RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v13]

[Vladimir Ivanov]

On Fri, 12 May 2023 21:09:01 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 incrementally with one additional commit since the last revision:
> 
>   Address PR review 5: refactor on rematerialization & add tests.

Very nice, Cesar. I like how the code shapes now.

I verified that the new test cases do trigger SR+NSR scenario. 

How do you test that deoptimization works as expected? 

Diagnostic output is still hard to read. On one hand, it's too verbose when it comes to PcDesc/ScopeDesc sections  ("pc-bytecode offsets" and "scopes")  in nmethod output (enabled either w/ `-XX:+PrintAssembly` or `-XX:CompileCommand=print,...`). On the other hand, it lacks some important details, like `selector` and `merge_ptr` location information which is essential to make sense of debug information at a safepoint in the code.

FTR `_skip_rematerialization` flag is unused now.

Speaking of `_only_merge_candidate` flag, I find it easier about the code when the property being tracked is whether the `ObjectValue` is referenced from corresponding JVM state or not. (Maybe call it `is_root()`?) So, `ScopeDesc::objects_to_rematerialize()` would skip everything not referenced from JVM state, but then unconditionally accept anything returned by `ObjectMergeValue::select()` which doesn't need to adjust the flag before returning selected object. Also, it's safer to track the flag status for every `ObjectValues`, even for `ObjectMergeValue`. 

Are you sure there's no way to end up with nested `ObjectMergeValue`s in presence of iterative EA?

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PR Comment: https://git.openjdk.org/jdk/pull/12897#issuecomment-1553966589