RFR: 8347499: C2: Make `PhaseIdealLoop` eliminate more redundant safepoints in loops [v2]

[Qizheng Xing]

On Wed, 5 Feb 2025 09:00:38 GMT, Qizheng Xing <qxing at openjdk.org> wrote:

>> In `PhaseIdealLoop`, `IdealLoopTree::check_safepts` method checks if any call that is guaranteed to have a safepoint dominates the tail of the loop. In the previous implementation, `check_safepts` would stop if it found a local non-call safepoint. At this time, if there was a call before the safepoint in the dom-path, this safepoint would not be eliminated.
>> 
>> <img width="353" alt="loop-safepoint" src="https://github.com/user-attachments/assets/c220e103-aaba-4e3f-98ac-1ddb6465c309" />
>> 
>> This patch changes the behavior of `check_safepts` to not stop when it finds a non-local safepoint. This makes simple loops with one method call ~3.8% faster (on aarch64).
>> 
>> 
>> Benchmark                Mode  Cnt       Score      Error  Units
>> LoopSafepoint.loopVar    avgt   15  208296.259 ± 1350.409  ns/op   # baseline
>> LoopSafepoint.loopVar    avgt   15  200692.874 ±  616.770  ns/op   # this patch
>> 
>> 
>> Testing: tier1-2 on x86_64 and aarch64.
>
> Qizheng Xing 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:
> 
>  - Merge branch 'master' into enhance-loop-safepoint-elim
>  - Add IR test and microbench.
>  - Make `PhaseIdealLoop` eliminate more redundant safepoints in loops.

Hi all,

This patch has now passed all GHA tests and is ready for further reviews.

If there are any other suggestions for this PR, please let me know.

Thanks!

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PR Comment: https://git.openjdk.org/jdk/pull/23057#issuecomment-2731505522