source: trunk/essentials/app-arch/tar/lib/memchr.c

Last change on this file was 3342, checked in by bird, 18 years ago

tar 1.16.1
File size: 6.2 KB
Line 
1/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006 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
10NOTE: The canonical source of this file is maintained with the GNU C Library.
11Bugs can be reported to bug-glibc@prep.ai.mit.edu.
12
13This program is free software; you can redistribute it and/or modify it
14under the terms of the GNU General Public License as published by the
15Free Software Foundation; either version 2, or (at your option) any
16later version.
17
18This program is distributed in the hope that it will be useful,
19but WITHOUT ANY WARRANTY; without even the implied warranty of
20MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
21GNU General Public License for more details.
22
23You should have received a copy of the GNU General Public License
24along with this program; if not, write to the Free Software Foundation,
25Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
26
27#ifndef _LIBC
28# include <config.h>
29#endif
30
31#include <string.h>
32
33#include <stddef.h>
34
35#if defined _LIBC
36# include <memcopy.h>
37#else
38# define reg_char char
39#endif
40
41#include <limits.h>
42
43#if HAVE_BP_SYM_H || defined _LIBC
44# include <bp-sym.h>
45#else
46# define BP_SYM(sym) sym
47#endif
48
49#undef memchr
50#undef __memchr
51
52/* Search no more than N bytes of S for C. */
53void *
54__memchr (void const *s, int c_in, size_t n)
55{
56 const unsigned char *char_ptr;
57 const unsigned long int *longword_ptr;
58 unsigned long int longword, magic_bits, charmask;
59 unsigned reg_char c;
60 int i;
61
62 c = (unsigned char) c_in;
63
64 /* Handle the first few characters by reading one character at a time.
65 Do this until CHAR_PTR is aligned on a longword boundary. */
66 for (char_ptr = (const unsigned char *) s;
67 n > 0 && (size_t) char_ptr % sizeof longword != 0;
68 --n, ++char_ptr)
69 if (*char_ptr == c)
70 return (void *) char_ptr;
71
72 /* All these elucidatory comments refer to 4-byte longwords,
73 but the theory applies equally well to any size longwords. */
74
75 longword_ptr = (const unsigned long int *) char_ptr;
76
77 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
78 the "holes." Note that there is a hole just to the left of
79 each byte, with an extra at the end:
80
81 bits: 01111110 11111110 11111110 11111111
82 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
83
84 The 1-bits make sure that carries propagate to the next 0-bit.
85 The 0-bits provide holes for carries to fall into. */
86
87 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
88 Set CHARMASK to be a longword, each of whose bytes is C. */
89
90 magic_bits = 0xfefefefe;
91 charmask = c | (c << 8);
92 charmask |= charmask << 16;
93#if 0xffffffffU < ULONG_MAX
94 magic_bits |= magic_bits << 32;
95 charmask |= charmask << 32;
96 if (8 < sizeof longword)
97 for (i = 64; i < sizeof longword * 8; i *= 2)
98 {
99 magic_bits |= magic_bits << i;
100 charmask |= charmask << i;
101 }
102#endif
103 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
104
105 /* Instead of the traditional loop which tests each character,
106 we will test a longword at a time. The tricky part is testing
107 if *any of the four* bytes in the longword in question are zero. */
108 while (n >= sizeof longword)
109 {
110 /* We tentatively exit the loop if adding MAGIC_BITS to
111 LONGWORD fails to change any of the hole bits of LONGWORD.
112
113 1) Is this safe? Will it catch all the zero bytes?
114 Suppose there is a byte with all zeros. Any carry bits
115 propagating from its left will fall into the hole at its
116 least significant bit and stop. Since there will be no
117 carry from its most significant bit, the LSB of the
118 byte to the left will be unchanged, and the zero will be
119 detected.
120
121 2) Is this worthwhile? Will it ignore everything except
122 zero bytes? Suppose every byte of LONGWORD has a bit set
123 somewhere. There will be a carry into bit 8. If bit 8
124 is set, this will carry into bit 16. If bit 8 is clear,
125 one of bits 9-15 must be set, so there will be a carry
126 into bit 16. Similarly, there will be a carry into bit
127 24. If one of bits 24-30 is set, there will be a carry
128 into bit 31, so all of the hole bits will be changed.
129
130 The one misfire occurs when bits 24-30 are clear and bit
131 31 is set; in this case, the hole at bit 31 is not
132 changed. If we had access to the processor carry flag,
133 we could close this loophole by putting the fourth hole
134 at bit 32!
135
136 So it ignores everything except 128's, when they're aligned
137 properly.
138
139 3) But wait! Aren't we looking for C, not zero?
140 Good point. So what we do is XOR LONGWORD with a longword,
141 each of whose bytes is C. This turns each byte that is C
142 into a zero. */
143
144 longword = *longword_ptr++ ^ charmask;
145
146 /* Add MAGIC_BITS to LONGWORD. */
147 if ((((longword + magic_bits)
148
149 /* Set those bits that were unchanged by the addition. */
150 ^ ~longword)
151
152 /* Look at only the hole bits. If any of the hole bits
153 are unchanged, most likely one of the bytes was a
154 zero. */
155 & ~magic_bits) != 0)
156 {
157 /* Which of the bytes was C? If none of them were, it was
158 a misfire; continue the search. */
159
160 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
161
162 if (cp[0] == c)
163 return (void *) cp;
164 if (cp[1] == c)
165 return (void *) &cp[1];
166 if (cp[2] == c)
167 return (void *) &cp[2];
168 if (cp[3] == c)
169 return (void *) &cp[3];
170 if (4 < sizeof longword && cp[4] == c)
171 return (void *) &cp[4];
172 if (5 < sizeof longword && cp[5] == c)
173 return (void *) &cp[5];
174 if (6 < sizeof longword && cp[6] == c)
175 return (void *) &cp[6];
176 if (7 < sizeof longword && cp[7] == c)
177 return (void *) &cp[7];
178 if (8 < sizeof longword)
179 for (i = 8; i < sizeof longword; i++)
180 if (cp[i] == c)
181 return (void *) &cp[i];
182 }
183
184 n -= sizeof longword;
185 }
186
187 char_ptr = (const unsigned char *) longword_ptr;
188
189 while (n-- > 0)
190 {
191 if (*char_ptr == c)
192 return (void *) char_ptr;
193 else
194 ++char_ptr;
195 }
196
197 return 0;
198}
199#ifdef weak_alias
200weak_alias (__memchr, BP_SYM (memchr))
201#endif
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