string/memrchr.c

   1 /* memrchr -- find the last occurrence of a byte in a memory block
   2    Copyright (C) 1991-2022 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    <https://www.gnu.org/licenses/>.  */
  23
  24 #include <stdlib.h>
  25
  26 #ifdef HAVE_CONFIG_H
  27 # include <config.h>
  28 #endif
  29
  30 #if defined _LIBC
  31 # include <string.h>
  32 # include <memcopy.h>
  33 #endif
  34
  35 #if defined HAVE_LIMITS_H || defined _LIBC
  36 # include <limits.h>
  37 #endif
  38
  39 #define LONG_MAX_32_BITS 2147483647
  40
  41 #ifndef LONG_MAX
  42 # define LONG_MAX LONG_MAX_32_BITS
  43 #endif
  44
  45 #include <sys/types.h>
  46
  47 #undef __memrchr
  48 #undef memrchr
  49
  50 #ifndef weak_alias
  51 # define __memrchr memrchr
  52 #endif
  53
  54 /* Search no more than N bytes of S for C.  */
  55 void *
  56 #ifndef MEMRCHR
  57 __memrchr
  58 #else
  59 MEMRCHR
  60 #endif
  61      (const void *s, int c_in, size_t n)
  62 {
  63   const unsigned char *char_ptr;
  64   const unsigned long int *longword_ptr;
  65   unsigned long int longword, magic_bits, charmask;
  66   unsigned char c;
  67
  68   c = (unsigned char) c_in;
  69
  70   /* Handle the last few characters by reading one character at a time.
  71      Do this until CHAR_PTR is aligned on a longword boundary.  */
  72   for (char_ptr = (const unsigned char *) s + n;
  73        n > 0 && ((unsigned long int) char_ptr
  74                  & (sizeof (longword) - 1)) != 0;
  75        --n)
  76     if (*--char_ptr == c)
  77       return (void *) char_ptr;
  78
  79   /* All these elucidatory comments refer to 4-byte longwords,
  80      but the theory applies equally well to 8-byte longwords.  */
  81
  82   longword_ptr = (const unsigned long int *) char_ptr;
  83
  84   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  85      the "holes."  Note that there is a hole just to the left of
  86      each byte, with an extra at the end:
  87
  88      bits:  01111110 11111110 11111110 11111111
  89      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  90
  91      The 1-bits make sure that carries propagate to the next 0-bit.
  92      The 0-bits provide holes for carries to fall into.  */
  93   magic_bits = -1;
  94   magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
  95
  96   /* Set up a longword, each of whose bytes is C.  */
  97   charmask = c | (c << 8);
  98   charmask |= charmask << 16;
  99 #if LONG_MAX > LONG_MAX_32_BITS
 100   charmask |= charmask << 32;
 101 #endif
 102
 103   /* Instead of the traditional loop which tests each character,
 104      we will test a longword at a time.  The tricky part is testing
 105      if *any of the four* bytes in the longword in question are zero.  */
 106   while (n >= sizeof (longword))
 107     {
 108       /* We tentatively exit the loop if adding MAGIC_BITS to
 109          LONGWORD fails to change any of the hole bits of LONGWORD.
 110
 111          1) Is this safe?  Will it catch all the zero bytes?
 112          Suppose there is a byte with all zeros.  Any carry bits
 113          propagating from its left will fall into the hole at its
 114          least significant bit and stop.  Since there will be no
 115          carry from its most significant bit, the LSB of the
 116          byte to the left will be unchanged, and the zero will be
 117          detected.
 118
 119          2) Is this worthwhile?  Will it ignore everything except
 120          zero bytes?  Suppose every byte of LONGWORD has a bit set
 121          somewhere.  There will be a carry into bit 8.  If bit 8
 122          is set, this will carry into bit 16.  If bit 8 is clear,
 123          one of bits 9-15 must be set, so there will be a carry
 124          into bit 16.  Similarly, there will be a carry into bit
 125          24.  If one of bits 24-30 is set, there will be a carry
 126          into bit 31, so all of the hole bits will be changed.
 127
 128          The one misfire occurs when bits 24-30 are clear and bit
 129          31 is set; in this case, the hole at bit 31 is not
 130          changed.  If we had access to the processor carry flag,
 131          we could close this loophole by putting the fourth hole
 132          at bit 32!
 133
 134          So it ignores everything except 128's, when they're aligned
 135          properly.
 136
 137          3) But wait!  Aren't we looking for C, not zero?
 138          Good point.  So what we do is XOR LONGWORD with a longword,
 139          each of whose bytes is C.  This turns each byte that is C
 140          into a zero.  */
 141
 142       longword = *--longword_ptr ^ charmask;
 143
 144       /* Add MAGIC_BITS to LONGWORD.  */
 145       if ((((longword + magic_bits)
 146
 147             /* Set those bits that were unchanged by the addition.  */
 148             ^ ~longword)
 149
 150            /* Look at only the hole bits.  If any of the hole bits
 151               are unchanged, most likely one of the bytes was a
 152               zero.  */
 153            & ~magic_bits) != 0)
 154         {
 155           /* Which of the bytes was C?  If none of them were, it was
 156              a misfire; continue the search.  */
 157
 158           const unsigned char *cp = (const unsigned char *) longword_ptr;
 159
 160 #if LONG_MAX > 2147483647
 161           if (cp[7] == c)
 162             return (void *) &cp[7];
 163           if (cp[6] == c)
 164             return (void *) &cp[6];
 165           if (cp[5] == c)
 166             return (void *) &cp[5];
 167           if (cp[4] == c)
 168             return (void *) &cp[4];
 169 #endif
 170           if (cp[3] == c)
 171             return (void *) &cp[3];
 172           if (cp[2] == c)
 173             return (void *) &cp[2];
 174           if (cp[1] == c)
 175             return (void *) &cp[1];
 176           if (cp[0] == c)
 177             return (void *) cp;
 178         }
 179
 180       n -= sizeof (longword);
 181     }
 182
 183   char_ptr = (const unsigned char *) longword_ptr;
 184
 185   while (n-- > 0)
 186     {
 187       if (*--char_ptr == c)
 188         return (void *) char_ptr;
 189     }
 190
 191   return 0;
 192 }
 193 #ifndef MEMRCHR
 194 # ifdef weak_alias
 195 weak_alias (__memrchr, memrchr)
 196 # endif
 197 #endif