gnu/dist/gettext/gettext-tools/libgrep/memchr.c

   1 /* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004 Free
   2    Software Foundation, Inc.
   3
   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 NOTE: The canonical source of this file is maintained with the GNU C Library.
  11 Bugs can be reported to bug-glibc@prep.ai.mit.edu.
  12
  13 This program is free software; you can redistribute it and/or modify it
  14 under the terms of the GNU General Public License as published by the
  15 Free Software Foundation; either version 2, or (at your option) any
  16 later version.
  17
  18 This program is distributed in the hope that it will be useful,
  19 but WITHOUT ANY WARRANTY; without even the implied warranty of
  20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  21 GNU General Public License for more details.
  22
  23 You should have received a copy of the GNU General Public License
  24 along with this program; if not, write to the Free Software
  25 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
  26 USA.  */
  27
  28 #ifdef HAVE_CONFIG_H
  29 # include <config.h>
  30 #endif
  31
  32 #include <string.h>
  33
  34 #include <stddef.h>
  35
  36 #if defined _LIBC
  37 # include <memcopy.h>
  38 #else
  39 # define reg_char char
  40 #endif
  41
  42 #include <limits.h>
  43
  44 #if HAVE_BP_SYM_H || defined _LIBC
  45 # include <bp-sym.h>
  46 #else
  47 # define BP_SYM(sym) sym
  48 #endif
  49
  50 #undef memchr
  51 #undef __memchr
  52
  53 /* Search no more than N bytes of S for C.  */
  54 void *
  55 __memchr (void const *s, int c_in, size_t n)
  56 {
  57   const unsigned char *char_ptr;
  58   const unsigned long int *longword_ptr;
  59   unsigned long int longword, magic_bits, charmask;
  60   unsigned reg_char c;
  61   int i;
  62
  63   c = (unsigned char) c_in;
  64
  65   /* Handle the first few characters by reading one character at a time.
  66      Do this until CHAR_PTR is aligned on a longword boundary.  */
  67   for (char_ptr = (const unsigned char *) s;
  68        n > 0 && (size_t) char_ptr % sizeof longword != 0;
  69        --n, ++char_ptr)
  70     if (*char_ptr == c)
  71       return (void *) char_ptr;
  72
  73   /* All these elucidatory comments refer to 4-byte longwords,
  74      but the theory applies equally well to any size longwords.  */
  75
  76   longword_ptr = (const unsigned long int *) char_ptr;
  77
  78   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  79      the "holes."  Note that there is a hole just to the left of
  80      each byte, with an extra at the end:
  81
  82      bits:  01111110 11111110 11111110 11111111
  83      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  84
  85      The 1-bits make sure that carries propagate to the next 0-bit.
  86      The 0-bits provide holes for carries to fall into.  */
  87
  88   /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
  89      Set CHARMASK to be a longword, each of whose bytes is C.  */
  90
  91   magic_bits = 0xfefefefe;
  92   charmask = c | (c << 8);
  93   charmask |= charmask << 16;
  94 #if 0xffffffffU < ULONG_MAX
  95   magic_bits |= magic_bits << 32;
  96   charmask |= charmask << 32;
  97   if (8 < sizeof longword)
  98     for (i = 64; i < sizeof longword * 8; i *= 2)
  99       {
 100         magic_bits |= magic_bits << i;
 101         charmask |= charmask << i;
 102       }
 103 #endif
 104   magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
 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 - 1);
 162
 163           if (cp[0] == c)
 164             return (void *) cp;
 165           if (cp[1] == c)
 166             return (void *) &cp[1];
 167           if (cp[2] == c)
 168             return (void *) &cp[2];
 169           if (cp[3] == c)
 170             return (void *) &cp[3];
 171           if (4 < sizeof longword && cp[4] == c)
 172             return (void *) &cp[4];
 173           if (5 < sizeof longword && cp[5] == c)
 174             return (void *) &cp[5];
 175           if (6 < sizeof longword && cp[6] == c)
 176             return (void *) &cp[6];
 177           if (7 < sizeof longword && cp[7] == c)
 178             return (void *) &cp[7];
 179           if (8 < sizeof longword)
 180             for (i = 8; i < sizeof longword; i++)
 181               if (cp[i] == c)
 182                 return (void *) &cp[i];
 183         }
 184
 185       n -= sizeof longword;
 186     }
 187
 188   char_ptr = (const unsigned char *) longword_ptr;
 189
 190   while (n-- > 0)
 191     {
 192       if (*char_ptr == c)
 193         return (void *) char_ptr;
 194       else
 195         ++char_ptr;
 196     }
 197
 198   return 0;
 199 }
 200 #ifdef weak_alias
 201 weak_alias (__memchr, BP_SYM (memchr))
 202 #endif