import std;

​

struct m128 { ulong lo, hi; }

​

// REX.W + F7 /4    MUL r/m64   (RDX:RAX ← RAX ∗ r/m64).

// https://en.wikipedia.org/wiki/X86_calling_conventions#x86-64_calling_conventions

// Windows RCX, RDX, R8, R9. returns RAX only

// Linux RDI, RSI, RDX, RCX, R8, R9. returns RDX:RAX

m128 mul128( ulong a, ulong b ) pure nothrow @nogc @trusted {

    asm pure @nogc nothrow {

        naked;

        mov RAX, RDI;

        mul RSI;

        ret;

    }

}

// probably u can do it inline

​

void main()

{

    ulong a = 0x0102030405060708, b = 0x0101010101010101;

    auto r = mul128( a, b );

    "%016x %016x".writefln( r.hi, r.lo );

    BigInt ba = a, bb = b, bc = ba * bb;

    "%016x %016x".writefln( bc.getDigit( 1), bc.getDigit( 0));

}

​