RFR: 8325252: C2 SuperWord: refactor the packset [v6]

[Christian Hagedorn]

On Wed, 27 Mar 2024 15:48:50 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> I'm refactoring the packset, separating the details of packset-manupulation from the SuperWord algorithm.
>> 
>> Most importantly: I split it into two classes: `PairSet` and `PackSet`.
>> `combine_pairs_to_longer_packs` converts the first into the second.
>> 
>> I was able to simplify the combining, and remove the pack-sorting.
>> I now walk "pair-chains" directly with `PairSetIterator`. One such pair-chain is equivalent to a pack.
>> 
>> I moved all the `filter / split` functionality to the `PackSet`, which allows hiding a lot of packset-manipulation from the SuperWord algorithm.
>> 
>> I ran into some issues when I was extending the pairset in `extend_pairset_with_more_pairs_by_following_use_and_def`:
>> Using the PairSetIterator changed the order of extension, and that messed with the packing heuristic, and quite a few examples did not vectorize, because we would pack up the wrong 2 nodes out of a choice of 4 (e.g. we would pack `ac bd` instead of `ab cd`). Hence, I now still have to keep the insertion order for the pairs, and this basically means we are extending with a BFS order. Maybe this issue can be removed, if I improve the packing heuristic with some look-ahead expansion approach (but that is for another day [JDK-8309908](https://bugs.openjdk.org/browse/JDK-8309908)).
>> 
>> But since I already spent some time on some of the packing heuristic (reordering and cost estimate), I did a light refactoring, and added extra tests for MulAddS2I.
>> 
>> More details are described in the annotations in the code.
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   more updates for Christian, batch 2

Thanks for doing all the updates. It looks good to me now!

src/hotspot/share/opto/superword.hpp line 353:

> 351:       unmap_node_in_pack(old_pack->at(i));
> 352:     }
> 353:   }

You even added a `remap` method and added some asserts, nice!

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18276#pullrequestreview-1963810973
PR Review Comment: https://git.openjdk.org/jdk/pull/18276#discussion_r1541412358