--- /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.
+ 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 Library General Public License as
+ published by the Free Software Foundation; either version 2 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
+ Library General Public License for more details.
+
+ You should have received a copy of the GNU Library General Public
+ License along with the GNU C Library; see the file COPYING.LIB. 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 = (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
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