Use of branch weight metadata in enhanced inliner

Thank you Easwaran for landing a great diff that enables the use of block frequency information in inliner. As of now, it seems that inliner can exploit the information only when profile data is available. However, instructions such as __builtin_expect may set branch weight metadata as well, which is useful for inliner.

I think this problem can be addressed by letting llvm::getInlineCost function to use BlockFrequencyAnalysis regardless of availability of profile data, and making CallAnalyzer::updateThreshold to use callsite frequency instead of callsite count. Is there a reason that we allow inliner to use block frequency information only when profile data is given?

Thanks,
Taewook

Hi Taewook,

There is no reason not do use BFI in non-PGO mode. My current priority is to improve PGO performance and I wanted to isolate this as much as possible - hence it is turned on only in PGO mode.

  • Easwaran

Right.

For non PGO use cases, we also need to measure more carefully compile time and memory overhead (especially in LTO, thinLTO contexts) of this change and decide what is right strategy (e.g, by default at O2 or higher opt level).

David