sysdeps/generic/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 The GNU C Library is free software; you can redistribute it and/or
   9 modify it under the terms of the GNU Library General Public License as
  10 published by the Free Software Foundation; either version 2 of the
  11 License, or (at your option) any later version.
  12
  13 The GNU C Library is distributed in the hope that it will be useful,
  14 but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16 Library General Public License for more details.
  17
  18 You should have received a copy of the GNU Library General Public
  19 License along with the GNU C Library; see the file COPYING.LIB.  If
  20 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  21 Cambridge, MA 02139, USA.  */
  22
  23 #include <ansidecl.h>
  24 #include <string.h>
  25
  26
  27 /* Search no more than N bytes of S for C.  */
  28
  29 PTR
  30 DEFUN(memchr, (s, c, n), CONST PTR s AND int c AND size_t n)
  31 {
  32   CONST unsigned char *char_ptr;
  33   CONST unsigned long int *longword_ptr;
  34   unsigned long int longword, magic_bits, charmask;
  35
  36   c = (unsigned char) c;
  37
  38   /* Handle the first few characters by reading one character at a time.
  39      Do this until CHAR_PTR is aligned on a longword boundary.  */
  40   for (char_ptr = s; n > 0 && ((unsigned long int) char_ptr
  41                                & (sizeof (longword) - 1)) != 0;
  42        --n, ++char_ptr)
  43     if (*char_ptr == c)
  44       return (PTR) char_ptr;
  45
  46   /* All these elucidatory comments refer to 4-byte longwords,
  47      but the theory applies equally well to 8-byte longwords.  */
  48
  49   longword_ptr = (unsigned long int *) char_ptr;
  50
  51   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  52      the "holes."  Note that there is a hole just to the left of
  53      each byte, with an extra at the end:
  54
  55      bits:  01111110 11111110 11111110 11111111
  56      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  57
  58      The 1-bits make sure that carries propagate to the next 0-bit.
  59      The 0-bits provide holes for carries to fall into.  */
  60   switch (sizeof (longword))
  61     {
  62     case 4: magic_bits = 0x7efefeffL; break;
  63     case 8: magic_bits = (0x7efefefeL << 32) | 0xfefefeffL; break;
  64     default:
  65       abort ();
  66     }
  67
  68   /* Set up a longword, each of whose bytes is C.  */
  69   charmask = c | (c << 8);
  70   charmask |= charmask << 16;
  71   if (sizeof (longword) > 4)
  72     charmask |= charmask << 32;
  73   if (sizeof (longword) > 8)
  74     abort ();
  75
  76   /* Instead of the traditional loop which tests each character,
  77      we will test a longword at a time.  The tricky part is testing
  78      if *any of the four* bytes in the longword in question are zero.  */
  79   while (n >= sizeof (longword))
  80     {
  81       /* We tentatively exit the loop if adding MAGIC_BITS to
  82          LONGWORD fails to change any of the hole bits of LONGWORD.
  83
  84          1) Is this safe?  Will it catch all the zero bytes?
  85          Suppose there is a byte with all zeros.  Any carry bits
  86          propagating from its left will fall into the hole at its
  87          least significant bit and stop.  Since there will be no
  88          carry from its most significant bit, the LSB of the
  89          byte to the left will be unchanged, and the zero will be
  90          detected.
  91
  92          2) Is this worthwhile?  Will it ignore everything except
  93          zero bytes?  Suppose every byte of LONGWORD has a bit set
  94          somewhere.  There will be a carry into bit 8.  If bit 8
  95          is set, this will carry into bit 16.  If bit 8 is clear,
  96          one of bits 9-15 must be set, so there will be a carry
  97          into bit 16.  Similarly, there will be a carry into bit
  98          24.  If one of bits 24-30 is set, there will be a carry
  99          into bit 31, so all of the hole bits will be changed.
 100
 101          The one misfire occurs when bits 24-30 are clear and bit
 102          31 is set; in this case, the hole at bit 31 is not
 103          changed.  If we had access to the processor carry flag,
 104          we could close this loophole by putting the fourth hole
 105          at bit 32!
 106
 107          So it ignores everything except 128's, when they're aligned
 108          properly.
 109
 110          3) But wait!  Aren't we looking for C, not zero?
 111          Good point.  So what we do is XOR LONGWORD with a longword,
 112          each of whose bytes is C.  This turns each byte that is C
 113          into a zero.  */
 114
 115       longword = *longword_ptr++ ^ charmask;
 116
 117       /* Add MAGIC_BITS to LONGWORD.  */
 118       if ((((longword + magic_bits)
 119
 120             /* Set those bits that were unchanged by the addition.  */
 121             ^ ~longword)
 122
 123            /* Look at only the hole bits.  If any of the hole bits
 124               are unchanged, most likely one of the bytes was a
 125               zero.  */
 126            & ~magic_bits) != 0)
 127         {
 128           /* Which of the bytes was C?  If none of them were, it was
 129              a misfire; continue the search.  */
 130
 131           CONST unsigned char *cp = (CONST unsigned char *) (longword_ptr - 1);
 132
 133           if (cp[0] == c)
 134             return (PTR) cp;
 135           if (cp[1] == c)
 136             return (PTR) &cp[1];
 137           if (cp[2] == c)
 138             return (PTR) &cp[2];
 139           if (cp[3] == c)
 140             return (PTR) &cp[3];
 141           if (sizeof (longword) > 4)
 142             {
 143               if (cp[4] == c)
 144                 return (PTR) &cp[4];
 145               if (cp[5] == c)
 146                 return (PTR) &cp[5];
 147               if (cp[6] == c)
 148                 return (PTR) &cp[6];
 149               if (cp[7] == c)
 150                 return (PTR) &cp[7];
 151             }
 152         }
 153
 154       n -= sizeof (longword);
 155     }
 156
 157   char_ptr = (CONST unsigned char *) longword_ptr;
 158
 159   while (n-- > 0)
 160     {
 161       if (*char_ptr == c)
 162         return (PTR) char_ptr;
 163       else
 164         ++char_ptr;
 165     }
 166
 167   return NULL;
 168 }