Annotation of gforth/engine/strchrnul.c, revision 1.1

1.1     ! anton       1: /* Copyright (C) 1991, 93, 94, 95, 96, 97, 99 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:    bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
        !             5:    adaptation to strchr 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: #include <string.h>
        !            24: 
        !            25: #undef __strchrnul
        !            26: #undef strchrnul
        !            27: 
        !            28: /* Find the first occurrence of C in S or the final NUL byte.  */
        !            29: char *
        !            30: __strchrnul (s, c)
        !            31:      const char *s;
        !            32:      int c;
        !            33: {
        !            34:   const unsigned char *char_ptr;
        !            35:   const unsigned long int *longword_ptr;
        !            36:   unsigned long int longword, magic_bits, charmask;
        !            37: 
        !            38:   c = (unsigned char) c;
        !            39: 
        !            40:   /* Handle the first few characters by reading one character at a time.
        !            41:      Do this until CHAR_PTR is aligned on a longword boundary.  */
        !            42:   for (char_ptr = s; ((unsigned long int) char_ptr
        !            43:                      & (sizeof (longword) - 1)) != 0;
        !            44:        ++char_ptr)
        !            45:     if (*char_ptr == c || *char_ptr == '\0')
        !            46:       return (void *) char_ptr;
        !            47: 
        !            48:   /* All these elucidatory comments refer to 4-byte longwords,
        !            49:      but the theory applies equally well to 8-byte longwords.  */
        !            50: 
        !            51:   longword_ptr = (unsigned long int *) char_ptr;
        !            52: 
        !            53:   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
        !            54:      the "holes."  Note that there is a hole just to the left of
        !            55:      each byte, with an extra at the end:
        !            56: 
        !            57:      bits:  01111110 11111110 11111110 11111111
        !            58:      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
        !            59: 
        !            60:      The 1-bits make sure that carries propagate to the next 0-bit.
        !            61:      The 0-bits provide holes for carries to fall into.  */
        !            62:   switch (sizeof (longword))
        !            63:     {
        !            64:     case 4: magic_bits = 0x7efefeffL; break;
        !            65:     case 8: magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; break;
        !            66:     default:
        !            67:       abort ();
        !            68:     }
        !            69: 
        !            70:   /* Set up a longword, each of whose bytes is C.  */
        !            71:   charmask = c | (c << 8);
        !            72:   charmask |= charmask << 16;
        !            73:   if (sizeof (longword) > 4)
        !            74:     /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
        !            75:     charmask |= (charmask << 16) << 16;
        !            76:   if (sizeof (longword) > 8)
        !            77:     abort ();
        !            78: 
        !            79:   /* Instead of the traditional loop which tests each character,
        !            80:      we will test a longword at a time.  The tricky part is testing
        !            81:      if *any of the four* bytes in the longword in question are zero.  */
        !            82:   for (;;)
        !            83:     {
        !            84:       /* We tentatively exit the loop if adding MAGIC_BITS to
        !            85:         LONGWORD fails to change any of the hole bits of LONGWORD.
        !            86: 
        !            87:         1) Is this safe?  Will it catch all the zero bytes?
        !            88:         Suppose there is a byte with all zeros.  Any carry bits
        !            89:         propagating from its left will fall into the hole at its
        !            90:         least significant bit and stop.  Since there will be no
        !            91:         carry from its most significant bit, the LSB of the
        !            92:         byte to the left will be unchanged, and the zero will be
        !            93:         detected.
        !            94: 
        !            95:         2) Is this worthwhile?  Will it ignore everything except
        !            96:         zero bytes?  Suppose every byte of LONGWORD has a bit set
        !            97:         somewhere.  There will be a carry into bit 8.  If bit 8
        !            98:         is set, this will carry into bit 16.  If bit 8 is clear,
        !            99:         one of bits 9-15 must be set, so there will be a carry
        !           100:         into bit 16.  Similarly, there will be a carry into bit
        !           101:         24.  If one of bits 24-30 is set, there will be a carry
        !           102:         into bit 31, so all of the hole bits will be changed.
        !           103: 
        !           104:         The one misfire occurs when bits 24-30 are clear and bit
        !           105:         31 is set; in this case, the hole at bit 31 is not
        !           106:         changed.  If we had access to the processor carry flag,
        !           107:         we could close this loophole by putting the fourth hole
        !           108:         at bit 32!
        !           109: 
        !           110:         So it ignores everything except 128's, when they're aligned
        !           111:         properly.
        !           112: 
        !           113:         3) But wait!  Aren't we looking for C as well as zero?
        !           114:         Good point.  So what we do is XOR LONGWORD with a longword,
        !           115:         each of whose bytes is C.  This turns each byte that is C
        !           116:         into a zero.  */
        !           117: 
        !           118:       longword = *longword_ptr++;
        !           119: 
        !           120:       /* Add MAGIC_BITS to LONGWORD.  */
        !           121:       if ((((longword + magic_bits)
        !           122: 
        !           123:            /* Set those bits that were unchanged by the addition.  */
        !           124:            ^ ~longword)
        !           125: 
        !           126:           /* Look at only the hole bits.  If any of the hole bits
        !           127:              are unchanged, most likely one of the bytes was a
        !           128:              zero.  */
        !           129:           & ~magic_bits) != 0 ||
        !           130: 
        !           131:          /* That caught zeroes.  Now test for C.  */
        !           132:          ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))
        !           133:           & ~magic_bits) != 0)
        !           134:        {
        !           135:          /* Which of the bytes was C or zero?
        !           136:             If none of them were, it was a misfire; continue the search.  */
        !           137: 
        !           138:          const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
        !           139: 
        !           140:          if (*cp == c || *cp == '\0')
        !           141:            return (char *) cp;
        !           142:          if (*++cp == c || *cp == '\0')
        !           143:            return (char *) cp;
        !           144:          if (*++cp == c || *cp == '\0')
        !           145:            return (char *) cp;
        !           146:          if (*++cp == c || *cp == '\0')
        !           147:            return (char *) cp;
        !           148:          if (sizeof (longword) > 4)
        !           149:            {
        !           150:              if (*++cp == c || *cp == '\0')
        !           151:                return (char *) cp;
        !           152:              if (*++cp == c || *cp == '\0')
        !           153:                return (char *) cp;
        !           154:              if (*++cp == c || *cp == '\0')
        !           155:                return (char *) cp;
        !           156:              if (*++cp == c || *cp == '\0')
        !           157:                return (char *) cp;
        !           158:            }
        !           159:        }
        !           160:     }
        !           161: 
        !           162:   /* This should never happen.  */
        !           163:   return NULL;
        !           164: }

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