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1/* SPDX-License-Identifier: GPL-2.0-only */
2/*
3 * Copyright (c) 2012-2021, Arm Limited.
4 *
5 * Adapted from the original at:
6 * https://github.com/ARM-software/optimized-routines/blob/afd6244a1f8d9229/string/aarch64/strcmp.S
7 */
8
9#include <linux/linkage.h>
10#include <asm/assembler.h>
11
12/* Assumptions:
13 *
14 * ARMv8-a, AArch64
15 */
16
17#define L(label) .L ## label
18
19#define REP8_01 0x0101010101010101
20#define REP8_7f 0x7f7f7f7f7f7f7f7f
21#define REP8_80 0x8080808080808080
22
23/* Parameters and result. */
24#define src1 x0
25#define src2 x1
26#define result x0
27
28/* Internal variables. */
29#define data1 x2
30#define data1w w2
31#define data2 x3
32#define data2w w3
33#define has_nul x4
34#define diff x5
35#define syndrome x6
36#define tmp1 x7
37#define tmp2 x8
38#define tmp3 x9
39#define zeroones x10
40#define pos x11
41
42 /* Start of performance-critical section -- one 64B cache line. */
43 .align 6
44SYM_FUNC_START_WEAK_PI(strcmp)
45 eor tmp1, src1, src2
46 mov zeroones, #REP8_01
47 tst tmp1, #7
48 b.ne L(misaligned8)
49 ands tmp1, src1, #7
50 b.ne L(mutual_align)
51 /* NUL detection works on the principle that (X - 1) & (~X) & 0x80
52 (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
53 can be done in parallel across the entire word. */
54L(loop_aligned):
55 ldr data1, [src1], #8
56 ldr data2, [src2], #8
57L(start_realigned):
58 sub tmp1, data1, zeroones
59 orr tmp2, data1, #REP8_7f
60 eor diff, data1, data2 /* Non-zero if differences found. */
61 bic has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
62 orr syndrome, diff, has_nul
63 cbz syndrome, L(loop_aligned)
64 /* End of performance-critical section -- one 64B cache line. */
65
66L(end):
67#ifndef __AARCH64EB__
68 rev syndrome, syndrome
69 rev data1, data1
70 /* The MS-non-zero bit of the syndrome marks either the first bit
71 that is different, or the top bit of the first zero byte.
72 Shifting left now will bring the critical information into the
73 top bits. */
74 clz pos, syndrome
75 rev data2, data2
76 lsl data1, data1, pos
77 lsl data2, data2, pos
78 /* But we need to zero-extend (char is unsigned) the value and then
79 perform a signed 32-bit subtraction. */
80 lsr data1, data1, #56
81 sub result, data1, data2, lsr #56
82 ret
83#else
84 /* For big-endian we cannot use the trick with the syndrome value
85 as carry-propagation can corrupt the upper bits if the trailing
86 bytes in the string contain 0x01. */
87 /* However, if there is no NUL byte in the dword, we can generate
88 the result directly. We can't just subtract the bytes as the
89 MSB might be significant. */
90 cbnz has_nul, 1f
91 cmp data1, data2
92 cset result, ne
93 cneg result, result, lo
94 ret
951:
96 /* Re-compute the NUL-byte detection, using a byte-reversed value. */
97 rev tmp3, data1
98 sub tmp1, tmp3, zeroones
99 orr tmp2, tmp3, #REP8_7f
100 bic has_nul, tmp1, tmp2
101 rev has_nul, has_nul
102 orr syndrome, diff, has_nul
103 clz pos, syndrome
104 /* The MS-non-zero bit of the syndrome marks either the first bit
105 that is different, or the top bit of the first zero byte.
106 Shifting left now will bring the critical information into the
107 top bits. */
108 lsl data1, data1, pos
109 lsl data2, data2, pos
110 /* But we need to zero-extend (char is unsigned) the value and then
111 perform a signed 32-bit subtraction. */
112 lsr data1, data1, #56
113 sub result, data1, data2, lsr #56
114 ret
115#endif
116
117L(mutual_align):
118 /* Sources are mutually aligned, but are not currently at an
119 alignment boundary. Round down the addresses and then mask off
120 the bytes that preceed the start point. */
121 bic src1, src1, #7
122 bic src2, src2, #7
123 lsl tmp1, tmp1, #3 /* Bytes beyond alignment -> bits. */
124 ldr data1, [src1], #8
125 neg tmp1, tmp1 /* Bits to alignment -64. */
126 ldr data2, [src2], #8
127 mov tmp2, #~0
128#ifdef __AARCH64EB__
129 /* Big-endian. Early bytes are at MSB. */
130 lsl tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */
131#else
132 /* Little-endian. Early bytes are at LSB. */
133 lsr tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */
134#endif
135 orr data1, data1, tmp2
136 orr data2, data2, tmp2
137 b L(start_realigned)
138
139L(misaligned8):
140 /* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
141 checking to make sure that we don't access beyond page boundary in
142 SRC2. */
143 tst src1, #7
144 b.eq L(loop_misaligned)
145L(do_misaligned):
146 ldrb data1w, [src1], #1
147 ldrb data2w, [src2], #1
148 cmp data1w, #1
149 ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
150 b.ne L(done)
151 tst src1, #7
152 b.ne L(do_misaligned)
153
154L(loop_misaligned):
155 /* Test if we are within the last dword of the end of a 4K page. If
156 yes then jump back to the misaligned loop to copy a byte at a time. */
157 and tmp1, src2, #0xff8
158 eor tmp1, tmp1, #0xff8
159 cbz tmp1, L(do_misaligned)
160 ldr data1, [src1], #8
161 ldr data2, [src2], #8
162
163 sub tmp1, data1, zeroones
164 orr tmp2, data1, #REP8_7f
165 eor diff, data1, data2 /* Non-zero if differences found. */
166 bic has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
167 orr syndrome, diff, has_nul
168 cbz syndrome, L(loop_misaligned)
169 b L(end)
170
171L(done):
172 sub result, data1, data2
173 ret
174
175SYM_FUNC_END_PI(strcmp)
176EXPORT_SYMBOL_NOHWKASAN(strcmp)