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

[Emanuel Peter]

On Mon, 3 Jul 2023 14:34:00 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> 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?

I guess you would have to separate out the loop-internal uses and the outside uses of the `incr`. The inside uses would use the `stride` (or is there an exception?) and the outside ones could use the `VectorMaskTrueCountNode`.

Doing something like that could have better performance.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1250976621