RFR: 8275167: x86 intrinsic for unsignedMultiplyHigh

[Andrew Haley]

On Wed, 13 Oct 2021 18:55:10 GMT, Vamsi Parasa <duke at openjdk.java.net> wrote:

> Optimize the new Math.unsignedMultiplyHigh using the x86 mul instruction. This change show 1.87X improvement on a micro benchmark.

test/micro/org/openjdk/bench/java/lang/MathBench.java line 547:

> 545:         return  Math.unsignedMultiplyHigh(long747, long13);
> 546:     }
> 547: 

As far as I can tell, the JMH overhead dominates when trying to measure the latency of events in the nanosecond range. `unsignedMultiplyHigh` should have a latency of maybe 1.5-2ns. Is that what you saw?

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PR: https://git.openjdk.java.net/jdk/pull/5933