mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-30 08:40:07 +00:00
223 lines
8.5 KiB
ArmAsm
223 lines
8.5 KiB
ArmAsm
/* rawmemchr (str, ch) -- Return pointer to first occurrence of CH in STR.
|
|
For Intel 80x86, x>=3.
|
|
Copyright (C) 1994-2014 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
|
|
Optimised a little by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>
|
|
This version is developed using the same algorithm as the fast C
|
|
version which carries the following introduction:
|
|
Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
|
|
with help from Dan Sahlin (dan@sics.se) and
|
|
commentary by Jim Blandy (jimb@ai.mit.edu);
|
|
adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
|
|
and implemented by Roland McGrath (roland@ai.mit.edu).
|
|
|
|
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
|
|
<http://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 (__rawmemchr)
|
|
|
|
/* Save callee-safe register used in this function. */
|
|
pushl %edi
|
|
cfi_adjust_cfa_offset (4)
|
|
cfi_rel_offset (edi, 0)
|
|
|
|
/* Load parameters into registers. */
|
|
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 c|c|0|0 */
|
|
movw %cx, %dx /* And finally c|c|c|c */
|
|
|
|
/* Better performance can be achieved if the word (32
|
|
bit) memory access is aligned on a four-byte-boundary.
|
|
So process first bytes one by one until boundary is
|
|
reached. Don't use a loop for better performance. */
|
|
|
|
testb $3, %al /* correctly aligned ? */
|
|
je L(1) /* yes => begin loop */
|
|
cmpb %dl, (%eax) /* compare byte */
|
|
je L(9) /* target found => return */
|
|
incl %eax /* increment source pointer */
|
|
|
|
testb $3, %al /* correctly aligned ? */
|
|
je L(1) /* yes => begin loop */
|
|
cmpb %dl, (%eax) /* compare byte */
|
|
je L(9) /* target found => return */
|
|
incl %eax /* increment source pointer */
|
|
|
|
testb $3, %al /* correctly aligned ? */
|
|
je L(1) /* yes => begin loop */
|
|
cmpb %dl, (%eax) /* compare byte */
|
|
je L(9) /* target found => return */
|
|
incl %eax /* increment source pointer */
|
|
|
|
/* 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): movl (%eax), %ecx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
addl %ecx, %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(8)
|
|
|
|
/* 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(8) /* found it => return pointer */
|
|
|
|
/* This process is unfolded four times for better performance.
|
|
we don't increment the source pointer each time. Instead we
|
|
use offsets and increment by 16 in each run of the loop. But
|
|
before probing for the matching byte we need some extra code
|
|
(following LL(13) below). Even the len can be compared with
|
|
constants instead of decrementing each time. */
|
|
|
|
movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
addl %ecx, %edi /* 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 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(7) /* found it => return pointer */
|
|
|
|
movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
addl %ecx, %edi /* 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 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(6) /* found it => return pointer */
|
|
|
|
movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
|
|
movl $0xfefefeff, %edi /* magic value */
|
|
xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
|
|
are now 0 */
|
|
addl %ecx, %edi /* 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 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(5) /* found it => return pointer */
|
|
|
|
/* Adjust both counters for a full round, i.e. 16 bytes. */
|
|
addl $16, %eax
|
|
jmp L(1)
|
|
/* add missing source pointer increments */
|
|
L(5): addl $4, %eax
|
|
L(6): addl $4, %eax
|
|
L(7): addl $4, %eax
|
|
|
|
/* Test for the matching byte in the word. %ecx contains a NUL
|
|
char in the byte which originally was the byte we are looking
|
|
at. */
|
|
L(8): testb %cl, %cl /* test first byte in dword */
|
|
jz L(9) /* if zero => return pointer */
|
|
incl %eax /* increment source pointer */
|
|
|
|
testb %ch, %ch /* test second byte in dword */
|
|
jz L(9) /* if zero => return pointer */
|
|
incl %eax /* increment source pointer */
|
|
|
|
testl $0xff0000, %ecx /* test third byte in dword */
|
|
jz L(9) /* if zero => return pointer */
|
|
incl %eax /* increment source pointer */
|
|
|
|
/* No further test needed we we know it is one of the four bytes. */
|
|
|
|
L(9):
|
|
popl %edi /* pop saved register */
|
|
cfi_adjust_cfa_offset (-4)
|
|
cfi_restore (edi)
|
|
|
|
ret
|
|
END (__rawmemchr)
|
|
|
|
libc_hidden_def (__rawmemchr)
|
|
weak_alias (__rawmemchr, rawmemchr)
|