Mercurial > hg > octave-kai > gnulib-hg
annotate lib/memchr.c @ 448:5bc485858505
merge with 1.11.4a
| author | Jim Meyering <jim@meyering.net> |
|---|---|
| date | Sun, 28 May 1995 01:55:00 +0000 |
| parents | 08929473d399 |
| children | de90fd3a635a |
| rev | line source |
|---|---|
|
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ae0780daedf2
* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
14
diff
changeset
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1 /* Copyright (C) 1991, 1993 Free Software Foundation, Inc. |
| 14 | 2 Based on strlen implemention by Torbjorn Granlund (tege@sics.se), |
| 3 with help from Dan Sahlin (dan@sics.se) and | |
| 4 commentary by Jim Blandy (jimb@ai.mit.edu); | |
| 5 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), | |
| 6 and implemented by Roland McGrath (roland@ai.mit.edu). | |
| 7 | |
| 394 | 8 NOTE: The canonical source of this file is maintained with the GNU C Library. |
| 9 Bugs can be reported to bug-glibc@prep.ai.mit.edu. | |
| 14 | 10 |
| 394 | 11 This program is free software; you can redistribute it and/or modify it |
| 12 under the terms of the GNU General Public License as published by the | |
| 13 Free Software Foundation; either version 2, or (at your option) any | |
| 14 later version. | |
| 15 | |
| 16 This program is distributed in the hope that it will be useful, | |
| 14 | 17 but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 394 | 18 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 19 GNU General Public License for more details. | |
| 14 | 20 |
| 394 | 21 You should have received a copy of the GNU General Public License |
| 22 along with this program; if not, write to the Free Software | |
| 23 Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. */ | |
| 14 | 24 |
| 166 | 25 #ifdef HAVE_CONFIG_H |
| 26 #include <config.h> | |
| 27 #endif | |
| 28 | |
| 394 | 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 | |
| 448 | 36 #if defined (_LIBC) |
| 37 # include <string.h> | |
| 38 #endif | |
| 39 | |
| 443 | 40 #if defined (HAVE_LIMITS_H) || defined (_LIBC) |
| 41 # include <limits.h> | |
| 394 | 42 #endif |
| 43 | |
| 44 #define LONG_MAX_32_BITS 2147483647 | |
| 45 | |
| 46 #ifndef LONG_MAX | |
| 47 #define LONG_MAX LONG_MAX_32_BITS | |
| 48 #endif | |
| 49 | |
| 50 #include <sys/types.h> | |
| 51 | |
| 52 | |
| 14 | 53 /* Search no more than N bytes of S for C. */ |
| 54 | |
| 394 | 55 __ptr_t |
|
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56 memchr (s, c, n) |
| 394 | 57 const __ptr_t s; |
|
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58 int c; |
| 394 | 59 size_t n; |
| 14 | 60 { |
|
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
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61 const unsigned char *char_ptr; |
|
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
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62 const unsigned long int *longword_ptr; |
| 14 | 63 unsigned long int longword, magic_bits, charmask; |
| 64 | |
| 65 c = (unsigned char) c; | |
| 66 | |
| 67 /* Handle the first few characters by reading one character at a time. | |
|
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68 Do this until CHAR_PTR is aligned on a longword boundary. */ |
| 448 | 69 for (char_ptr = (const unsigned char *) s; |
| 70 n > 0 && ((unsigned long int) char_ptr | |
| 71 & (sizeof (longword) - 1)) != 0; | |
| 14 | 72 --n, ++char_ptr) |
| 73 if (*char_ptr == c) | |
| 394 | 74 return (__ptr_t) char_ptr; |
| 14 | 75 |
|
165
ae0780daedf2
* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
14
diff
changeset
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76 /* All these elucidatory comments refer to 4-byte longwords, |
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ae0780daedf2
* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
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77 but the theory applies equally well to 8-byte longwords. */ |
|
ae0780daedf2
* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
14
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78 |
| 14 | 79 longword_ptr = (unsigned long int *) char_ptr; |
| 80 | |
| 81 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits | |
| 82 the "holes." Note that there is a hole just to the left of | |
| 83 each byte, with an extra at the end: | |
| 448 | 84 |
| 14 | 85 bits: 01111110 11111110 11111110 11111111 |
| 448 | 86 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD |
| 14 | 87 |
| 88 The 1-bits make sure that carries propagate to the next 0-bit. | |
| 89 The 0-bits provide holes for carries to fall into. */ | |
| 394 | 90 |
| 91 if (sizeof (longword) != 4 && sizeof (longword) != 8) | |
| 92 abort (); | |
| 93 | |
| 94 #if LONG_MAX <= LONG_MAX_32_BITS | |
| 95 magic_bits = 0x7efefeff; | |
|
165
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
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96 #else |
| 394 | 97 magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff; |
| 98 #endif | |
| 14 | 99 |
| 100 /* Set up a longword, each of whose bytes is C. */ | |
| 101 charmask = c | (c << 8); | |
| 102 charmask |= charmask << 16; | |
| 394 | 103 #if LONG_MAX > LONG_MAX_32_BITS |
| 400 | 104 charmask |= charmask << 32; |
| 394 | 105 #endif |
| 14 | 106 |
| 107 /* Instead of the traditional loop which tests each character, | |
| 108 we will test a longword at a time. The tricky part is testing | |
| 109 if *any of the four* bytes in the longword in question are zero. */ | |
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
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110 while (n >= sizeof (longword)) |
| 14 | 111 { |
| 112 /* We tentatively exit the loop if adding MAGIC_BITS to | |
| 113 LONGWORD fails to change any of the hole bits of LONGWORD. | |
| 114 | |
| 115 1) Is this safe? Will it catch all the zero bytes? | |
| 116 Suppose there is a byte with all zeros. Any carry bits | |
| 117 propagating from its left will fall into the hole at its | |
| 118 least significant bit and stop. Since there will be no | |
| 119 carry from its most significant bit, the LSB of the | |
| 120 byte to the left will be unchanged, and the zero will be | |
| 121 detected. | |
| 122 | |
| 123 2) Is this worthwhile? Will it ignore everything except | |
| 124 zero bytes? Suppose every byte of LONGWORD has a bit set | |
| 125 somewhere. There will be a carry into bit 8. If bit 8 | |
| 126 is set, this will carry into bit 16. If bit 8 is clear, | |
| 127 one of bits 9-15 must be set, so there will be a carry | |
| 128 into bit 16. Similarly, there will be a carry into bit | |
| 129 24. If one of bits 24-30 is set, there will be a carry | |
| 130 into bit 31, so all of the hole bits will be changed. | |
| 131 | |
| 132 The one misfire occurs when bits 24-30 are clear and bit | |
| 133 31 is set; in this case, the hole at bit 31 is not | |
| 134 changed. If we had access to the processor carry flag, | |
| 135 we could close this loophole by putting the fourth hole | |
| 136 at bit 32! | |
| 137 | |
| 138 So it ignores everything except 128's, when they're aligned | |
| 139 properly. | |
| 140 | |
| 141 3) But wait! Aren't we looking for C, not zero? | |
| 142 Good point. So what we do is XOR LONGWORD with a longword, | |
| 143 each of whose bytes is C. This turns each byte that is C | |
| 144 into a zero. */ | |
| 145 | |
| 146 longword = *longword_ptr++ ^ charmask; | |
| 147 | |
| 148 /* Add MAGIC_BITS to LONGWORD. */ | |
| 149 if ((((longword + magic_bits) | |
| 448 | 150 |
| 394 | 151 /* Set those bits that were unchanged by the addition. */ |
| 14 | 152 ^ ~longword) |
| 448 | 153 |
| 394 | 154 /* Look at only the hole bits. If any of the hole bits |
| 14 | 155 are unchanged, most likely one of the bytes was a |
| 156 zero. */ | |
| 157 & ~magic_bits) != 0) | |
| 158 { | |
| 159 /* Which of the bytes was C? If none of them were, it was | |
| 160 a misfire; continue the search. */ | |
| 161 | |
|
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
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162 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); |
| 14 | 163 |
| 164 if (cp[0] == c) | |
| 394 | 165 return (__ptr_t) cp; |
| 14 | 166 if (cp[1] == c) |
| 394 | 167 return (__ptr_t) &cp[1]; |
| 14 | 168 if (cp[2] == c) |
| 394 | 169 return (__ptr_t) &cp[2]; |
| 14 | 170 if (cp[3] == c) |
| 394 | 171 return (__ptr_t) &cp[3]; |
| 172 #if LONG_MAX > 2147483647 | |
|
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
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173 if (cp[4] == c) |
| 394 | 174 return (__ptr_t) &cp[4]; |
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
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parents:
14
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175 if (cp[5] == c) |
| 394 | 176 return (__ptr_t) &cp[5]; |
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
14
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|
177 if (cp[6] == c) |
| 394 | 178 return (__ptr_t) &cp[6]; |
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165
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
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parents:
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179 if (cp[7] == c) |
| 394 | 180 return (__ptr_t) &cp[7]; |
| 181 #endif | |
| 14 | 182 } |
| 183 | |
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
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184 n -= sizeof (longword); |
| 14 | 185 } |
| 186 | |
|
165
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* memchr.c (memchr): Do the 32-bit assignment only if !LONG_64_BITS.
Jim Meyering <jim@meyering.net>
parents:
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187 char_ptr = (const unsigned char *) longword_ptr; |
| 14 | 188 |
| 189 while (n-- > 0) | |
| 190 { | |
| 191 if (*char_ptr == c) | |
| 394 | 192 return (__ptr_t) char_ptr; |
| 14 | 193 else |
| 194 ++char_ptr; | |
| 195 } | |
| 196 | |
| 197 return 0; | |
| 198 } |