RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v21]

[Vladimir Kozlov]

On Thu, 6 Jul 2023 13:06:30 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:
>> 
>> ![image](https://user-images.githubusercontent.com/2249648/222280517-4dcf5871-2564-4207-b49e-22aee47fa49d.png)
>> 
>> 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:
>> 
>> ![image](https://user-images.githubusercontent.com/2249648/222280608-ca742a4e-1622-4e69-a778-e4db6805ea02.png)
>> 
>> 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 22 commits:
> 
>  - Merge branch 'openjdk:master' into rematerialization-of-merges
>  - Addressing PR feedback.
>  - Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
>  - 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.
>  - ... and 12 more: https://git.openjdk.org/jdk/compare/97e99f01...25b683d6

The final version looks good to me too.

-------------

Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/12897#pullrequestreview-1522912018