1/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006 Free 2 Software Foundation, Inc. 3 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 10NOTE: The canonical source of this file is maintained with the GNU C Library. 11Bugs can be reported to bug-glibc@prep.ai.mit.edu. 12 13This program is free software: you can redistribute it and/or modify it 14under the terms of the GNU General Public License as published by the 15Free Software Foundation; either version 3 of the License, or any 16later version. 17 18This program is distributed in the hope that it will be useful, 19but WITHOUT ANY WARRANTY; without even the implied warranty of 20MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 21GNU General Public License for more details. 22 23You should have received a copy of the GNU General Public License 24along with this program. If not, see <http://www.gnu.org/licenses/>. */ 25 26#ifndef _LIBC 27# include <config.h> 28#endif 29 30#include <string.h> 31 32#include <stddef.h> 33 34#if defined _LIBC 35# include <memcopy.h> 36#else 37# define reg_char char 38#endif 39 40#include <limits.h> 41 42#if HAVE_BP_SYM_H || defined _LIBC 43# include <bp-sym.h> 44#else 45# define BP_SYM(sym) sym 46#endif 47 48#undef memchr 49#undef __memchr 50 51/* Search no more than N bytes of S for C. */ 52void * 53__memchr (void const *s, int c_in, size_t n) 54{ 55 const unsigned char *char_ptr; 56 const unsigned long int *longword_ptr; 57 unsigned long int longword, magic_bits, charmask; 58 unsigned reg_char c; 59 int i; 60 61 c = (unsigned char) c_in; 62 63 /* Handle the first few characters by reading one character at a time. 64 Do this until CHAR_PTR is aligned on a longword boundary. */ 65 for (char_ptr = (const unsigned char *) s; 66 n > 0 && (size_t) char_ptr % sizeof longword != 0; 67 --n, ++char_ptr) 68 if (*char_ptr == c) 69 return (void *) char_ptr; 70 71 /* All these elucidatory comments refer to 4-byte longwords, 72 but the theory applies equally well to any size longwords. */ 73 74 longword_ptr = (const unsigned long int *) char_ptr; 75 76 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits 77 the "holes." Note that there is a hole just to the left of 78 each byte, with an extra at the end: 79 80 bits: 01111110 11111110 11111110 11111111 81 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD 82 83 The 1-bits make sure that carries propagate to the next 0-bit. 84 The 0-bits provide holes for carries to fall into. */ 85 86 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. 87 Set CHARMASK to be a longword, each of whose bytes is C. */ 88 89 magic_bits = 0xfefefefe; 90 charmask = c | (c << 8); 91 charmask |= charmask << 16; 92#if 0xffffffffU < ULONG_MAX 93 magic_bits |= magic_bits << 32; 94 charmask |= charmask << 32; 95 if (8 < sizeof longword) 96 for (i = 64; i < sizeof longword * 8; i *= 2) 97 { 98 magic_bits |= magic_bits << i; 99 charmask |= charmask << i; 100 } 101#endif 102 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1); 103 104 /* Instead of the traditional loop which tests each character, 105 we will test a longword at a time. The tricky part is testing 106 if *any of the four* bytes in the longword in question are zero. */ 107 while (n >= sizeof longword) 108 { 109 /* We tentatively exit the loop if adding MAGIC_BITS to 110 LONGWORD fails to change any of the hole bits of LONGWORD. 111 112 1) Is this safe? Will it catch all the zero bytes? 113 Suppose there is a byte with all zeros. Any carry bits 114 propagating from its left will fall into the hole at its 115 least significant bit and stop. Since there will be no 116 carry from its most significant bit, the LSB of the 117 byte to the left will be unchanged, and the zero will be 118 detected. 119 120 2) Is this worthwhile? Will it ignore everything except 121 zero bytes? Suppose every byte of LONGWORD has a bit set 122 somewhere. There will be a carry into bit 8. If bit 8 123 is set, this will carry into bit 16. If bit 8 is clear, 124 one of bits 9-15 must be set, so there will be a carry 125 into bit 16. Similarly, there will be a carry into bit 126 24. If one of bits 24-30 is set, there will be a carry 127 into bit 31, so all of the hole bits will be changed. 128 129 The one misfire occurs when bits 24-30 are clear and bit 130 31 is set; in this case, the hole at bit 31 is not 131 changed. If we had access to the processor carry flag, 132 we could close this loophole by putting the fourth hole 133 at bit 32! 134 135 So it ignores everything except 128's, when they're aligned 136 properly. 137 138 3) But wait! Aren't we looking for C, not zero? 139 Good point. So what we do is XOR LONGWORD with a longword, 140 each of whose bytes is C. This turns each byte that is C 141 into a zero. */ 142 143 longword = *longword_ptr++ ^ charmask; 144 145 /* Add MAGIC_BITS to LONGWORD. */ 146 if ((((longword + magic_bits) 147 148 /* Set those bits that were unchanged by the addition. */ 149 ^ ~longword) 150 151 /* Look at only the hole bits. If any of the hole bits 152 are unchanged, most likely one of the bytes was a 153 zero. */ 154 & ~magic_bits) != 0) 155 { 156 /* Which of the bytes was C? If none of them were, it was 157 a misfire; continue the search. */ 158 159 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); 160 161 if (cp[0] == c) 162 return (void *) cp; 163 if (cp[1] == c) 164 return (void *) &cp[1]; 165 if (cp[2] == c) 166 return (void *) &cp[2]; 167 if (cp[3] == c) 168 return (void *) &cp[3]; 169 if (4 < sizeof longword && cp[4] == c) 170 return (void *) &cp[4]; 171 if (5 < sizeof longword && cp[5] == c) 172 return (void *) &cp[5]; 173 if (6 < sizeof longword && cp[6] == c) 174 return (void *) &cp[6]; 175 if (7 < sizeof longword && cp[7] == c) 176 return (void *) &cp[7]; 177 if (8 < sizeof longword) 178 for (i = 8; i < sizeof longword; i++) 179 if (cp[i] == c) 180 return (void *) &cp[i]; 181 } 182 183 n -= sizeof longword; 184 } 185 186 char_ptr = (const unsigned char *) longword_ptr; 187 188 while (n-- > 0) 189 { 190 if (*char_ptr == c) 191 return (void *) char_ptr; 192 else 193 ++char_ptr; 194 } 195 196 return 0; 197} 198#ifdef weak_alias 199weak_alias (__memchr, BP_SYM (memchr)) 200#endif 201