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/superword.hpp line 666:

> 664:   IdealLoopTree*  lpt() const   { return _lpt; }
> 665:   PhiNode* iv() const {
> 666:     return _slp ? _slp->iv() : _lpt->_head->as_CountedLoop()->phi()->as_Phi();

I'd suggest either cache it directly from `_lpt->_head->as_CountedLoop()->phi()->as_Phi()`, or just query it directly. Reduce dependence on `_slp`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239693729