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