+++ /dev/null
-/* memrchr -- find the last occurrence of a byte in a memory block
- Copyright (C) 1991, 93, 96, 97, 99, 2000 Free Software Foundation, Inc.
- This file is part of the GNU C Library.
- Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
- with help from Dan Sahlin (dan@sics.se) and
- commentary by Jim Blandy (jimb@ai.mit.edu);
- adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
- and implemented by Roland McGrath (roland@ai.mit.edu).
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library; if not, write to the Free
- Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
- 02111-1307 USA. */
-
-#include <stdlib.h>
-
-#ifdef HAVE_CONFIG_H
-# include <config.h>
-#endif
-
-#undef __ptr_t
-#if defined __cplusplus || (defined __STDC__ && __STDC__)
-# define __ptr_t void *
-#else /* Not C++ or ANSI C. */
-# define __ptr_t char *
-#endif /* C++ or ANSI C. */
-
-#if defined _LIBC
-# include <string.h>
-# include <memcopy.h>
-#else
-# define reg_char char
-#endif
-
-#if defined HAVE_LIMITS_H || defined _LIBC
-# include <limits.h>
-#endif
-
-#define LONG_MAX_32_BITS 2147483647
-
-#ifndef LONG_MAX
-# define LONG_MAX LONG_MAX_32_BITS
-#endif
-
-#ifdef HAVE_SYS_TYPES_H
-# include <sys/types.h>
-#endif
-
-#undef __memrchr
-#undef memrchr
-
-#ifndef weak_alias
-# define __memrchr memrchr
-#endif
-
-/* Search no more than N bytes of S for C. */
-__ptr_t
-__memrchr (s, c_in, n)
- const __ptr_t s;
- int c_in;
- size_t n;
-{
- const unsigned char *char_ptr;
- const unsigned long int *longword_ptr;
- unsigned long int longword, magic_bits, charmask;
- unsigned reg_char c;
-
- c = (unsigned char) c_in;
-
- /* Handle the last few characters by reading one character at a time.
- Do this until CHAR_PTR is aligned on a longword boundary. */
- for (char_ptr = (const unsigned char *) s + n;
- n > 0 && ((unsigned long int) char_ptr
- & (sizeof (longword) - 1)) != 0;
- --n)
- if (*--char_ptr == c)
- return (__ptr_t) char_ptr;
-
- /* All these elucidatory comments refer to 4-byte longwords,
- but the theory applies equally well to 8-byte longwords. */
-
- longword_ptr = (const unsigned long int *) char_ptr;
-
- /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
- the "holes." Note that there is a hole just to the left of
- each byte, with an extra at the end:
-
- bits: 01111110 11111110 11111110 11111111
- bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
-
- The 1-bits make sure that carries propagate to the next 0-bit.
- The 0-bits provide holes for carries to fall into. */
-
- if (sizeof (longword) != 4 && sizeof (longword) != 8)
- abort ();
-
-#if LONG_MAX <= LONG_MAX_32_BITS
- magic_bits = 0x7efefeff;
-#else
- magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
-#endif
-
- /* Set up a longword, each of whose bytes is C. */
- charmask = c | (c << 8);
- charmask |= charmask << 16;
-#if LONG_MAX > LONG_MAX_32_BITS
- charmask |= charmask << 32;
-#endif
-
- /* Instead of the traditional loop which tests each character,
- we will test a longword at a time. The tricky part is testing
- if *any of the four* bytes in the longword in question are zero. */
- while (n >= sizeof (longword))
- {
- /* We tentatively exit the loop if adding MAGIC_BITS to
- LONGWORD fails to change any of the hole bits of LONGWORD.
-
- 1) Is this safe? Will it catch all the zero bytes?
- Suppose there is a byte with all zeros. Any carry bits
- propagating from its left will fall into the hole at its
- least significant bit and stop. Since there will be no
- carry from its most significant bit, the LSB of the
- byte to the left will be unchanged, and the zero will be
- detected.
-
- 2) Is this worthwhile? Will it ignore everything except
- zero bytes? Suppose every byte of LONGWORD has a bit set
- somewhere. There will be a carry into bit 8. If bit 8
- is set, this will carry into bit 16. If bit 8 is clear,
- one of bits 9-15 must be set, so there will be a carry
- into bit 16. Similarly, there will be a carry into bit
- 24. If one of bits 24-30 is set, there will be a carry
- into bit 31, so all of the hole bits will be changed.
-
- The one misfire occurs when bits 24-30 are clear and bit
- 31 is set; in this case, the hole at bit 31 is not
- changed. If we had access to the processor carry flag,
- we could close this loophole by putting the fourth hole
- at bit 32!
-
- So it ignores everything except 128's, when they're aligned
- properly.
-
- 3) But wait! Aren't we looking for C, not zero?
- Good point. So what we do is XOR LONGWORD with a longword,
- each of whose bytes is C. This turns each byte that is C
- into a zero. */
-
- longword = *--longword_ptr ^ charmask;
-
- /* Add MAGIC_BITS to LONGWORD. */
- if ((((longword + magic_bits)
-
- /* Set those bits that were unchanged by the addition. */
- ^ ~longword)
-
- /* Look at only the hole bits. If any of the hole bits
- are unchanged, most likely one of the bytes was a
- zero. */
- & ~magic_bits) != 0)
- {
- /* Which of the bytes was C? If none of them were, it was
- a misfire; continue the search. */
-
- const unsigned char *cp = (const unsigned char *) longword_ptr;
-
-#if LONG_MAX > 2147483647
- if (cp[7] == c)
- return (__ptr_t) &cp[7];
- if (cp[6] == c)
- return (__ptr_t) &cp[6];
- if (cp[5] == c)
- return (__ptr_t) &cp[5];
- if (cp[4] == c)
- return (__ptr_t) &cp[4];
-#endif
- if (cp[3] == c)
- return (__ptr_t) &cp[3];
- if (cp[2] == c)
- return (__ptr_t) &cp[2];
- if (cp[1] == c)
- return (__ptr_t) &cp[1];
- if (cp[0] == c)
- return (__ptr_t) cp;
- }
-
- n -= sizeof (longword);
- }
-
- char_ptr = (const unsigned char *) longword_ptr;
-
- while (n-- > 0)
- {
- if (*--char_ptr == c)
- return (__ptr_t) char_ptr;
- }
-
- return 0;
-}
-#ifdef weak_alias
-weak_alias (__memrchr, memrchr)
-#endif
projects / gpgme.git / commitdiff