RFR: 8188044: We need Math.unsignedMultiplyHigh [v2]

[Andrew Haley]

On Fri, 2 Jul 2021 11:06:06 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

> You can also do that branchlessly which might prove better
> 
> ```
>      long result = Math.multiplyHigh(x, y);
>      result += (y & (x >> 63));
>      result += (x & (y >> 63));
>      return result;
> ```
I doubt very much that it would be better, because these days branch prediction is excellent, and we also have conditional select instructions. Exposing the condition helps C2 to eliminate it if the range of args is known. The `if` code is easier to understand.

Benchmark results, with one of the operands changing signs every iteration, 1000 iterations:


Benchmark                  Mode  Cnt     Score    Error  Units
MulHiTest.mulHiTest1   (aph)     avgt    3  1570.587 ± 16.602  ns/op
MulHiTest.mulHiTest2   (adinn)   avgt    3  2237.637 ±  4.740  ns/op

In any case, note that with this optimization the unsigned mulHi is in the nanosecond range, so Good Enough. IMO.

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PR: https://git.openjdk.java.net/jdk/pull/4644