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

[Magnus Ihse Bursie]

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!

Build changes look good (but would be slightly better without the extra blank line). I have not reviewed the actual hotspot changes.

make/hotspot/gensrc/GensrcAdlc.gmk line 60:

> 58: 
> 59:   ADLC_CFLAGS += -D$(HOTSPOT_TARGET_CPU_DEFINE)
> 60: 

Maybe skip this blank line?

src/hotspot/cpu/aarch64/c2_lowering_aarch64.cpp line 31:

> 29: Node* PhaseLowering::lower_node(Node* in) {
> 30:   return nullptr;
> 31: }

Note that this, and several other of the new files, are missing a trailing newline on the last line (marked by the red circle/dash icon). I thought this was checked by jcheck, but apparently not. It is still not recommended, though.

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Marked as reviewed by ihse (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21599#pullrequestreview-2381995734
PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1808752950
PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1808763526