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/* SPDX-License-Identifier: GPL-2.0-only */
* Copyright (c) 2013-2022, Arm Limited.
* Adapted from the original at:
#include <linux/linkage.h>
#include <asm/assembler.h>
/* Assumptions:
* ARMv8-a, AArch64.
* MTE compatible.
#define L(label) .L ## label
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
/* 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 mask x13
#define endloop x14
#define count mask
#define offset pos
#define neg_offset x15
/* Define endian dependent shift operations.
On big-endian early bytes are at MSB and on little-endian LSB.
LS_FW means shifting towards early bytes.
LS_BK means shifting towards later bytes.
#ifdef __AARCH64EB__
#define LS_FW lsl
#define LS_BK lsr
#define LS_FW lsr
#define LS_BK lsl
cbz limit, L(ret0)
eor tmp1, src1, src2
mov zeroones, #REP8_01
tst tmp1, #7
and count, src1, #7 L(misaligned8)
cbnz count, L(mutual_align)
/* 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. */
.p2align 4
ldr data1, [src1], #8
ldr data2, [src2], #8
subs limit, limit, #8
sub tmp1, data1, zeroones
orr tmp2, data1, #REP8_7f
eor diff, data1, data2 /* Non-zero if differences found. */
csinv endloop, diff, xzr, hi /* 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 main loop */
#ifndef __AARCH64EB__
orr syndrome, diff, has_nul
add limit, limit, 8 /* Rewind limit to before last subs. */
/* Limit was reached. Check if the NUL byte or the difference
is before the limit. */
rev syndrome, syndrome
rev data1, data1
clz pos, syndrome
rev data2, data2
lsl data1, data1, pos
cmp limit, pos, lsr #3
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
csel result, result, xzr, hi
/* Not reached the limit, must have found the end or a diff. */
tbz limit, #63, L(not_limit)
add tmp1, limit, 8
cbz limit, L(not_limit)
lsl limit, tmp1, #3 /* Bits -> bytes. */
mov mask, #~0
lsr mask, mask, limit
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
/* 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
/* 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 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, 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
/* 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
LS_FW tmp2, tmp2, tmp3 /* Shift (count & 63). */
/* Adjust the limit and ensure it doesn't overflow. */
adds limit, limit, count
csinv limit, limit, xzr, lo
orr data1, data1, tmp2
orr data2, data2, tmp2
b L(start_realigned)
.p2align 4
/* Don't bother with dwords for up to 16 bytes. */
cmp limit, #16
b.hs L(try_misaligned_words)
/* 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)
sub result, data1, data2
/* Align the SRC1 to a dword by doing a bytewise compare and then do
the dword loop. */
cbz count, L(src1_aligned)
neg count, count
and count, count, #7
sub limit, limit, count
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
cmp data1w, #1
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */ L(done)
subs count, count, #1
b.hi L(page_end_loop)
/* The following diagram explains the comparison of misaligned strings.
The bytes are shown in natural order. For little-endian, it is
reversed in the registers. The "x" bytes are before the string.
The "|" separates data that is loaded at one time.
src1 | a a a a a a a a | b b b c c c c c | . . .
src2 | x x x x x a a a a a a a a b b b | c c c c c . . .
After shifting in each step, the data looks like this:
data1 a a a a a a a a b b b c c c c c b b b c c c c c
data2 a a a a a a a a b b b 0 0 0 0 0 0 0 0 c c c c c
The bytes with "0" are eliminated from the syndrome via mask.
Align SRC2 down to 16 bytes. This way we can read 16 bytes at a
time from SRC2. The comparison happens in 3 steps. After each step
the loop can exit, or read from SRC1 or SRC2. */
/* Calculate offset from 8 byte alignment to string start in bits. No
need to mask offset since shifts are ignoring upper bits. */
lsl offset, src2, #3
bic src2, src2, #0xf
mov mask, -1
neg neg_offset, offset
ldr data1, [src1], #8
ldp tmp1, tmp2, [src2], #16
LS_BK mask, mask, neg_offset
and neg_offset, neg_offset, #63 /* Need actual value for cmp later. */
/* Skip the first compare if data in tmp1 is irrelevant. */
tbnz offset, 6, L(misaligned_mid_loop)
/* STEP_A: Compare full 8 bytes when there is enough data from SRC2.*/
LS_FW data2, tmp1, offset
LS_BK tmp1, tmp2, neg_offset
subs limit, limit, #8
orr data2, data2, tmp1 /* 8 bytes from SRC2 combined from two regs.*/
sub has_nul, data1, zeroones
eor diff, data1, data2 /* Non-zero if differences found. */
orr tmp3, data1, #REP8_7f
csinv endloop, diff, xzr, hi /* If limit, set to all ones. */
bic has_nul, has_nul, tmp3 /* Non-zero if NUL byte found in SRC1. */
orr tmp3, endloop, has_nul
cbnz tmp3, L(full_check)
ldr data1, [src1], #8
/* STEP_B: Compare first part of data1 to second part of tmp2. */
LS_FW data2, tmp2, offset
#ifdef __AARCH64EB__
/* For big-endian we do a byte reverse to avoid carry-propagation
problem described above. This way we can reuse the has_nul in the
next step and also use syndrome value trick at the end. */
rev tmp3, data1
#define data1_fixed tmp3
#define data1_fixed data1
sub has_nul, data1_fixed, zeroones
orr tmp3, data1_fixed, #REP8_7f
eor diff, data2, data1 /* Non-zero if differences found. */
bic has_nul, has_nul, tmp3 /* Non-zero if NUL terminator. */
#ifdef __AARCH64EB__
rev has_nul, has_nul
cmp limit, neg_offset, lsr #3
orr syndrome, diff, has_nul
bic syndrome, syndrome, mask /* Ignore later bytes. */
csinv tmp3, syndrome, xzr, hi /* If limit, set to all ones. */
cbnz tmp3, L(syndrome_check)
/* STEP_C: Compare second part of data1 to first part of tmp1. */
ldp tmp1, tmp2, [src2], #16
cmp limit, #8
LS_BK data2, tmp1, neg_offset
eor diff, data2, data1 /* Non-zero if differences found. */
orr syndrome, diff, has_nul
and syndrome, syndrome, mask /* Ignore earlier bytes. */
csinv tmp3, syndrome, xzr, hi /* If limit, set to all ones. */
cbnz tmp3, L(syndrome_check)
ldr data1, [src1], #8
sub limit, limit, #8
b L(loop_misaligned)
#ifdef __AARCH64EB__
clz pos, syndrome
cmp pos, limit, lsl #3
b.lo L(end_quick)
mov result, #0
SYM_FUNC_ALIAS_WEAK(strncmp, __pi_strncmp)