RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v10]

[Vladimir Ivanov]

On Thu, 27 Apr 2023 20:33:38 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> src/hotspot/share/code/debugInfo.hpp line 199:
>> 
>>> 197: // ObjectValue describing an object that was scalar replaced.
>>> 198: 
>>> 199: class ObjectMergeValue: public ObjectValue {
>> 
>> I find the decision to subclass`ObjectValue` confusing and error prone: now `is_object()` returns true for `ObjectMergeValue`, but you have to apply the selector first to turn it into `ObjectValue`. And now the order of checks matter, so you always have to perform `is_object_merge()` first and then follow it with `is_object()` guard.
>> 
>> You have 3 flavors of `ObjectValue` now:
>> * good old `ObjectValue`;
>> * `ObjectMergeValue`
>> * merge candidates (`ObjectMergeCandidateValue`?)
>> 
>> Does it make sense to introduce 3 different subclasses under `ObjectValue` to clearly distinguish the scenarios?
>
> Hi @iwanowww . I finished implementing a version of this like the illustration below (I didn't add a Candidate class). 
> 
> 
> ScopeValue
>     ObjectValue
>         ObjectAllocationValue
>             AutoBoxObjectValue
>         ObjectMergeValue
> 
>  
> Here are some observations:
> 
> - I don't think ObjectMergeValue should be under ObjectValue. The two classes only have two fields in common (_id and _visited). I think it should be a subclass of ScopeValue.
> - ObjectCandidateValue would need to go under ObjectAllocationValue because it essentially _is_ an ObjectAllocationValue in most aspects.
> - I didn't add a ObjectCandidateValue class because that class would need to go under ObjectAllocationValue and we would still need to do an "is_object_candidate" before all "is_object_allocation" and we would end up in much the situation that we want to avoid - needing to do is_object_merge before is_object.
> - It seems the best place to flag an object as candidate is really in ObjectAllocationValue.
> 
> What do you think? As I said, I already have the code, if you want I can push it and you take a look.

Can `ObjectCandidateValue` be a wrapper around a `ObjectAllocationValue`?

It does make sense to separate `ObjectMergeValue` and `ObjectValue`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/12897#discussion_r1179798496