RFR: 8308994: C2: Re-implement experimental post loop vectorization [v2]

[Emanuel Peter]

On Mon, 3 Jul 2023 07:37:22 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.
>
> Pengfei Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address part of comments from Emanuel

src/hotspot/share/opto/vmaskloop.cpp line 978:

> 976: 
> 977:   // Update loop increment/decrement to the vector mask true count
> 978:   Node* true_cnt = new VectorMaskTrueCountNode(root_vmask, TypeInt::INT);

This seems expensive to have to use inside the loop. Is there a way we could move this outside the loop? Because if we do take the backedge then we know that we have to take the full `stride`, right?

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1250973547