1 /* $NetBSD: memchr.c,v 1.1.1.1 2016/01/10 21:36:18 christos Exp $ */
2
3 /* Copyright (C) 1991, 1993, 1997, 1999 Free Software Foundation, Inc.
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
25 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
26 USA. */
27
28 #ifdef HAVE_CONFIG_H
29 # include <config.h>
30 #endif
31
32 #undef __ptr_t
33 #if defined (__cplusplus) || (defined (__STDC__) && __STDC__)
34 # define __ptr_t void *
35 #else /* Not C++ or ANSI C. */
36 # define __ptr_t char *
37 #endif /* C++ or ANSI C. */
38
39 #if defined (_LIBC)
40 # include <string.h>
41 #endif
42
43 #if defined (HAVE_LIMITS_H) || defined (_LIBC)
44 # include <limits.h>
45 #endif
46
47 #define LONG_MAX_32_BITS 2147483647
48
49 #ifndef LONG_MAX
50 # define LONG_MAX LONG_MAX_32_BITS
51 #endif
52
53 #include <sys/types.h>
54
55
56 /* Search no more than N bytes of S for C. */
57
58 __ptr_t
memchr(s,c,n)59 memchr (s, c, n)
60 const __ptr_t s;
61 int c;
62 size_t n;
63 {
64 const unsigned char *char_ptr;
65 const unsigned long int *longword_ptr;
66 unsigned long int longword, magic_bits, charmask;
67
68 c = (unsigned char) c;
69
70 /* Handle the first few characters by reading one character at a time.
71 Do this until CHAR_PTR is aligned on a longword boundary. */
72 for (char_ptr = (const unsigned char *) s;
73 n > 0 && ((unsigned long int) char_ptr
74 & (sizeof (longword) - 1)) != 0;
75 --n, ++char_ptr)
76 if (*char_ptr == c)
77 return (__ptr_t) char_ptr;
78
79 /* All these elucidatory comments refer to 4-byte longwords,
80 but the theory applies equally well to 8-byte longwords. */
81
82 longword_ptr = (unsigned long int *) char_ptr;
83
84 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
85 the "holes." Note that there is a hole just to the left of
86 each byte, with an extra at the end:
87
88 bits: 01111110 11111110 11111110 11111111
89 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
90
91 The 1-bits make sure that carries propagate to the next 0-bit.
92 The 0-bits provide holes for carries to fall into. */
93
94 if (sizeof (longword) != 4 && sizeof (longword) != 8)
95 abort ();
96
97 #if LONG_MAX <= LONG_MAX_32_BITS
98 magic_bits = 0x7efefeff;
99 #else
100 magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
101 #endif
102
103 /* Set up a longword, each of whose bytes is C. */
104 charmask = c | (c << 8);
105 charmask |= charmask << 16;
106 #if LONG_MAX > LONG_MAX_32_BITS
107 charmask |= charmask << 32;
108 #endif
109
110 /* Instead of the traditional loop which tests each character,
111 we will test a longword at a time. The tricky part is testing
112 if *any of the four* bytes in the longword in question are zero. */
113 while (n >= sizeof (longword))
114 {
115 /* We tentatively exit the loop if adding MAGIC_BITS to
116 LONGWORD fails to change any of the hole bits of LONGWORD.
117
118 1) Is this safe? Will it catch all the zero bytes?
119 Suppose there is a byte with all zeros. Any carry bits
120 propagating from its left will fall into the hole at its
121 least significant bit and stop. Since there will be no
122 carry from its most significant bit, the LSB of the
123 byte to the left will be unchanged, and the zero will be
124 detected.
125
126 2) Is this worthwhile? Will it ignore everything except
127 zero bytes? Suppose every byte of LONGWORD has a bit set
128 somewhere. There will be a carry into bit 8. If bit 8
129 is set, this will carry into bit 16. If bit 8 is clear,
130 one of bits 9-15 must be set, so there will be a carry
131 into bit 16. Similarly, there will be a carry into bit
132 24. If one of bits 24-30 is set, there will be a carry
133 into bit 31, so all of the hole bits will be changed.
134
135 The one misfire occurs when bits 24-30 are clear and bit
136 31 is set; in this case, the hole at bit 31 is not
137 changed. If we had access to the processor carry flag,
138 we could close this loophole by putting the fourth hole
139 at bit 32!
140
141 So it ignores everything except 128's, when they're aligned
142 properly.
143
144 3) But wait! Aren't we looking for C, not zero?
145 Good point. So what we do is XOR LONGWORD with a longword,
146 each of whose bytes is C. This turns each byte that is C
147 into a zero. */
148
149 longword = *longword_ptr++ ^ charmask;
150
151 /* Add MAGIC_BITS to LONGWORD. */
152 if ((((longword + magic_bits)
153
154 /* Set those bits that were unchanged by the addition. */
155 ^ ~longword)
156
157 /* Look at only the hole bits. If any of the hole bits
158 are unchanged, most likely one of the bytes was a
159 zero. */
160 & ~magic_bits) != 0)
161 {
162 /* Which of the bytes was C? If none of them were, it was
163 a misfire; continue the search. */
164
165 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
166
167 if (cp[0] == c)
168 return (__ptr_t) cp;
169 if (cp[1] == c)
170 return (__ptr_t) &cp[1];
171 if (cp[2] == c)
172 return (__ptr_t) &cp[2];
173 if (cp[3] == c)
174 return (__ptr_t) &cp[3];
175 #if LONG_MAX > 2147483647
176 if (cp[4] == c)
177 return (__ptr_t) &cp[4];
178 if (cp[5] == c)
179 return (__ptr_t) &cp[5];
180 if (cp[6] == c)
181 return (__ptr_t) &cp[6];
182 if (cp[7] == c)
183 return (__ptr_t) &cp[7];
184 #endif
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 (__ptr_t) char_ptr;
196 else
197 ++char_ptr;
198 }
199
200 return 0;
201 }
202