/*
* strncmp - compare two strings
*
* Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
* See https://llvm.org/LICENSE.txt for license information.
* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
*/
/* Assumptions:
*
* ARMv8-a, AArch64
*/
#include "../asmdefs.h"
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080
/* Parameters and result. */
#define src1 x0
#define src2 x1
#define limit x2
#define result x0
/* Internal variables. */
#define data1 x3
#define data1w w3
#define data2 x4
#define data2w w4
#define has_nul x5
#define diff x6
#define syndrome x7
#define tmp1 x8
#define tmp2 x9
#define tmp3 x10
#define zeroones x11
#define pos x12
#define limit_wd x13
#define mask x14
#define endloop x15
#define count mask
.text
.p2align 6
.rep 7
nop /* Pad so that the loop below fits a cache line. */
.endr
ENTRY_ALIGN (__strncmp_aarch64, 0)
cbz limit, L(ret0)
eor tmp1, src1, src2
mov zeroones, #REP8_01
tst tmp1, #7
and count, src1, #7
b.ne L(misaligned8)
cbnz count, L(mutual_align)
/* Calculate the number of full and partial words -1. */
sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
lsr limit_wd, limit_wd, #3 /* Convert to Dwords. */
/* 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. */
/* Start of performance-critical section -- one 64B cache line. */
L(loop_aligned):
ldr data1, [src1], #8
ldr data2, [src2], #8
L(start_realigned):
subs limit_wd, limit_wd, #1
sub tmp1, data1, zeroones
orr tmp2, data1, #REP8_7f
eor diff, data1, data2 /* Non-zero if differences found. */
csinv endloop, diff, xzr, pl /* Last Dword or differences. */
bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
ccmp endloop, #0, #0, eq
b.eq L(loop_aligned)
/* End of performance-critical section -- one 64B cache line. */
/* Not reached the limit, must have found the end or a diff. */
tbz limit_wd, #63, L(not_limit)
/* Limit % 8 == 0 => all bytes significant. */
ands limit, limit, #7
b.eq L(not_limit)
lsl limit, limit, #3 /* Bits -> bytes. */
mov mask, #~0
#ifdef __AARCH64EB__
lsr mask, mask, limit
#else
lsl mask, mask, limit
#endif
bic data1, data1, mask
bic data2, data2, mask
/* Make sure that the NUL byte is marked in the syndrome. */
orr has_nul, has_nul, mask
L(not_limit):
orr syndrome, diff, has_nul
#ifndef __AARCH64EB__
rev syndrome, syndrome
rev data1, data1
/* The MS-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. */
clz pos, syndrome
rev data2, data2
lsl data1, data1, pos
lsl data2, data2, pos
/* 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
#else
/* For big-endian we cannot use the trick with the syndrome value
as carry-propagation can corrupt the upper bits if the trailing
bytes in the string contain 0x01. */
/* However, if there is no NUL byte in the dword, we can generate
the result directly. We can't just subtract the bytes as the
MSB might be significant. */
cbnz has_nul, 1f
cmp data1, data2
cset result, ne
cneg result, result, lo
ret
1:
/* Re-compute the NUL-byte detection, using a byte-reversed value. */
rev tmp3, data1
sub tmp1, tmp3, zeroones
orr tmp2, tmp3, #REP8_7f
bic has_nul, tmp1, tmp2
rev has_nul, has_nul
orr syndrome, diff, has_nul
clz pos, syndrome
/* The MS-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, pos
lsl data2, data2, pos
/* 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
#endif
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.
We also need to adjust the limit calculations, but without
overflowing if the limit is near ULONG_MAX. */
bic src1, src1, #7
bic src2, src2, #7
ldr data1, [src1], #8
neg tmp3, count, lsl #3 /* 64 - bits(bytes beyond align). */
ldr data2, [src2], #8
mov tmp2, #~0
sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
#ifdef __AARCH64EB__
/* Big-endian. Early bytes are at MSB. */
lsl tmp2, tmp2, tmp3 /* Shift (count & 63). */
#else
/* Little-endian. Early bytes are at LSB. */
lsr tmp2, tmp2, tmp3 /* Shift (count & 63). */
#endif
and tmp3, limit_wd, #7
lsr limit_wd, limit_wd, #3
/* Adjust the limit. Only low 3 bits used, so overflow irrelevant. */
add limit, limit, count
add tmp3, tmp3, count
orr data1, data1, tmp2
orr data2, data2, tmp2
add limit_wd, limit_wd, tmp3, lsr #3
b L(start_realigned)
.p2align 6
/* Don't bother with dwords for up to 16 bytes. */
L(misaligned8):
cmp limit, #16
b.hs L(try_misaligned_words)
L(byte_loop):
/* Perhaps we can do better than this. */
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
subs limit, limit, #1
ccmp data1w, #1, #0, hi /* NZCV = 0b0000. */
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
b.eq L(byte_loop)
L(done):
sub result, data1, data2
ret
/* Align the SRC1 to a dword by doing a bytewise compare and then do
the dword loop. */
L(try_misaligned_words):
lsr limit_wd, limit, #3
cbz count, L(do_misaligned)
neg count, count
and count, count, #7
sub limit, limit, count
lsr limit_wd, limit, #3
L(page_end_loop):
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
cmp data1w, #1
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
b.ne L(done)
subs count, count, #1
b.hi L(page_end_loop)
L(do_misaligned):
/* Prepare ourselves for the next page crossing. Unlike the aligned
loop, we fetch 1 less dword because we risk crossing bounds on
SRC2. */
mov count, #8
subs limit_wd, limit_wd, #1
b.lo L(done_loop)
L(loop_misaligned):
and tmp2, src2, #0xff8
eor tmp2, tmp2, #0xff8
cbz tmp2, L(page_end_loop)
ldr data1, [src1], #8
ldr data2, [src2], #8
sub tmp1, data1, zeroones
orr tmp2, data1, #REP8_7f
eor diff, data1, data2 /* Non-zero if differences found. */
bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
ccmp diff, #0, #0, eq
b.ne L(not_limit)
subs limit_wd, limit_wd, #1
b.pl L(loop_misaligned)
L(done_loop):
/* We found a difference or a NULL before the limit was reached. */
and limit, limit, #7
cbz limit, L(not_limit)
/* Read the last word. */
sub src1, src1, 8
sub src2, src2, 8
ldr data1, [src1, limit]
ldr data2, [src2, limit]
sub tmp1, data1, zeroones
orr tmp2, data1, #REP8_7f
eor diff, data1, data2 /* Non-zero if differences found. */
bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
ccmp diff, #0, #0, eq
b.ne L(not_limit)
L(ret0):
mov result, #0
ret
END ( __strncmp_aarch64)
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