1999-10-04 Roland McGrath <roland@baalperazim.frob.com>

* sysdeps/generic/memrchr.c: New file, adapted from memchr.c.
	* sysdeps/i386/bits/string.h (__memrchr): New function.
	* string/string.h: Declare memrchr, __memrchr.
	* manual/string.texi (Search Functions): Document memrchr.

manual/string.texi

@@ -1062,6 +1062,13 @@ object beginning at @var{block}.  The return value is a pointer to the
 located byte, or a null pointer if no match was found.
 @end deftypefun
 
+@comment string.h
+@comment GNU
+@deftypefun {void *} memrchr (const void *@var{block}, int @var{c}, size_t @var{size})
+The function @code{memrchr} is like @code{memchr}, except that it searches
+backwards from the end of the block defined by @var{block} and @var{size}
+(instead of forwards from the front).
+
 @comment string.h
 @comment ISO
 @deftypefun {char *} strchr (const char *@var{string}, int @var{c})

string/string.h

@@ -64,6 +64,10 @@ extern __ptr_t memchr __P ((__const __ptr_t __s, int __c, size_t __n));
 /* Search in S for C.  This is similar to `memchr' but there is no
    length limit.  */
 extern __ptr_t rawmemchr __P ((__const __ptr_t __s, int __c));
+
+/* Search N bytes of S for the final occurrence of C.  */
+extern __ptr_t __memrchr __P ((__const __ptr_t __s, int __c, size_t __n));
+extern __ptr_t memrchr __P ((__const __ptr_t __s, int __c, size_t __n));
 #endif
 
 

sysdeps/generic/memrchr.c

@@ -0,0 +1,201 @@
+/* memrchr -- find the last occurrence of a byte in a memory block
+   Copyright (C) 1991, 1993, 1996, 1997, 1999 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.  */
+
+#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 memchr
+
+
+/* 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 (cp[0] == c)
+	    return (__ptr_t) cp;
+	  if (cp[1] == c)
+	    return (__ptr_t) &cp[1];
+	  if (cp[2] == c)
+	    return (__ptr_t) &cp[2];
+	  if (cp[3] == c)
+	    return (__ptr_t) &cp[3];
+#if LONG_MAX > 2147483647
+	  if (cp[4] == c)
+	    return (__ptr_t) &cp[4];
+	  if (cp[5] == c)
+	    return (__ptr_t) &cp[5];
+	  if (cp[6] == c)
+	    return (__ptr_t) &cp[6];
+	  if (cp[7] == c)
+	    return (__ptr_t) &cp[7];
+#endif
+	}
+
+      n -= sizeof (longword);
+    }
+
+  char_ptr = (const unsigned char *) longword_ptr;
+
+  while (n-- > 0)
+    {
+      if (*--char_ptr == c)
+	return (__ptr_t) char_ptr;
+    }
+
+  return 0;
+}
+weak_alias (__memrchr, memrchr)

sysdeps/i386/bits/string.h

@@ -305,6 +305,27 @@ memchr (__const void *__s, int __c, size_t __n)
 }
 #endif
 
+#define _HAVE_STRING_ARCH_memrchr 1
+#ifndef _FORCE_INLINES
+__STRING_INLINE void *
+__memrchr (__const void *__s, int __c, size_t __n)
+{
+  register unsigned long int __d0;
+  register void *__res;
+  if (__n == 0)
+    return NULL;
+  __asm__ __volatile__
+    ("std\n\t"
+     "repne; scasb\n\t"
+     "je 1f\n\t"
+     "movl $1,%0\n"
+     "1:"
+     : "=D" (__res), "=&c" (__d0)
+     : "a" (__c), "0" (__s), "1" (__n));
+  return __res - 1;
+}
+#endif
+
 /* Return the length of S.  */
 #define _HAVE_STRING_ARCH_strlen 1
 #ifndef _FORCE_INLINES