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

[Quan Anh Mai]

On Wed, 23 Oct 2024 08:10:38 GMT, Jatin Bhateja <jbhateja 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!
>
> src/hotspot/share/opto/compile.cpp line 2466:
> 
>> 2464:     print_method(PHASE_BEFORE_LOWERING, 3);
>> 2465: 
>> 2466:     PhaseLowering lower(&igvn);
> 
> Any specific reason to have lowering after loop optimizations ?
> Lowered nodes may change the loop body size thereby impacting unrolling decisions.

Because lowering is a transformation that increases the complexity of the graph.

- A `d = ExtractD(z,  4)` expanded into `x = VectorExtract(z, 2); d = ExtractD(x, 0)` increases the number of nodes by 1.
- A logic cone transformed into a `MacroLogicV` introduces another kind of node that may not be recognized by other nodes.

As a result, we should do this as the last step when other transformation has finished their jobs. For the concerns regarding loop body size, we still have a function in `Matcher` for that purpose.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1813233875