RFR: 8342662: C2: Add new phase for backend-specific lowering [v3]

[Quan Anh Mai]

On Thu, 31 Oct 2024 02:19:50 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> src/hotspot/share/opto/phaseX.cpp line 2289:
>> 
>>> 2287:   _worklist.ensure_empty();
>>> 2288: 
>>> 2289:   C->identify_useful_nodes(_worklist);
>> 
>> To address @iwanowww 's concern, you can have a backend-specific method `PhaseLowering::do_lowering()` that will decide whether we should perform lowering on a particular graph.
>
> I think if we're planning on moving optimizations like `DivMod` to the platform-independent part of this phase we may end up processing nodes more generally, since the graph patterns we expect would be pretty wide and shared between multiple backends. What do you think?

But there are still backends where we don't do such optimization, so it is still worth it to have a predicate for it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1823707056