mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-05 03:01:05 +00:00
41bdb6e20c
2001-07-06 Paul Eggert <eggert@twinsun.com> * manual/argp.texi: Remove ignored LGPL copyright notice; it's not appropriate for documentation anyway. * manual/libc-texinfo.sh: "Library General Public License" -> "Lesser General Public License". 2001-07-06 Andreas Jaeger <aj@suse.de> * All files under GPL/LGPL version 2: Place under LGPL version 2.1.
211 lines
6.3 KiB
C
211 lines
6.3 KiB
C
/* 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
|
|
|
|
#include <sys/types.h>
|
|
|
|
#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
|