RFR: 8341137: Optimize long vector multiplication using x86 VPMUL[U]DQ instruction

[Quan Anh Mai]

On Fri, 18 Oct 2024 04:16:15 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> I see this is as a twostep optimization, in the first step we do analysis and annotate additional information on existing IR, which is later used by instruction selector. I plan to subsume first stage with enhanced dataflow analysis going forward.

The issue is that a node is not immutable. This puts a burden on every place to keep the annotation sane when doing transformations, which is easily missed when there are a lot of kinds of `Node`s out there. That's why I think it is most suitable to be done only right before matching. `Node::Ideal` is invoked in a really generous manner so I would prefer not to add analysis to it that can be done more efficiently somewhere else. Additionally, if you have a separate IR node for this operation, you can do some more beneficial transformations such as `MulVL (AndV x max_juint) (AndV y max_juint)` into `MulVLowIToL x y`.

My suggestions are based on this PR as a standalone, so they may not be optimal when looking at a wider perspective, in case you think this approach would fit more nicely into a larger landscape of your planned enhancements please let us know. Thanks for your patience.

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PR Comment: https://git.openjdk.org/jdk/pull/21244#issuecomment-2421376285