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/* strchr -- find the first instance of C in a nul-terminated string.
Copyright (C) 2013-2023 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>
.syntax unified
.text
ENTRY (strchr)
@ r0 = start of string
@ r1 = character to match
@ returns NULL for no match, or a pointer to the match
ldrb r2, [r0] @ load the first byte asap
uxtb r1, r1
@ To cater to long strings, we want to search through a few
@ characters until we reach an aligned pointer. To cater to
@ small strings, we don't want to start doing word operations
@ immediately. The compromise is a maximum of 16 bytes less
@ whatever is required to end with an aligned pointer.
@ r3 = number of characters to search in alignment loop
and r3, r0, #7
rsb r3, r3, #15 @ 16 - 1 peeled loop iteration
cmp r2, r1 @ Found C?
it ne
cmpne r2, #0 @ Found EOS?
beq 99f
@ Loop until we find ...
1: ldrb r2, [r0, #1]!
subs r3, r3, #1 @ ... the aligment point
it ne
cmpne r2, r1 @ ... or the character
it ne
cmpne r2, #0 @ ... or EOS
bne 1b
@ Disambiguate the exit possibilites above
cmp r2, r1 @ Found the character
it ne
cmpne r2, #0 @ Found EOS
beq 99f
add r0, r0, #1
@ So now we're aligned. Now we actually need a stack frame.
push { r4, r5, r6, r7 }
cfi_adjust_cfa_offset (16)
cfi_rel_offset (r4, 0)
cfi_rel_offset (r5, 4)
cfi_rel_offset (r6, 8)
cfi_rel_offset (r7, 12)
ldrd r2, r3, [r0], #8
orr r1, r1, r1, lsl #8 @ Replicate C to all bytes
#ifdef ARCH_HAS_T2
movw ip, #0x0101
pld [r0, #64]
movt ip, #0x0101
#else
ldr ip, =0x01010101
pld [r0, #64]
#endif
orr r1, r1, r1, lsl #16
@ Loop searching for EOS or C, 8 bytes at a time.
2:
@ Subtracting (unsigned saturating) from 1 means result of 1 for
@ any byte that was originally zero and 0 otherwise. Therefore
@ we consider the lsb of each byte the "found" bit.
uqsub8 r4, ip, r2 @ Find EOS
eor r6, r2, r1 @ Convert C bytes to 0
uqsub8 r5, ip, r3
eor r7, r3, r1
uqsub8 r6, ip, r6 @ Find C
pld [r0, #128] @ Prefetch 2 lines ahead
uqsub8 r7, ip, r7
orr r4, r4, r6 @ Combine found for EOS and C
orr r5, r5, r7
orrs r6, r4, r5 @ Combine the two words
it eq
ldrdeq r2, r3, [r0], #8
beq 2b
@ Found something. Disambiguate between first and second words.
@ Adjust r0 to point to the word containing the match.
@ Adjust r2 to the contents of the word containing the match.
@ Adjust r4 to the found bits for the word containing the match.
cmp r4, #0
sub r0, r0, #4
itte eq
moveq r4, r5
moveq r2, r3
subne r0, r0, #4
@ Find the bit-offset of the match within the word.
#if defined(__ARMEL__)
@ For LE, swap the found word so clz searches from the little end.
rev r4, r4
#else
@ For BE, byte swap the word to make it easier to extract the byte.
rev r2, r2
#endif
@ We're counting 0x01 (not 0x80), so the bit offset is 7 too high.
clz r3, r4
sub r3, r3, #7
lsr r2, r2, r3 @ Shift down found byte
uxtb r1, r1 @ Undo replication of C
uxtb r2, r2 @ Extract found byte
add r0, r0, r3, lsr #3 @ Adjust the pointer to the found byte
pop { r4, r5, r6, r7 }
cfi_adjust_cfa_offset (-16)
cfi_restore (r4)
cfi_restore (r5)
cfi_restore (r6)
cfi_restore (r7)
@ Disambiguate between EOS and C.
99:
cmp r2, r1
it ne
movne r0, #0 @ Found EOS, return NULL
bx lr
END (strchr)
weak_alias (strchr, index)
libc_hidden_builtin_def (strchr)
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