Reasonably portable way to get top 64-bits from 64x64 bit multiply? [duplicate]

Answer 1

A little algebra never hurt:

#include 

uint64_t top64(uint64_t x, uint64_t y) {
    uint64_t a = x >> 32, b = x & 0xFFFFFFFF;
    uint64_t c = y >> 32, d = y & 0xFFFFFFFF;

    return a * c + ((b * d >> 32) + (a * d) + (b * c)) >> 32 +
    ((((a * d) & 0xFFFFFFFF) + ((b *c) & 0xFFFFFFFF) + ((b * d) >> 32)) >> 32);
}