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100 lines
2.9 KiB
ArmAsm
100 lines
2.9 KiB
ArmAsm
/* strlen -- find the length of a nul-terminated string.
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Copyright (C) 2013-2024 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library. If not, see
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<https://www.gnu.org/licenses/>. */
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#include <sysdep.h>
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.syntax unified
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.text
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ENTRY (strlen)
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@ r0 = start of string
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ldrb r2, [r0] @ load the first byte asap
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@ To cater to long strings, we want to search through a few
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@ characters until we reach an aligned pointer. To cater to
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@ small strings, we don't want to start doing word operations
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@ immediately. The compromise is a maximum of 16 bytes less
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@ whatever is required to end with an aligned pointer.
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@ r3 = number of characters to search in alignment loop
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and r3, r0, #7
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mov r1, r0 @ Save the input pointer
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rsb r3, r3, #15 @ 16 - 1 peeled loop iteration
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cmp r2, #0
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beq 99f
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@ Loop until we find ...
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1: ldrb r2, [r0, #1]!
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subs r3, r3, #1 @ ... the alignment point
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it ne
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cmpne r2, #0 @ ... or EOS
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bne 1b
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@ Disambiguate the exit possibilities above
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cmp r2, #0 @ Found EOS
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beq 99f
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add r0, r0, #1
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@ So now we're aligned.
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ldrd r2, r3, [r0], #8
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#ifdef ARCH_HAS_T2
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movw ip, #0x0101
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pld [r0, #64]
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movt ip, #0x0101
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#else
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ldr ip, =0x01010101
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pld [r0, #64]
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#endif
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@ Loop searching for EOS, 8 bytes at a time.
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@ Subtracting (unsigned saturating) from 1 for any byte means that
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@ we get 1 for any byte that was originally zero and 0 otherwise.
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@ Therefore we consider the lsb of each byte the "found" bit.
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.balign 16
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2: uqsub8 r2, ip, r2 @ Find EOS
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uqsub8 r3, ip, r3
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pld [r0, #128] @ Prefetch 2 lines ahead
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orrs r3, r3, r2 @ Combine the two words
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it eq
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ldrdeq r2, r3, [r0], #8
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beq 2b
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@ Found something. Disambiguate between first and second words.
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@ Adjust r0 to point to the word containing the match.
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@ Adjust r2 to the found bits for the word containing the match.
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cmp r2, #0
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sub r0, r0, #4
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ite eq
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moveq r2, r3
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subne r0, r0, #4
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@ Find the bit-offset of the match within the word. Note that the
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@ bit result from clz will be 7 higher than "true", but we'll
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@ immediately discard those bits converting to a byte offset.
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#ifdef __ARMEL__
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rev r2, r2 @ For LE, count from the little end
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#endif
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clz r2, r2
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add r0, r0, r2, lsr #3 @ Adjust the pointer to the found byte
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99:
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sub r0, r0, r1 @ Subtract input to compute length
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bx lr
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END (strlen)
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libc_hidden_builtin_def (strlen)
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