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[glibc/history.git] / string / memrchr.c
blob21662b1bd792784728f02b6f837a10b01a224b9b
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 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).
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.
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.
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. */
25 #include <stdlib.h>
27 #ifdef HAVE_CONFIG_H
28 # include <config.h>
29 #endif
31 #undef __ptr_t
32 #if defined __cplusplus || (defined __STDC__ && __STDC__)
33 # define __ptr_t void *
34 #else /* Not C++ or ANSI C. */
35 # define __ptr_t char *
36 #endif /* C++ or ANSI C. */
38 #if defined _LIBC
39 # include <string.h>
40 # include <memcopy.h>
41 #else
42 # define reg_char char
43 #endif
45 #if defined HAVE_LIMITS_H || defined _LIBC
46 # include <limits.h>
47 #endif
49 #define LONG_MAX_32_BITS 2147483647
51 #ifndef LONG_MAX
52 # define LONG_MAX LONG_MAX_32_BITS
53 #endif
55 #include <sys/types.h>
57 #undef __memrchr
58 #undef memrchr
60 #ifndef weak_alias
61 # define __memrchr memrchr
62 #endif
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;
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;
76 c = (unsigned char) c_in;
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;
87 /* All these elucidatory comments refer to 4-byte longwords,
88 but the theory applies equally well to 8-byte longwords. */
90 longword_ptr = (const unsigned long int *) char_ptr;
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:
96 bits: 01111110 11111110 11111110 11111111
97 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
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. */
102 if (sizeof (longword) != 4 && sizeof (longword) != 8)
103 abort ();
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
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
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))
123 /* We tentatively exit the loop if adding MAGIC_BITS to
124 LONGWORD fails to change any of the hole bits of LONGWORD.
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.
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.
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!
149 So it ignores everything except 128's, when they're aligned
150 properly.
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. */
157 longword = *--longword_ptr ^ charmask;
159 /* Add MAGIC_BITS to LONGWORD. */
160 if ((((longword + magic_bits)
162 /* Set those bits that were unchanged by the addition. */
163 ^ ~longword)
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)
170 /* Which of the bytes was C? If none of them were, it was
171 a misfire; continue the search. */
173 const unsigned char *cp = (const unsigned char *) longword_ptr;
175 #if LONG_MAX > 2147483647
176 if (cp[7] == c)
177 return (__ptr_t) &cp[7];
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];
184 #endif
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;
195 n -= sizeof (longword);
198 char_ptr = (const unsigned char *) longword_ptr;
200 while (n-- > 0)
202 if (*--char_ptr == c)
203 return (__ptr_t) char_ptr;
206 return 0;
208 #ifdef weak_alias
209 weak_alias (__memrchr, memrchr)
210 #endif