8332163: C2 SuperWord: refactor PacksetGraph and SuperWord::output into VTransformGraph

[eme64]

The original PR was here, it got too chaotic.

I added some extra tests for this in: https://github.com/openjdk/jdk/pull/19558 I extracted some refactorings to: https://github.com/openjdk/jdk/pull/19573

We used to have:

• PacksetGraph: this detects cycles introduces by packs, and schedules/reorders the memops.
• SuperWord::apply_vectorization: creates VectorNodes directly from the PackSet.

In my blog, I have published lots of ideas for SuperWord / AutoVectorization improvements: https://eme64.github.io/blog/2023/11/03/C2-AutoVectorizer-Improvement-Ideas.html

Many ideas are based on the "VectorTransform IR": cost-model, if-conversion, direct widening of scalars to vectors, additional optimizations/features with shuffle/pack/extract, handling more reduction patterns, etc.

I now decided to name it VTransform, which is essencially a graph VtransformGraph of nodes VTransformNodes that resemble the C2 Node on purpose, because the VTransform models the C2 graph after vectorization. We can now model the transformation from scalar-loop to vectorized-loop without modifying the C2 graph yet.

The new code has these steps:

• Given the PackSet from SuperWord, we create a VTransformGraph with SuperWordVTransformBuilder.
• [Not yet: all sorts of optimizations / checks on the VTransformGraph, in future RFE's]
• We then schedule the VTransformGraph, and check for cycles.
• Once we are ready to commit to vectorization, we call VTransformGraph::apply_vectorization which lets each individual VTransformNode::apply generate the new vectorized C2 nodes.

Testing

Regression testing passed.

Performance testing: no significant change in performance (as expected).

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Progress

• [ ] Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• [x] Change must not contain extraneous whitespace
• [x] Commit message must refer to an issue

Issue

• JDK-8332163: C2 SuperWord: refactor PacksetGraph and SuperWord::output into VTransformGraph (Enhancement - P4)

Reviewers

• Vladimir Kozlov (@vnkozlov - Reviewer) 🔄 Re-review required (review applies to be2ea66c)

Reviewing

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