1/* Copyright (C) 2012-2021 Free Software Foundation, Inc.
2
3   This file is part of the GNU C Library.
4
5   The GNU C Library is free software; you can redistribute it and/or
6   modify it under the terms of the GNU Lesser General Public
7   License as published by the Free Software Foundation; either
8   version 2.1 of the License, or (at your option) any later version.
9
10   The GNU C Library is distributed in the hope that it will be useful,
11   but WITHOUT ANY WARRANTY; without even the implied warranty of
12   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
13   Lesser General Public License for more details.
14
15   You should have received a copy of the GNU Lesser General Public
16   License along with the GNU C Library.  If not, see
17   <https://www.gnu.org/licenses/>.  */
18
19/* Assumptions:
20 *
21 * ARMv8-a, AArch64.
22 * MTE compatible.
23 */
24
25#include <sysdep.h>
26
27#define REP8_01 0x0101010101010101
28#define REP8_7f 0x7f7f7f7f7f7f7f7f
29
30/* Parameters and result.  */
31#define src1		x0
32#define src2		x1
33#define result		x0
34
35/* Internal variables.  */
36#define data1		x2
37#define data1w		w2
38#define data2		x3
39#define data2w		w3
40#define has_nul		x4
41#define diff		x5
42#define off1		x5
43#define syndrome	x6
44#define tmp		x6
45#define data3		x7
46#define zeroones	x8
47#define shift		x9
48#define off2		x10
49
50/* On big-endian early bytes are at MSB and on little-endian LSB.
51   LS_FW means shifting towards early bytes.  */
52#ifdef __AARCH64EB__
53# define LS_FW lsl
54#else
55# define LS_FW lsr
56#endif
57
58/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
59   (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
60   can be done in parallel across the entire word.
61   Since carry propagation makes 0x1 bytes before a NUL byte appear
62   NUL too in big-endian, byte-reverse the data before the NUL check.  */
63
64ENTRY(strcmp)
65	PTR_ARG (0)
66	PTR_ARG (1)
67	sub	off2, src2, src1
68	mov	zeroones, REP8_01
69	and	tmp, src1, 7
70	tst	off2, 7
71	b.ne	L(misaligned8)
72	cbnz	tmp, L(mutual_align)
73
74	.p2align 4
75
76L(loop_aligned):
77	ldr	data2, [src1, off2]
78	ldr	data1, [src1], 8
79L(start_realigned):
80#ifdef __AARCH64EB__
81	rev	tmp, data1
82	sub	has_nul, tmp, zeroones
83	orr	tmp, tmp, REP8_7f
84#else
85	sub	has_nul, data1, zeroones
86	orr	tmp, data1, REP8_7f
87#endif
88	bics	has_nul, has_nul, tmp	/* Non-zero if NUL terminator.  */
89	ccmp	data1, data2, 0, eq
90	b.eq	L(loop_aligned)
91#ifdef __AARCH64EB__
92	rev	has_nul, has_nul
93#endif
94	eor	diff, data1, data2
95	orr	syndrome, diff, has_nul
96L(end):
97#ifndef __AARCH64EB__
98	rev	syndrome, syndrome
99	rev	data1, data1
100	rev	data2, data2
101#endif
102	clz	shift, syndrome
103	/* The most-significant-non-zero bit of the syndrome marks either the
104	   first bit that is different, or the top bit of the first zero byte.
105	   Shifting left now will bring the critical information into the
106	   top bits.  */
107	lsl	data1, data1, shift
108	lsl	data2, data2, shift
109	/* But we need to zero-extend (char is unsigned) the value and then
110	   perform a signed 32-bit subtraction.  */
111	lsr	data1, data1, 56
112	sub	result, data1, data2, lsr 56
113	ret
114
115	.p2align 4
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 precede the start point.  */
121	bic	src1, src1, 7
122	ldr	data2, [src1, off2]
123	ldr	data1, [src1], 8
124	neg	shift, src2, lsl 3	/* Bits to alignment -64.  */
125	mov	tmp, -1
126	LS_FW	tmp, tmp, shift
127	orr	data1, data1, tmp
128	orr	data2, data2, tmp
129	b	L(start_realigned)
130
131L(misaligned8):
132	/* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
133	   checking to make sure that we don't access beyond the end of SRC2.  */
134	cbz	tmp, L(src1_aligned)
135L(do_misaligned):
136	ldrb	data1w, [src1], 1
137	ldrb	data2w, [src2], 1
138	cmp	data1w, 0
139	ccmp	data1w, data2w, 0, ne	/* NZCV = 0b0000.  */
140	b.ne	L(done)
141	tst	src1, 7
142	b.ne	L(do_misaligned)
143
144L(src1_aligned):
145	neg	shift, src2, lsl 3
146	bic	src2, src2, 7
147	ldr	data3, [src2], 8
148#ifdef __AARCH64EB__
149	rev	data3, data3
150#endif
151	lsr	tmp, zeroones, shift
152	orr	data3, data3, tmp
153	sub	has_nul, data3, zeroones
154	orr	tmp, data3, REP8_7f
155	bics	has_nul, has_nul, tmp
156	b.ne	L(tail)
157
158	sub	off1, src2, src1
159
160	.p2align 4
161
162L(loop_unaligned):
163	ldr	data3, [src1, off1]
164	ldr	data2, [src1, off2]
165#ifdef __AARCH64EB__
166	rev	data3, data3
167#endif
168	sub	has_nul, data3, zeroones
169	orr	tmp, data3, REP8_7f
170	ldr	data1, [src1], 8
171	bics	has_nul, has_nul, tmp
172	ccmp	data1, data2, 0, eq
173	b.eq	L(loop_unaligned)
174
175	lsl	tmp, has_nul, shift
176#ifdef __AARCH64EB__
177	rev	tmp, tmp
178#endif
179	eor	diff, data1, data2
180	orr	syndrome, diff, tmp
181	cbnz	syndrome, L(end)
182L(tail):
183	ldr	data1, [src1]
184	neg	shift, shift
185	lsr	data2, data3, shift
186	lsr	has_nul, has_nul, shift
187#ifdef __AARCH64EB__
188	rev     data2, data2
189	rev	has_nul, has_nul
190#endif
191	eor	diff, data1, data2
192	orr	syndrome, diff, has_nul
193	b	L(end)
194
195L(done):
196	sub	result, data1, data2
197	ret
198
199END(strcmp)
200libc_hidden_builtin_def (strcmp)
201