RFR: 8324794: C2 SuperWord: do not ignore reductions in SuperWord::unrolling_analysis

[Vladimir Kozlov]

On Sun, 28 Jan 2024 09:44:14 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> I can only speculate: maybe the idea was that reductions are not profitable, unless there are other nodes, like stores and loads. So if we only find reductions, then we would not adjust the unrolling, since we are not expecting vectorization anyway. Again: only speculation. You reviewed the code in 2015, maybe you still remember the reason ;)

I looked and there was no discussion about that during review. Originally it was not SuperWord analysis - it only looked for arithmetic Phi nodes in loop. Last year we changed it: [1be80a44](https://github.com/openjdk/jdk/commit/1be80a4445cf74adc9b2cd5bf262a897f9ede74f)
I think the check simplify `unrolling_analysis` code since we skip nodes we already know about them.

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PR Comment: https://git.openjdk.org/jdk/pull/17604#issuecomment-1915247383