string/memrchr.c

   1 /* memrchr -- find the last occurrence of a byte in a memory block
   2    Copyright (C) 1991-2017 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      (const __ptr_t s, int c_in, 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 last 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 + n;
  76        n > 0 && ((unsigned long int) char_ptr
  77                  & (sizeof (longword) - 1)) != 0;
  78        --n)
  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 = (const 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   magic_bits = -1;
  97   magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
  98
  99   /* Set up a longword, each of whose bytes is C.  */
 100   charmask = c | (c << 8);
 101   charmask |= charmask << 16;
 102 #if LONG_MAX > LONG_MAX_32_BITS
 103   charmask |= charmask << 32;
 104 #endif
 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;
 162
 163 #if LONG_MAX > 2147483647
 164           if (cp[7] == c)
 165             return (__ptr_t) &cp[7];
 166           if (cp[6] == c)
 167             return (__ptr_t) &cp[6];
 168           if (cp[5] == c)
 169             return (__ptr_t) &cp[5];
 170           if (cp[4] == c)
 171             return (__ptr_t) &cp[4];
 172 #endif
 173           if (cp[3] == c)
 174             return (__ptr_t) &cp[3];
 175           if (cp[2] == c)
 176             return (__ptr_t) &cp[2];
 177           if (cp[1] == c)
 178             return (__ptr_t) &cp[1];
 179           if (cp[0] == c)
 180             return (__ptr_t) cp;
 181         }
 182
 183       n -= sizeof (longword);
 184     }
 185
 186   char_ptr = (const unsigned char *) longword_ptr;
 187
 188   while (n-- > 0)
 189     {
 190       if (*--char_ptr == c)
 191         return (__ptr_t) char_ptr;
 192     }
 193
 194   return 0;
 195 }
 196 #ifndef MEMRCHR
 197 # ifdef weak_alias
 198 weak_alias (__memrchr, memrchr)
 199 # endif
 200 #endif