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22312b71 AD |
1 | /* memrchr -- find the last occurrence of a byte in a memory block |
2 | Copyright (C) 1991, 93, 96, 97, 99, 2000 Free Software Foundation, Inc. | |
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 | ||
9742c552 PE |
9 | This program is free software; you can redistribute it and/or modify |
10 | it under the terms of the GNU General Public License as published by | |
11 | the Free Software Foundation; either version 2, or (at your option) | |
12 | any later version. | |
22312b71 | 13 | |
9742c552 | 14 | This program is distributed in the hope that it will be useful, |
22312b71 | 15 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
9742c552 PE |
16 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
17 | GNU General Public License for more details. | |
22312b71 | 18 | |
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19 | You should have received a copy of the GNU General Public License |
20 | along with this program; if not, write to the Free Software Foundation, | |
21 | Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ | |
22312b71 AD |
22 | |
23 | #ifdef HAVE_CONFIG_H | |
24 | # include <config.h> | |
25 | #endif | |
26 | ||
27 | #include <stdlib.h> | |
28 | ||
29 | #undef __ptr_t | |
30 | #if defined (__cplusplus) || (defined (__STDC__) && __STDC__) | |
31 | # define __ptr_t void * | |
32 | #else /* Not C++ or ANSI C. */ | |
33 | # define __ptr_t char * | |
34 | #endif /* C++ or ANSI C. */ | |
35 | ||
36 | #if defined (_LIBC) | |
37 | # include <string.h> | |
38 | # include <memcopy.h> | |
39 | #else | |
40 | # define reg_char char | |
41 | #endif | |
42 | ||
43 | #if defined (HAVE_LIMITS_H) || defined (_LIBC) | |
44 | # include <limits.h> | |
45 | #endif | |
46 | ||
47 | #define LONG_MAX_32_BITS 2147483647 | |
48 | ||
49 | #ifndef LONG_MAX | |
50 | # define LONG_MAX LONG_MAX_32_BITS | |
51 | #endif | |
52 | ||
53 | #include <sys/types.h> | |
54 | ||
55 | #undef __memrchr | |
56 | #undef memrchr | |
57 | ||
58 | #ifndef weak_alias | |
59 | # define __memrchr memrchr | |
60 | #endif | |
61 | ||
62 | /* Search no more than N bytes of S for C. */ | |
63 | __ptr_t | |
64 | __memrchr (s, c_in, n) | |
65 | const __ptr_t s; | |
66 | int c_in; | |
67 | size_t n; | |
68 | { | |
69 | const unsigned char *char_ptr; | |
70 | const unsigned long int *longword_ptr; | |
71 | unsigned long int longword, magic_bits, charmask; | |
72 | unsigned reg_char c; | |
73 | ||
74 | c = (unsigned char) c_in; | |
75 | ||
76 | /* Handle the last few characters by reading one character at a time. | |
77 | Do this until CHAR_PTR is aligned on a longword boundary. */ | |
78 | for (char_ptr = (const unsigned char *) s + n; | |
79 | n > 0 && ((unsigned long int) char_ptr | |
80 | & (sizeof (longword) - 1)) != 0; | |
81 | --n) | |
82 | if (*--char_ptr == c) | |
83 | return (__ptr_t) char_ptr; | |
84 | ||
85 | /* All these elucidatory comments refer to 4-byte longwords, | |
86 | but the theory applies equally well to 8-byte longwords. */ | |
87 | ||
88 | longword_ptr = (unsigned long int *) char_ptr; | |
89 | ||
90 | /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits | |
91 | the "holes." Note that there is a hole just to the left of | |
92 | each byte, with an extra at the end: | |
93 | ||
94 | bits: 01111110 11111110 11111110 11111111 | |
95 | bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD | |
96 | ||
97 | The 1-bits make sure that carries propagate to the next 0-bit. | |
98 | The 0-bits provide holes for carries to fall into. */ | |
99 | ||
100 | if (sizeof (longword) != 4 && sizeof (longword) != 8) | |
101 | abort (); | |
102 | ||
103 | #if LONG_MAX <= LONG_MAX_32_BITS | |
104 | magic_bits = 0x7efefeff; | |
105 | #else | |
106 | magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff; | |
107 | #endif | |
108 | ||
109 | /* Set up a longword, each of whose bytes is C. */ | |
110 | charmask = c | (c << 8); | |
111 | charmask |= charmask << 16; | |
112 | #if LONG_MAX > LONG_MAX_32_BITS | |
113 | charmask |= charmask << 32; | |
114 | #endif | |
115 | ||
116 | /* Instead of the traditional loop which tests each character, | |
117 | we will test a longword at a time. The tricky part is testing | |
118 | if *any of the four* bytes in the longword in question are zero. */ | |
119 | while (n >= sizeof (longword)) | |
120 | { | |
121 | /* We tentatively exit the loop if adding MAGIC_BITS to | |
122 | LONGWORD fails to change any of the hole bits of LONGWORD. | |
123 | ||
124 | 1) Is this safe? Will it catch all the zero bytes? | |
125 | Suppose there is a byte with all zeros. Any carry bits | |
126 | propagating from its left will fall into the hole at its | |
127 | least significant bit and stop. Since there will be no | |
128 | carry from its most significant bit, the LSB of the | |
129 | byte to the left will be unchanged, and the zero will be | |
130 | detected. | |
131 | ||
132 | 2) Is this worthwhile? Will it ignore everything except | |
133 | zero bytes? Suppose every byte of LONGWORD has a bit set | |
134 | somewhere. There will be a carry into bit 8. If bit 8 | |
135 | is set, this will carry into bit 16. If bit 8 is clear, | |
136 | one of bits 9-15 must be set, so there will be a carry | |
137 | into bit 16. Similarly, there will be a carry into bit | |
138 | 24. If one of bits 24-30 is set, there will be a carry | |
139 | into bit 31, so all of the hole bits will be changed. | |
140 | ||
141 | The one misfire occurs when bits 24-30 are clear and bit | |
142 | 31 is set; in this case, the hole at bit 31 is not | |
143 | changed. If we had access to the processor carry flag, | |
144 | we could close this loophole by putting the fourth hole | |
145 | at bit 32! | |
146 | ||
147 | So it ignores everything except 128's, when they're aligned | |
148 | properly. | |
149 | ||
150 | 3) But wait! Aren't we looking for C, not zero? | |
151 | Good point. So what we do is XOR LONGWORD with a longword, | |
152 | each of whose bytes is C. This turns each byte that is C | |
153 | into a zero. */ | |
154 | ||
155 | longword = *--longword_ptr ^ charmask; | |
156 | ||
157 | /* Add MAGIC_BITS to LONGWORD. */ | |
158 | if ((((longword + magic_bits) | |
159 | ||
160 | /* Set those bits that were unchanged by the addition. */ | |
161 | ^ ~longword) | |
162 | ||
163 | /* Look at only the hole bits. If any of the hole bits | |
164 | are unchanged, most likely one of the bytes was a | |
165 | zero. */ | |
166 | & ~magic_bits) != 0) | |
167 | { | |
168 | /* Which of the bytes was C? If none of them were, it was | |
169 | a misfire; continue the search. */ | |
170 | ||
171 | const unsigned char *cp = (const unsigned char *) longword_ptr; | |
172 | ||
173 | #if LONG_MAX > 2147483647 | |
174 | if (cp[7] == c) | |
175 | return (__ptr_t) &cp[7]; | |
176 | if (cp[6] == c) | |
177 | return (__ptr_t) &cp[6]; | |
178 | if (cp[5] == c) | |
179 | return (__ptr_t) &cp[5]; | |
180 | if (cp[4] == c) | |
181 | return (__ptr_t) &cp[4]; | |
182 | #endif | |
183 | if (cp[3] == c) | |
184 | return (__ptr_t) &cp[3]; | |
185 | if (cp[2] == c) | |
186 | return (__ptr_t) &cp[2]; | |
187 | if (cp[1] == c) | |
188 | return (__ptr_t) &cp[1]; | |
189 | if (cp[0] == c) | |
190 | return (__ptr_t) cp; | |
191 | } | |
192 | ||
193 | n -= sizeof (longword); | |
194 | } | |
195 | ||
196 | char_ptr = (const unsigned char *) longword_ptr; | |
197 | ||
198 | while (n-- > 0) | |
199 | { | |
200 | if (*--char_ptr == c) | |
201 | return (__ptr_t) char_ptr; | |
202 | } | |
203 | ||
204 | return 0; | |
205 | } | |
206 | #ifdef weak_alias | |
207 | weak_alias (__memrchr, memrchr) | |
208 | #endif |