gnu/dist/diffutils/lib/memchr.c

   1 /*      $NetBSD$        */
   2
   3 /* Copyright (C) 1991,93,96,97,99,2000 Free Software Foundation, Inc.
   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 #undef __ptr_t
  33 #if defined (__cplusplus) || (defined (__STDC__) && __STDC__)
  34 # define __ptr_t void *
  35 #else /* Not C++ or ANSI C.  */
  36 # define __ptr_t char *
  37 #endif /* C++ or ANSI C.  */
  38
  39 #if defined _LIBC
  40 # include <string.h>
  41 # include <memcopy.h>
  42 #else
  43 # define reg_char char
  44 #endif
  45
  46 #if HAVE_STDLIB_H || defined _LIBC
  47 # include <stdlib.h>
  48 #endif
  49
  50 #if HAVE_LIMITS_H || defined _LIBC
  51 # include <limits.h>
  52 #endif
  53
  54 #define LONG_MAX_32_BITS 2147483647
  55
  56 #ifndef LONG_MAX
  57 # define LONG_MAX LONG_MAX_32_BITS
  58 #endif
  59
  60 #include <sys/types.h>
  61 #if HAVE_BP_SYM_H || defined _LIBC
  62 # include <bp-sym.h>
  63 #else
  64 # define BP_SYM(sym) sym
  65 #endif
  66
  67 #undef memchr
  68 #undef __memchr
  69
  70 /* Search no more than N bytes of S for C.  */
  71 __ptr_t
  72 __memchr (s, c_in, n)
  73      const __ptr_t s;
  74      int c_in;
  75      size_t n;
  76 {
  77   const unsigned char *char_ptr;
  78   const unsigned long int *longword_ptr;
  79   unsigned long int longword, magic_bits, charmask;
  80   unsigned reg_char c;
  81
  82   c = (unsigned char) c_in;
  83
  84   /* Handle the first few characters by reading one character at a time.
  85      Do this until CHAR_PTR is aligned on a longword boundary.  */
  86   for (char_ptr = (const unsigned char *) s;
  87        n > 0 && ((unsigned long int) char_ptr
  88                  & (sizeof (longword) - 1)) != 0;
  89        --n, ++char_ptr)
  90     if (*char_ptr == c)
  91       return (__ptr_t) char_ptr;
  92
  93   /* All these elucidatory comments refer to 4-byte longwords,
  94      but the theory applies equally well to 8-byte longwords.  */
  95
  96   longword_ptr = (unsigned long int *) char_ptr;
  97
  98   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  99      the "holes."  Note that there is a hole just to the left of
 100      each byte, with an extra at the end:
 101
 102      bits:  01111110 11111110 11111110 11111111
 103      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
 104
 105      The 1-bits make sure that carries propagate to the next 0-bit.
 106      The 0-bits provide holes for carries to fall into.  */
 107
 108   if (sizeof (longword) != 4 && sizeof (longword) != 8)
 109     abort ();
 110
 111 #if LONG_MAX <= LONG_MAX_32_BITS
 112   magic_bits = 0x7efefeff;
 113 #else
 114   magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
 115 #endif
 116
 117   /* Set up a longword, each of whose bytes is C.  */
 118   charmask = c | (c << 8);
 119   charmask |= charmask << 16;
 120 #if LONG_MAX > LONG_MAX_32_BITS
 121   charmask |= charmask << 32;
 122 #endif
 123
 124   /* Instead of the traditional loop which tests each character,
 125      we will test a longword at a time.  The tricky part is testing
 126      if *any of the four* bytes in the longword in question are zero.  */
 127   while (n >= sizeof (longword))
 128     {
 129       /* We tentatively exit the loop if adding MAGIC_BITS to
 130          LONGWORD fails to change any of the hole bits of LONGWORD.
 131
 132          1) Is this safe?  Will it catch all the zero bytes?
 133          Suppose there is a byte with all zeros.  Any carry bits
 134          propagating from its left will fall into the hole at its
 135          least significant bit and stop.  Since there will be no
 136          carry from its most significant bit, the LSB of the
 137          byte to the left will be unchanged, and the zero will be
 138          detected.
 139
 140          2) Is this worthwhile?  Will it ignore everything except
 141          zero bytes?  Suppose every byte of LONGWORD has a bit set
 142          somewhere.  There will be a carry into bit 8.  If bit 8
 143          is set, this will carry into bit 16.  If bit 8 is clear,
 144          one of bits 9-15 must be set, so there will be a carry
 145          into bit 16.  Similarly, there will be a carry into bit
 146          24.  If one of bits 24-30 is set, there will be a carry
 147          into bit 31, so all of the hole bits will be changed.
 148
 149          The one misfire occurs when bits 24-30 are clear and bit
 150          31 is set; in this case, the hole at bit 31 is not
 151          changed.  If we had access to the processor carry flag,
 152          we could close this loophole by putting the fourth hole
 153          at bit 32!
 154
 155          So it ignores everything except 128's, when they're aligned
 156          properly.
 157
 158          3) But wait!  Aren't we looking for C, not zero?
 159          Good point.  So what we do is XOR LONGWORD with a longword,
 160          each of whose bytes is C.  This turns each byte that is C
 161          into a zero.  */
 162
 163       longword = *longword_ptr++ ^ charmask;
 164
 165       /* Add MAGIC_BITS to LONGWORD.  */
 166       if ((((longword + magic_bits)
 167
 168             /* Set those bits that were unchanged by the addition.  */
 169             ^ ~longword)
 170
 171            /* Look at only the hole bits.  If any of the hole bits
 172               are unchanged, most likely one of the bytes was a
 173               zero.  */
 174            & ~magic_bits) != 0)
 175         {
 176           /* Which of the bytes was C?  If none of them were, it was
 177              a misfire; continue the search.  */
 178
 179           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 180
 181           if (cp[0] == c)
 182             return (__ptr_t) cp;
 183           if (cp[1] == c)
 184             return (__ptr_t) &cp[1];
 185           if (cp[2] == c)
 186             return (__ptr_t) &cp[2];
 187           if (cp[3] == c)
 188             return (__ptr_t) &cp[3];
 189 #if LONG_MAX > 2147483647
 190           if (cp[4] == c)
 191             return (__ptr_t) &cp[4];
 192           if (cp[5] == c)
 193             return (__ptr_t) &cp[5];
 194           if (cp[6] == c)
 195             return (__ptr_t) &cp[6];
 196           if (cp[7] == c)
 197             return (__ptr_t) &cp[7];
 198 #endif
 199         }
 200
 201       n -= sizeof (longword);
 202     }
 203
 204   char_ptr = (const unsigned char *) longword_ptr;
 205
 206   while (n-- > 0)
 207     {
 208       if (*char_ptr == c)
 209         return (__ptr_t) char_ptr;
 210       else
 211         ++char_ptr;
 212     }
 213
 214   return 0;
 215 }
 216 #ifdef weak_alias
 217 weak_alias (__memchr, BP_SYM (memchr))
 218 #endif