RFR: 8289943: Simplify some object allocation merges [v14]

[Cesar Soares Lucas]

On Tue, 3 Jan 2023 20:27:41 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> Hi there, can I please get some feedback on this approach to simplify object allocation merges in order to promote Scalar Replacement of the objects involved in the merge?
>> 
>> The basic idea for this [approach was discussed in this thread](https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2022-April/055189.html) and it consists of: 
>> 1) Identify Phi nodes that merge object allocations and replace them with a new IR node called ReducedAllocationMergeNode (RAM node).
>> 2) Scalar Replace the incoming allocations to the RAM node.
>> 3) Scalar Replace the RAM node itself.
>> 
>> There are a few conditions for doing the replacement of the Phi by a RAM node though - Although I plan to work on removing them in subsequent PRs:
>> 
>> - The only supported users of the original Phi are AddP->Load, SafePoints/Traps, DecodeN.
>> 
>> These are the critical parts of the implementation and I'd appreciate it very much if you could tell me if what I implemented isn't violating any C2 IR constraints:
>> 
>> - The way I identify/use the memory edges that will be used to find the last stored values to the merged object fields.
>> - The way I check if there is an incoming Allocate node to the original Phi node.
>> - The way I check if there is no store to the merged objects after they are merged.
>> 
>> Testing:
>> - Windows/Linux/MAC fastdebug/release 
>>   - hotspot_all
>>   - tier1
>>   - Renaissance
>>   - dacapo
>>   - new IR-based tests
>
> 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 43 commits:
> 
>  - updating with master branch
>  - Fix x86 tests.
>  - Fix code style.
>  - Address PR feedback. Fix test & one bug. Set RAM parameter to true by default.
>  - Addressing PR feedback. Added new constraint for case of merging SR and NSR allocations.
>  - Merge branch 'openjdk:master' into allocation-merges
>  - Remove debug messages.
>  - Add functional tests, micro benchmarks and fix some bugs.
>  - fix 32 bit execution.
>  - Back out on fixing existing issue. Some tests depend on it.
>  - ... and 33 more: https://git.openjdk.org/jdk/compare/ea25a561...d26909fa

Hi all! I'm back from leave and I'm resuming the work on this project. I'll keep you updated. Thanks again for all the comments and feel free to add more in the meantime.

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PR: https://git.openjdk.org/jdk/pull/9073