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/* Copyright (C) 2012-2024 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/>. */
/* Assumptions:
*
* ARMv8-a, AArch64.
* MTE compatible.
*/
#include <sysdep.h>
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
/* Parameters and result. */
#define src1 x0
#define src2 x1
#define result x0
/* Internal variables. */
#define data1 x2
#define data1w w2
#define data2 x3
#define data2w w3
#define has_nul x4
#define diff x5
#define off1 x5
#define syndrome x6
#define tmp x6
#define data3 x7
#define zeroones x8
#define shift x9
#define off2 x10
/* On big-endian early bytes are at MSB and on little-endian LSB.
LS_FW means shifting towards early bytes. */
#ifdef __AARCH64EB__
# define LS_FW lsl
#else
# define LS_FW lsr
#endif
/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
(=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
can be done in parallel across the entire word.
Since carry propagation makes 0x1 bytes before a NUL byte appear
NUL too in big-endian, byte-reverse the data before the NUL check. */
ENTRY(strcmp)
PTR_ARG (0)
PTR_ARG (1)
sub off2, src2, src1
mov zeroones, REP8_01
and tmp, src1, 7
tst off2, 7
b.ne L(misaligned8)
cbnz tmp, L(mutual_align)
.p2align 4
L(loop_aligned):
ldr data2, [src1, off2]
ldr data1, [src1], 8
L(start_realigned):
#ifdef __AARCH64EB__
rev tmp, data1
sub has_nul, tmp, zeroones
orr tmp, tmp, REP8_7f
#else
sub has_nul, data1, zeroones
orr tmp, data1, REP8_7f
#endif
bics has_nul, has_nul, tmp /* Non-zero if NUL terminator. */
ccmp data1, data2, 0, eq
b.eq L(loop_aligned)
#ifdef __AARCH64EB__
rev has_nul, has_nul
#endif
eor diff, data1, data2
orr syndrome, diff, has_nul
L(end):
#ifndef __AARCH64EB__
rev syndrome, syndrome
rev data1, data1
rev data2, data2
#endif
clz shift, syndrome
/* The most-significant-non-zero bit of the syndrome marks either the
first bit that is different, or the top bit of the first zero byte.
Shifting left now will bring the critical information into the
top bits. */
lsl data1, data1, shift
lsl data2, data2, shift
/* But we need to zero-extend (char is unsigned) the value and then
perform a signed 32-bit subtraction. */
lsr data1, data1, 56
sub result, data1, data2, lsr 56
ret
.p2align 4
L(mutual_align):
/* Sources are mutually aligned, but are not currently at an
alignment boundary. Round down the addresses and then mask off
the bytes that precede the start point. */
bic src1, src1, 7
ldr data2, [src1, off2]
ldr data1, [src1], 8
neg shift, src2, lsl 3 /* Bits to alignment -64. */
mov tmp, -1
LS_FW tmp, tmp, shift
orr data1, data1, tmp
orr data2, data2, tmp
b L(start_realigned)
L(misaligned8):
/* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
checking to make sure that we don't access beyond the end of SRC2. */
cbz tmp, L(src1_aligned)
L(do_misaligned):
ldrb data1w, [src1], 1
ldrb data2w, [src2], 1
cmp data1w, 0
ccmp data1w, data2w, 0, ne /* NZCV = 0b0000. */
b.ne L(done)
tst src1, 7
b.ne L(do_misaligned)
L(src1_aligned):
neg shift, src2, lsl 3
bic src2, src2, 7
ldr data3, [src2], 8
#ifdef __AARCH64EB__
rev data3, data3
#endif
lsr tmp, zeroones, shift
orr data3, data3, tmp
sub has_nul, data3, zeroones
orr tmp, data3, REP8_7f
bics has_nul, has_nul, tmp
b.ne L(tail)
sub off1, src2, src1
.p2align 4
L(loop_unaligned):
ldr data3, [src1, off1]
ldr data2, [src1, off2]
#ifdef __AARCH64EB__
rev data3, data3
#endif
sub has_nul, data3, zeroones
orr tmp, data3, REP8_7f
ldr data1, [src1], 8
bics has_nul, has_nul, tmp
ccmp data1, data2, 0, eq
b.eq L(loop_unaligned)
lsl tmp, has_nul, shift
#ifdef __AARCH64EB__
rev tmp, tmp
#endif
eor diff, data1, data2
orr syndrome, diff, tmp
cbnz syndrome, L(end)
L(tail):
ldr data1, [src1]
neg shift, shift
lsr data2, data3, shift
lsr has_nul, has_nul, shift
#ifdef __AARCH64EB__
rev data2, data2
rev has_nul, has_nul
#endif
eor diff, data1, data2
orr syndrome, diff, has_nul
b L(end)
L(done):
sub result, data1, data2
ret
END(strcmp)
libc_hidden_builtin_def (strcmp)
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