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
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
|
/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006 Free
Software Foundation, Inc.
Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
with help from Dan Sahlin (dan@sics.se) and
commentary by Jim Blandy (jimb@ai.mit.edu);
adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
and implemented by Roland McGrath (roland@ai.mit.edu).
NOTE: The canonical source of this file is maintained with the GNU C Library.
Bugs can be reported to bug-glibc@prep.ai.mit.edu.
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2, or (at your option) any
later version.
This program 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 General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
#ifndef _LIBC
# include <config.h>
#endif
#include <string.h>
#include <stddef.h>
#if defined _LIBC
# include <memcopy.h>
#else
# define reg_char char
#endif
#include <limits.h>
#if HAVE_BP_SYM_H || defined _LIBC
# include <bp-sym.h>
#else
# define BP_SYM(sym) sym
#endif
#undef memchr
#undef __memchr
/* Search no more than N bytes of S for C. */
void *
__memchr (void const *s, int c_in, size_t n)
{
const unsigned char *char_ptr;
const unsigned long int *longword_ptr;
unsigned long int longword, magic_bits, charmask;
unsigned reg_char c;
int i;
c = (unsigned char) c_in;
/* Handle the first few characters by reading one character at a time.
Do this until CHAR_PTR is aligned on a longword boundary. */
for (char_ptr = (const unsigned char *) s;
n > 0 && (size_t) char_ptr % sizeof longword != 0;
--n, ++char_ptr)
if (*char_ptr == c)
return (void *) char_ptr;
/* All these elucidatory comments refer to 4-byte longwords,
but the theory applies equally well to any size longwords. */
longword_ptr = (const unsigned long int *) char_ptr;
/* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
the "holes." Note that there is a hole just to the left of
each byte, with an extra at the end:
bits: 01111110 11111110 11111110 11111111
bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
The 1-bits make sure that carries propagate to the next 0-bit.
The 0-bits provide holes for carries to fall into. */
/* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
Set CHARMASK to be a longword, each of whose bytes is C. */
magic_bits = 0xfefefefe;
charmask = c | (c << 8);
charmask |= charmask << 16;
#if 0xffffffffU < ULONG_MAX
magic_bits |= magic_bits << 32;
charmask |= charmask << 32;
if (8 < sizeof longword)
for (i = 64; i < sizeof longword * 8; i *= 2)
{
magic_bits |= magic_bits << i;
charmask |= charmask << i;
}
#endif
magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
/* Instead of the traditional loop which tests each character,
we will test a longword at a time. The tricky part is testing
if *any of the four* bytes in the longword in question are zero. */
while (n >= sizeof longword)
{
/* We tentatively exit the loop if adding MAGIC_BITS to
LONGWORD fails to change any of the hole bits of LONGWORD.
1) Is this safe? Will it catch all the zero bytes?
Suppose there is a byte with all zeros. Any carry bits
propagating from its left will fall into the hole at its
least significant bit and stop. Since there will be no
carry from its most significant bit, the LSB of the
byte to the left will be unchanged, and the zero will be
detected.
2) Is this worthwhile? Will it ignore everything except
zero bytes? Suppose every byte of LONGWORD has a bit set
somewhere. There will be a carry into bit 8. If bit 8
is set, this will carry into bit 16. If bit 8 is clear,
one of bits 9-15 must be set, so there will be a carry
into bit 16. Similarly, there will be a carry into bit
24. If one of bits 24-30 is set, there will be a carry
into bit 31, so all of the hole bits will be changed.
The one misfire occurs when bits 24-30 are clear and bit
31 is set; in this case, the hole at bit 31 is not
changed. If we had access to the processor carry flag,
we could close this loophole by putting the fourth hole
at bit 32!
So it ignores everything except 128's, when they're aligned
properly.
3) But wait! Aren't we looking for C, not zero?
Good point. So what we do is XOR LONGWORD with a longword,
each of whose bytes is C. This turns each byte that is C
into a zero. */
longword = *longword_ptr++ ^ charmask;
/* Add MAGIC_BITS to LONGWORD. */
if ((((longword + magic_bits)
/* Set those bits that were unchanged by the addition. */
^ ~longword)
/* Look at only the hole bits. If any of the hole bits
are unchanged, most likely one of the bytes was a
zero. */
& ~magic_bits) != 0)
{
/* Which of the bytes was C? If none of them were, it was
a misfire; continue the search. */
const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
if (cp[0] == c)
return (void *) cp;
if (cp[1] == c)
return (void *) &cp[1];
if (cp[2] == c)
return (void *) &cp[2];
if (cp[3] == c)
return (void *) &cp[3];
if (4 < sizeof longword && cp[4] == c)
return (void *) &cp[4];
if (5 < sizeof longword && cp[5] == c)
return (void *) &cp[5];
if (6 < sizeof longword && cp[6] == c)
return (void *) &cp[6];
if (7 < sizeof longword && cp[7] == c)
return (void *) &cp[7];
if (8 < sizeof longword)
for (i = 8; i < sizeof longword; i++)
if (cp[i] == c)
return (void *) &cp[i];
}
n -= sizeof longword;
}
char_ptr = (const unsigned char *) longword_ptr;
while (n-- > 0)
{
if (*char_ptr == c)
return (void *) char_ptr;
else
++char_ptr;
}
return 0;
}
#ifdef weak_alias
weak_alias (__memchr, BP_SYM (memchr))
#endif
|