RFC: MI strlen()

[Xin LI]

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Hi,

Here is a new implementation of strlen() which employed the bitmask
skill in order to achieve better performance on modern hardware.  For
common case, this would be a 5.2x boost on FreeBSD/amd64.  The code is
intended for MI use when there is no hand-optimized assembly.

	http://people.freebsd.org/~delphij/for_review/strlen.diff

Note that this version of strlen() has suboptimal performance if there
are a lot of characters that has their highest bit set (we can change it
to have uniform performance at the expense of about ~30% performance
penalty).

Comments?

Cheers,
- --
Xin LI <delphij at delphij.net>	http://www.delphij.net/
FreeBSD - The Power to Serve!
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Index: strlen.c
===================================================================
--- strlen.c	(revision 186910)
+++ strlen.c	(working copy)
@@ -1,6 +1,6 @@
 /*-
- * Copyright (c) 1990, 1993
- *	The Regents of the University of California.  All rights reserved.
+ * Copyright (c) 2009 Xin LI <delphij at FreeBSD.org>
+ * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -10,14 +10,11 @@
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
- * 4. Neither the name of the University nor the names of its contributors
- *    may be used to endorse or promote products derived from this software
- *    without specific prior written permission.
  *
- * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
@@ -27,21 +24,93 @@
  * SUCH DAMAGE.
  */
 
-#if defined(LIBC_SCCS) && !defined(lint)
-static char sccsid[] = "@(#)strlen.c	8.1 (Berkeley) 6/4/93";
-#endif /* LIBC_SCCS and not lint */
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <sys/limits.h>
+#include <sys/types.h>
 #include <string.h>
 
+#ifndef CTASSERT
+#define CTASSERT(x)		_CTASSERT(x, __LINE__)
+#define _CTASSERT(x, y)		__CTASSERT(x, y)
+#define __CTASSERT(x, y)	typedef char __assert_ ## y [(x) ? 1 : -1]
+#endif
+
+CTASSERT(LONG_BIT == 32 || LONG_BIT == 64);
+
+/*
+ * Portable strlen() for 32-bit and 64-bit systems.
+ *
+ * Rationale: it is generally much more efficient to do word length
+ * operations and avoid branches on modern computer systems, as
+ * compared to byte-length operations with a lot of branches.
+ *
+ * The expression:
+ *
+ *	((x - 0x01....01) & ~x & 0x80....80)
+ *
+ * would evaluate to a non-zero value iff any of the bytes in the
+ * original word is zero.  However, we can further reduce ~1/3 of
+ * time if we consider that strlen() usually operate on 7-bit ASCII
+ * by employing the following expression, which allows false positive
+ * when high bit of 1 and use the tail case to catch these case:
+ *
+ *	((x - 0x01....01) & 0x80....80)
+ *
+ * This is more than 5.2 times as compared to the raw implementation
+ * on Intel T7300 under EM64T mode.
+ */
+
+/* Magic numbers for the algorithm */
+#if LONG_BIT == 32
+static const unsigned long mask01 = 0x01010101;
+static const unsigned long mask80 = 0x80808080;
+#elif LONG_BIT == 64
+static const unsigned long mask01 = 0x0101010101010101;
+static const unsigned long mask80 = 0x8080808080808080;
+#endif
+
+#define	LONGPTR_MASK (sizeof(long) - 1)
+
+/*
+ * Helper macro to return string length if we caught the zero
+ * byte.
+ */
+#define testbyte(x)				\
+	do {					\
+		if (p[x] == '\0')		\
+		    return (p - str + x);	\
+	} while (0);
+
 size_t
-strlen(str)
-	const char *str;
+strlen(const char *str)
 {
-	const char *s;
+	const char *p;
+	const unsigned long *lp;
 
-	for (s = str; *s; ++s);
-	return(s - str);
+	/* Skip the first few bytes until we have an aligned p */
+	for (p = str; (uintptr_t)p & LONGPTR_MASK; ++p)
+	    if (*p == 0)
+		return (p - str);
+
+	/* Scan the rest of the string using word sized operation */
+	for (lp = (const unsigned long *)p; ; lp++)
+	    if ((*lp - mask01) & mask80) {
+		p = (const char *)(lp);
+		testbyte(0);
+		testbyte(1);
+		testbyte(2);
+		testbyte(3);
+#if (LONG_BIT >= 64)
+		testbyte(4);
+		testbyte(5);
+		testbyte(6);
+		testbyte(7);
+#endif
+	    }
+
+	/* NOTREACHED */
+	return 0;
 }