1 /* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004 Free
2 Software Foundation, Inc.
3
4 Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
5 with help from Dan Sahlin (dan@sics.se) and
6 commentary by Jim Blandy (jimb@ai.mit.edu);
7 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
8 and implemented by Roland McGrath (roland@ai.mit.edu).
9
10 NOTE: The canonical source of this file is maintained with the GNU C Library.
11 Bugs can be reported to bug-glibc@prep.ai.mit.edu.
12
13 This program is free software; you can redistribute it and/or modify it
14 under the terms of the GNU General Public License as published by the
15 Free Software Foundation; either version 2, or (at your option) any
16 later version.
17
18 This program is distributed in the hope that it will be useful,
19 but WITHOUT ANY WARRANTY; without even the implied warranty of
20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
21 GNU General Public License for more details.
22
23 You should have received a copy of the GNU General Public License
24 along with this program; if not, write to the Free Software Foundation,
25 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
26 #include <sys/cdefs.h>
27 __RCSID("$NetBSD: memchr.c,v 1.2 2016/05/17 14:00:09 christos Exp $");
28
29
30 #ifdef HAVE_CONFIG_H
31 # include <config.h>
32 #endif
33
34 #include <string.h>
35
36 #include <stddef.h>
37
38 #if defined _LIBC
39 # include <memcopy.h>
40 #else
41 # define reg_char char
42 #endif
43
44 #include <limits.h>
45
46 #if HAVE_BP_SYM_H || defined _LIBC
47 # include <bp-sym.h>
48 #else
49 # define BP_SYM(sym) sym
50 #endif
51
52 #undef memchr
53 #undef __memchr
54
55 /* Search no more than N bytes of S for C. */
56 void *
__memchr(void const * s,int c_in,size_t n)57 __memchr (void const *s, int c_in, size_t n)
58 {
59 const unsigned char *char_ptr;
60 const unsigned long int *longword_ptr;
61 unsigned long int longword, magic_bits, charmask;
62 unsigned reg_char c;
63 int i;
64
65 c = (unsigned char) c_in;
66
67 /* Handle the first few characters by reading one character at a time.
68 Do this until CHAR_PTR is aligned on a longword boundary. */
69 for (char_ptr = (const unsigned char *) s;
70 n > 0 && (size_t) char_ptr % sizeof longword != 0;
71 --n, ++char_ptr)
72 if (*char_ptr == c)
73 return (void *) char_ptr;
74
75 /* All these elucidatory comments refer to 4-byte longwords,
76 but the theory applies equally well to any size longwords. */
77
78 longword_ptr = (const unsigned long int *) char_ptr;
79
80 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
81 the "holes." Note that there is a hole just to the left of
82 each byte, with an extra at the end:
83
84 bits: 01111110 11111110 11111110 11111111
85 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
86
87 The 1-bits make sure that carries propagate to the next 0-bit.
88 The 0-bits provide holes for carries to fall into. */
89
90 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
91 Set CHARMASK to be a longword, each of whose bytes is C. */
92
93 magic_bits = 0xfefefefe;
94 charmask = c | (c << 8);
95 charmask |= charmask << 16;
96 #if 0xffffffffU < ULONG_MAX
97 magic_bits |= magic_bits << 32;
98 charmask |= charmask << 32;
99 if (8 < sizeof longword)
100 for (i = 64; i < sizeof longword * 8; i *= 2)
101 {
102 magic_bits |= magic_bits << i;
103 charmask |= charmask << i;
104 }
105 #endif
106 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
107
108 /* Instead of the traditional loop which tests each character,
109 we will test a longword at a time. The tricky part is testing
110 if *any of the four* bytes in the longword in question are zero. */
111 while (n >= sizeof longword)
112 {
113 /* We tentatively exit the loop if adding MAGIC_BITS to
114 LONGWORD fails to change any of the hole bits of LONGWORD.
115
116 1) Is this safe? Will it catch all the zero bytes?
117 Suppose there is a byte with all zeros. Any carry bits
118 propagating from its left will fall into the hole at its
119 least significant bit and stop. Since there will be no
120 carry from its most significant bit, the LSB of the
121 byte to the left will be unchanged, and the zero will be
122 detected.
123
124 2) Is this worthwhile? Will it ignore everything except
125 zero bytes? Suppose every byte of LONGWORD has a bit set
126 somewhere. There will be a carry into bit 8. If bit 8
127 is set, this will carry into bit 16. If bit 8 is clear,
128 one of bits 9-15 must be set, so there will be a carry
129 into bit 16. Similarly, there will be a carry into bit
130 24. If one of bits 24-30 is set, there will be a carry
131 into bit 31, so all of the hole bits will be changed.
132
133 The one misfire occurs when bits 24-30 are clear and bit
134 31 is set; in this case, the hole at bit 31 is not
135 changed. If we had access to the processor carry flag,
136 we could close this loophole by putting the fourth hole
137 at bit 32!
138
139 So it ignores everything except 128's, when they're aligned
140 properly.
141
142 3) But wait! Aren't we looking for C, not zero?
143 Good point. So what we do is XOR LONGWORD with a longword,
144 each of whose bytes is C. This turns each byte that is C
145 into a zero. */
146
147 longword = *longword_ptr++ ^ charmask;
148
149 /* Add MAGIC_BITS to LONGWORD. */
150 if ((((longword + magic_bits)
151
152 /* Set those bits that were unchanged by the addition. */
153 ^ ~longword)
154
155 /* Look at only the hole bits. If any of the hole bits
156 are unchanged, most likely one of the bytes was a
157 zero. */
158 & ~magic_bits) != 0)
159 {
160 /* Which of the bytes was C? If none of them were, it was
161 a misfire; continue the search. */
162
163 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
164
165 if (cp[0] == c)
166 return (void *) cp;
167 if (cp[1] == c)
168 return (void *) &cp[1];
169 if (cp[2] == c)
170 return (void *) &cp[2];
171 if (cp[3] == c)
172 return (void *) &cp[3];
173 if (4 < sizeof longword && cp[4] == c)
174 return (void *) &cp[4];
175 if (5 < sizeof longword && cp[5] == c)
176 return (void *) &cp[5];
177 if (6 < sizeof longword && cp[6] == c)
178 return (void *) &cp[6];
179 if (7 < sizeof longword && cp[7] == c)
180 return (void *) &cp[7];
181 if (8 < sizeof longword)
182 for (i = 8; i < sizeof longword; i++)
183 if (cp[i] == c)
184 return (void *) &cp[i];
185 }
186
187 n -= sizeof longword;
188 }
189
190 char_ptr = (const unsigned char *) longword_ptr;
191
192 while (n-- > 0)
193 {
194 if (*char_ptr == c)
195 return (void *) char_ptr;
196 else
197 ++char_ptr;
198 }
199
200 return 0;
201 }
202 #ifdef weak_alias
203 weak_alias (__memchr, BP_SYM (memchr))
204 #endif
205