1/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004 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 25Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, 26USA. */ 27 28#ifdef HAVE_CONFIG_H 29# include <config.h> 30#endif 31 32#include <string.h> 33 34#include <stddef.h> 35 36#if defined _LIBC 37# include <memcopy.h> 38#else 39# define reg_char char 40#endif 41 42#include <limits.h> 43 44#if HAVE_BP_SYM_H || defined _LIBC 45# include <bp-sym.h> 46#else 47# define BP_SYM(sym) sym 48#endif 49 50#undef memchr 51#undef __memchr 52 53/* Search no more than N bytes of S for C. */ 54void * 55__memchr (void const *s, int c_in, size_t n) 56{ 57 const unsigned char *char_ptr; 58 const unsigned long int *longword_ptr; 59 unsigned long int longword, magic_bits, charmask; 60 unsigned reg_char c; 61 int i; 62 63 c = (unsigned char) c_in; 64 65 /* Handle the first few characters by reading one character at a time. 66 Do this until CHAR_PTR is aligned on a longword boundary. */ 67 for (char_ptr = (const unsigned char *) s; 68 n > 0 && (size_t) char_ptr % sizeof longword != 0; 69 --n, ++char_ptr) 70 if (*char_ptr == c) 71 return (void *) char_ptr; 72 73 /* All these elucidatory comments refer to 4-byte longwords, 74 but the theory applies equally well to any size longwords. */ 75 76 longword_ptr = (const unsigned long int *) char_ptr; 77 78 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits 79 the "holes." Note that there is a hole just to the left of 80 each byte, with an extra at the end: 81 82 bits: 01111110 11111110 11111110 11111111 83 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD 84 85 The 1-bits make sure that carries propagate to the next 0-bit. 86 The 0-bits provide holes for carries to fall into. */ 87 88 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. 89 Set CHARMASK to be a longword, each of whose bytes is C. */ 90 91 magic_bits = 0xfefefefe; 92 charmask = c | (c << 8); 93 charmask |= charmask << 16; 94#if 0xffffffffU < ULONG_MAX 95 magic_bits |= magic_bits << 32; 96 charmask |= charmask << 32; 97 if (8 < sizeof longword) 98 for (i = 64; i < sizeof longword * 8; i *= 2) 99 { 100 magic_bits |= magic_bits << i; 101 charmask |= charmask << i; 102 } 103#endif 104 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1); 105 106 /* Instead of the traditional loop which tests each character, 107 we will test a longword at a time. The tricky part is testing 108 if *any of the four* bytes in the longword in question are zero. */ 109 while (n >= sizeof longword) 110 { 111 /* We tentatively exit the loop if adding MAGIC_BITS to 112 LONGWORD fails to change any of the hole bits of LONGWORD. 113 114 1) Is this safe? Will it catch all the zero bytes? 115 Suppose there is a byte with all zeros. Any carry bits 116 propagating from its left will fall into the hole at its 117 least significant bit and stop. Since there will be no 118 carry from its most significant bit, the LSB of the 119 byte to the left will be unchanged, and the zero will be 120 detected. 121 122 2) Is this worthwhile? Will it ignore everything except 123 zero bytes? Suppose every byte of LONGWORD has a bit set 124 somewhere. There will be a carry into bit 8. If bit 8 125 is set, this will carry into bit 16. If bit 8 is clear, 126 one of bits 9-15 must be set, so there will be a carry 127 into bit 16. Similarly, there will be a carry into bit 128 24. If one of bits 24-30 is set, there will be a carry 129 into bit 31, so all of the hole bits will be changed. 130 131 The one misfire occurs when bits 24-30 are clear and bit 132 31 is set; in this case, the hole at bit 31 is not 133 changed. If we had access to the processor carry flag, 134 we could close this loophole by putting the fourth hole 135 at bit 32! 136 137 So it ignores everything except 128's, when they're aligned 138 properly. 139 140 3) But wait! Aren't we looking for C, not zero? 141 Good point. So what we do is XOR LONGWORD with a longword, 142 each of whose bytes is C. This turns each byte that is C 143 into a zero. */ 144 145 longword = *longword_ptr++ ^ charmask; 146 147 /* Add MAGIC_BITS to LONGWORD. */ 148 if ((((longword + magic_bits) 149 150 /* Set those bits that were unchanged by the addition. */ 151 ^ ~longword) 152 153 /* Look at only the hole bits. If any of the hole bits 154 are unchanged, most likely one of the bytes was a 155 zero. */ 156 & ~magic_bits) != 0) 157 { 158 /* Which of the bytes was C? If none of them were, it was 159 a misfire; continue the search. */ 160 161 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); 162 163 if (cp[0] == c) 164 return (void *) cp; 165 if (cp[1] == c) 166 return (void *) &cp[1]; 167 if (cp[2] == c) 168 return (void *) &cp[2]; 169 if (cp[3] == c) 170 return (void *) &cp[3]; 171 if (4 < sizeof longword && cp[4] == c) 172 return (void *) &cp[4]; 173 if (5 < sizeof longword && cp[5] == c) 174 return (void *) &cp[5]; 175 if (6 < sizeof longword && cp[6] == c) 176 return (void *) &cp[6]; 177 if (7 < sizeof longword && cp[7] == c) 178 return (void *) &cp[7]; 179 if (8 < sizeof longword) 180 for (i = 8; i < sizeof longword; i++) 181 if (cp[i] == c) 182 return (void *) &cp[i]; 183 } 184 185 n -= sizeof longword; 186 } 187 188 char_ptr = (const unsigned char *) longword_ptr; 189 190 while (n-- > 0) 191 { 192 if (*char_ptr == c) 193 return (void *) char_ptr; 194 else 195 ++char_ptr; 196 } 197 198 return 0; 199} 200#ifdef weak_alias 201weak_alias (__memchr, BP_SYM (memchr)) 202#endif 203