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/* Copyright (C) 2013-2014 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
<http://www.gnu.org/licenses/>. */
#include <sysdep.h>
/* Assumptions:
*
* ARMv8-a, AArch64
*/
#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
ENTRY_ALIGN_AND_PAD (strncmp, 6, 7)
cbz limit, L(ret0)
eor tmp1, src1, src2
mov zeroones, #REP8_01
tst tmp1, #7
b.ne L(misaligned8)
ands tmp1, src1, #7
b.ne 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, tmp1, 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 (tmp1 & 63). */
#else
/* Little-endian. Early bytes are at LSB. */
lsr tmp2, tmp2, tmp3 /* Shift (tmp1 & 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, tmp1
add tmp3, tmp3, tmp1
orr data1, data1, tmp2
orr data2, data2, tmp2
add limit_wd, limit_wd, tmp3, lsr #3
b L(start_realigned)
L(ret0):
mov result, #0
RET
.p2align 6
L(misaligned8):
sub limit, limit, #1
1:
/* Perhaps we can do better than this. */
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
subs limit, limit, #1
ccmp data1w, #1, #0, cs /* NZCV = 0b0000. */
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
b.eq 1b
sub result, data1, data2
RET
END (strncmp)
libc_hidden_builtin_def (strncmp)
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