summary refs log tree commit diff
path: root/sysdeps/aarch64/memchr.S
blob: e422aef090d10074b64f7148651270dbce04154d (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
/* memchr - find a character in a memory zone

   Copyright (C) 2015-2018 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
 * Neon Available.
 */

/* Arguments and results.  */
#define srcin		x0
#define chrin		w1
#define cntin		x2

#define result		x0

#define src		x3
#define	tmp		x4
#define wtmp2		w5
#define synd		x6
#define soff		x9
#define cntrem		x10

#define vrepchr		v0
#define vdata1		v1
#define vdata2		v2
#define vhas_chr1	v3
#define vhas_chr2	v4
#define vrepmask	v5
#define vend		v6

/*
 * Core algorithm:
 *
 * For each 32-byte chunk we calculate a 64-bit syndrome value, with two bits
 * per byte. For each tuple, bit 0 is set if the relevant byte matched the
 * requested character and bit 1 is not used (faster than using a 32bit
 * syndrome). Since the bits in the syndrome reflect exactly the order in which
 * things occur in the original string, counting trailing zeros allows to
 * identify exactly which byte has matched.
 */

ENTRY (__memchr)
	/* Do not dereference srcin if no bytes to compare.  */
	cbz	cntin, L(zero_length)
	/*
	 * Magic constant 0x40100401 allows us to identify which lane matches
	 * the requested byte.
	 */
	mov	wtmp2, #0x0401
	movk	wtmp2, #0x4010, lsl #16
	dup	vrepchr.16b, chrin
	/* Work with aligned 32-byte chunks */
	bic	src, srcin, #31
	dup	vrepmask.4s, wtmp2
	ands	soff, srcin, #31
	and	cntrem, cntin, #31
	b.eq	L(loop)

	/*
	 * Input string is not 32-byte aligned. We calculate the syndrome
	 * value for the aligned 32 bytes block containing the first bytes
	 * and mask the irrelevant part.
	 */

	ld1	{vdata1.16b, vdata2.16b}, [src], #32
	sub	tmp, soff, #32
	adds	cntin, cntin, tmp
	cmeq	vhas_chr1.16b, vdata1.16b, vrepchr.16b
	cmeq	vhas_chr2.16b, vdata2.16b, vrepchr.16b
	and	vhas_chr1.16b, vhas_chr1.16b, vrepmask.16b
	and	vhas_chr2.16b, vhas_chr2.16b, vrepmask.16b
	addp	vend.16b, vhas_chr1.16b, vhas_chr2.16b		/* 256->128 */
	addp	vend.16b, vend.16b, vend.16b			/* 128->64 */
	mov	synd, vend.2d[0]
	/* Clear the soff*2 lower bits */
	lsl	tmp, soff, #1
	lsr	synd, synd, tmp
	lsl	synd, synd, tmp
	/* The first block can also be the last */
	b.ls	L(masklast)
	/* Have we found something already? */
	cbnz	synd, L(tail)

L(loop):
	ld1	{vdata1.16b, vdata2.16b}, [src], #32
	subs	cntin, cntin, #32
	cmeq	vhas_chr1.16b, vdata1.16b, vrepchr.16b
	cmeq	vhas_chr2.16b, vdata2.16b, vrepchr.16b
	/* If we're out of data we finish regardless of the result */
	b.ls	L(end)
	/* Use a fast check for the termination condition */
	orr	vend.16b, vhas_chr1.16b, vhas_chr2.16b
	addp	vend.2d, vend.2d, vend.2d
	mov	synd, vend.2d[0]
	/* We're not out of data, loop if we haven't found the character */
	cbz	synd, L(loop)

L(end):
	/* Termination condition found, let's calculate the syndrome value */
	and	vhas_chr1.16b, vhas_chr1.16b, vrepmask.16b
	and	vhas_chr2.16b, vhas_chr2.16b, vrepmask.16b
	addp	vend.16b, vhas_chr1.16b, vhas_chr2.16b		/* 256->128 */
	addp	vend.16b, vend.16b, vend.16b			/* 128->64 */
	mov	synd, vend.2d[0]
	/* Only do the clear for the last possible block */
	b.hi	L(tail)

L(masklast):
	/* Clear the (32 - ((cntrem + soff) % 32)) * 2 upper bits */
	add	tmp, cntrem, soff
	and	tmp, tmp, #31
	sub	tmp, tmp, #32
	neg	tmp, tmp, lsl #1
	lsl	synd, synd, tmp
	lsr	synd, synd, tmp

L(tail):
	/* Count the trailing zeros using bit reversing */
	rbit	synd, synd
	/* Compensate the last post-increment */
	sub	src, src, #32
	/* Check that we have found a character */
	cmp	synd, #0
	/* And count the leading zeros */
	clz	synd, synd
	/* Compute the potential result */
	add	result, src, synd, lsr #1
	/* Select result or NULL */
	csel	result, xzr, result, eq
	ret

L(zero_length):
	mov	result, #0
	ret
END (__memchr)
weak_alias (__memchr, memchr)
libc_hidden_builtin_def (memchr)