svn commit: r243809 - stable/9/sys/libkern

[Xin LI]

Author: delphij
Date: Mon Dec  3 18:01:53 2012
New Revision: 243809
URL: http://svnweb.freebsd.org/changeset/base/243809

Log:
  MFC r242506:
  
  Sync strlen with userland implementation.

Modified:
  stable/9/sys/libkern/strlen.c
Directory Properties:
  stable/9/sys/   (props changed)

Modified: stable/9/sys/libkern/strlen.c
==============================================================================
--- stable/9/sys/libkern/strlen.c	Mon Dec  3 05:03:33 2012	(r243808)
+++ stable/9/sys/libkern/strlen.c	Mon Dec  3 18:01:53 2012	(r243809)
@@ -1,6 +1,6 @@
 /*-
- * Copyright (c) 1990, 1993
- *	The Regents of the University of California.  All rights reserved.
+ * Copyright (c) 2009, 2010 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)
@@ -31,14 +28,102 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/libkern.h>
+#include <sys/limits.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.
+ *
+ * On multi-issue processors, we can divide the above expression into:
+ *	a)  (x - 0x01....01)
+ *	b) (~x & 0x80....80)
+ *	c) a & b
+ *
+ * Where, a) and b) can be partially computed in parallel.
+ *
+ * The algorithm above is found on "Hacker's Delight" by
+ * Henry S. Warren, Jr.
+ */
+
+/* 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)
 {
-	register const char *s;
+	const char *p;
+	const unsigned long *lp;
+	long va, vb;
 
-	for (s = str; *s; ++s);
-	return(s - str);
-}
+	/*
+	 * Before trying the hard (unaligned byte-by-byte access) way
+	 * to figure out whether there is a nul character, try to see
+	 * if there is a nul character is within this accessible word
+	 * first.
+	 *
+	 * p and (p & ~LONGPTR_MASK) must be equally accessible since
+	 * they always fall in the same memory page, as long as page
+	 * boundaries is integral multiple of word size.
+	 */
+	lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
+	va = (*lp - mask01);
+	vb = ((~*lp) & mask80);
+	lp++;
+	if (va & vb)
+		/* Check if we have \0 in the first part */
+		for (p = str; p < (const char *)lp; p++)
+			if (*p == '\0')
+				return (p - str);
 
+	/* Scan the rest of the string using word sized operation */
+	for (; ; lp++) {
+		va = (*lp - mask01);
+		vb = ((~*lp) & mask80);
+		if (va & vb) {
+			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);
+}