HelenOS sources

root/kernel/genarch/src/softint/multiplication.c

DEFINITIONS

1. mul
2. __muldi3

/*
 * Copyright (c) 2009 Josef Cejka
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - 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.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 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.
 */

/** @addtogroup kernel_genarch
 * @{
 */
/**
 * @file
 * SW implementation of 32 and 64 bit multiplication.
 */

#include <genarch/softint/multiplication.h>

/** Set 1 to return MAX_INT64 or MIN_INT64 on overflow */
#ifndef SOFTINT_CHECK_OF
# define SOFTINT_CHECK_OF 0
#endif

#define MAX_UINT16 (0xFFFFu)
#define MAX_UINT32 (0xFFFFFFFFu)
#define MAX_INT64 (0x7FFFFFFFFFFFFFFFll)
#define MIN_INT64 (0x8000000000000000ll)

/**
 * Multiply two integers and return long long as result.
 * This function is overflow safe.
 * @param a
 * @param b
 * @result
 */
static unsigned long long mul(unsigned int a, unsigned int b)
{
        unsigned int a1, a2, b1, b2;
        unsigned long long t1, t2, t3;

        a1 = a >> 16;
        a2 = a & MAX_UINT16;
        b1 = b >> 16;
        b2 = b & MAX_UINT16;

        t1 = a1 * b1;
        t2 = a1 * b2;
        t2 += a2 * b1;
        t3 = a2 * b2;

        t3 = (((t1 << 16) + t2) << 16) + t3;

        return t3;
}

/** Emulate multiplication of two 64-bit long long integers.
 *
 */
long long __muldi3 (long long a, long long b)
{
        long long result;
        unsigned long long t1, t2;
        unsigned long long a1, a2, b1, b2;
        char neg = 0;

        if (a < 0) {
                neg = !neg;
                a = -a;
        }

        if (b < 0) {
                neg = !neg;
                b = -b;
        }

        a1 = a >> 32;
        b1 = b >> 32;

        a2 = a & (MAX_UINT32);
        b2 = b & (MAX_UINT32);

        if (SOFTINT_CHECK_OF && (a1 != 0) && (b1 != 0)) {
                // error, overflow
                return (neg ? MIN_INT64 : MAX_INT64);
        }

        // (if OF checked) a1 or b1 is zero => result fits in 64 bits, no need to another overflow check
        t1 = mul(a1, b2) + mul(b1, a2);

        if (SOFTINT_CHECK_OF && t1 > MAX_UINT32) {
                // error, overflow
                return (neg ? MIN_INT64 : MAX_INT64);
        }

        t1 = t1 << 32;
        t2 = mul(a2, b2);
        t2 += t1;

        /*
         * t2 & (1ull << 63) - if this bit is set in unsigned long long,
         * result does not fit in signed one
         */
        if (SOFTINT_CHECK_OF && ((t2 < t1) || (t2 & (1ull << 63)))) {
                // error, overflow
                return (neg ? MIN_INT64 : MAX_INT64);
        }

        result = t2;

        if (neg) {
                result = -result;
        }

        return result;
}

/** @}
 */