xref: /freebsd-10.0-release/sys/libkern/strlen.c

/*-
 * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 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.
 *
 * 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 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)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
__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(const char *str)
{
	const char *p;
	const unsigned long *lp;
	long va, vb;

	/*
	 * 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);
}