svn commit: r187700 - head/lib/libc/string

[Xin LI]

Author: delphij
Date: Sun Jan 25 23:08:47 2009
New Revision: 187700
URL: http://svn.freebsd.org/changeset/base/187700

Log:
  Rewrite of MI strlen(3) in a way that can better utilize modern hardware by
  reducing branches and doing word-sized operation.
  
  The idea is taken from J.T. Conklin's x86_64 optimized version of strlen(3)
  for NetBSD, and reimplemented in C by me.
  
  Discussed on:	-arch@

Modified:
  head/lib/libc/string/strlen.c

Modified: head/lib/libc/string/strlen.c
==============================================================================
--- head/lib/libc/string/strlen.c	Sun Jan 25 21:27:31 2009	(r187699)
+++ head/lib/libc/string/strlen.c	Sun Jan 25 23:08:47 2009	(r187700)
@@ -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 +24,87 @@
  * 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>
 
+/*
+ * 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 for strings longer than word length.
+ */
+
+/* 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;
+#else
+#error Unsupported word size
+#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;
+
+	/* 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
+	    }
 
-	for (s = str; *s; ++s);
-	return(s - str);
+	/* NOTREACHED */
+	return 0;
 }