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ca747856 | 1 | /* memrchr -- find the last occurrence of a byte in a memory block |
ef52edfc | 2 | Copyright (C) 1991, 93, 96, 97, 99, 2000 Free Software Foundation, Inc. |
41bdb6e2 | 3 | This file is part of the GNU C Library. |
ca747856 RM |
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 | |
41bdb6e2 AJ |
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. | |
ca747856 RM |
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 | |
41bdb6e2 | 18 | Lesser General Public License for more details. |
ca747856 | 19 | |
41bdb6e2 AJ |
20 | You should have received a copy of the GNU Lesser General Public |
21 | License along with the GNU C Library; if not, write to the Free | |
22 | Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA | |
23 | 02111-1307 USA. */ | |
ca747856 | 24 | |
ef52edfc UD |
25 | #include <stdlib.h> |
26 | ||
ca747856 | 27 | #ifdef HAVE_CONFIG_H |
298ea0ff | 28 | # include <config.h> |
ca747856 RM |
29 | #endif |
30 | ||
31 | #undef __ptr_t | |
298ea0ff | 32 | #if defined __cplusplus || (defined __STDC__ && __STDC__) |
ca747856 RM |
33 | # define __ptr_t void * |
34 | #else /* Not C++ or ANSI C. */ | |
35 | # define __ptr_t char * | |
36 | #endif /* C++ or ANSI C. */ | |
37 | ||
298ea0ff | 38 | #if defined _LIBC |
ca747856 RM |
39 | # include <string.h> |
40 | # include <memcopy.h> | |
41 | #else | |
42 | # define reg_char char | |
43 | #endif | |
44 | ||
298ea0ff | 45 | #if defined HAVE_LIMITS_H || defined _LIBC |
ca747856 RM |
46 | # include <limits.h> |
47 | #endif | |
48 | ||
49 | #define LONG_MAX_32_BITS 2147483647 | |
50 | ||
51 | #ifndef LONG_MAX | |
4572f6c2 | 52 | # define LONG_MAX LONG_MAX_32_BITS |
ca747856 RM |
53 | #endif |
54 | ||
55 | #include <sys/types.h> | |
56 | ||
298ea0ff | 57 | #undef __memrchr |
fbda91b1 UD |
58 | #undef memrchr |
59 | ||
298ea0ff UD |
60 | #ifndef weak_alias |
61 | # define __memrchr memrchr | |
62 | #endif | |
ca747856 RM |
63 | |
64 | /* Search no more than N bytes of S for C. */ | |
65 | __ptr_t | |
66 | __memrchr (s, c_in, n) | |
67 | const __ptr_t s; | |
68 | int c_in; | |
69 | size_t n; | |
70 | { | |
71 | const unsigned char *char_ptr; | |
72 | const unsigned long int *longword_ptr; | |
73 | unsigned long int longword, magic_bits, charmask; | |
74 | unsigned reg_char c; | |
75 | ||
76 | c = (unsigned char) c_in; | |
77 | ||
78 | /* Handle the last few characters by reading one character at a time. | |
79 | Do this until CHAR_PTR is aligned on a longword boundary. */ | |
80 | for (char_ptr = (const unsigned char *) s + n; | |
81 | n > 0 && ((unsigned long int) char_ptr | |
82 | & (sizeof (longword) - 1)) != 0; | |
83 | --n) | |
84 | if (*--char_ptr == c) | |
85 | return (__ptr_t) char_ptr; | |
86 | ||
87 | /* All these elucidatory comments refer to 4-byte longwords, | |
88 | but the theory applies equally well to 8-byte longwords. */ | |
89 | ||
c3301189 | 90 | longword_ptr = (const unsigned long int *) char_ptr; |
ca747856 RM |
91 | |
92 | /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits | |
93 | the "holes." Note that there is a hole just to the left of | |
94 | each byte, with an extra at the end: | |
95 | ||
96 | bits: 01111110 11111110 11111110 11111111 | |
97 | bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD | |
98 | ||
99 | The 1-bits make sure that carries propagate to the next 0-bit. | |
100 | The 0-bits provide holes for carries to fall into. */ | |
101 | ||
102 | if (sizeof (longword) != 4 && sizeof (longword) != 8) | |
103 | abort (); | |
104 | ||
105 | #if LONG_MAX <= LONG_MAX_32_BITS | |
106 | magic_bits = 0x7efefeff; | |
107 | #else | |
108 | magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff; | |
109 | #endif | |
110 | ||
111 | /* Set up a longword, each of whose bytes is C. */ | |
112 | charmask = c | (c << 8); | |
113 | charmask |= charmask << 16; | |
114 | #if LONG_MAX > LONG_MAX_32_BITS | |
115 | charmask |= charmask << 32; | |
116 | #endif | |
117 | ||
118 | /* Instead of the traditional loop which tests each character, | |
119 | we will test a longword at a time. The tricky part is testing | |
120 | if *any of the four* bytes in the longword in question are zero. */ | |
121 | while (n >= sizeof (longword)) | |
122 | { | |
123 | /* We tentatively exit the loop if adding MAGIC_BITS to | |
124 | LONGWORD fails to change any of the hole bits of LONGWORD. | |
125 | ||
126 | 1) Is this safe? Will it catch all the zero bytes? | |
127 | Suppose there is a byte with all zeros. Any carry bits | |
128 | propagating from its left will fall into the hole at its | |
129 | least significant bit and stop. Since there will be no | |
130 | carry from its most significant bit, the LSB of the | |
131 | byte to the left will be unchanged, and the zero will be | |
132 | detected. | |
133 | ||
134 | 2) Is this worthwhile? Will it ignore everything except | |
135 | zero bytes? Suppose every byte of LONGWORD has a bit set | |
136 | somewhere. There will be a carry into bit 8. If bit 8 | |
137 | is set, this will carry into bit 16. If bit 8 is clear, | |
138 | one of bits 9-15 must be set, so there will be a carry | |
139 | into bit 16. Similarly, there will be a carry into bit | |
140 | 24. If one of bits 24-30 is set, there will be a carry | |
141 | into bit 31, so all of the hole bits will be changed. | |
142 | ||
143 | The one misfire occurs when bits 24-30 are clear and bit | |
144 | 31 is set; in this case, the hole at bit 31 is not | |
145 | changed. If we had access to the processor carry flag, | |
146 | we could close this loophole by putting the fourth hole | |
147 | at bit 32! | |
148 | ||
149 | So it ignores everything except 128's, when they're aligned | |
150 | properly. | |
151 | ||
152 | 3) But wait! Aren't we looking for C, not zero? | |
153 | Good point. So what we do is XOR LONGWORD with a longword, | |
154 | each of whose bytes is C. This turns each byte that is C | |
155 | into a zero. */ | |
156 | ||
157 | longword = *--longword_ptr ^ charmask; | |
158 | ||
159 | /* Add MAGIC_BITS to LONGWORD. */ | |
160 | if ((((longword + magic_bits) | |
161 | ||
162 | /* Set those bits that were unchanged by the addition. */ | |
163 | ^ ~longword) | |
164 | ||
165 | /* Look at only the hole bits. If any of the hole bits | |
166 | are unchanged, most likely one of the bytes was a | |
167 | zero. */ | |
168 | & ~magic_bits) != 0) | |
169 | { | |
170 | /* Which of the bytes was C? If none of them were, it was | |
171 | a misfire; continue the search. */ | |
172 | ||
173 | const unsigned char *cp = (const unsigned char *) longword_ptr; | |
174 | ||
ca747856 | 175 | #if LONG_MAX > 2147483647 |
ca747856 RM |
176 | if (cp[7] == c) |
177 | return (__ptr_t) &cp[7]; | |
fbda91b1 UD |
178 | if (cp[6] == c) |
179 | return (__ptr_t) &cp[6]; | |
180 | if (cp[5] == c) | |
181 | return (__ptr_t) &cp[5]; | |
182 | if (cp[4] == c) | |
183 | return (__ptr_t) &cp[4]; | |
ca747856 | 184 | #endif |
fbda91b1 UD |
185 | if (cp[3] == c) |
186 | return (__ptr_t) &cp[3]; | |
187 | if (cp[2] == c) | |
188 | return (__ptr_t) &cp[2]; | |
189 | if (cp[1] == c) | |
190 | return (__ptr_t) &cp[1]; | |
191 | if (cp[0] == c) | |
192 | return (__ptr_t) cp; | |
ca747856 RM |
193 | } |
194 | ||
195 | n -= sizeof (longword); | |
196 | } | |
197 | ||
198 | char_ptr = (const unsigned char *) longword_ptr; | |
199 | ||
200 | while (n-- > 0) | |
201 | { | |
202 | if (*--char_ptr == c) | |
203 | return (__ptr_t) char_ptr; | |
204 | } | |
205 | ||
206 | return 0; | |
207 | } | |
298ea0ff | 208 | #ifdef weak_alias |
ca747856 | 209 | weak_alias (__memrchr, memrchr) |
298ea0ff | 210 | #endif |