[llvm-dev] Problem using BlockFrequencyInfo's getBlockProfileCount (original) (raw)

Dennis Fischer via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 17 08:02:29 PST 2019

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(); 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(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

Thanks for the feedback! I've tried debugging but this didn't get me far. I found a different solution. First, you're right that my code is correct. The problem existed elsewhere. I've upgraded to the latest LLVM 8 commit and - after recompiling my projects - the above code executed without segfaults. Going back to my LLVM 7.01 source build, and again I could no longer observe any segfaults. I know that one of my projects was compiled with clang 7, while LLVM build from source was done with the system's default gcc/g++ 8. Not sure if combining differently compiled libraries is considered UB in C++, but after using the same compiler for both parts errors stopped occurring.

 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<mailto:tejohnson at google.com> |

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