[llvm-dev] Problem using BlockFrequencyInfo's getBlockProfileCount

[Teresa Johnson via llvm-dev]

On Sun, Jan 13, 2019 at 9:24 AM Dennis Fischer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hey,
>
> I am trying to use the BlockFrequencyInfoWrapperPass to obtain hotness
> information in my LLVM pass. Attached is a minimal example of code which
> creates a SIGSEGV. The pass calls
> AU.addRequired<BlockFrequencyInfoWrapperPass>(); in
> getAnalysisUsage(..). The problem exists with changed and unchanged IR.
>
> The binary is instrumented like this: clang input.bc -fprofile-generate
> -o output
>
> The binary runs with selected inputs and the profiles are merged:
> llvm-profdata merge input.profraw -output=output.prof
>
> Then opt runs with arguments: opt input.bc -o output.bc -load mypass.so
> -block-freq -pgo-instr-use -pgo-test-profile-file=output.prof
> -profile-sample-accurate -mypass
>
> Is this a bug or am can someone provide an example on how to use
> BlockFrequencyInfo correctly?
>
>
> Example code:
>
>
> for (auto& F : M) {
>      if (F.isDeclaration()) {
>          continue;
>      }
>      auto& bfiPass = getAnalysis<BlockFrequencyInfoWrapperPass>(F);
>      llvm::BlockFrequencyInfo* BFI = &bfiPass.getBFI();
>
>      //Works - shows all info I want
>      BFI->print(llvm::dbgs());
>

I don't know offhand why your code below is segfaulting, but note that
BFI->print() will dereference the F pointer just as *getFunction below
will. I would look in the debugger to see what the value of F is during
print() since it seems to be ok there, and then see if you can figure out
where it is changing if it is returning a bad value below.
Teresa


>      for (const llvm::BasicBlock& B : F) {
>          //Fails with SIGSEGV -> *getFunction() returns 0xa
>          dbgs() << BFI->getBlockProfileCount(&B).getValueOr(0) << "\n";
>      }
> }
>
>
> lib/Analysis/BlockFrequencyInfo.cpp#L211
>
> return BFI->getBlockProfileCount(*getFunction(), BB);
>
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-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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