string/strchr.c

   1 /* Copyright (C) 1991,1993-1997,1999,2000,2003,2006
   2    Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   5    with help from Dan Sahlin (dan@sics.se) and
   6    bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
   7    adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   8    and implemented by Roland McGrath (roland@ai.mit.edu).
   9
  10    The GNU C Library is free software; you can redistribute it and/or
  11    modify it under the terms of the GNU Lesser General Public
  12    License as published by the Free Software Foundation; either
  13    version 2.1 of the License, or (at your option) any later version.
  14
  15    The GNU C Library is distributed in the hope that it will be useful,
  16    but WITHOUT ANY WARRANTY; without even the implied warranty of
  17    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  18    Lesser General Public License for more details.
  19
  20    You should have received a copy of the GNU Lesser General Public
  21    License along with the GNU C Library; if not, write to the Free
  22    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  23    02111-1307 USA.  */
  24
  25 #include <string.h>
  26 #include <memcopy.h>
  27 #include <stdlib.h>
  28
  29 #undef strchr
  30
  31 /* Find the first occurrence of C in S.  */
  32 char *
  33 strchr (s, c_in)
  34      const char *s;
  35      int c_in;
  36 {
  37   const unsigned char *char_ptr;
  38   const unsigned long int *longword_ptr;
  39   unsigned long int longword, magic_bits, charmask;
  40   unsigned reg_char c;
  41
  42   c = (unsigned char) c_in;
  43
  44   /* Handle the first few characters by reading one character at a time.
  45      Do this until CHAR_PTR is aligned on a longword boundary.  */
  46   for (char_ptr = (const unsigned char *) s;
  47        ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
  48        ++char_ptr)
  49     if (*char_ptr == c)
  50       return (void *) char_ptr;
  51     else if (*char_ptr == '\0')
  52       return NULL;
  53
  54   /* All these elucidatory comments refer to 4-byte longwords,
  55      but the theory applies equally well to 8-byte longwords.  */
  56
  57   longword_ptr = (unsigned long int *) char_ptr;
  58
  59   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  60      the "holes."  Note that there is a hole just to the left of
  61      each byte, with an extra at the end:
  62
  63      bits:  01111110 11111110 11111110 11111111
  64      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  65
  66      The 1-bits make sure that carries propagate to the next 0-bit.
  67      The 0-bits provide holes for carries to fall into.  */
  68   switch (sizeof (longword))
  69     {
  70     case 4: magic_bits = 0x7efefeffL; break;
  71     case 8: magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; break;
  72     default:
  73       abort ();
  74     }
  75
  76   /* Set up a longword, each of whose bytes is C.  */
  77   charmask = c | (c << 8);
  78   charmask |= charmask << 16;
  79   if (sizeof (longword) > 4)
  80     /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
  81     charmask |= (charmask << 16) << 16;
  82   if (sizeof (longword) > 8)
  83     abort ();
  84
  85   /* Instead of the traditional loop which tests each character,
  86      we will test a longword at a time.  The tricky part is testing
  87      if *any of the four* bytes in the longword in question are zero.  */
  88   for (;;)
  89     {
  90       /* We tentatively exit the loop if adding MAGIC_BITS to
  91          LONGWORD fails to change any of the hole bits of LONGWORD.
  92
  93          1) Is this safe?  Will it catch all the zero bytes?
  94          Suppose there is a byte with all zeros.  Any carry bits
  95          propagating from its left will fall into the hole at its
  96          least significant bit and stop.  Since there will be no
  97          carry from its most significant bit, the LSB of the
  98          byte to the left will be unchanged, and the zero will be
  99          detected.
 100
 101          2) Is this worthwhile?  Will it ignore everything except
 102          zero bytes?  Suppose every byte of LONGWORD has a bit set
 103          somewhere.  There will be a carry into bit 8.  If bit 8
 104          is set, this will carry into bit 16.  If bit 8 is clear,
 105          one of bits 9-15 must be set, so there will be a carry
 106          into bit 16.  Similarly, there will be a carry into bit
 107          24.  If one of bits 24-30 is set, there will be a carry
 108          into bit 31, so all of the hole bits will be changed.
 109
 110          The one misfire occurs when bits 24-30 are clear and bit
 111          31 is set; in this case, the hole at bit 31 is not
 112          changed.  If we had access to the processor carry flag,
 113          we could close this loophole by putting the fourth hole
 114          at bit 32!
 115
 116          So it ignores everything except 128's, when they're aligned
 117          properly.
 118
 119          3) But wait!  Aren't we looking for C as well as zero?
 120          Good point.  So what we do is XOR LONGWORD with a longword,
 121          each of whose bytes is C.  This turns each byte that is C
 122          into a zero.  */
 123
 124       longword = *longword_ptr++;
 125
 126       /* Add MAGIC_BITS to LONGWORD.  */
 127       if ((((longword + magic_bits)
 128
 129             /* Set those bits that were unchanged by the addition.  */
 130             ^ ~longword)
 131
 132            /* Look at only the hole bits.  If any of the hole bits
 133               are unchanged, most likely one of the bytes was a
 134               zero.  */
 135            & ~magic_bits) != 0 ||
 136
 137           /* That caught zeroes.  Now test for C.  */
 138           ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))
 139            & ~magic_bits) != 0)
 140         {
 141           /* Which of the bytes was C or zero?
 142              If none of them were, it was a misfire; continue the search.  */
 143
 144           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 145
 146           if (*cp == c)
 147             return (char *) cp;
 148           else if (*cp == '\0')
 149             return NULL;
 150           if (*++cp == c)
 151             return (char *) cp;
 152           else if (*cp == '\0')
 153             return NULL;
 154           if (*++cp == c)
 155             return (char *) cp;
 156           else if (*cp == '\0')
 157             return NULL;
 158           if (*++cp == c)
 159             return (char *) cp;
 160           else if (*cp == '\0')
 161             return NULL;
 162           if (sizeof (longword) > 4)
 163             {
 164               if (*++cp == c)
 165                 return (char *) cp;
 166               else if (*cp == '\0')
 167                 return NULL;
 168               if (*++cp == c)
 169                 return (char *) cp;
 170               else if (*cp == '\0')
 171                 return NULL;
 172               if (*++cp == c)
 173                 return (char *) cp;
 174               else if (*cp == '\0')
 175                 return NULL;
 176               if (*++cp == c)
 177                 return (char *) cp;
 178               else if (*cp == '\0')
 179                 return NULL;
 180             }
 181         }
 182     }
 183
 184   return NULL;
 185 }
 186
 187 #ifdef weak_alias
 188 #undef index
 189 weak_alias (strchr, index)
 190 #endif
 191 libc_hidden_builtin_def (strchr)