mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-09 23:00:07 +00:00
333 lines
13 KiB
ArmAsm
333 lines
13 KiB
ArmAsm
/* strrchr (str, ch) -- Return pointer to last 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+8 /* space for 2 saved regs */
|
|
#define RTN PARMS
|
|
#define STR RTN
|
|
#define CHR STR+4
|
|
|
|
.text
|
|
ENTRY (strrchr)
|
|
|
|
pushl %edi /* Save callee-safe registers used here. */
|
|
cfi_adjust_cfa_offset (4)
|
|
cfi_rel_offset (edi, 0)
|
|
pushl %esi
|
|
cfi_adjust_cfa_offset (4)
|
|
|
|
xorl %eax, %eax
|
|
movl STR(%esp), %esi
|
|
cfi_rel_offset (esi, 0)
|
|
movl CHR(%esp), %ecx
|
|
|
|
/* At the moment %ecx 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 %cl, %ch /* now it is 0|0|c|c */
|
|
movl %ecx, %edx
|
|
shll $16, %ecx /* now it is c|c|0|0 */
|
|
movw %dx, %cx /* 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. */
|
|
|
|
testl $3, %esi /* correctly aligned ? */
|
|
jz L(19) /* yes => begin loop */
|
|
movb (%esi), %dl /* load byte in question (we need it twice) */
|
|
cmpb %dl, %cl /* compare byte */
|
|
jne L(11) /* target found => return */
|
|
movl %esi, %eax /* remember pointer as possible result */
|
|
L(11): orb %dl, %dl /* is NUL? */
|
|
jz L(2) /* yes => return NULL */
|
|
incl %esi /* increment pointer */
|
|
|
|
testl $3, %esi /* correctly aligned ? */
|
|
jz L(19) /* yes => begin loop */
|
|
movb (%esi), %dl /* load byte in question (we need it twice) */
|
|
cmpb %dl, %cl /* compare byte */
|
|
jne L(12) /* target found => return */
|
|
movl %esi, %eax /* remember pointer as result */
|
|
L(12): orb %dl, %dl /* is NUL? */
|
|
jz L(2) /* yes => return NULL */
|
|
incl %esi /* increment pointer */
|
|
|
|
testl $3, %esi /* correctly aligned ? */
|
|
jz L(19) /* yes => begin loop */
|
|
movb (%esi), %dl /* load byte in question (we need it twice) */
|
|
cmpb %dl, %cl /* compare byte */
|
|
jne L(13) /* target found => return */
|
|
movl %esi, %eax /* remember pointer as result */
|
|
L(13): orb %dl, %dl /* is NUL? */
|
|
jz L(2) /* yes => return NULL */
|
|
incl %esi /* increment pointer */
|
|
|
|
/* No we have reached alignment. */
|
|
jmp L(19) /* 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. */
|
|
|
|
/* Jump to here when the character is detected. We chose this
|
|
way around because the character one is looking for is not
|
|
as frequent as the rest and taking a conditional jump is more
|
|
expensive than ignoring it.
|
|
|
|
Some more words to the code below: it might not be obvious why
|
|
we decrement the source pointer here. In the loop the pointer
|
|
is not pre-incremented and so it still points before the word
|
|
we are looking at. But you should take a look at the instruction
|
|
which gets executed before we get into the loop: `addl $16, %esi'.
|
|
This makes the following subs into adds. */
|
|
|
|
/* These fill bytes make the main loop be correctly aligned.
|
|
We cannot use align because it is not the following instruction
|
|
which should be aligned. */
|
|
.byte 0, 0
|
|
#ifndef PROF
|
|
/* Profiling adds some code and so changes the alignment. */
|
|
.byte 0
|
|
#endif
|
|
|
|
L(4): subl $4, %esi /* adjust pointer */
|
|
L(41): subl $4, %esi
|
|
L(42): subl $4, %esi
|
|
L(43): testl $0xff000000, %edx /* is highest byte == C? */
|
|
jnz L(33) /* no => try other bytes */
|
|
leal 15(%esi), %eax /* store address as result */
|
|
jmp L(1) /* and start loop again */
|
|
|
|
L(3): subl $4, %esi /* adjust pointer */
|
|
L(31): subl $4, %esi
|
|
L(32): subl $4, %esi
|
|
L(33): testl $0xff0000, %edx /* is C in third byte? */
|
|
jnz L(51) /* no => try other bytes */
|
|
leal 14(%esi), %eax /* store address as result */
|
|
jmp L(1) /* and start loop again */
|
|
|
|
L(51):
|
|
/* At this point we know that the byte is in one of the lower bytes.
|
|
We make a guess and correct it if necessary. This reduces the
|
|
number of necessary jumps. */
|
|
leal 12(%esi), %eax /* guess address of lowest byte as result */
|
|
testb %dh, %dh /* is guess correct? */
|
|
jnz L(1) /* yes => start loop */
|
|
leal 13(%esi), %eax /* correct guess to second byte */
|
|
|
|
L(1): addl $16, %esi /* increment pointer for full round */
|
|
|
|
L(19): movl (%esi), %edx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
|
|
/* 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(20) /* found NUL => check last word */
|
|
|
|
/* We are only interested in carry bits that change due to the
|
|
previous add, so remove original bits */
|
|
xorl %edx, %edi /* (word+magic)^word */
|
|
|
|
/* 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(20) /* found NUL => check last word */
|
|
|
|
/* 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 %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(4) /* highest byte is C => examine dword */
|
|
xorl %edx, %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(3) /* C is detected in the word => examine it */
|
|
|
|
movl 4(%esi), %edx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(21) /* found NUL => check last word */
|
|
xorl %edx, %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(21) /* found NUL => check last word */
|
|
xorl %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(41) /* highest byte is C => examine dword */
|
|
xorl %edx, %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(31) /* C is detected in the word => examine it */
|
|
|
|
movl 8(%esi), %edx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(22) /* found NUL => check last word */
|
|
xorl %edx, %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(22) /* found NUL => check last word */
|
|
xorl %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(42) /* highest byte is C => examine dword */
|
|
xorl %edx, %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(32) /* C is detected in the word => examine it */
|
|
|
|
movl 12(%esi), %edx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(23) /* found NUL => check last word */
|
|
xorl %edx, %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(23) /* found NUL => check last word */
|
|
xorl %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
addl %edx, %edi /* add the magic value to the word. We get
|
|
carry bits reported for each byte which
|
|
is *not* 0 */
|
|
jnc L(43) /* highest byte is C => examine dword */
|
|
xorl %edx, %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. */
|
|
jz L(1) /* C is not detected => restart loop */
|
|
jmp L(33) /* examine word */
|
|
|
|
L(23): addl $4, %esi /* adjust pointer */
|
|
L(22): addl $4, %esi
|
|
L(21): addl $4, %esi
|
|
|
|
/* What remains to do is to test which byte the NUL char is and
|
|
whether the searched character appears in one of the bytes
|
|
before. A special case is that the searched byte maybe NUL.
|
|
In this case a pointer to the terminating NUL char has to be
|
|
returned. */
|
|
|
|
L(20): cmpb %cl, %dl /* is first byte == C? */
|
|
jne L(24) /* no => skip */
|
|
movl %esi, %eax /* store address as result */
|
|
L(24): testb %dl, %dl /* is first byte == NUL? */
|
|
jz L(2) /* yes => return */
|
|
|
|
cmpb %cl, %dh /* is second byte == C? */
|
|
jne L(25) /* no => skip */
|
|
leal 1(%esi), %eax /* store address as result */
|
|
L(25): testb %dh, %dh /* is second byte == NUL? */
|
|
jz L(2) /* yes => return */
|
|
|
|
shrl $16,%edx /* make upper bytes accessible */
|
|
cmpb %cl, %dl /* is third byte == C */
|
|
jne L(26) /* no => skip */
|
|
leal 2(%esi), %eax /* store address as result */
|
|
L(26): testb %dl, %dl /* is third byte == NUL */
|
|
jz L(2) /* yes => return */
|
|
|
|
cmpb %cl, %dh /* is fourth byte == C */
|
|
jne L(2) /* no => skip */
|
|
leal 3(%esi), %eax /* store address as result */
|
|
|
|
L(2): popl %esi /* restore saved register content */
|
|
cfi_adjust_cfa_offset (-4)
|
|
cfi_restore (esi)
|
|
popl %edi
|
|
cfi_adjust_cfa_offset (-4)
|
|
cfi_restore (edi)
|
|
|
|
ret
|
|
END (strrchr)
|
|
|
|
weak_alias (strrchr, rindex)
|
|
libc_hidden_builtin_def (strrchr)
|