high bits of long multiplication in Java?

Question

Is there any way to get the high half of the multiplication of two longs in Java? I.e. the part that vanishes due to overflow. (So the upper 64 bits of the 128-

Answer 1

If either x or y can be negative, you should use Hacker's Delight function (Henry S. Warren, Hacker's Delight, Addison-Wesley, 2nd edition, Fig. 8.2):

long x_high = x >>> 32;
long x_low = x & 0xFFFFFFFFL;
long y_high = y >>> 32;
long y_low = y & 0xFFFFFFFFL;
long z2 = x_low * y_low;
long t = x_high * y_low + (z2 >>> 32);
long z1 = t & 0xFFFFFFFFL;
long z0 = t >>> 32;
z1 += x_low * y_high;
return x_high * y_high + z0 + (z1 >>> 32);