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
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
|
/* Copyright (C) 2010-2023 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
<https://www.gnu.org/licenses/>. */
#include <string.h>
typedef unsigned long word;
static inline word
ldq_u(const void *s)
{
return *(const word *)((word)s & -8);
}
#define unlikely(X) __builtin_expect ((X), 0)
#define prefetch(X) __builtin_prefetch ((void *)(X), 0)
#define cmpbeq0(X) __builtin_alpha_cmpbge(0, (X))
#define find(X, Y) cmpbeq0 ((X) ^ (Y))
/* Search no more than N bytes of S for C. */
void *
__memchr (const void *s, int xc, size_t n)
{
const word *s_align;
word t, current, found, mask, offset;
if (unlikely (n == 0))
return 0;
current = ldq_u (s);
/* Replicate low byte of XC into all bytes of C. */
t = xc & 0xff; /* 0000000c */
t = (t << 8) | t; /* 000000cc */
t = (t << 16) | t; /* 0000cccc */
const word c = (t << 32) | t; /* cccccccc */
/* Align the source, and decrement the count by the number
of bytes searched in the first word. */
s_align = (const word *)((word)s & -8);
{
size_t inc = n + ((word)s & 7);
n = inc | -(inc < n);
}
/* Deal with misalignment in the first word for the comparison. */
mask = (1ul << ((word)s & 7)) - 1;
/* If the entire string fits within one word, we may need masking
at both the front and the back of the string. */
if (unlikely (n <= 8))
{
mask |= -1ul << n;
goto last_quad;
}
found = find (current, c) & ~mask;
if (unlikely (found))
goto found_it;
s_align++;
n -= 8;
/* If the block is sufficiently large, align to cacheline and prefetch. */
if (unlikely (n >= 256))
{
/* Prefetch 3 cache lines beyond the one we're working on. */
prefetch (s_align + 8);
prefetch (s_align + 16);
prefetch (s_align + 24);
while ((word)s_align & 63)
{
current = *s_align;
found = find (current, c);
if (found)
goto found_it;
s_align++;
n -= 8;
}
/* Within each cacheline, advance the load for the next word
before the test for the previous word is complete. This
allows us to hide the 3 cycle L1 cache load latency. We
only perform this advance load within a cacheline to prevent
reading across page boundary. */
#define CACHELINE_LOOP \
do { \
word i, next = s_align[0]; \
for (i = 0; i < 7; ++i) \
{ \
current = next; \
next = s_align[1]; \
found = find (current, c); \
if (unlikely (found)) \
goto found_it; \
s_align++; \
} \
current = next; \
found = find (current, c); \
if (unlikely (found)) \
goto found_it; \
s_align++; \
n -= 64; \
} while (0)
/* While there's still lots more data to potentially be read,
continue issuing prefetches for the 4th cacheline out. */
while (n >= 256)
{
prefetch (s_align + 24);
CACHELINE_LOOP;
}
/* Up to 3 cache lines remaining. Continue issuing advanced
loads, but stop prefetching. */
while (n >= 64)
CACHELINE_LOOP;
/* We may have exhausted the buffer. */
if (n == 0)
return NULL;
}
/* Quadword aligned loop. */
current = *s_align;
while (n > 8)
{
found = find (current, c);
if (unlikely (found))
goto found_it;
current = *++s_align;
n -= 8;
}
/* The last word may need masking at the tail of the compare. */
mask = -1ul << n;
last_quad:
found = find (current, c) & ~mask;
if (found == 0)
return NULL;
found_it:
#ifdef __alpha_cix__
offset = __builtin_alpha_cttz (found);
#else
/* Extract LSB. */
found &= -found;
/* Binary search for the LSB. */
offset = (found & 0x0f ? 0 : 4);
offset += (found & 0x33 ? 0 : 2);
offset += (found & 0x55 ? 0 : 1);
#endif
return (void *)((word)s_align + offset);
}
#ifdef weak_alias
weak_alias (__memchr, memchr)
#endif
libc_hidden_builtin_def (memchr)
|
