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 775:

> 773: 
> 774:   void collect_loop_core_nodes(PhaseIdealLoop* phase, Unique_Node_List& wq) const;
> 775: 

nit: why move it?

src/hotspot/share/opto/superword.cpp line 179:

> 177:     assert(_packset.length() == 0, "packset must be empty");
> 178:     success = SLP_extract();
> 179:     if (PostLoopMultiversioning) {

Could we now have an assert for `cl->is_main_loop()` at the beginning of `SuperWord::transform_loop`, and remove all checks for it in SuperWord?

src/hotspot/share/opto/superword.cpp line 632:

> 630:       cl->set_slp_pack_count(_packset.length());
> 631:     }
> 632:   } else {

Again: Could we now have an assert for `cl->is_main_loop()` at the beginning of `SuperWord::SLP_extract`, and remove all checks for it in SuperWord?

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239670056
PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239672094
PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239673798