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/* Optimized strlen implementation using SIMD.
Copyright (C) 2018-2021 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>
/* Assumptions:
*
* ARMv8-a, AArch64, Advanced SIMD, unaligned accesses.
* Not MTE compatible.
*/
#define srcin x0
#define len x0
#define src x1
#define data1 x2
#define data2 x3
#define has_nul1 x4
#define has_nul2 x5
#define tmp1 x4
#define tmp2 x5
#define tmp3 x6
#define tmp4 x7
#define zeroones x8
#define maskv v0
#define maskd d0
#define dataq1 q1
#define dataq2 q2
#define datav1 v1
#define datav2 v2
#define tmp x2
#define tmpw w2
#define synd x3
#define syndw w3
#define shift x4
/* For the first 32 bytes, NUL detection works on the principle that
(X - 1) & (~X) & 0x80 (=> (X - 1) & ~(X | 0x7f)) is non-zero if a
byte is zero, and can be done in parallel across the entire word. */
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
/* To test the page crossing code path more thoroughly, compile with
-DTEST_PAGE_CROSS - this will force all calls through the slower
entry path. This option is not intended for production use. */
#ifdef TEST_PAGE_CROSS
# define MIN_PAGE_SIZE 32
#else
# define MIN_PAGE_SIZE 4096
#endif
/* Core algorithm:
Since strings are short on average, we check the first 32 bytes of the
string for a NUL character without aligning the string. In order to use
unaligned loads safely we must do a page cross check first.
If there is a NUL byte we calculate the length from the 2 8-byte words
using conditional select to reduce branch mispredictions (it is unlikely
strlen will be repeatedly called on strings with the same length).
If the string is longer than 32 bytes, align src so we don't need further
page cross checks, and process 32 bytes per iteration using a fast SIMD
loop.
If the page cross check fails, we read 32 bytes from an aligned address,
and ignore any characters before the string. If it contains a NUL
character, return the length, if not, continue in the main loop. */
ENTRY (__strlen_asimd)
PTR_ARG (0)
and tmp1, srcin, MIN_PAGE_SIZE - 1
cmp tmp1, MIN_PAGE_SIZE - 32
b.hi L(page_cross)
/* Look for a NUL byte in the first 16 bytes. */
ldp data1, data2, [srcin]
mov zeroones, REP8_01
#ifdef __AARCH64EB__
/* For big-endian, carry propagation (if the final byte in the
string is 0x01) means we cannot use has_nul1/2 directly.
Since we expect strings to be small and early-exit,
byte-swap the data now so has_null1/2 will be correct. */
rev data1, data1
rev data2, data2
#endif
sub tmp1, data1, zeroones
orr tmp2, data1, REP8_7f
sub tmp3, data2, zeroones
orr tmp4, data2, REP8_7f
bics has_nul1, tmp1, tmp2
bic has_nul2, tmp3, tmp4
ccmp has_nul2, 0, 0, eq
b.eq L(bytes16_31)
/* Find the exact offset of the first NUL byte in the first 16 bytes
from the string start. Enter with C = has_nul1 == 0. */
csel has_nul1, has_nul1, has_nul2, cc
mov len, 8
rev has_nul1, has_nul1
csel len, xzr, len, cc
clz tmp1, has_nul1
add len, len, tmp1, lsr 3
ret
/* Look for a NUL byte at offset 16..31 in the string. */
L(bytes16_31):
ldp data1, data2, [srcin, 16]
#ifdef __AARCH64EB__
rev data1, data1
rev data2, data2
#endif
sub tmp1, data1, zeroones
orr tmp2, data1, REP8_7f
sub tmp3, data2, zeroones
orr tmp4, data2, REP8_7f
bics has_nul1, tmp1, tmp2
bic has_nul2, tmp3, tmp4
ccmp has_nul2, 0, 0, eq
b.eq L(loop_entry)
/* Find the exact offset of the first NUL byte at offset 16..31 from
the string start. Enter with C = has_nul1 == 0. */
csel has_nul1, has_nul1, has_nul2, cc
mov len, 24
rev has_nul1, has_nul1
mov tmp3, 16
clz tmp1, has_nul1
csel len, tmp3, len, cc
add len, len, tmp1, lsr 3
ret
nop
L(loop_entry):
bic src, srcin, 31
.p2align 5
L(loop):
ldp dataq1, dataq2, [src, 32]!
uminp maskv.16b, datav1.16b, datav2.16b
uminp maskv.16b, maskv.16b, maskv.16b
cmeq maskv.8b, maskv.8b, 0
fmov synd, maskd
cbz synd, L(loop)
/* Low 32 bits of synd are non-zero if a NUL was found in datav1. */
cmeq maskv.16b, datav1.16b, 0
sub len, src, srcin
cbnz syndw, 1f
cmeq maskv.16b, datav2.16b, 0
add len, len, 16
1:
/* Generate a bitmask and compute correct byte offset. */
shrn maskv.8b, maskv.8h, 4
fmov synd, maskd
#ifndef __AARCH64EB__
rbit synd, synd
#endif
clz tmp, synd
add len, len, tmp, lsr 2
ret
L(page_cross):
bic src, srcin, 31
mov tmpw, 0x0c03
movk tmpw, 0xc030, lsl 16
ld1 {datav1.16b, datav2.16b}, [src]
dup maskv.4s, tmpw
cmeq datav1.16b, datav1.16b, 0
cmeq datav2.16b, datav2.16b, 0
and datav1.16b, datav1.16b, maskv.16b
and datav2.16b, datav2.16b, maskv.16b
addp maskv.16b, datav1.16b, datav2.16b
addp maskv.16b, maskv.16b, maskv.16b
fmov synd, maskd
lsl shift, srcin, 1
lsr synd, synd, shift
cbz synd, L(loop)
rbit synd, synd
clz len, synd
lsr len, len, 1
ret
END (__strlen_asimd)
libc_hidden_builtin_def (__strlen_asimd)
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