1 /* $NetBSD: memchr.c,v 1.1.1.1 2016/01/13 03:15:30 christos Exp $ */
2
3 /* Copyright (C) 1991,93,96,97,99,2000 Free Software Foundation, Inc.
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 NOTE: The canonical source of this file is maintained with the GNU C Library.
11 Bugs can be reported to bug-glibc@prep.ai.mit.edu.
12
13 This program is free software; you can redistribute it and/or modify it
14 under the terms of the GNU General Public License as published by the
15 Free Software Foundation; either version 2, or (at your option) any
16 later version.
17
18 This program is distributed in the hope that it will be useful,
19 but WITHOUT ANY WARRANTY; without even the implied warranty of
20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
21 GNU General Public License for more details.
22
23 You should have received a copy of the GNU General Public License
24 along with this program; if not, write to the Free Software
25 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
26 USA. */
27
28 #ifdef HAVE_CONFIG_H
29 # include <config.h>
30 #endif
31
32 #undef __ptr_t
33 #if defined (__cplusplus) || (defined (__STDC__) && __STDC__)
34 # define __ptr_t void *
35 #else /* Not C++ or ANSI C. */
36 # define __ptr_t char *
37 #endif /* C++ or ANSI C. */
38
39 #if defined _LIBC
40 # include <string.h>
41 # include <memcopy.h>
42 #else
43 # define reg_char char
44 #endif
45
46 #if HAVE_STDLIB_H || defined _LIBC
47 # include <stdlib.h>
48 #endif
49
50 #if HAVE_LIMITS_H || defined _LIBC
51 # include <limits.h>
52 #endif
53
54 #define LONG_MAX_32_BITS 2147483647
55
56 #ifndef LONG_MAX
57 # define LONG_MAX LONG_MAX_32_BITS
58 #endif
59
60 #include <sys/types.h>
61 #if HAVE_BP_SYM_H || defined _LIBC
62 # include <bp-sym.h>
63 #else
64 # define BP_SYM(sym) sym
65 #endif
66
67 #undef memchr
68 #undef __memchr
69
70 /* Search no more than N bytes of S for C. */
71 __ptr_t
__memchr(s,c_in,n)72 __memchr (s, c_in, n)
73 const __ptr_t s;
74 int c_in;
75 size_t n;
76 {
77 const unsigned char *char_ptr;
78 const unsigned long int *longword_ptr;
79 unsigned long int longword, magic_bits, charmask;
80 unsigned reg_char c;
81
82 c = (unsigned char) c_in;
83
84 /* Handle the first few characters by reading one character at a time.
85 Do this until CHAR_PTR is aligned on a longword boundary. */
86 for (char_ptr = (const unsigned char *) s;
87 n > 0 && ((unsigned long int) char_ptr
88 & (sizeof (longword) - 1)) != 0;
89 --n, ++char_ptr)
90 if (*char_ptr == c)
91 return (__ptr_t) char_ptr;
92
93 /* All these elucidatory comments refer to 4-byte longwords,
94 but the theory applies equally well to 8-byte longwords. */
95
96 longword_ptr = (unsigned long int *) char_ptr;
97
98 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
99 the "holes." Note that there is a hole just to the left of
100 each byte, with an extra at the end:
101
102 bits: 01111110 11111110 11111110 11111111
103 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
104
105 The 1-bits make sure that carries propagate to the next 0-bit.
106 The 0-bits provide holes for carries to fall into. */
107
108 if (sizeof (longword) != 4 && sizeof (longword) != 8)
109 abort ();
110
111 #if LONG_MAX <= LONG_MAX_32_BITS
112 magic_bits = 0x7efefeff;
113 #else
114 magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
115 #endif
116
117 /* Set up a longword, each of whose bytes is C. */
118 charmask = c | (c << 8);
119 charmask |= charmask << 16;
120 #if LONG_MAX > LONG_MAX_32_BITS
121 charmask |= charmask << 32;
122 #endif
123
124 /* Instead of the traditional loop which tests each character,
125 we will test a longword at a time. The tricky part is testing
126 if *any of the four* bytes in the longword in question are zero. */
127 while (n >= sizeof (longword))
128 {
129 /* We tentatively exit the loop if adding MAGIC_BITS to
130 LONGWORD fails to change any of the hole bits of LONGWORD.
131
132 1) Is this safe? Will it catch all the zero bytes?
133 Suppose there is a byte with all zeros. Any carry bits
134 propagating from its left will fall into the hole at its
135 least significant bit and stop. Since there will be no
136 carry from its most significant bit, the LSB of the
137 byte to the left will be unchanged, and the zero will be
138 detected.
139
140 2) Is this worthwhile? Will it ignore everything except
141 zero bytes? Suppose every byte of LONGWORD has a bit set
142 somewhere. There will be a carry into bit 8. If bit 8
143 is set, this will carry into bit 16. If bit 8 is clear,
144 one of bits 9-15 must be set, so there will be a carry
145 into bit 16. Similarly, there will be a carry into bit
146 24. If one of bits 24-30 is set, there will be a carry
147 into bit 31, so all of the hole bits will be changed.
148
149 The one misfire occurs when bits 24-30 are clear and bit
150 31 is set; in this case, the hole at bit 31 is not
151 changed. If we had access to the processor carry flag,
152 we could close this loophole by putting the fourth hole
153 at bit 32!
154
155 So it ignores everything except 128's, when they're aligned
156 properly.
157
158 3) But wait! Aren't we looking for C, not zero?
159 Good point. So what we do is XOR LONGWORD with a longword,
160 each of whose bytes is C. This turns each byte that is C
161 into a zero. */
162
163 longword = *longword_ptr++ ^ charmask;
164
165 /* Add MAGIC_BITS to LONGWORD. */
166 if ((((longword + magic_bits)
167
168 /* Set those bits that were unchanged by the addition. */
169 ^ ~longword)
170
171 /* Look at only the hole bits. If any of the hole bits
172 are unchanged, most likely one of the bytes was a
173 zero. */
174 & ~magic_bits) != 0)
175 {
176 /* Which of the bytes was C? If none of them were, it was
177 a misfire; continue the search. */
178
179 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
180
181 if (cp[0] == c)
182 return (__ptr_t) cp;
183 if (cp[1] == c)
184 return (__ptr_t) &cp[1];
185 if (cp[2] == c)
186 return (__ptr_t) &cp[2];
187 if (cp[3] == c)
188 return (__ptr_t) &cp[3];
189 #if LONG_MAX > 2147483647
190 if (cp[4] == c)
191 return (__ptr_t) &cp[4];
192 if (cp[5] == c)
193 return (__ptr_t) &cp[5];
194 if (cp[6] == c)
195 return (__ptr_t) &cp[6];
196 if (cp[7] == c)
197 return (__ptr_t) &cp[7];
198 #endif
199 }
200
201 n -= sizeof (longword);
202 }
203
204 char_ptr = (const unsigned char *) longword_ptr;
205
206 while (n-- > 0)
207 {
208 if (*char_ptr == c)
209 return (__ptr_t) char_ptr;
210 else
211 ++char_ptr;
212 }
213
214 return 0;
215 }
216 #ifdef weak_alias
217 weak_alias (__memchr, BP_SYM (memchr))
218 #endif
219