previous add, so remove original bits */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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/* Now test for the other three overflow bits. */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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/* If at least one byte of the word is C we don't get 0 in %edi. */
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jnz L(7) /* found it => return pointer */
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/* Now we made sure the dword does not contain the character we are
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looking for. But because we deal with strings we have to check
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for the end of string before testing the next dword. */
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xorl %edx, %ecx /* restore original dword without reload */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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(2) /* highest byte is NUL => return NULL */
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xorl %ecx, %edi /* (word+magic)^word */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(2) /* found NUL => return NULL */
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movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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* C */
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jnc L(71) /* highest byte is C => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(71) /* found it => return pointer */
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xorl %edx, %ecx /* restore original dword without reload */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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(2) /* highest byte is NUL => return NULL */
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xorl %ecx, %edi /* (word+magic)^word */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(2) /* found NUL => return NULL */
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movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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* C */
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jnc L(72) /* highest byte is C => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(72) /* found it => return pointer */
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xorl %edx, %ecx /* restore original dword without reload */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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(2) /* highest byte is NUL => return NULL */
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xorl %ecx, %edi /* (word+magic)^word */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(2) /* found NUL => return NULL */
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movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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* C */
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jnc L(73) /* highest byte is C => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(73) /* found it => return pointer */
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xorl %edx, %ecx /* restore original dword without reload */
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movl $0xfefefeff, %edi /* magic value */
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addl %ecx, %edi /* 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(2) /* highest byte is NUL => return NULL */
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xorl %ecx, %edi /* (word+magic)^word */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* 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(1) /* no NUL found => restart loop */
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L(2): /* Return NULL. */
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xorl %eax, %eax
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popl %edi /* restore saved register content */
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cfi_adjust_cfa_offset (-4)
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cfi_restore (edi)
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ret
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cfi_adjust_cfa_offset (4)
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cfi_rel_offset (edi, 0)
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L(73): addl $4, %eax /* adjust pointer */
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L(72): addl $4, %eax
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L(71): addl $4, %eax
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/* We now scan for the byte in which the character was matched.
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But we have to take care of the case that a NUL char is
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found before this in the dword. Note that we XORed %ecx
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with the byte we're looking for, therefore the tests below look
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reversed. */
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L(7): testb %cl, %cl /* is first byte C? */
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jz L(6) /* yes => return pointer */
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cmpb %dl, %cl /* is first byte NUL? */
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je L(2) /* yes => return NULL */
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incl %eax /* it's not in the first byte */
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testb %ch, %ch /* is second byte C? */
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jz L(6) /* yes => return pointer */
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cmpb %dl, %ch /* is second byte NUL? */
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je L(2) /* yes => return NULL? */
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incl %eax /* it's not in the second byte */
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shrl $16, %ecx /* make upper byte accessible */
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testb %cl, %cl /* is third byte C? */
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jz L(6) /* yes => return pointer */
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cmpb %dl, %cl /* is third byte NUL? */
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je L(2) /* yes => return NULL */
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/* It must be in the fourth byte and it cannot be NUL. */
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incl %eax
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L(6):
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popl %edi /* restore saved register content */
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cfi_adjust_cfa_offset (-4)
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cfi_restore (edi)
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ret
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END (strchr)
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weak_alias (strchr, index)
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libc_hidden_builtin_def (strchr)
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