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 | 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 |
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 | #ifndef _LIBC |
24 | # include <config.h> |
25 | #endif |
26 | |
27 | #include <string.h> |
28 | |
29 | #include <stddef.h> |
30 | |
31 | #include <limits.h> |
32 | |
33 | #undef __memchr |
34 | #ifdef _LIBC |
35 | # undef memchr |
36 | #endif |
37 | |
38 | #ifndef weak_alias |
39 | # define __memchr memchr |
40 | #endif |
41 | |
42 | #ifndef MEMCHR |
43 | # define MEMCHR __memchr |
44 | #endif |
45 | |
46 | /* Search no more than N bytes of S for C. */ |
47 | void * |
48 | MEMCHR (void const *s, int c_in, size_t n) |
49 | { |
50 | /* On 32-bit hardware, choosing longword to be a 32-bit unsigned |
51 | long instead of a 64-bit uintmax_t tends to give better |
52 | performance. On 64-bit hardware, unsigned long is generally 64 |
53 | bits already. Change this typedef to experiment with |
54 | performance. */ |
55 | typedef unsigned long int longword; |
56 | |
57 | const unsigned char *char_ptr; |
58 | const longword *longword_ptr; |
59 | longword repeated_one; |
60 | longword repeated_c; |
61 | unsigned char c; |
62 | |
63 | c = (unsigned char) c_in; |
64 | |
65 | /* Handle the first few bytes by reading one byte at a time. |
66 | Do this until CHAR_PTR is aligned on a longword boundary. */ |
67 | for (char_ptr = (const unsigned char *) s; |
68 | n > 0 && (size_t) char_ptr % sizeof (longword) != 0; |
69 | --n, ++char_ptr) |
70 | if (*char_ptr == c) |
71 | return (void *) char_ptr; |
72 | |
73 | longword_ptr = (const longword *) char_ptr; |
74 | |
75 | /* All these elucidatory comments refer to 4-byte longwords, |
76 | but the theory applies equally well to any size longwords. */ |
77 | |
78 | /* Compute auxiliary longword values: |
79 | repeated_one is a value which has a 1 in every byte. |
80 | repeated_c has c in every byte. */ |
81 | repeated_one = 0x01010101; |
82 | repeated_c = c | (c << 8); |
83 | repeated_c |= repeated_c << 16; |
84 | if (0xffffffffU < (longword) -1) |
85 | { |
86 | repeated_one |= repeated_one << 31 << 1; |
87 | repeated_c |= repeated_c << 31 << 1; |
88 | if (8 < sizeof (longword)) |
89 | { |
90 | size_t i; |
91 | |
92 | for (i = 64; i < sizeof (longword) * 8; i *= 2) |
93 | { |
94 | repeated_one |= repeated_one << i; |
95 | repeated_c |= repeated_c << i; |
96 | } |
97 | } |
98 | } |
99 | |
100 | /* Instead of the traditional loop which tests each byte, we will test a |
101 | longword at a time. The tricky part is testing if *any of the four* |
102 | bytes in the longword in question are equal to c. We first use an xor |
103 | with repeated_c. This reduces the task to testing whether *any of the |
104 | four* bytes in longword1 is zero. |
105 | |
106 | We compute tmp = |
107 | ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7). |
108 | That is, we perform the following operations: |
109 | 1. Subtract repeated_one. |
110 | 2. & ~longword1. |
111 | 3. & a mask consisting of 0x80 in every byte. |
112 | Consider what happens in each byte: |
113 | - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff, |
114 | and step 3 transforms it into 0x80. A carry can also be propagated |
115 | to more significant bytes. |
116 | - If a byte of longword1 is nonzero, let its lowest 1 bit be at |
117 | position k (0 <= k <= 7); so the lowest k bits are 0. After step 1, |
118 | the byte ends in a single bit of value 0 and k bits of value 1. |
119 | After step 2, the result is just k bits of value 1: 2^k - 1. After |
120 | step 3, the result is 0. And no carry is produced. |
121 | So, if longword1 has only non-zero bytes, tmp is zero. |
122 | Whereas if longword1 has a zero byte, call j the position of the least |
123 | significant zero byte. Then the result has a zero at positions 0, ..., |
124 | j-1 and a 0x80 at position j. We cannot predict the result at the more |
125 | significant bytes (positions j+1..3), but it does not matter since we |
126 | already have a non-zero bit at position 8*j+7. |
127 | |
128 | So, the test whether any byte in longword1 is zero is equivalent to |
129 | testing whether tmp is nonzero. */ |
130 | |
131 | while (n >= sizeof (longword)) |
132 | { |
133 | longword longword1 = *longword_ptr ^ repeated_c; |
134 | |
135 | if ((((longword1 - repeated_one) & ~longword1) |
136 | & (repeated_one << 7)) != 0) |
137 | break; |
138 | longword_ptr++; |
139 | n -= sizeof (longword); |
140 | } |
141 | |
142 | char_ptr = (const unsigned char *) longword_ptr; |
143 | |
144 | /* At this point, we know that either n < sizeof (longword), or one of the |
145 | sizeof (longword) bytes starting at char_ptr is == c. On little-endian |
146 | machines, we could determine the first such byte without any further |
147 | memory accesses, just by looking at the tmp result from the last loop |
148 | iteration. But this does not work on big-endian machines. Choose code |
149 | that works in both cases. */ |
150 | |
151 | for (; n > 0; --n, ++char_ptr) |
152 | { |
153 | if (*char_ptr == c) |
154 | return (void *) char_ptr; |
155 | } |
156 | |
157 | return NULL; |
158 | } |
159 | #ifdef weak_alias |
160 | weak_alias (__memchr, memchr) |
161 | #endif |
162 | libc_hidden_builtin_def (memchr) |
163 | |