string/memchr.c

   1 /* Copyright (C) 1991-2013 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
  51 #undef memchr
  52 #undef __memchr
  53
  54 #ifndef MEMCHR
  55 # define MEMCHR __memchr
  56 #endif
  57
  58 /* Search no more than N bytes of S for C.  */
  59 __ptr_t
  60 MEMCHR (s, c_in, n)
  61      const __ptr_t s;
  62      int c_in;
  63      size_t n;
  64 {
  65   const unsigned char *char_ptr;
  66   const unsigned long int *longword_ptr;
  67   unsigned long int longword, magic_bits, charmask;
  68   unsigned char c;
  69
  70   c = (unsigned char) c_in;
  71
  72   /* Handle the first few characters by reading one character at a time.
  73      Do this until CHAR_PTR is aligned on a longword boundary.  */
  74   for (char_ptr = (const unsigned char *) s;
  75        n > 0 && ((unsigned long int) char_ptr
  76                  & (sizeof (longword) - 1)) != 0;
  77        --n, ++char_ptr)
  78     if (*char_ptr == c)
  79       return (__ptr_t) char_ptr;
  80
  81   /* All these elucidatory comments refer to 4-byte longwords,
  82      but the theory applies equally well to 8-byte longwords.  */
  83
  84   longword_ptr = (unsigned long int *) char_ptr;
  85
  86   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  87      the "holes."  Note that there is a hole just to the left of
  88      each byte, with an extra at the end:
  89
  90      bits:  01111110 11111110 11111110 11111111
  91      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  92
  93      The 1-bits make sure that carries propagate to the next 0-bit.
  94      The 0-bits provide holes for carries to fall into.  */
  95
  96   if (sizeof (longword) != 4 && sizeof (longword) != 8)
  97     abort ();
  98
  99 #if LONG_MAX <= LONG_MAX_32_BITS
 100   magic_bits = 0x7efefeff;
 101 #else
 102   magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
 103 #endif
 104
 105   /* Set up a longword, each of whose bytes is C.  */
 106   charmask = c | (c << 8);
 107   charmask |= charmask << 16;
 108 #if LONG_MAX > LONG_MAX_32_BITS
 109   charmask |= charmask << 32;
 110 #endif
 111
 112   /* Instead of the traditional loop which tests each character,
 113      we will test a longword at a time.  The tricky part is testing
 114      if *any of the four* bytes in the longword in question are zero.  */
 115   while (n >= sizeof (longword))
 116     {
 117       /* We tentatively exit the loop if adding MAGIC_BITS to
 118          LONGWORD fails to change any of the hole bits of LONGWORD.
 119
 120          1) Is this safe?  Will it catch all the zero bytes?
 121          Suppose there is a byte with all zeros.  Any carry bits
 122          propagating from its left will fall into the hole at its
 123          least significant bit and stop.  Since there will be no
 124          carry from its most significant bit, the LSB of the
 125          byte to the left will be unchanged, and the zero will be
 126          detected.
 127
 128          2) Is this worthwhile?  Will it ignore everything except
 129          zero bytes?  Suppose every byte of LONGWORD has a bit set
 130          somewhere.  There will be a carry into bit 8.  If bit 8
 131          is set, this will carry into bit 16.  If bit 8 is clear,
 132          one of bits 9-15 must be set, so there will be a carry
 133          into bit 16.  Similarly, there will be a carry into bit
 134          24.  If one of bits 24-30 is set, there will be a carry
 135          into bit 31, so all of the hole bits will be changed.
 136
 137          The one misfire occurs when bits 24-30 are clear and bit
 138          31 is set; in this case, the hole at bit 31 is not
 139          changed.  If we had access to the processor carry flag,
 140          we could close this loophole by putting the fourth hole
 141          at bit 32!
 142
 143          So it ignores everything except 128's, when they're aligned
 144          properly.
 145
 146          3) But wait!  Aren't we looking for C, not zero?
 147          Good point.  So what we do is XOR LONGWORD with a longword,
 148          each of whose bytes is C.  This turns each byte that is C
 149          into a zero.  */
 150
 151       longword = *longword_ptr++ ^ charmask;
 152
 153       /* Add MAGIC_BITS to LONGWORD.  */
 154       if ((((longword + magic_bits)
 155
 156             /* Set those bits that were unchanged by the addition.  */
 157             ^ ~longword)
 158
 159            /* Look at only the hole bits.  If any of the hole bits
 160               are unchanged, most likely one of the bytes was a
 161               zero.  */
 162            & ~magic_bits) != 0)
 163         {
 164           /* Which of the bytes was C?  If none of them were, it was
 165              a misfire; continue the search.  */
 166
 167           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 168
 169           if (cp[0] == c)
 170             return (__ptr_t) cp;
 171           if (cp[1] == c)
 172             return (__ptr_t) &cp[1];
 173           if (cp[2] == c)
 174             return (__ptr_t) &cp[2];
 175           if (cp[3] == c)
 176             return (__ptr_t) &cp[3];
 177 #if LONG_MAX > 2147483647
 178           if (cp[4] == c)
 179             return (__ptr_t) &cp[4];
 180           if (cp[5] == c)
 181             return (__ptr_t) &cp[5];
 182           if (cp[6] == c)
 183             return (__ptr_t) &cp[6];
 184           if (cp[7] == c)
 185             return (__ptr_t) &cp[7];
 186 #endif
 187         }
 188
 189       n -= sizeof (longword);
 190     }
 191
 192   char_ptr = (const unsigned char *) longword_ptr;
 193
 194   while (n-- > 0)
 195     {
 196       if (*char_ptr == c)
 197         return (__ptr_t) char_ptr;
 198       else
 199         ++char_ptr;
 200     }
 201
 202   return 0;
 203 }
 204 #ifdef weak_alias
 205 weak_alias (__memchr, memchr)
 206 #endif
 207 libc_hidden_builtin_def (memchr)