strlen.c

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/*-
 * Copyright (c) 2009 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 <limits.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 fast as the raw implementation on
 * Intel T7300 under long 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 ((uintptr_t)p - (uintptr_t)str + x); \
    } while(0)

size_t strlen(const char* str)
{
    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 ((uintptr_t)p - (uintptr_t)str);
        }
    }

    /* Scan the rest of the string using word sized operation */
    // Cast to void to prevent alignment warning
    for(lp = (const unsigned long*)(const void*)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);
}