RFR: 8334431: C2 SuperWord: fix performance regression due to store-to-load-forwarding failures [v2]

[Emanuel Peter]

On Tue, 19 Nov 2024 12:45:03 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> Should we also mention here that it also works when the loaded data is fully contained in the stored data.

fully contained, as in `strict subset`? I mentioned that already... and sadly it works on some platforms, but not others... quite complex. That is why I make the "conservative assumption".

> src/hotspot/share/opto/vtransform.cpp line 278:
> 
>> 276: // Performance measurements with the JMH benchmark StoreToLoadForwarding.java have indicated
>> 277: // that there is some iteration threshold: if the failure happens between a store and load that
>> 278: // have an iteration distance below this threshold, the latency is the limiting factor, and we
> 
> It's probably clear what you mean by "iteration distance" but maybe to be sure, you can add at your example above that the "iteration distance" is 3 there.

done

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PR Review Comment: https://git.openjdk.org/jdk/pull/21521#discussion_r1848557462
PR Review Comment: https://git.openjdk.org/jdk/pull/21521#discussion_r1848561078