lib/memchr.c

   1 /* Copyright (C) 1991, 1993 Free Software Foundation, Inc.
   2    Based on strlen implemention by Torbjorn Granlund (tege@sics.se),
   3    with help from Dan Sahlin (dan@sics.se) and
   4    commentary by Jim Blandy (jimb@ai.mit.edu);
   5    adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   6    and implemented by Roland McGrath (roland@ai.mit.edu).
   7
   8 NOTE: The canonical source of this file is maintained with the GNU C Library.
   9 Bugs can be reported to bug-glibc@prep.ai.mit.edu.
  10
  11 This program is free software; you can redistribute it and/or modify it
  12 under the terms of the GNU General Public License as published by the
  13 Free Software Foundation; either version 2, or (at your option) any
  14 later version.
  15
  16 This program is distributed in the hope that it will be useful,
  17 but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 GNU General Public License for more details.
  20
  21 You should have received a copy of the GNU General Public License
  22 along with this program; if not, write to the Free Software
  23 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
  24 USA.  */
  25
  26 #ifdef HAVE_CONFIG_H
  27 #include <config.h>
  28 #endif
  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 #endif
  40
  41 #if defined (HAVE_LIMITS_H) || defined (_LIBC)
  42 # include <limits.h>
  43 #endif
  44
  45 #define LONG_MAX_32_BITS 2147483647
  46
  47 #ifndef LONG_MAX
  48 #define LONG_MAX LONG_MAX_32_BITS
  49 #endif
  50
  51 #include <sys/types.h>
  52
  53
  54 /* Search no more than N bytes of S for C.  */
  55
  56 __ptr_t
  57 memchr (s, c, n)
  58      const __ptr_t s;
  59      int c;
  60      size_t n;
  61 {
  62   const unsigned char *char_ptr;
  63   const unsigned long int *longword_ptr;
  64   unsigned long int longword, magic_bits, charmask;
  65
  66   c = (unsigned char) c;
  67
  68   /* Handle the first few characters by reading one character at a time.
  69      Do this until CHAR_PTR is aligned on a longword boundary.  */
  70   for (char_ptr = (const unsigned char *) s;
  71        n > 0 && ((unsigned long int) char_ptr
  72                  & (sizeof (longword) - 1)) != 0;
  73        --n, ++char_ptr)
  74     if (*char_ptr == c)
  75       return (__ptr_t) char_ptr;
  76
  77   /* All these elucidatory comments refer to 4-byte longwords,
  78      but the theory applies equally well to 8-byte longwords.  */
  79
  80   longword_ptr = (unsigned long int *) char_ptr;
  81
  82   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  83      the "holes."  Note that there is a hole just to the left of
  84      each byte, with an extra at the end:
  85
  86      bits:  01111110 11111110 11111110 11111111
  87      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  88
  89      The 1-bits make sure that carries propagate to the next 0-bit.
  90      The 0-bits provide holes for carries to fall into.  */
  91
  92   if (sizeof (longword) != 4 && sizeof (longword) != 8)
  93     abort ();
  94
  95 #if LONG_MAX <= LONG_MAX_32_BITS
  96   magic_bits = 0x7efefeff;
  97 #else
  98   magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
  99 #endif
 100
 101   /* Set up a longword, each of whose bytes is C.  */
 102   charmask = c | (c << 8);
 103   charmask |= charmask << 16;
 104 #if LONG_MAX > LONG_MAX_32_BITS
 105   charmask |= charmask << 32;
 106 #endif
 107
 108   /* Instead of the traditional loop which tests each character,
 109      we will test a longword at a time.  The tricky part is testing
 110      if *any of the four* bytes in the longword in question are zero.  */
 111   while (n >= sizeof (longword))
 112     {
 113       /* We tentatively exit the loop if adding MAGIC_BITS to
 114          LONGWORD fails to change any of the hole bits of LONGWORD.
 115
 116          1) Is this safe?  Will it catch all the zero bytes?
 117          Suppose there is a byte with all zeros.  Any carry bits
 118          propagating from its left will fall into the hole at its
 119          least significant bit and stop.  Since there will be no
 120          carry from its most significant bit, the LSB of the
 121          byte to the left will be unchanged, and the zero will be
 122          detected.
 123
 124          2) Is this worthwhile?  Will it ignore everything except
 125          zero bytes?  Suppose every byte of LONGWORD has a bit set
 126          somewhere.  There will be a carry into bit 8.  If bit 8
 127          is set, this will carry into bit 16.  If bit 8 is clear,
 128          one of bits 9-15 must be set, so there will be a carry
 129          into bit 16.  Similarly, there will be a carry into bit
 130          24.  If one of bits 24-30 is set, there will be a carry
 131          into bit 31, so all of the hole bits will be changed.
 132
 133          The one misfire occurs when bits 24-30 are clear and bit
 134          31 is set; in this case, the hole at bit 31 is not
 135          changed.  If we had access to the processor carry flag,
 136          we could close this loophole by putting the fourth hole
 137          at bit 32!
 138
 139          So it ignores everything except 128's, when they're aligned
 140          properly.
 141
 142          3) But wait!  Aren't we looking for C, not zero?
 143          Good point.  So what we do is XOR LONGWORD with a longword,
 144          each of whose bytes is C.  This turns each byte that is C
 145          into a zero.  */
 146
 147       longword = *longword_ptr++ ^ charmask;
 148
 149       /* Add MAGIC_BITS to LONGWORD.  */
 150       if ((((longword + magic_bits)
 151
 152             /* Set those bits that were unchanged by the addition.  */
 153             ^ ~longword)
 154
 155            /* Look at only the hole bits.  If any of the hole bits
 156               are unchanged, most likely one of the bytes was a
 157               zero.  */
 158            & ~magic_bits) != 0)
 159         {
 160           /* Which of the bytes was C?  If none of them were, it was
 161              a misfire; continue the search.  */
 162
 163           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 164
 165           if (cp[0] == c)
 166             return (__ptr_t) cp;
 167           if (cp[1] == c)
 168             return (__ptr_t) &cp[1];
 169           if (cp[2] == c)
 170             return (__ptr_t) &cp[2];
 171           if (cp[3] == c)
 172             return (__ptr_t) &cp[3];
 173 #if LONG_MAX > 2147483647
 174           if (cp[4] == c)
 175             return (__ptr_t) &cp[4];
 176           if (cp[5] == c)
 177             return (__ptr_t) &cp[5];
 178           if (cp[6] == c)
 179             return (__ptr_t) &cp[6];
 180           if (cp[7] == c)
 181             return (__ptr_t) &cp[7];
 182 #endif
 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 (__ptr_t) char_ptr;
 194       else
 195         ++char_ptr;
 196     }
 197
 198   return 0;
 199 }