1 | /* Copyright (C) 1991-2022 Free Software Foundation, Inc. |
2 | This file is part of the GNU C Library. |
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 |
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. |
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 |
17 | Lesser General Public License for more details. |
18 | |
19 | You should have received a copy of the GNU Lesser General Public |
20 | License along with the GNU C Library; if not, see |
21 | <https://www.gnu.org/licenses/>. */ |
22 | |
23 | #include <string.h> |
24 | #include <memcopy.h> |
25 | #include <stdlib.h> |
26 | |
27 | #undef __strchrnul |
28 | #undef strchrnul |
29 | |
30 | #ifndef STRCHRNUL |
31 | # define STRCHRNUL __strchrnul |
32 | #endif |
33 | |
34 | /* Find the first occurrence of C in S or the final NUL byte. */ |
35 | char * |
36 | STRCHRNUL (const char *s, int c_in) |
37 | { |
38 | const unsigned char *char_ptr; |
39 | const unsigned long int *longword_ptr; |
40 | unsigned long int longword, magic_bits, charmask; |
41 | unsigned char c; |
42 | |
43 | c = (unsigned char) c_in; |
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. */ |
47 | for (char_ptr = (const unsigned char *) s; |
48 | ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0; |
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. */ |
67 | magic_bits = -1; |
68 | magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1; |
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) |
167 | |