sysdeps/generic/rawmemchr.c

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