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 2, or (at your option) 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, write to the Free Software Foundation, 25Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ 26 27#ifndef _LIBC 28# include <config.h> 29#endif 30 31#include <string.h> 32 33#include <stddef.h> 34 35#if defined _LIBC 36# include <memcopy.h> 37#else 38# define reg_char char 39#endif 40 41#include <limits.h> 42 43#if HAVE_BP_SYM_H || defined _LIBC 44# include <bp-sym.h> 45#else 46# define BP_SYM(sym) sym 47#endif 48 49#undef memchr 50#undef __memchr 51 52/* Search no more than N bytes of S for C. */ 53void * 54__memchr (void const *s, int c_in, size_t n) 55{ 56 const unsigned char *char_ptr; 57 const unsigned long int *longword_ptr; 58 unsigned long int longword, magic_bits, charmask; 59 unsigned reg_char c; 60 int i; 61 62 c = (unsigned char) c_in; 63 64 /* Handle the first few characters by reading one character at a time. 65 Do this until CHAR_PTR is aligned on a longword boundary. */ 66 for (char_ptr = (const unsigned char *) s; 67 n > 0 && (size_t) char_ptr % sizeof longword != 0; 68 --n, ++char_ptr) 69 if (*char_ptr == c) 70 return (void *) char_ptr; 71 72 /* All these elucidatory comments refer to 4-byte longwords, 73 but the theory applies equally well to any size longwords. */ 74 75 longword_ptr = (const unsigned long int *) char_ptr; 76 77 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits 78 the "holes." Note that there is a hole just to the left of 79 each byte, with an extra at the end: 80 81 bits: 01111110 11111110 11111110 11111111 82 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD 83 84 The 1-bits make sure that carries propagate to the next 0-bit. 85 The 0-bits provide holes for carries to fall into. */ 86 87 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. 88 Set CHARMASK to be a longword, each of whose bytes is C. */ 89 90 magic_bits = 0xfefefefe; 91 charmask = c | (c << 8); 92 charmask |= charmask << 16; 93#if 0xffffffffU < ULONG_MAX 94 magic_bits |= magic_bits << 32; 95 charmask |= charmask << 32; 96 if (8 < sizeof longword) 97 for (i = 64; i < sizeof longword * 8; i *= 2) 98 { 99 magic_bits |= magic_bits << i; 100 charmask |= charmask << i; 101 } 102#endif 103 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1); 104 105 /* Instead of the traditional loop which tests each character, 106 we will test a longword at a time. The tricky part is testing 107 if *any of the four* bytes in the longword in question are zero. */ 108 while (n >= sizeof longword) 109 { 110 /* We tentatively exit the loop if adding MAGIC_BITS to 111 LONGWORD fails to change any of the hole bits of LONGWORD. 112 113 1) Is this safe? Will it catch all the zero bytes? 114 Suppose there is a byte with all zeros. Any carry bits 115 propagating from its left will fall into the hole at its 116 least significant bit and stop. Since there will be no 117 carry from its most significant bit, the LSB of the 118 byte to the left will be unchanged, and the zero will be 119 detected. 120 121 2) Is this worthwhile? Will it ignore everything except 122 zero bytes? Suppose every byte of LONGWORD has a bit set 123 somewhere. There will be a carry into bit 8. If bit 8 124 is set, this will carry into bit 16. If bit 8 is clear, 125 one of bits 9-15 must be set, so there will be a carry 126 into bit 16. Similarly, there will be a carry into bit 127 24. If one of bits 24-30 is set, there will be a carry 128 into bit 31, so all of the hole bits will be changed. 129 130 The one misfire occurs when bits 24-30 are clear and bit 131 31 is set; in this case, the hole at bit 31 is not 132 changed. If we had access to the processor carry flag, 133 we could close this loophole by putting the fourth hole 134 at bit 32! 135 136 So it ignores everything except 128's, when they're aligned 137 properly. 138 139 3) But wait! Aren't we looking for C, not zero? 140 Good point. So what we do is XOR LONGWORD with a longword, 141 each of whose bytes is C. This turns each byte that is C 142 into a zero. */ 143 144 longword = *longword_ptr++ ^ charmask; 145 146 /* Add MAGIC_BITS to LONGWORD. */ 147 if ((((longword + magic_bits) 148 149 /* Set those bits that were unchanged by the addition. */ 150 ^ ~longword) 151 152 /* Look at only the hole bits. If any of the hole bits 153 are unchanged, most likely one of the bytes was a 154 zero. */ 155 & ~magic_bits) != 0) 156 { 157 /* Which of the bytes was C? If none of them were, it was 158 a misfire; continue the search. */ 159 160 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); 161 162 if (cp[0] == c) 163 return (void *) cp; 164 if (cp[1] == c) 165 return (void *) &cp[1]; 166 if (cp[2] == c) 167 return (void *) &cp[2]; 168 if (cp[3] == c) 169 return (void *) &cp[3]; 170 if (4 < sizeof longword && cp[4] == c) 171 return (void *) &cp[4]; 172 if (5 < sizeof longword && cp[5] == c) 173 return (void *) &cp[5]; 174 if (6 < sizeof longword && cp[6] == c) 175 return (void *) &cp[6]; 176 if (7 < sizeof longword && cp[7] == c) 177 return (void *) &cp[7]; 178 if (8 < sizeof longword) 179 for (i = 8; i < sizeof longword; i++) 180 if (cp[i] == c) 181 return (void *) &cp[i]; 182 } 183 184 n -= sizeof longword; 185 } 186 187 char_ptr = (const unsigned char *) longword_ptr; 188 189 while (n-- > 0) 190 { 191 if (*char_ptr == c) 192 return (void *) char_ptr; 193 else 194 ++char_ptr; 195 } 196 197 return 0; 198} 199#ifdef weak_alias 200weak_alias (__memchr, BP_SYM (memchr)) 201#endif 202