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

[Emanuel Peter]

On Fri, 23 Jun 2023 11:28:02 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/vectornode.hpp line 1839:
> 
>> 1837:   virtual bool cmp(const Node& n) const {
>> 1838:     return TypeNode::cmp(n) &&
>> 1839:            _max_trips == ((LoopVectorMaskNode&)n)._max_trips;
> 
> Is this cast really safe? Can you use `as_LoopVectorMaskNode()` instead, so we at least have an assert if it fails to be true? I fear this may get us into undefined behavior otherwise...

Oh dear, I just saw the same pattern in:

bool TypeNode::cmp(const Node& n) const {
  return !Type::cmp(_type, ((TypeNode&)n)._type);
}

We should try to avoid doing that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239710563