string/memrchr.c

   1 /* memrchr -- find the last occurrence of a byte in a memory block
   2    Copyright (C) 1991-2015 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   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
  10    The GNU C Library is free software; you can redistribute it and/or
  11    modify it under the terms of the GNU Lesser General Public
  12    License as published by the Free Software Foundation; either
  13    version 2.1 of the License, or (at your option) any later version.
  14
  15    The GNU C Library is distributed in the hope that it will be useful,
  16    but WITHOUT ANY WARRANTY; without even the implied warranty of
  17    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  18    Lesser General Public License for more details.
  19
  20    You should have received a copy of the GNU Lesser General Public
  21    License along with the GNU C Library; if not, see
  22    <http://www.gnu.org/licenses/>.  */
  23
  24 #include <stdlib.h>
  25
  26 #ifdef HAVE_CONFIG_H
  27 # include <config.h>
  28 #endif
  29
  30 #undef __ptr_t
  31 #define __ptr_t void *
  32
  33 #if defined _LIBC
  34 # include <string.h>
  35 # include <memcopy.h>
  36 #endif
  37
  38 #if defined HAVE_LIMITS_H || defined _LIBC
  39 # include <limits.h>
  40 #endif
  41
  42 #define LONG_MAX_32_BITS 2147483647
  43
  44 #ifndef LONG_MAX
  45 # define LONG_MAX LONG_MAX_32_BITS
  46 #endif
  47
  48 #include <sys/types.h>
  49
  50 #undef __memrchr
  51 #undef memrchr
  52
  53 #ifndef weak_alias
  54 # define __memrchr memrchr
  55 #endif
  56
  57 /* Search no more than N bytes of S for C.  */
  58 __ptr_t
  59 #ifndef MEMRCHR
  60 __memrchr
  61 #else
  62 MEMRCHR
  63 #endif
  64      (s, c_in, n)
  65      const __ptr_t s;
  66      int c_in;
  67      size_t n;
  68 {
  69   const unsigned char *char_ptr;
  70   const unsigned long int *longword_ptr;
  71   unsigned long int longword, magic_bits, charmask;
  72   unsigned char c;
  73
  74   c = (unsigned char) c_in;
  75
  76   /* Handle the last few characters by reading one character at a time.
  77      Do this until CHAR_PTR is aligned on a longword boundary.  */
  78   for (char_ptr = (const unsigned char *) s + n;
  79        n > 0 && ((unsigned long int) char_ptr
  80                  & (sizeof (longword) - 1)) != 0;
  81        --n)
  82     if (*--char_ptr == c)
  83       return (__ptr_t) char_ptr;
  84
  85   /* All these elucidatory comments refer to 4-byte longwords,
  86      but the theory applies equally well to 8-byte longwords.  */
  87
  88   longword_ptr = (const unsigned long int *) char_ptr;
  89
  90   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  91      the "holes."  Note that there is a hole just to the left of
  92      each byte, with an extra at the end:
  93
  94      bits:  01111110 11111110 11111110 11111111
  95      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  96
  97      The 1-bits make sure that carries propagate to the next 0-bit.
  98      The 0-bits provide holes for carries to fall into.  */
  99   magic_bits = -1;
 100   magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
 101
 102   /* Set up a longword, each of whose bytes is C.  */
 103   charmask = c | (c << 8);
 104   charmask |= charmask << 16;
 105 #if LONG_MAX > LONG_MAX_32_BITS
 106   charmask |= charmask << 32;
 107 #endif
 108
 109   /* Instead of the traditional loop which tests each character,
 110      we will test a longword at a time.  The tricky part is testing
 111      if *any of the four* bytes in the longword in question are zero.  */
 112   while (n >= sizeof (longword))
 113     {
 114       /* We tentatively exit the loop if adding MAGIC_BITS to
 115          LONGWORD fails to change any of the hole bits of LONGWORD.
 116
 117          1) Is this safe?  Will it catch all the zero bytes?
 118          Suppose there is a byte with all zeros.  Any carry bits
 119          propagating from its left will fall into the hole at its
 120          least significant bit and stop.  Since there will be no
 121          carry from its most significant bit, the LSB of the
 122          byte to the left will be unchanged, and the zero will be
 123          detected.
 124
 125          2) Is this worthwhile?  Will it ignore everything except
 126          zero bytes?  Suppose every byte of LONGWORD has a bit set
 127          somewhere.  There will be a carry into bit 8.  If bit 8
 128          is set, this will carry into bit 16.  If bit 8 is clear,
 129          one of bits 9-15 must be set, so there will be a carry
 130          into bit 16.  Similarly, there will be a carry into bit
 131          24.  If one of bits 24-30 is set, there will be a carry
 132          into bit 31, so all of the hole bits will be changed.
 133
 134          The one misfire occurs when bits 24-30 are clear and bit
 135          31 is set; in this case, the hole at bit 31 is not
 136          changed.  If we had access to the processor carry flag,
 137          we could close this loophole by putting the fourth hole
 138          at bit 32!
 139
 140          So it ignores everything except 128's, when they're aligned
 141          properly.
 142
 143          3) But wait!  Aren't we looking for C, not zero?
 144          Good point.  So what we do is XOR LONGWORD with a longword,
 145          each of whose bytes is C.  This turns each byte that is C
 146          into a zero.  */
 147
 148       longword = *--longword_ptr ^ charmask;
 149
 150       /* Add MAGIC_BITS to LONGWORD.  */
 151       if ((((longword + magic_bits)
 152
 153             /* Set those bits that were unchanged by the addition.  */
 154             ^ ~longword)
 155
 156            /* Look at only the hole bits.  If any of the hole bits
 157               are unchanged, most likely one of the bytes was a
 158               zero.  */
 159            & ~magic_bits) != 0)
 160         {
 161           /* Which of the bytes was C?  If none of them were, it was
 162              a misfire; continue the search.  */
 163
 164           const unsigned char *cp = (const unsigned char *) longword_ptr;
 165
 166 #if LONG_MAX > 2147483647
 167           if (cp[7] == c)
 168             return (__ptr_t) &cp[7];
 169           if (cp[6] == c)
 170             return (__ptr_t) &cp[6];
 171           if (cp[5] == c)
 172             return (__ptr_t) &cp[5];
 173           if (cp[4] == c)
 174             return (__ptr_t) &cp[4];
 175 #endif
 176           if (cp[3] == c)
 177             return (__ptr_t) &cp[3];
 178           if (cp[2] == c)
 179             return (__ptr_t) &cp[2];
 180           if (cp[1] == c)
 181             return (__ptr_t) &cp[1];
 182           if (cp[0] == c)
 183             return (__ptr_t) cp;
 184         }
 185
 186       n -= sizeof (longword);
 187     }
 188
 189   char_ptr = (const unsigned char *) longword_ptr;
 190
 191   while (n-- > 0)
 192     {
 193       if (*--char_ptr == c)
 194         return (__ptr_t) char_ptr;
 195     }
 196
 197   return 0;
 198 }
 199 #ifndef MEMRCHR
 200 # ifdef weak_alias
 201 weak_alias (__memrchr, memrchr)
 202 # endif
 203 #endif