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

[Vladimir Kozlov]

On Fri, 14 Jun 2024 10:34:38 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> The original PR was [here](https://github.com/openjdk/jdk/pull/19261), 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).

I did not look on changes in details but in general they seems fine.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19719#pullrequestreview-2128780964