[PATCH] D83013: [LPM] Port CGProfilePass from NPM to LPM

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/test/Other/opt-O2-pipeline.ll:289
+; CHECK-NEXT:         Branch Probability Analysis
+; CHECK-NEXT:         Block Frequency Analysis
 ; CHECK-NEXT:     FunctionPass Manager
----------------
hans wrote:
> nikic wrote:
> > Is it possible to switch this pass to use LazyBPI / LazyBFA, only fetched if PGO is actually in use?
> > 
> > PGO functionality that most people don't use adding expensive analysis passes like PDT should be avoided.
> I wonder if just switching to LazyBlockFrequencyInfo would help though. It looks to me like the CGProfile would request info about each function anyway.
> 
> I was surprised to see that Clang sets Opts.CallGraphProfile solely based on whether the integrated assembler is used. Maybe a better fix is to only set that to true when a profile is actually being used?
> I wonder if just switching to LazyBlockFrequencyInfo would help though. It looks to me like the CGProfile would request info about each function anyway.

It would only help if there is some way to only fetch the analysis conditionally. I believe many PGO passes use something like PSI.hasProfileSummary() or F.hasProfileData() for that.

> I was surprised to see that Clang sets Opts.CallGraphProfile solely based on whether the integrated assembler is used. Maybe a better fix is to only set that to true when a profile is actually being used?

Right, just disabling this by default in clang/opt would also work.

For reference, the current compile-time numbers for this patch: https://llvm-compile-time-tracker.com/compare.php?from=516ff1d4baee28b1911737e47b42973567adf8ff&to=8df840660bb764b6653fcfd9ac7a72cc6adebde6&stat=instructions Not huge, but it adds up (some similar regressions have been introduced in LLVM 10).


Repository:
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  https://reviews.llvm.org/D83013/new/

https://reviews.llvm.org/D83013