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/vmaskloop.cpp line 104:

> 102:   _core_set.clear();
> 103:   _body_set.clear();
> 104:   _body_nodes.clear();

Would it make sense to somehow reserve the space, so that we do not allocate multiple times when growing these data structures later?

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239767741