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

[Emanuel Peter]

On Fri, 23 Jun 2023 11:06:13 GMT, Emanuel Peter <epeter 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.cpp line 4297:
> 
>> 4295: 
>> 4296: bool SWPointer::Tracer::slp_trace_alignment() {
>> 4297:   return _slp && _slp->is_trace_alignment();
> 
> Aha, here you wrap it. You have some uses above that could be replaced with this now. But again, even better would be if we had a general trace flag that could trace it for any context, not just SuperWord.

After all, should the `VectorizeDebug` flag not apply everywhere? See `phase->C->directive()->VectorizeDebugOption`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239687685