lib/memrchr.c

   1 /* memrchr -- find the last occurrence of a byte in a memory block
   2    Copyright (C) 1991, 93, 96, 97, 99, 2000 Free Software Foundation, Inc.
   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 Library General Public License as
  11    published by the Free Software Foundation; either version 2 of the
  12    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    Library General Public License for more details.
  18
  19    You should have received a copy of the GNU Library General Public
  20    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  21    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  22    Boston, MA 02111-1307, USA.  */
  23
  24 #ifdef HAVE_CONFIG_H
  25 # include <config.h>
  26 #endif
  27
  28 #include <stdlib.h>
  29
  30 #undef __ptr_t
  31 #if defined (__cplusplus) || (defined (__STDC__) && __STDC__)
  32 # define __ptr_t void *
  33 #else /* Not C++ or ANSI C.  */
  34 # define __ptr_t char *
  35 #endif /* C++ or ANSI C.  */
  36
  37 #if defined (_LIBC)
  38 # include <string.h>
  39 # include <memcopy.h>
  40 #else
  41 # define reg_char char
  42 #endif
  43
  44 #if defined (HAVE_LIMITS_H) || defined (_LIBC)
  45 # include <limits.h>
  46 #endif
  47
  48 #define LONG_MAX_32_BITS 2147483647
  49
  50 #ifndef LONG_MAX
  51 # define LONG_MAX LONG_MAX_32_BITS
  52 #endif
  53
  54 #include <sys/types.h>
  55
  56 #undef __memrchr
  57 #undef memrchr
  58
  59 #ifndef weak_alias
  60 # define __memrchr memrchr
  61 #endif
  62
  63 /* Search no more than N bytes of S for C.  */
  64 __ptr_t
  65 __memrchr (s, c_in, n)
  66      const __ptr_t s;
  67      int c_in;
  68      size_t n;
  69 {
  70   const unsigned char *char_ptr;
  71   const unsigned long int *longword_ptr;
  72   unsigned long int longword, magic_bits, charmask;
  73   unsigned reg_char c;
  74
  75   c = (unsigned char) c_in;
  76
  77   /* Handle the last few characters by reading one character at a time.
  78      Do this until CHAR_PTR is aligned on a longword boundary.  */
  79   for (char_ptr = (const unsigned char *) s + n;
  80        n > 0 && ((unsigned long int) char_ptr
  81                  & (sizeof (longword) - 1)) != 0;
  82        --n)
  83     if (*--char_ptr == c)
  84       return (__ptr_t) char_ptr;
  85
  86   /* All these elucidatory comments refer to 4-byte longwords,
  87      but the theory applies equally well to 8-byte longwords.  */
  88
  89   longword_ptr = (unsigned long int *) char_ptr;
  90
  91   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  92      the "holes."  Note that there is a hole just to the left of
  93      each byte, with an extra at the end:
  94
  95      bits:  01111110 11111110 11111110 11111111
  96      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  97
  98      The 1-bits make sure that carries propagate to the next 0-bit.
  99      The 0-bits provide holes for carries to fall into.  */
 100
 101   if (sizeof (longword) != 4 && sizeof (longword) != 8)
 102     abort ();
 103
 104 #if LONG_MAX <= LONG_MAX_32_BITS
 105   magic_bits = 0x7efefeff;
 106 #else
 107   magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
 108 #endif
 109
 110   /* Set up a longword, each of whose bytes is C.  */
 111   charmask = c | (c << 8);
 112   charmask |= charmask << 16;
 113 #if LONG_MAX > LONG_MAX_32_BITS
 114   charmask |= charmask << 32;
 115 #endif
 116
 117   /* Instead of the traditional loop which tests each character,
 118      we will test a longword at a time.  The tricky part is testing
 119      if *any of the four* bytes in the longword in question are zero.  */
 120   while (n >= sizeof (longword))
 121     {
 122       /* We tentatively exit the loop if adding MAGIC_BITS to
 123          LONGWORD fails to change any of the hole bits of LONGWORD.
 124
 125          1) Is this safe?  Will it catch all the zero bytes?
 126          Suppose there is a byte with all zeros.  Any carry bits
 127          propagating from its left will fall into the hole at its
 128          least significant bit and stop.  Since there will be no
 129          carry from its most significant bit, the LSB of the
 130          byte to the left will be unchanged, and the zero will be
 131          detected.
 132
 133          2) Is this worthwhile?  Will it ignore everything except
 134          zero bytes?  Suppose every byte of LONGWORD has a bit set
 135          somewhere.  There will be a carry into bit 8.  If bit 8
 136          is set, this will carry into bit 16.  If bit 8 is clear,
 137          one of bits 9-15 must be set, so there will be a carry
 138          into bit 16.  Similarly, there will be a carry into bit
 139          24.  If one of bits 24-30 is set, there will be a carry
 140          into bit 31, so all of the hole bits will be changed.
 141
 142          The one misfire occurs when bits 24-30 are clear and bit
 143          31 is set; in this case, the hole at bit 31 is not
 144          changed.  If we had access to the processor carry flag,
 145          we could close this loophole by putting the fourth hole
 146          at bit 32!
 147
 148          So it ignores everything except 128's, when they're aligned
 149          properly.
 150
 151          3) But wait!  Aren't we looking for C, not zero?
 152          Good point.  So what we do is XOR LONGWORD with a longword,
 153          each of whose bytes is C.  This turns each byte that is C
 154          into a zero.  */
 155
 156       longword = *--longword_ptr ^ charmask;
 157
 158       /* Add MAGIC_BITS to LONGWORD.  */
 159       if ((((longword + magic_bits)
 160
 161             /* Set those bits that were unchanged by the addition.  */
 162             ^ ~longword)
 163
 164            /* Look at only the hole bits.  If any of the hole bits
 165               are unchanged, most likely one of the bytes was a
 166               zero.  */
 167            & ~magic_bits) != 0)
 168         {
 169           /* Which of the bytes was C?  If none of them were, it was
 170              a misfire; continue the search.  */
 171
 172           const unsigned char *cp = (const unsigned char *) longword_ptr;
 173
 174 #if LONG_MAX > 2147483647
 175           if (cp[7] == c)
 176             return (__ptr_t) &cp[7];
 177           if (cp[6] == c)
 178             return (__ptr_t) &cp[6];
 179           if (cp[5] == c)
 180             return (__ptr_t) &cp[5];
 181           if (cp[4] == c)
 182             return (__ptr_t) &cp[4];
 183 #endif
 184           if (cp[3] == c)
 185             return (__ptr_t) &cp[3];
 186           if (cp[2] == c)
 187             return (__ptr_t) &cp[2];
 188           if (cp[1] == c)
 189             return (__ptr_t) &cp[1];
 190           if (cp[0] == c)
 191             return (__ptr_t) cp;
 192         }
 193
 194       n -= sizeof (longword);
 195     }
 196
 197   char_ptr = (const unsigned char *) longword_ptr;
 198
 199   while (n-- > 0)
 200     {
 201       if (*--char_ptr == c)
 202         return (__ptr_t) char_ptr;
 203     }
 204
 205   return 0;
 206 }
 207 #ifdef weak_alias
 208 weak_alias (__memrchr, memrchr)
 209 #endif