1 /* memrchr -- find the last occurrence of a byte in a memory block
2
3 Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2005
4 Free Software Foundation, Inc.
5
6 Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
7 with help from Dan Sahlin (dan@sics.se) and
8 commentary by Jim Blandy (jimb@ai.mit.edu);
9 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
10 and implemented by Roland McGrath (roland@ai.mit.edu).
11
12 This program is free software; you can redistribute it and/or modify
13 it under the terms of the GNU General Public License as published by
14 the Free Software Foundation; either version 2, or (at your option)
15 any later version.
16
17 This program is distributed in the hope that it will be useful,
18 but WITHOUT ANY WARRANTY; without even the implied warranty of
19 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
20 GNU General Public License for more details.
21
22 You should have received a copy of the GNU General Public License along
23 with this program; if not, write to the Free Software Foundation,
24 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
25 #include <sys/cdefs.h>
26 __RCSID("$NetBSD: memrchr.c,v 1.2 2016/05/17 14:00:09 christos Exp $");
27
28
29 #ifdef HAVE_CONFIG_H
30 # include <config.h>
31 #endif
32
33 #if defined _LIBC
34 # include <string.h>
35 # include <memcopy.h>
36 #else
37 # include "memrchr.h"
38 # define reg_char char
39 #endif
40
41 #include <limits.h>
42
43 #undef __memrchr
44 #undef memrchr
45
46 #ifndef weak_alias
47 # define __memrchr memrchr
48 #endif
49
50 /* Search no more than N bytes of S for C. */
51 void *
__memrchr(void const * s,int c_in,size_t n)52 __memrchr (void const *s, int c_in, size_t n)
53 {
54 const unsigned char *char_ptr;
55 const unsigned long int *longword_ptr;
56 unsigned long int longword, magic_bits, charmask;
57 unsigned reg_char c;
58 int i;
59
60 c = (unsigned char) c_in;
61
62 /* Handle the last few characters by reading one character at a time.
63 Do this until CHAR_PTR is aligned on a longword boundary. */
64 for (char_ptr = (const unsigned char *) s + n;
65 n > 0 && (size_t) char_ptr % sizeof longword != 0;
66 --n)
67 if (*--char_ptr == c)
68 return (void *) char_ptr;
69
70 /* All these elucidatory comments refer to 4-byte longwords,
71 but the theory applies equally well to any size longwords. */
72
73 longword_ptr = (const unsigned long int *) char_ptr;
74
75 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
76 the "holes." Note that there is a hole just to the left of
77 each byte, with an extra at the end:
78
79 bits: 01111110 11111110 11111110 11111111
80 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
81
82 The 1-bits make sure that carries propagate to the next 0-bit.
83 The 0-bits provide holes for carries to fall into. */
84
85 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
86 Set CHARMASK to be a longword, each of whose bytes is C. */
87
88 magic_bits = 0xfefefefe;
89 charmask = c | (c << 8);
90 charmask |= charmask << 16;
91 #if 0xffffffffU < ULONG_MAX
92 magic_bits |= magic_bits << 32;
93 charmask |= charmask << 32;
94 if (8 < sizeof longword)
95 for (i = 64; i < sizeof longword * 8; i *= 2)
96 {
97 magic_bits |= magic_bits << i;
98 charmask |= charmask << i;
99 }
100 #endif
101 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
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 (8 < sizeof longword)
161 for (i = sizeof longword - 1; 8 <= i; i--)
162 if (cp[i] == c)
163 return (void *) &cp[i];
164 if (7 < sizeof longword && cp[7] == c)
165 return (void *) &cp[7];
166 if (6 < sizeof longword && cp[6] == c)
167 return (void *) &cp[6];
168 if (5 < sizeof longword && cp[5] == c)
169 return (void *) &cp[5];
170 if (4 < sizeof longword && cp[4] == c)
171 return (void *) &cp[4];
172 if (cp[3] == c)
173 return (void *) &cp[3];
174 if (cp[2] == c)
175 return (void *) &cp[2];
176 if (cp[1] == c)
177 return (void *) &cp[1];
178 if (cp[0] == c)
179 return (void *) cp;
180 }
181
182 n -= sizeof longword;
183 }
184
185 char_ptr = (const unsigned char *) longword_ptr;
186
187 while (n-- > 0)
188 {
189 if (*--char_ptr == c)
190 return (void *) char_ptr;
191 }
192
193 return 0;
194 }
195 #ifdef weak_alias
196 weak_alias (__memrchr, memrchr)
197 #endif
198