Do we need an unsigned multiplyHigh? (original) (raw)

Andrew Haley aph at redhat.com
Mon Sep 25 14:50:01 UTC 2017

We now have a multiplyHigh intrinsic, but it is signed. Unsigned multiplyHigh is in general a more useful primitive for crypto than signed, and I wonder if we need an intrinsic for that as well. I've looked at cooking up an unsigned multiplyHigh in Java, and I think the fastest way is this:

private static final long unsignedMultiplyHigh(long a, long b) {
    long result = Math.multiplyHigh(a, b);
    if (a < 0)  result += b;
    if (b < 0)  result += a;
    // Can also be written as:
    // result += (a >> 63) & b;
    // result += (b >> 63) & a;
    return result;
}

It's still about 50% slower than the signed multiplyHigh, though. Thoughts?

-- Andrew Haley Java Platform Lead Engineer Red Hat UK Ltd. <https://www.redhat.com> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671

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