Integrated: 8366702: C2 SuperWord: refactor VTransform vector nodes

[Emanuel Peter]

On Tue, 2 Sep 2025 15:30:06 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> I'm working on cost-modeling, and am integrating some smaller changes from this proof-of-concept PR:
> https://github.com/openjdk/jdk/pull/20964
> [See plan overfiew.](https://bugs.openjdk.org/browse/JDK-8340093)
> 
> This is a pure refactoring - no change in behaviour. I'm presenting it like this because it will make reviews easier.
> 
> ---------------------------------
> 
> I have to say: I'm very sorry for this refactoring. I took some decisions in https://github.com/openjdk/jdk/pull/19719 that I'm now partially undoing. I moved too much logic from `SuperWord::output` (now called `SuperWordVTransformBuilder::make_vector_vtnode_for_pack`) to the `VTransform...Node::apply`. https://github.com/openjdk/jdk/pull/19719  was a roughly 1.5k line change, and I took about a 0.3k misstep that I'm now correcting here ;)
> 
> I had accidentially made the `VTransformGraph` too close to the `PackSet`, and not close enough to the future vectorized C2 Graph. And that makes some future changes hard.
> 
> My vision:
> - VLoop / VLoopAnalyzer look at the scalar loop and prepare it for SuperWord
> - SuperWord creates the `PackSet`: some nodes are packed, all others are scalar.
> - `SuperWordVTransformBuilder` converts the `PackSet` into the `VTransformGraph`
> - The `VTransformGraph` very closely represents the C2 vectorized loop after vectorization
>   - It does not need to know which `nodes` it packs, it rather just needs to know how to generate the new vector nodes
>   - That means it is straight-forward to compute cost
>   - And it also makes optimizations on that graph easier
>   - And the `apply` methods are simpler too
> 
> ----------------------------------
> 
> So therefore, the main goal was to make the `VTransform...Node::apply` calls simpler again. And move the logic back to `SuperWordVTransformBuilder::make_vector_vtnode_for_pack`.
> 
> One important step to making the the `VTransformGraph` less of a `PackSet` is to remove reliance on `nodes` for the vector nodes.
> 
> What I did:
> - Moving a lot of the logic in `VTransformElementWiseVectorNode::apply` to `SuperWordVTransformBuilder::make_vector_vtnode_for_pack`.
>   - Will make it easier to optimize and compute cost in future RFE's.
> - `VTransformVectorNodePrototype`: packs a lot of the info for `VTransformVectorNode`.
>   - pass info about `bt`, `vlen`, `sopc` instead of the `pack` -> allows us to eventually remove the dependency on `nodes`.
> - New vector nodes, they are special cases I split away from `VTransformElementWiseVectorNode`:
>   - `VTransformReinterpretVectorN...

This pull request has now been integrated.

Changeset: 4cc75be8
Author:    Emanuel Peter <epeter at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/4cc75be80e6a89e0ed293e2f8bbb6d0f94189468
Stats:     352 lines in 4 files changed: 173 ins; 65 del; 114 mod

8366702: C2 SuperWord: refactor VTransform vector nodes

Reviewed-by: chagedorn, galder

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PR: https://git.openjdk.org/jdk/pull/27056