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

[Zequan Wu via Phabricator via cfe-commits]

zequanwu 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
----------------
MaskRay wrote:
> zequanwu wrote:
> > zequanwu wrote:
> > > nikic wrote:
> > > > 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).
> > > Do you mean disabling it just for LPM or both?
> > > 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?
> > For Clang, a better fix I think is that `Opts.CallGraphProfile` should based on both whether the integrated assembler is used and whether profile instrumentation is turned on. What do you think?
> I'd prefer not having `CallGraphProfile`
> 
> * `-no-integrated-as -S` => no .cgprofile (.llvm_addrsig behaves this way)
> * `-S` -> .cgprofile
As discussed above, I think `CGProfilePass` should be disabled by default in clang unless `-no-integrated-as` is not given and `-fprofile-instrument-use-path=` is given. So, `Opts.CallGraphProfile` is a convenient switch for that.


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https://reviews.llvm.org/D83013