RFR: 8319451: PhaseIdealLoop::conditional_move is too conservative [v2]

[Claes Redestad]

On Tue, 12 Dec 2023 16:39:17 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> When transforming a Phi into a CMove, the threshold is set to be approximately BlockLayoutMinDiamondPercentage, the reason is given:
>> 
>>     // BlockLayoutByFrequency optimization moves infrequent branch
>>     // from hot path. No point in CMOV'ing in such case
>> 
>> This sets the default value of the threshold to be around 18%, which is too conservative. The reason also does not make a lot of sense since the important property which makes jumping expensive is not code layout. We should remove this.
>> 
>> Please kindly review, thank you very much.
>
> Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - change freq to double
>  - Merge branch 'master' into cmovethreshold
>  - adjust threshold, add benchmark

Thank you for the work on this and for picking up on my suggestions for the microbenchmark! Looking forward to see the effects of this on a wider selection of benchmarks.

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Marked as reviewed by redestad (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16524#pullrequestreview-1782059077