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/* strlen -- find the length of a nul-terminated string.
Copyright (C) 2013-2021 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 (strlen)
@ r0 = start of string
ldrb r2, [r0] @ load the first byte asap
@ 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
mov r1, r0 @ Save the input pointer
rsb r3, r3, #15 @ 16 - 1 peeled loop iteration
cmp r2, #0
beq 99f
@ Loop until we find ...
1: ldrb r2, [r0, #1]!
subs r3, r3, #1 @ ... the aligment point
it ne
cmpne r2, #0 @ ... or EOS
bne 1b
@ Disambiguate the exit possibilites above
cmp r2, #0 @ Found EOS
beq 99f
add r0, r0, #1
@ So now we're aligned.
ldrd r2, r3, [r0], #8
#ifdef ARCH_HAS_T2
movw ip, #0x0101
pld [r0, #64]
movt ip, #0x0101
#else
ldr ip, =0x01010101
pld [r0, #64]
#endif
@ Loop searching for EOS, 8 bytes at a time.
@ Subtracting (unsigned saturating) from 1 for any byte means that
@ we get 1 for any byte that was originally zero and 0 otherwise.
@ Therefore we consider the lsb of each byte the "found" bit.
.balign 16
2: uqsub8 r2, ip, r2 @ Find EOS
uqsub8 r3, ip, r3
pld [r0, #128] @ Prefetch 2 lines ahead
orrs r3, r3, r2 @ 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 found bits for the word containing the match.
cmp r2, #0
sub r0, r0, #4
ite eq
moveq r2, r3
subne r0, r0, #4
@ Find the bit-offset of the match within the word. Note that the
@ bit result from clz will be 7 higher than "true", but we'll
@ immediately discard those bits converting to a byte offset.
#ifdef __ARMEL__
rev r2, r2 @ For LE, count from the little end
#endif
clz r2, r2
add r0, r0, r2, lsr #3 @ Adjust the pointer to the found byte
99:
sub r0, r0, r1 @ Subtract input to compute length
bx lr
END (strlen)
libc_hidden_builtin_def (strlen)
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