Blame sysdeps/arm/armv6/strlen.S

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/* strlen -- find the length of a nul-terminated string.
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   Copyright (C) 2013-2018 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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   <http://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 aligment 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 possibilites 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)