string/memchr.c

   1 /* Copyright (C) 1991,93,96,97,99,2000,2003,2012 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   4    with help from Dan Sahlin (dan@sics.se) and
   5    commentary by Jim Blandy (jimb@ai.mit.edu);
   6    adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   7    and implemented by Roland McGrath (roland@ai.mit.edu).
   8
   9    The GNU C Library is free software; you can redistribute it and/or
  10    modify it under the terms of the GNU Lesser General Public
  11    License as published by the Free Software Foundation; either
  12    version 2.1 of the License, or (at your option) any later version.
  13
  14    The GNU C Library is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17    Lesser General Public License for more details.
  18
  19    You should have received a copy of the GNU Lesser General Public
  20    License along with the GNU C Library; if not, see
  21    <http://www.gnu.org/licenses/>.  */
  22
  23 #ifdef HAVE_CONFIG_H
  24 #include <config.h>
  25 #endif
  26
  27 #undef __ptr_t
  28 #define __ptr_t void *
  29
  30 #if defined _LIBC
  31 # include <string.h>
  32 # include <memcopy.h>
  33 #endif
  34
  35 #if HAVE_STDLIB_H || defined _LIBC
  36 # include <stdlib.h>
  37 #endif
  38
  39 #if HAVE_LIMITS_H || defined _LIBC
  40 # include <limits.h>
  41 #endif
  42
  43 #define LONG_MAX_32_BITS 2147483647
  44
  45 #ifndef LONG_MAX
  46 #define LONG_MAX LONG_MAX_32_BITS
  47 #endif
  48
  49 #include <sys/types.h>
  50 #if HAVE_BP_SYM_H || defined _LIBC
  51 #include <bp-sym.h>
  52 #else
  53 # define BP_SYM(sym) sym
  54 #endif
  55
  56 #undef memchr
  57 #undef __memchr
  58
  59 /* Search no more than N bytes of S for C.  */
  60 __ptr_t
  61 __memchr (s, c_in, n)
  62      const __ptr_t s;
  63      int c_in;
  64      size_t n;
  65 {
  66   const unsigned char *char_ptr;
  67   const unsigned long int *longword_ptr;
  68   unsigned long int longword, magic_bits, charmask;
  69   unsigned char c;
  70
  71   c = (unsigned char) c_in;
  72
  73   /* Handle the first few characters by reading one character at a time.
  74      Do this until CHAR_PTR is aligned on a longword boundary.  */
  75   for (char_ptr = (const unsigned char *) s;
  76        n > 0 && ((unsigned long int) char_ptr
  77                  & (sizeof (longword) - 1)) != 0;
  78        --n, ++char_ptr)
  79     if (*char_ptr == c)
  80       return (__ptr_t) char_ptr;
  81
  82   /* All these elucidatory comments refer to 4-byte longwords,
  83      but the theory applies equally well to 8-byte longwords.  */
  84
  85   longword_ptr = (unsigned long int *) char_ptr;
  86
  87   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  88      the "holes."  Note that there is a hole just to the left of
  89      each byte, with an extra at the end:
  90
  91      bits:  01111110 11111110 11111110 11111111
  92      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  93
  94      The 1-bits make sure that carries propagate to the next 0-bit.
  95      The 0-bits provide holes for carries to fall into.  */
  96
  97   if (sizeof (longword) != 4 && sizeof (longword) != 8)
  98     abort ();
  99
 100 #if LONG_MAX <= LONG_MAX_32_BITS
 101   magic_bits = 0x7efefeff;
 102 #else
 103   magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
 104 #endif
 105
 106   /* Set up a longword, each of whose bytes is C.  */
 107   charmask = c | (c << 8);
 108   charmask |= charmask << 16;
 109 #if LONG_MAX > LONG_MAX_32_BITS
 110   charmask |= charmask << 32;
 111 #endif
 112
 113   /* Instead of the traditional loop which tests each character,
 114      we will test a longword at a time.  The tricky part is testing
 115      if *any of the four* bytes in the longword in question are zero.  */
 116   while (n >= sizeof (longword))
 117     {
 118       /* We tentatively exit the loop if adding MAGIC_BITS to
 119          LONGWORD fails to change any of the hole bits of LONGWORD.
 120
 121          1) Is this safe?  Will it catch all the zero bytes?
 122          Suppose there is a byte with all zeros.  Any carry bits
 123          propagating from its left will fall into the hole at its
 124          least significant bit and stop.  Since there will be no
 125          carry from its most significant bit, the LSB of the
 126          byte to the left will be unchanged, and the zero will be
 127          detected.
 128
 129          2) Is this worthwhile?  Will it ignore everything except
 130          zero bytes?  Suppose every byte of LONGWORD has a bit set
 131          somewhere.  There will be a carry into bit 8.  If bit 8
 132          is set, this will carry into bit 16.  If bit 8 is clear,
 133          one of bits 9-15 must be set, so there will be a carry
 134          into bit 16.  Similarly, there will be a carry into bit
 135          24.  If one of bits 24-30 is set, there will be a carry
 136          into bit 31, so all of the hole bits will be changed.
 137
 138          The one misfire occurs when bits 24-30 are clear and bit
 139          31 is set; in this case, the hole at bit 31 is not
 140          changed.  If we had access to the processor carry flag,
 141          we could close this loophole by putting the fourth hole
 142          at bit 32!
 143
 144          So it ignores everything except 128's, when they're aligned
 145          properly.
 146
 147          3) But wait!  Aren't we looking for C, not zero?
 148          Good point.  So what we do is XOR LONGWORD with a longword,
 149          each of whose bytes is C.  This turns each byte that is C
 150          into a zero.  */
 151
 152       longword = *longword_ptr++ ^ charmask;
 153
 154       /* Add MAGIC_BITS to LONGWORD.  */
 155       if ((((longword + magic_bits)
 156
 157             /* Set those bits that were unchanged by the addition.  */
 158             ^ ~longword)
 159
 160            /* Look at only the hole bits.  If any of the hole bits
 161               are unchanged, most likely one of the bytes was a
 162               zero.  */
 163            & ~magic_bits) != 0)
 164         {
 165           /* Which of the bytes was C?  If none of them were, it was
 166              a misfire; continue the search.  */
 167
 168           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 169
 170           if (cp[0] == c)
 171             return (__ptr_t) cp;
 172           if (cp[1] == c)
 173             return (__ptr_t) &cp[1];
 174           if (cp[2] == c)
 175             return (__ptr_t) &cp[2];
 176           if (cp[3] == c)
 177             return (__ptr_t) &cp[3];
 178 #if LONG_MAX > 2147483647
 179           if (cp[4] == c)
 180             return (__ptr_t) &cp[4];
 181           if (cp[5] == c)
 182             return (__ptr_t) &cp[5];
 183           if (cp[6] == c)
 184             return (__ptr_t) &cp[6];
 185           if (cp[7] == c)
 186             return (__ptr_t) &cp[7];
 187 #endif
 188         }
 189
 190       n -= sizeof (longword);
 191     }
 192
 193   char_ptr = (const unsigned char *) longword_ptr;
 194
 195   while (n-- > 0)
 196     {
 197       if (*char_ptr == c)
 198         return (__ptr_t) char_ptr;
 199       else
 200         ++char_ptr;
 201     }
 202
 203   return 0;
 204 }
 205 #ifdef weak_alias
 206 weak_alias (__memchr, BP_SYM (memchr))
 207 #endif
 208 libc_hidden_builtin_def (memchr)