150 lines
5.0 KiB
C
150 lines
5.0 KiB
C
/* Copyright (C) 1991 Free Software Foundation, Inc.
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Based on strlen implemention by Torbjorn Granlund (tege@sics.se),
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with help from Dan Sahlin (dan@sics.se) and
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commentary by Jim Blandy (jimb@ai.mit.edu);
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adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
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and implemented by Roland McGrath (roland@ai.mit.edu).
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Library General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Library General Public License for more details.
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You should have received a copy of the GNU Library General Public
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License along with the GNU C Library; see the file COPYING.LIB. If
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not, write to the Free Software Foundation, Inc., 675 Mass Ave,
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Cambridge, MA 02139, USA.
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This file was modified slightly by Ian Lance Taylor, May 1992, for
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Taylor UUCP. It assumes 32 bit longs. I'm willing to trust that any
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system which does not have 32 bit longs will have its own
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implementation of memchr. */
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#include "uucp.h"
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/* Search no more than N bytes of S for C. */
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pointer
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memchr (s, c, n)
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constpointer s;
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int c;
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size_t n;
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{
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const char *char_ptr;
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const unsigned long int *longword_ptr;
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unsigned long int longword, magic_bits, charmask;
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c = BUCHAR (c);
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/* Handle the first few characters by reading one character at a time.
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Do this until CHAR_PTR is aligned on a 4-byte border. */
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for (char_ptr = s; n > 0 && ((unsigned long int) char_ptr & 3) != 0;
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--n, ++char_ptr)
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if (BUCHAR (*char_ptr) == c)
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return (pointer) char_ptr;
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longword_ptr = (unsigned long int *) char_ptr;
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/* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
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the "holes." Note that there is a hole just to the left of
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each byte, with an extra at the end:
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bits: 01111110 11111110 11111110 11111111
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bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
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The 1-bits make sure that carries propagate to the next 0-bit.
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The 0-bits provide holes for carries to fall into. */
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magic_bits = 0x7efefeff;
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/* Set up a longword, each of whose bytes is C. */
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charmask = c | (c << 8);
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charmask |= charmask << 16;
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/* Instead of the traditional loop which tests each character,
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we will test a longword at a time. The tricky part is testing
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if *any of the four* bytes in the longword in question are zero. */
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while (n >= 4)
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{
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/* We tentatively exit the loop if adding MAGIC_BITS to
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LONGWORD fails to change any of the hole bits of LONGWORD.
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1) Is this safe? Will it catch all the zero bytes?
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Suppose there is a byte with all zeros. Any carry bits
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propagating from its left will fall into the hole at its
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least significant bit and stop. Since there will be no
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carry from its most significant bit, the LSB of the
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byte to the left will be unchanged, and the zero will be
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detected.
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2) Is this worthwhile? Will it ignore everything except
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zero bytes? Suppose every byte of LONGWORD has a bit set
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somewhere. There will be a carry into bit 8. If bit 8
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is set, this will carry into bit 16. If bit 8 is clear,
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one of bits 9-15 must be set, so there will be a carry
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into bit 16. Similarly, there will be a carry into bit
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24. If one of bits 24-30 is set, there will be a carry
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into bit 31, so all of the hole bits will be changed.
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The one misfire occurs when bits 24-30 are clear and bit
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31 is set; in this case, the hole at bit 31 is not
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changed. If we had access to the processor carry flag,
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we could close this loophole by putting the fourth hole
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at bit 32!
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So it ignores everything except 128's, when they're aligned
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properly.
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3) But wait! Aren't we looking for C, not zero?
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Good point. So what we do is XOR LONGWORD with a longword,
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each of whose bytes is C. This turns each byte that is C
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into a zero. */
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longword = *longword_ptr++ ^ charmask;
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/* Add MAGIC_BITS to LONGWORD. */
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if ((((longword + magic_bits)
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/* Set those bits that were unchanged by the addition. */
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^ ~longword)
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/* Look at only the hole bits. If any of the hole bits
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are unchanged, most likely one of the bytes was a
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zero. */
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& ~magic_bits) != 0)
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{
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/* Which of the bytes was C? If none of them were, it was
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a misfire; continue the search. */
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const char *cp = (const char *) (longword_ptr - 1);
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if (BUCHAR (cp[0]) == c)
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return (pointer) cp;
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if (BUCHAR (cp[1]) == c)
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return (pointer) &cp[1];
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if (BUCHAR (cp[2]) == c)
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return (pointer) &cp[2];
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if (BUCHAR (cp[3]) == c)
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return (pointer) &cp[3];
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}
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n -= 4;
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}
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char_ptr = (const char *) longword_ptr;
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while (n-- > 0)
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{
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if (BUCHAR (*char_ptr) == c)
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return (pointer) char_ptr;
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else
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++char_ptr;
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}
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return NULL;
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}
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