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d4697bc9 | 1 | /* Copyright (C) 1991-2014 Free Software Foundation, Inc. |
41bdb6e2 | 2 | This file is part of the GNU C Library. |
482eec0d UD |
3 | Based on strlen implementation by Torbjorn Granlund (tege@sics.se), |
4 | with help from Dan Sahlin (dan@sics.se) and | |
5 | commentary by Jim Blandy (jimb@ai.mit.edu); | |
6 | adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), | |
7 | and implemented by Roland McGrath (roland@ai.mit.edu). | |
8 | ||
9 | The GNU C Library is free software; you can redistribute it and/or | |
41bdb6e2 AJ |
10 | modify it under the terms of the GNU Lesser General Public |
11 | License as published by the Free Software Foundation; either | |
12 | version 2.1 of the License, or (at your option) any later version. | |
482eec0d UD |
13 | |
14 | The GNU C Library is distributed in the hope that it will be useful, | |
15 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
16 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
41bdb6e2 | 17 | Lesser General Public License for more details. |
482eec0d | 18 | |
41bdb6e2 | 19 | You should have received a copy of the GNU Lesser General Public |
59ba27a6 PE |
20 | License along with the GNU C Library; if not, see |
21 | <http://www.gnu.org/licenses/>. */ | |
482eec0d UD |
22 | |
23 | #ifdef HAVE_CONFIG_H | |
24 | #include <config.h> | |
25 | #endif | |
26 | ||
27 | #undef __ptr_t | |
c8a89e7d | 28 | #define __ptr_t void * |
482eec0d UD |
29 | |
30 | #if defined (_LIBC) | |
31 | # include <string.h> | |
28e35124 | 32 | # include <memcopy.h> |
a2616aed | 33 | # include <stdlib.h> |
482eec0d UD |
34 | #endif |
35 | ||
36 | #if defined (HAVE_LIMITS_H) || defined (_LIBC) | |
37 | # include <limits.h> | |
38 | #endif | |
39 | ||
40 | #define LONG_MAX_32_BITS 2147483647 | |
41 | ||
42 | #ifndef LONG_MAX | |
43 | #define LONG_MAX LONG_MAX_32_BITS | |
44 | #endif | |
45 | ||
46 | #include <sys/types.h> | |
47 | ||
48 | #undef memchr | |
49 | ||
7af8b946 AZ |
50 | #ifndef RAWMEMCHR |
51 | # define RAWMEMCHR __rawmemchr | |
52 | #endif | |
482eec0d | 53 | |
28e35124 | 54 | /* Find the first occurrence of C in S. */ |
482eec0d | 55 | __ptr_t |
7af8b946 | 56 | RAWMEMCHR (s, c_in) |
482eec0d | 57 | const __ptr_t s; |
28e35124 | 58 | int c_in; |
482eec0d UD |
59 | { |
60 | const unsigned char *char_ptr; | |
61 | const unsigned long int *longword_ptr; | |
62 | unsigned long int longword, magic_bits, charmask; | |
50f81fd7 | 63 | unsigned char c; |
482eec0d | 64 | |
28e35124 | 65 | c = (unsigned char) c_in; |
482eec0d UD |
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 | ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0; | |
71 | ++char_ptr) | |
72 | if (*char_ptr == c) | |
73 | return (__ptr_t) char_ptr; | |
74 | ||
75 | /* All these elucidatory comments refer to 4-byte longwords, | |
76 | but the theory applies equally well to 8-byte longwords. */ | |
77 | ||
78 | longword_ptr = (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 | if (sizeof (longword) != 4 && sizeof (longword) != 8) | |
91 | abort (); | |
92 | ||
93 | #if LONG_MAX <= LONG_MAX_32_BITS | |
94 | magic_bits = 0x7efefeff; | |
95 | #else | |
96 | magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff; | |
97 | #endif | |
98 | ||
99 | /* Set up a longword, each of whose bytes is C. */ | |
100 | charmask = c | (c << 8); | |
101 | charmask |= charmask << 16; | |
102 | #if LONG_MAX > LONG_MAX_32_BITS | |
103 | charmask |= charmask << 32; | |
104 | #endif | |
105 | ||
106 | /* Instead of the traditional loop which tests each character, | |
107 | we will test a longword at a time. The tricky part is testing | |
108 | if *any of the four* bytes in the longword in question are zero. */ | |
109 | while (1) | |
110 | { | |
111 | /* We tentatively exit the loop if adding MAGIC_BITS to | |
112 | LONGWORD fails to change any of the hole bits of LONGWORD. | |
113 | ||
114 | 1) Is this safe? Will it catch all the zero bytes? | |
115 | Suppose there is a byte with all zeros. Any carry bits | |
116 | propagating from its left will fall into the hole at its | |
117 | least significant bit and stop. Since there will be no | |
118 | carry from its most significant bit, the LSB of the | |
119 | byte to the left will be unchanged, and the zero will be | |
120 | detected. | |
121 | ||
122 | 2) Is this worthwhile? Will it ignore everything except | |
123 | zero bytes? Suppose every byte of LONGWORD has a bit set | |
124 | somewhere. There will be a carry into bit 8. If bit 8 | |
125 | is set, this will carry into bit 16. If bit 8 is clear, | |
126 | one of bits 9-15 must be set, so there will be a carry | |
127 | into bit 16. Similarly, there will be a carry into bit | |
128 | 24. If one of bits 24-30 is set, there will be a carry | |
129 | into bit 31, so all of the hole bits will be changed. | |
130 | ||
131 | The one misfire occurs when bits 24-30 are clear and bit | |
132 | 31 is set; in this case, the hole at bit 31 is not | |
133 | changed. If we had access to the processor carry flag, | |
134 | we could close this loophole by putting the fourth hole | |
135 | at bit 32! | |
136 | ||
137 | So it ignores everything except 128's, when they're aligned | |
138 | properly. | |
139 | ||
140 | 3) But wait! Aren't we looking for C, not zero? | |
141 | Good point. So what we do is XOR LONGWORD with a longword, | |
142 | each of whose bytes is C. This turns each byte that is C | |
143 | into a zero. */ | |
144 | ||
145 | longword = *longword_ptr++ ^ charmask; | |
146 | ||
147 | /* Add MAGIC_BITS to LONGWORD. */ | |
148 | if ((((longword + magic_bits) | |
149 | ||
150 | /* Set those bits that were unchanged by the addition. */ | |
151 | ^ ~longword) | |
152 | ||
153 | /* Look at only the hole bits. If any of the hole bits | |
154 | are unchanged, most likely one of the bytes was a | |
155 | zero. */ | |
156 | & ~magic_bits) != 0) | |
157 | { | |
158 | /* Which of the bytes was C? If none of them were, it was | |
159 | a misfire; continue the search. */ | |
160 | ||
161 | const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); | |
162 | ||
163 | if (cp[0] == c) | |
164 | return (__ptr_t) cp; | |
165 | if (cp[1] == c) | |
166 | return (__ptr_t) &cp[1]; | |
167 | if (cp[2] == c) | |
168 | return (__ptr_t) &cp[2]; | |
169 | if (cp[3] == c) | |
170 | return (__ptr_t) &cp[3]; | |
171 | #if LONG_MAX > 2147483647 | |
172 | if (cp[4] == c) | |
173 | return (__ptr_t) &cp[4]; | |
174 | if (cp[5] == c) | |
175 | return (__ptr_t) &cp[5]; | |
176 | if (cp[6] == c) | |
177 | return (__ptr_t) &cp[6]; | |
178 | if (cp[7] == c) | |
179 | return (__ptr_t) &cp[7]; | |
180 | #endif | |
181 | } | |
182 | } | |
183 | } | |
37ba7d66 | 184 | libc_hidden_def (__rawmemchr) |
482eec0d | 185 | weak_alias (__rawmemchr, rawmemchr) |