RFR: JDK-8308994: C2: Re-implement experimental post loop vectorization

[Emanuel Peter]

On Wed, 21 Jun 2023 08:24:19 GMT, Pengfei Li <pli at openjdk.org> wrote:

> ## TL;DR
> 
> This patch completely re-implements C2's experimental post loop vectorization for better stability, maintainability and performance. Compared with the original implementation, this new implementation adds a standalone loop phase in C2's ideal loop phases and can vectorize more post loops. The original implementation and all code related to multi-versioned post loops are deleted in this patch. More details about this patch can be found in the document replied in this pull request.

src/hotspot/share/opto/loopnode.hpp line 143:

> 141:     if (is_vector_masked()) {
> 142:       return false;
> 143:     }

Does this mean that the post-loop has a `CountedLoop` node, but it does not adhere to the counted-loop assumptions, such as having a `incr`, `limit`, `phi` etc? With the old post-loop-vectorization, the LoopNode would always fold away, so it would disappear after IGVN. But now it would stick around, right? Could that turn out to be a problem?

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239666832