multiply.cc

来自「一个mips虚拟机非常好代码,使用C++来编写的,希望大家多学学,」· CC 代码 · 共 90 行

#include "inttypes.hh"


// Perform long multiplication of x by y.

template <> MulResult<UInt64>
multiply(UInt64 u, UInt64 v)
{
    UInt64 u0 = bits(u, 31,  0);
    UInt64 u1 = bits(u, 63, 32);

    UInt64 v0 = bits(v, 31,  0);
    UInt64 v1 = bits(v, 63, 32);

    UInt64 w0 = 0;
    UInt64 w1 = 0;
    UInt64 w2;
    UInt64 w3;

    UInt64 t, k;

    k = 0;

    // i == 0, j == 0
    t  = u0 * v0 + w0 + k;
    w0 = bits(t, 31,  0);
    k  = bits(t, 63, 32);

    // i == 1, j == 0
    t  = u1 * v0 + w1 + k;
    w1 = bits(t, 31,  0);
    k  = bits(t, 63, 32);

    w2 = k;
    k  = 0;

    // i == 0, j == 1
    t  = u0 * v1 + w1 + k;
    w1 = bits(t, 31,  0);
    k  = bits(t, 63, 32);

    // j == 1, i == 1
    t  = u1 * v1 + w2 + k;
    w2 = bits(t, 31,  0);
    k  = bits(t, 63, 32);

    w3 = k;

    // Glue the bits back into full words.
    UInt64 lo = w0 | (w1 << 32);
    UInt64 hi = w2 | (w3 << 32);

    return MulResult<UInt64>(hi, lo);
}

template <> MulResult<Int64>
multiply(Int64 u, Int64 v)
{
    // Compute the sign of the result;
    bool neg;
    if (u < 0) {
	if (v < 0) {
	    neg = false;
	    v = -v;
	}
	else {
	    neg = true;
	}
	u = -u;
    }
    else {
	if (v < 0) {
	    neg = true;
	    v = -v;
	}
	else {
	    neg = false;
	}
    }
    MulResult<UInt64> ret = multiply((UInt64)u, (UInt64)v);
    if (neg) {
	ret.lo = ~ret.lo;
	ret.hi = ~ret.hi;
	if (++ret.lo == 0) {
	    ++ret.hi;
	}
    }
    return MulResult<Int64>(ret.hi, ret.lo);
}