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

[Vladimir Kozlov]

On Wed, 31 May 2023 00:47:56 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

>> This PR fixes the problem with double reduction on x86_64. 
>> 
>> In the test compiler.loopopts.superword.ProdRed_Double, the product reduction loop in prodReductionImplement() was not getting vectorized when run as follows:
>> jtreg -XX:CompileCommand=PrintAssembly,compiler.loopopts.superword.ProdRed_Double::prodReductionImplement compiler/loopopts/superword/ProdRed_Double.java
>> The print assembly generated in the pid-xxx.log output in JTwork/scratch directory was not showing any vector_reduction_double node.
>> 
>> This was happening as the ReductionNode::implemented was passed a vector size of one element. For the vector reduction implemented we need to check with at least vector size of two elements.
>> 
>> With this PR the vector_reduction_double node is generated.
>> 
>> Please review.
>> 
>> Best Regards,
>> Sandhya
>
> Sandhya Viswanathan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add jmh test case

Looks good. You may regress for next case on KNL? when using `max vector size` (code is in x86.ad) which I think is fine.

    case Op_MinReductionV:
    case Op_MaxReductionV:
      if (UseAVX > 2 && (!VM_Version::supports_avx512dq() && size_in_bits == 512)) {
        return false;
      }

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14065#pullrequestreview-1454013018