RFR: 8287697: Limit auto vectorization to 32-byte vector on Cascade Lake [v2]

[Vladimir Kozlov]

On Thu, 2 Jun 2022 04:37:58 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

>> I think we missed the test with setting `MaxVectorSize` to 32 (vs 64) on Cascade Lake CPU. We should do that.
>> 
>> That may be preferable "simple fix" vs suggested changes for "short term solution".
>> 
>> The objection was that user may still want to use wide 64 bytes vectors for Vector API. But I agree with Jatin argument about that.
>> Limiting `MaxVectorSize` **will** affect our intrinsics/stubs code and may affect performance. That is why we need to test it. I will ask Eric.
>> 
>> BTW, `SuperWordMaxVectorSize` should be diagnostic or experimental since it is temporary solution.
>
> @vnkozlov I have made SuperWordMaxVectorSize as a develop option as you suggested. As far as I know, the only intrinsics/stubs that uses MaxVectorSize are for clear/copy. This is done in conjunction with AVX3Threshold so we are ok there for Cascade Lake.

> Hi @sviswa7 , #7806 implemented an interface for auto-vectorization to disable some unprofitable cases on aarch64. Can it also be applied to your case?

Maybe. But it would require more careful changes. And that changeset is not integrated yet.
Current changes are clean and serve their purpose good.

And, as Jatin and Sandhya said, we may do proper fix after JDK 19 fork. Then we can look on your proposal.

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PR: https://git.openjdk.java.net/jdk/pull/8877