RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v4]

[Cesar Soares Lucas]

On Mon, 20 Mar 2023 19:23:34 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 that are 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 in: 1) Extend SafePointScalarObjectNode to represent multiple SR objects; 2) Add a new Class to support rematerialization of SR objects part of merges; 3) Patch HotSpot to be able to serialize and deserialize debug information related to allocation merges; 4) 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 straight forward: 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 tested with several applications and didn't see any failure. I also run 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 incrementally with one additional commit since the last revision:
> 
>   Add support for SR'ing some inputs of merges used for field loads

src/hotspot/share/opto/escape.cpp line 3734:

> 3732:       if (reducible_merges.member(n)) {
> 3733:         // Split loads through phi
> 3734:         reduce_this_phi_on_field_access(n->as_Phi(), alloc_worklist);

I decided to do the split here so that Phase 1 of `split_unique_types` could assign a new instance type for the new loads created by `split_through_phi`. However, I'm considering doing the split at the end of `compute_escape` instead of here to keep all the code that does split close together.

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PR Review Comment: https://git.openjdk.org/jdk/pull/12897#discussion_r1148153569