RFR: 8342662: C2: Add new phase for backend-specific lowering [v6]

[Vladimir Ivanov]

On Mon, 16 Dec 2024 02:23:30 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 incrementally with one additional commit since the last revision:
> 
>   Implement apply_identity

Were there any experiments conducted to port existing lowering transformations to the new pass? 

As we discussed before, there are multiple places in the code where lowering takes place. It is still not clear to me how much proposed solution unifies across existing use cases. What I'd really like to avoid is yet another peculiar way to perform lowering transformations in C2.

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PR Comment: https://git.openjdk.org/jdk/pull/21599#issuecomment-2576372165