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

[Quan Anh Mai]

On Mon, 21 Oct 2024 04:11:03 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!

src/hotspot/cpu/aarch64/c2_lowering_aarch64.cpp line 29:

> 27: #include "opto/phaseX.hpp"
> 28: 
> 29: Node* PhaseLowering::lower_node(Node* in) {

You need to wrap all these in `#ifdef COMPILER2`

src/hotspot/share/opto/compile.cpp line 2464:

> 2462:   {
> 2463:     TracePhase tp("lower", &timers[_t_lower]);
> 2464:     print_method(PHASE_BEFORE_LOWERING, 3);

Isn't `BEFORE_LOWERING` the same as `AFTER_BARRIER_EXPANSION` right above?

src/hotspot/share/opto/phaseX.cpp line 2277:

> 2275: 
> 2276:   // Try to find an existing version of the same node
> 2277:   Node* existing = _igvn->hash_find_insert(n);

I think it would be easier if you have a switch in `gvn` that says you passed the point of doing `Ideal`, moving forward you will probably want to have a `IdealLowering` to transform nodes during this phase. `Identity` I think is fine since it returns an existing node.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1808154845
PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1808157225
PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1808156752