RFR: 8300865: C2: product reduction in ProdRed_Double is not vectorized [v4]

[Emanuel Peter]

On Wed, 31 May 2023 00:55:22 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

>> Sandhya Viswanathan has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add jmh test case
>
> The performance numbers on my desktop are:
> Base runs, no vectorization happens with superword:
> Benchmark                              (COUNT)  (seed)  Mode  Cnt    Score   Error  Units
> VectorReduction.NoSuperword.mulRedD        512       0  avgt    4  435.795 ± 0.082  ns/op
> VectorReduction.WithSuperword.mulRedD      512       0  avgt    4  434.154 ± 0.042  ns/op
> 
> With the PR reduction succeeds and vectorization of the loop happens when superword is enabled: 
> Benchmark                              (COUNT)  (seed)  Mode  Cnt    Score   Error  Units
> VectorReduction.NoSuperword.mulRedD        512       0  avgt    4  435.897 ± 0.137  ns/op
> VectorReduction.WithSuperword.mulRedD      512       0  avgt    4  405.479 ± 1.896  ns/op

@sviswa7 Thanks for adding the benchmark. The win is small, but that was to be expected given that the double reduction has to be performed in a linear order, and hence has quite a large latency.

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PR Comment: https://git.openjdk.org/jdk/pull/14065#issuecomment-1569572955