1 /* memrchr -- find the last occurrence of a byte in a memory block 2 3 Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2005 4 Free Software Foundation, Inc. 5 6 Based on strlen implementation by Torbjorn Granlund (tege@sics.se), 7 with help from Dan Sahlin (dan@sics.se) and 8 commentary by Jim Blandy (jimb@ai.mit.edu); 9 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), 10 and implemented by Roland McGrath (roland@ai.mit.edu). 11 12 This program is free software; you can redistribute it and/or modify 13 it under the terms of the GNU General Public License as published by 14 the Free Software Foundation; either version 2, or (at your option) 15 any later version. 16 17 This program is distributed in the hope that it will be useful, 18 but WITHOUT ANY WARRANTY; without even the implied warranty of 19 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 20 GNU General Public License for more details. 21 22 You should have received a copy of the GNU General Public License along 23 with this program; if not, write to the Free Software Foundation, 24 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ 25 26 #ifdef HAVE_CONFIG_H 27 # include <config.h> 28 #endif 29 30 #if defined _LIBC 31 # include <string.h> 32 # include <memcopy.h> 33 #else 34 # include "memrchr.h" 35 # define reg_char char 36 #endif 37 38 #include <limits.h> 39 40 #undef __memrchr 41 #undef memrchr 42 43 #ifndef weak_alias 44 # define __memrchr memrchr 45 #endif 46 47 /* Search no more than N bytes of S for C. */ 48 void * 49 __memrchr (void const *s, int c_in, size_t n) 50 { 51 const unsigned char *char_ptr; 52 const unsigned long int *longword_ptr; 53 unsigned long int longword, magic_bits, charmask; 54 unsigned reg_char c; 55 int i; 56 57 c = (unsigned char) c_in; 58 59 /* Handle the last few characters by reading one character at a time. 60 Do this until CHAR_PTR is aligned on a longword boundary. */ 61 for (char_ptr = (const unsigned char *) s + n; 62 n > 0 && (size_t) char_ptr % sizeof longword != 0; 63 --n) 64 if (*--char_ptr == c) 65 return (void *) char_ptr; 66 67 /* All these elucidatory comments refer to 4-byte longwords, 68 but the theory applies equally well to any size longwords. */ 69 70 longword_ptr = (const unsigned long int *) char_ptr; 71 72 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits 73 the "holes." Note that there is a hole just to the left of 74 each byte, with an extra at the end: 75 76 bits: 01111110 11111110 11111110 11111111 77 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD 78 79 The 1-bits make sure that carries propagate to the next 0-bit. 80 The 0-bits provide holes for carries to fall into. */ 81 82 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. 83 Set CHARMASK to be a longword, each of whose bytes is C. */ 84 85 magic_bits = 0xfefefefe; 86 charmask = c | (c << 8); 87 charmask |= charmask << 16; 88 #if 0xffffffffU < ULONG_MAX 89 magic_bits |= magic_bits << 32; 90 charmask |= charmask << 32; 91 if (8 < sizeof longword) 92 for (i = 64; i < sizeof longword * 8; i *= 2) 93 { 94 magic_bits |= magic_bits << i; 95 charmask |= charmask << i; 96 } 97 #endif 98 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1); 99 100 /* Instead of the traditional loop which tests each character, 101 we will test a longword at a time. The tricky part is testing 102 if *any of the four* bytes in the longword in question are zero. */ 103 while (n >= sizeof longword) 104 { 105 /* We tentatively exit the loop if adding MAGIC_BITS to 106 LONGWORD fails to change any of the hole bits of LONGWORD. 107 108 1) Is this safe? Will it catch all the zero bytes? 109 Suppose there is a byte with all zeros. Any carry bits 110 propagating from its left will fall into the hole at its 111 least significant bit and stop. Since there will be no 112 carry from its most significant bit, the LSB of the 113 byte to the left will be unchanged, and the zero will be 114 detected. 115 116 2) Is this worthwhile? Will it ignore everything except 117 zero bytes? Suppose every byte of LONGWORD has a bit set 118 somewhere. There will be a carry into bit 8. If bit 8 119 is set, this will carry into bit 16. If bit 8 is clear, 120 one of bits 9-15 must be set, so there will be a carry 121 into bit 16. Similarly, there will be a carry into bit 122 24. If one of bits 24-30 is set, there will be a carry 123 into bit 31, so all of the hole bits will be changed. 124 125 The one misfire occurs when bits 24-30 are clear and bit 126 31 is set; in this case, the hole at bit 31 is not 127 changed. If we had access to the processor carry flag, 128 we could close this loophole by putting the fourth hole 129 at bit 32! 130 131 So it ignores everything except 128's, when they're aligned 132 properly. 133 134 3) But wait! Aren't we looking for C, not zero? 135 Good point. So what we do is XOR LONGWORD with a longword, 136 each of whose bytes is C. This turns each byte that is C 137 into a zero. */ 138 139 longword = *--longword_ptr ^ charmask; 140 141 /* Add MAGIC_BITS to LONGWORD. */ 142 if ((((longword + magic_bits) 143 144 /* Set those bits that were unchanged by the addition. */ 145 ^ ~longword) 146 147 /* Look at only the hole bits. If any of the hole bits 148 are unchanged, most likely one of the bytes was a 149 zero. */ 150 & ~magic_bits) != 0) 151 { 152 /* Which of the bytes was C? If none of them were, it was 153 a misfire; continue the search. */ 154 155 const unsigned char *cp = (const unsigned char *) longword_ptr; 156 157 if (8 < sizeof longword) 158 for (i = sizeof longword - 1; 8 <= i; i--) 159 if (cp[i] == c) 160 return (void *) &cp[i]; 161 if (7 < sizeof longword && cp[7] == c) 162 return (void *) &cp[7]; 163 if (6 < sizeof longword && cp[6] == c) 164 return (void *) &cp[6]; 165 if (5 < sizeof longword && cp[5] == c) 166 return (void *) &cp[5]; 167 if (4 < sizeof longword && cp[4] == c) 168 return (void *) &cp[4]; 169 if (cp[3] == c) 170 return (void *) &cp[3]; 171 if (cp[2] == c) 172 return (void *) &cp[2]; 173 if (cp[1] == c) 174 return (void *) &cp[1]; 175 if (cp[0] == c) 176 return (void *) cp; 177 } 178 179 n -= sizeof longword; 180 } 181 182 char_ptr = (const unsigned char *) longword_ptr; 183 184 while (n-- > 0) 185 { 186 if (*--char_ptr == c) 187 return (void *) char_ptr; 188 } 189 190 return 0; 191 } 192 #ifdef weak_alias 193 weak_alias (__memrchr, memrchr) 194 #endif 195