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b168057a | 1 | /* Copyright (C) 1991-2015 Free Software Foundation, Inc. |
41bdb6e2 | 2 | This file is part of the GNU C Library. |
c4563d2d UD |
3 | Based on strlen implementation by Torbjorn Granlund (tege@sics.se), |
4 | with help from Dan Sahlin (dan@sics.se) and | |
5 | bug fix and commentary by Jim Blandy (jimb@ai.mit.edu); | |
6 | adaptation to strchr 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. | |
c4563d2d 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. |
c4563d2d | 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/>. */ | |
c4563d2d UD |
22 | |
23 | #include <string.h> | |
28e35124 | 24 | #include <memcopy.h> |
a2616aed | 25 | #include <stdlib.h> |
c4563d2d UD |
26 | |
27 | #undef __strchrnul | |
28 | #undef strchrnul | |
29 | ||
31c81aaa AZ |
30 | #ifndef STRCHRNUL |
31 | # define STRCHRNUL __strchrnul | |
32 | #endif | |
33 | ||
c4563d2d UD |
34 | /* Find the first occurrence of C in S or the final NUL byte. */ |
35 | char * | |
9d46370c | 36 | STRCHRNUL (const char *s, int c_in) |
c4563d2d UD |
37 | { |
38 | const unsigned char *char_ptr; | |
39 | const unsigned long int *longword_ptr; | |
40 | unsigned long int longword, magic_bits, charmask; | |
50f81fd7 | 41 | unsigned char c; |
c4563d2d | 42 | |
28e35124 | 43 | c = (unsigned char) c_in; |
c4563d2d UD |
44 | |
45 | /* Handle the first few characters by reading one character at a time. | |
46 | Do this until CHAR_PTR is aligned on a longword boundary. */ | |
3cc4a097 UD |
47 | for (char_ptr = (const unsigned char *) s; |
48 | ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0; | |
c4563d2d UD |
49 | ++char_ptr) |
50 | if (*char_ptr == c || *char_ptr == '\0') | |
51 | return (void *) char_ptr; | |
52 | ||
53 | /* All these elucidatory comments refer to 4-byte longwords, | |
54 | but the theory applies equally well to 8-byte longwords. */ | |
55 | ||
56 | longword_ptr = (unsigned long int *) char_ptr; | |
57 | ||
58 | /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits | |
59 | the "holes." Note that there is a hole just to the left of | |
60 | each byte, with an extra at the end: | |
61 | ||
62 | bits: 01111110 11111110 11111110 11111111 | |
63 | bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD | |
64 | ||
65 | The 1-bits make sure that carries propagate to the next 0-bit. | |
66 | The 0-bits provide holes for carries to fall into. */ | |
55422368 PE |
67 | magic_bits = -1; |
68 | magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1; | |
c4563d2d UD |
69 | |
70 | /* Set up a longword, each of whose bytes is C. */ | |
71 | charmask = c | (c << 8); | |
72 | charmask |= charmask << 16; | |
73 | if (sizeof (longword) > 4) | |
74 | /* Do the shift in two steps to avoid a warning if long has 32 bits. */ | |
75 | charmask |= (charmask << 16) << 16; | |
76 | if (sizeof (longword) > 8) | |
77 | abort (); | |
78 | ||
79 | /* Instead of the traditional loop which tests each character, | |
80 | we will test a longword at a time. The tricky part is testing | |
81 | if *any of the four* bytes in the longword in question are zero. */ | |
82 | for (;;) | |
83 | { | |
84 | /* We tentatively exit the loop if adding MAGIC_BITS to | |
85 | LONGWORD fails to change any of the hole bits of LONGWORD. | |
86 | ||
87 | 1) Is this safe? Will it catch all the zero bytes? | |
88 | Suppose there is a byte with all zeros. Any carry bits | |
89 | propagating from its left will fall into the hole at its | |
90 | least significant bit and stop. Since there will be no | |
91 | carry from its most significant bit, the LSB of the | |
92 | byte to the left will be unchanged, and the zero will be | |
93 | detected. | |
94 | ||
95 | 2) Is this worthwhile? Will it ignore everything except | |
96 | zero bytes? Suppose every byte of LONGWORD has a bit set | |
97 | somewhere. There will be a carry into bit 8. If bit 8 | |
98 | is set, this will carry into bit 16. If bit 8 is clear, | |
99 | one of bits 9-15 must be set, so there will be a carry | |
100 | into bit 16. Similarly, there will be a carry into bit | |
101 | 24. If one of bits 24-30 is set, there will be a carry | |
102 | into bit 31, so all of the hole bits will be changed. | |
103 | ||
104 | The one misfire occurs when bits 24-30 are clear and bit | |
105 | 31 is set; in this case, the hole at bit 31 is not | |
106 | changed. If we had access to the processor carry flag, | |
107 | we could close this loophole by putting the fourth hole | |
108 | at bit 32! | |
109 | ||
110 | So it ignores everything except 128's, when they're aligned | |
111 | properly. | |
112 | ||
113 | 3) But wait! Aren't we looking for C as well as zero? | |
114 | Good point. So what we do is XOR LONGWORD with a longword, | |
115 | each of whose bytes is C. This turns each byte that is C | |
116 | into a zero. */ | |
117 | ||
118 | longword = *longword_ptr++; | |
119 | ||
120 | /* Add MAGIC_BITS to LONGWORD. */ | |
121 | if ((((longword + magic_bits) | |
122 | ||
123 | /* Set those bits that were unchanged by the addition. */ | |
124 | ^ ~longword) | |
125 | ||
126 | /* Look at only the hole bits. If any of the hole bits | |
127 | are unchanged, most likely one of the bytes was a | |
128 | zero. */ | |
129 | & ~magic_bits) != 0 || | |
130 | ||
131 | /* That caught zeroes. Now test for C. */ | |
132 | ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask)) | |
133 | & ~magic_bits) != 0) | |
134 | { | |
135 | /* Which of the bytes was C or zero? | |
136 | If none of them were, it was a misfire; continue the search. */ | |
137 | ||
138 | const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); | |
139 | ||
140 | if (*cp == c || *cp == '\0') | |
141 | return (char *) cp; | |
142 | if (*++cp == c || *cp == '\0') | |
143 | return (char *) cp; | |
144 | if (*++cp == c || *cp == '\0') | |
145 | return (char *) cp; | |
146 | if (*++cp == c || *cp == '\0') | |
147 | return (char *) cp; | |
148 | if (sizeof (longword) > 4) | |
149 | { | |
150 | if (*++cp == c || *cp == '\0') | |
151 | return (char *) cp; | |
152 | if (*++cp == c || *cp == '\0') | |
153 | return (char *) cp; | |
154 | if (*++cp == c || *cp == '\0') | |
155 | return (char *) cp; | |
156 | if (*++cp == c || *cp == '\0') | |
157 | return (char *) cp; | |
158 | } | |
159 | } | |
160 | } | |
161 | ||
162 | /* This should never happen. */ | |
163 | return NULL; | |
164 | } | |
165 | ||
166 | weak_alias (__strchrnul, strchrnul) |