RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v18]
Vladimir Ivanov
vlivanov at openjdk.org
Sat Jun 17 00:44:23 UTC 2023
On Wed, 14 Jun 2023 19:29:45 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:
>> Can I please get reviews for this PR?
>>
>> The most common and frequent use of NonEscaping Phis merging object allocations is for debugging information. The two graphs below show numbers for Renaissance and DaCapo benchmarks - similar results are obtained for all other applications that I tested.
>>
>> With what frequency does each IR node type occurs as an allocation merge user? I.e., if the same node type uses a Phi N times the counter is incremented by N:
>>
>> 
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>> What are the most common users of allocation merges? I.e., if the same node type uses a Phi N times the counter is incremented by 1:
>>
>> 
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>> This PR adds support scalar replacing allocations participating in merges used as debug information OR as a base for field loads. I plan to create subsequent PRs to enable scalar replacement of merges used by other node types (CmpP is next on the list) subsequently.
>>
>> The approach I used for _rematerialization_ is pretty straightforward. It consists basically of the following. 1) New IR node (suggested by V. Kozlov), named SafePointScalarMergeNode, to represent a set of SafePointScalarObjectNode; 2) Each scalar replaceable input participating in a merge will get a SafePointScalarObjectNode like if it weren't part of a merge. 3) Add a new Class to support the rematerialization of SR objects that are part of a merge; 4) Patch HotSpot to be able to serialize and deserialize debug information related to allocation merges; 5) Patch C2 to generate unique types for SR objects participating in some allocation merges.
>>
>> The approach I used for _enabling the scalar replacement of some of the inputs of the allocation merge_ is also pretty straightforward: call `MemNode::split_through_phi` to, well, split AddP->Load* through the merge which will render the Phi useless.
>>
>> I tested this with JTREG tests tier 1-4 (Windows, Linux, and Mac) and didn't see regression. I also experimented with several applications and didn't see any failure. I also ran tests with "-ea -esa -Xbatch -Xcomp -XX:+UnlockExperimentalVMOptions -XX:-TieredCompilation -server -XX:+IgnoreUnrecognizedVMOptions -XX:+UnlockDiagnosticVMOptions -XX:+StressLCM -XX:+StressGCM -XX:+StressCCP" and didn't observe any related failures.
>
> Cesar Soares Lucas has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 19 commits:
>
> - Merge branch 'openjdk:master' into rematerialization-of-merges
> - Rome minor refactorings.
> - Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
> Catching up with master.
> - Address PR review 6: debug format output & some refactoring.
> - Catching up with master branch.
>
> Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
> - Address PR review 6: refactoring around rematerialization & improve test cases.
> - Address PR review 5: refactor on rematerialization & add tests.
> - Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
> - Address part of PR review 4 & fix a bug setting only_candidate
> - Catching up with master
>
> Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
> - ... and 9 more: https://git.openjdk.org/jdk/compare/57b82512...939dcffe
Testing results (both functional and performance) are good.
In addition, I tested with a `C.failure_reason_is(retry_no_reduce_allocation_merges()) == true` guarantee and there were no failures observed.
Once you address my latest comments I'll mark the PR as reviewed.
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PR Comment: https://git.openjdk.org/jdk/pull/12897#issuecomment-1595520571
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