1 /* memrchr -- find the last occurrence of a byte in a memory block
2 Copyright (C) 1991-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
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
10 The GNU C Library is free software; you can redistribute it and/or
11 modify it under the terms of the GNU Lesser General Public
12 License as published by the Free Software Foundation; either
13 version 2.1 of the License, or (at your option) any later version.
14
15 The GNU C Library is distributed in the hope that it will be useful,
16 but WITHOUT ANY WARRANTY; without even the implied warranty of
17 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
18 Lesser General Public License for more details.
19
20 You should have received a copy of the GNU Lesser General Public
21 License along with the GNU C Library; if not, see
22 <https://www.gnu.org/licenses/>. */
23
24 #include <stdlib.h>
25
26 #ifdef HAVE_CONFIG_H
27 # include <config.h>
28 #endif
29
30 #if defined _LIBC
31 # include <string.h>
32 # include <memcopy.h>
33 #endif
34
35 #if defined HAVE_LIMITS_H || defined _LIBC
36 # include <limits.h>
37 #endif
38
39 #define LONG_MAX_32_BITS 2147483647
40
41 #ifndef LONG_MAX
42 # define LONG_MAX LONG_MAX_32_BITS
43 #endif
44
45 #include <sys/types.h>
46
47 #undef __memrchr
48 #undef memrchr
49
50 #ifndef weak_alias
51 # define __memrchr memrchr
52 #endif
53
54 /* Search no more than N bytes of S for C. */
55 void *
56 #ifndef MEMRCHR
__memrchr(const void * s,int c_in,size_t n)57 __memrchr
58 #else
59 MEMRCHR
60 #endif
61 (const void *s, int c_in, size_t n)
62 {
63 const unsigned char *char_ptr;
64 const unsigned long int *longword_ptr;
65 unsigned long int longword, magic_bits, charmask;
66 unsigned char c;
67
68 c = (unsigned char) c_in;
69
70 /* Handle the last few characters by reading one character at a time.
71 Do this until CHAR_PTR is aligned on a longword boundary. */
72 for (char_ptr = (const unsigned char *) s + n;
73 n > 0 && ((unsigned long int) char_ptr
74 & (sizeof (longword) - 1)) != 0;
75 --n)
76 if (*--char_ptr == c)
77 return (void *) char_ptr;
78
79 /* All these elucidatory comments refer to 4-byte longwords,
80 but the theory applies equally well to 8-byte longwords. */
81
82 longword_ptr = (const unsigned long int *) char_ptr;
83
84 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
85 the "holes." Note that there is a hole just to the left of
86 each byte, with an extra at the end:
87
88 bits: 01111110 11111110 11111110 11111111
89 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
90
91 The 1-bits make sure that carries propagate to the next 0-bit.
92 The 0-bits provide holes for carries to fall into. */
93 magic_bits = -1;
94 magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
95
96 /* Set up a longword, each of whose bytes is C. */
97 charmask = c | (c << 8);
98 charmask |= charmask << 16;
99 #if LONG_MAX > LONG_MAX_32_BITS
100 charmask |= charmask << 32;
101 #endif
102
103 /* Instead of the traditional loop which tests each character,
104 we will test a longword at a time. The tricky part is testing
105 if *any of the four* bytes in the longword in question are zero. */
106 while (n >= sizeof (longword))
107 {
108 /* We tentatively exit the loop if adding MAGIC_BITS to
109 LONGWORD fails to change any of the hole bits of LONGWORD.
110
111 1) Is this safe? Will it catch all the zero bytes?
112 Suppose there is a byte with all zeros. Any carry bits
113 propagating from its left will fall into the hole at its
114 least significant bit and stop. Since there will be no
115 carry from its most significant bit, the LSB of the
116 byte to the left will be unchanged, and the zero will be
117 detected.
118
119 2) Is this worthwhile? Will it ignore everything except
120 zero bytes? Suppose every byte of LONGWORD has a bit set
121 somewhere. There will be a carry into bit 8. If bit 8
122 is set, this will carry into bit 16. If bit 8 is clear,
123 one of bits 9-15 must be set, so there will be a carry
124 into bit 16. Similarly, there will be a carry into bit
125 24. If one of bits 24-30 is set, there will be a carry
126 into bit 31, so all of the hole bits will be changed.
127
128 The one misfire occurs when bits 24-30 are clear and bit
129 31 is set; in this case, the hole at bit 31 is not
130 changed. If we had access to the processor carry flag,
131 we could close this loophole by putting the fourth hole
132 at bit 32!
133
134 So it ignores everything except 128's, when they're aligned
135 properly.
136
137 3) But wait! Aren't we looking for C, not zero?
138 Good point. So what we do is XOR LONGWORD with a longword,
139 each of whose bytes is C. This turns each byte that is C
140 into a zero. */
141
142 longword = *--longword_ptr ^ charmask;
143
144 /* Add MAGIC_BITS to LONGWORD. */
145 if ((((longword + magic_bits)
146
147 /* Set those bits that were unchanged by the addition. */
148 ^ ~longword)
149
150 /* Look at only the hole bits. If any of the hole bits
151 are unchanged, most likely one of the bytes was a
152 zero. */
153 & ~magic_bits) != 0)
154 {
155 /* Which of the bytes was C? If none of them were, it was
156 a misfire; continue the search. */
157
158 const unsigned char *cp = (const unsigned char *) longword_ptr;
159
160 #if LONG_MAX > 2147483647
161 if (cp[7] == c)
162 return (void *) &cp[7];
163 if (cp[6] == c)
164 return (void *) &cp[6];
165 if (cp[5] == c)
166 return (void *) &cp[5];
167 if (cp[4] == c)
168 return (void *) &cp[4];
169 #endif
170 if (cp[3] == c)
171 return (void *) &cp[3];
172 if (cp[2] == c)
173 return (void *) &cp[2];
174 if (cp[1] == c)
175 return (void *) &cp[1];
176 if (cp[0] == c)
177 return (void *) cp;
178 }
179
180 n -= sizeof (longword);
181 }
182
183 char_ptr = (const unsigned char *) longword_ptr;
184
185 while (n-- > 0)
186 {
187 if (*--char_ptr == c)
188 return (void *) char_ptr;
189 }
190
191 return 0;
192 }
193 #ifndef MEMRCHR
194 # ifdef weak_alias
195 weak_alias (__memrchr, memrchr)
196 # endif
197 #endif
198