RFR: 8342662: C2: Add new phase for backend-specific lowering [v5]

[Jasmine Karthikeyan]

On Wed, 11 Dec 2024 04:16:36 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> Hi all,
>> This patch adds a new pass to consolidate lowering of complex backend-specific code patterns, such as `MacroLogicV` and the optimization proposed by #21244. Moving these optimizations to backend code can simplify shared code, while also making it easier to develop more in-depth optimizations. The linked bug has an example of a new optimization this could enable. The new phase does GVN to de-duplicate nodes and calls nodes' `Value()` method, but it does not call `Identity()` or `Ideal()` to avoid undoing any changes done during lowering. It also reuses the IGVN worklist to avoid needing to re-create the notification mechanism.
>> 
>> In this PR only the skeleton code for the pass is added, moving `MacroLogicV` to this system will be done separately in a future patch. Tier 1 tests pass on my linux x64 machine. Feedback on this patch would be greatly appreciated!
>
> Jasmine Karthikeyan has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
> 
>  - Fix merge conflict
>  - Re-use optimize() and add backend-specific should_lower()
>  - Merge branch 'master' into phase-lowering
>  - Remove platform-dependent node definitions, rework PhaseLowering implementation
>  - Address some changes from code review
>  - Implement PhaseLowering

Thanks a lot for the ideas! I agree that running IGVN for the whole graph could become too expensive. I think that the suggestion with `PhaseGVN::apply_identity` is good, I'll make sure to implement it in the next commit.

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PR Comment: https://git.openjdk.org/jdk/pull/21599#issuecomment-2539831610