[llvm-dev] [RDF] Performance Issues for rdf::Liveness::computePhiInfo()

Constable, Scott D via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 30 13:49:00 PDT 2020

Hi Krzysztof,

I have been working to debug some performance issues in https://reviews.llvm.org/D75936, and I have found that the main issue is rdf::Liveness::computePhiInfo(). Specifically, a single line introduces 20-30% overhead for the entire llc X86 pipeline:


Linux perf shows that the culprit is MachineDominatorTree::dominates(), which is being invoked on nearly every less-than comparison between nodes. The documentation for dominates() reads "dominates -Returns true iff A dominates B. Note that this is not a constant time operation!"

Do you think that this could be fixed? Maybe some memorization could help?

Thanks and regards,

Scott Constable
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