strlen.c

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/*-
 * 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("$BSDSUniX$");

#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);
}