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Hey,<br>
<br>
I am trying to use the BlockFrequencyInfoWrapperPass to obtain hotness <br>
information in my LLVM pass. Attached is a minimal example of code which <br>
creates a SIGSEGV. The pass calls <br>
AU.addRequired<BlockFrequencyInfoWrapperPass>(); in <br>
getAnalysisUsage(..). The problem exists with changed and unchanged IR.<br>
<br>
The binary is instrumented like this: clang input.bc -fprofile-generate <br>
-o output<br>
<br>
The binary runs with selected inputs and the profiles are merged: <br>
llvm-profdata merge input.profraw -output=output.prof<br>
<br>
Then opt runs with arguments: opt input.bc -o output.bc -load mypass.so <br>
-block-freq -pgo-instr-use -pgo-test-profile-file=output.prof <br>
-profile-sample-accurate -mypass<br>
<br>
Is this a bug or am can someone provide an example on how to use <br>
BlockFrequencyInfo correctly?<br>
<br>
<br>
Example code:<br>
<br>
<br>
for (auto& F : M) {<br>
if (F.isDeclaration()) {<br>
continue;<br>
}<br>
auto& bfiPass = getAnalysis<BlockFrequencyInfoWrapperPass>(F);<br>
llvm::BlockFrequencyInfo* BFI = &bfiPass.getBFI();<br>
<br>
//Works - shows all info I want<br>
BFI->print(llvm::dbgs());<br>
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<div>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.</div>
<div>Teresa</div>
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<p>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.<br>
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.
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<blockquote type="cite" cite="mid:CAAe5K+VbZPvghBSLRYRdY_1BZwqARK5wv-g7FLdb=kCqFn91mQ@mail.gmail.com">
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<br>
for (const llvm::BasicBlock& B : F) {<br>
//Fails with SIGSEGV -> *getFunction() returns 0xa<br>
dbgs() << BFI->getBlockProfileCount(&B).getValueOr(0) << "\n";<br>
}<br>
}<br>
<br>
<br>
lib/Analysis/BlockFrequencyInfo.cpp#L211<br>
<br>
return BFI->getBlockProfileCount(*getFunction(), BB);<br>
<br>
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<td style="border-top:2px solid rgb(213,15,37)" nowrap="nowrap">Teresa Johnson |</td>
<td style="border-top:2px solid rgb(51,105,232)" nowrap="nowrap"> Software Engineer |</td>
<td style="border-top:2px solid rgb(0,153,57)" nowrap="nowrap"> <a href="mailto:tejohnson@google.com" target="_blank" moz-do-not-send="true">tejohnson@google.com</a> |</td>
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