How to get loopinfo in Module Pass

Question

I want to get loopinfo in each function by iterating through functions in Module Pass. My code is as follows:

for (auto &F:M) {   
  if(!F.isDeclaration()){
  LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>(F).getLoopInfo();   
  } 
}

However, there is an error, I think my variable Settings should conform to the first function definition, how should I resolve.

clang-12: /llvmtest/llvm/lib/IR/LegacyPassManager.cpp:1645: virtual std::tuple<llvm::Pass*, bool> {anonymous}::MPPassManager::getOnTheFlyPass(llvm::Pass*, llvm::AnalysisID, llvm::Function&): Assertion `FPP && “Unable to find on the fly pass”’ failed. PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.

Answer 1

You can not do this with the legacy pass manager. In the legacy pass manager, every pass could only get info from same-scoped passes -- module from module, function from function, loop from loop, plus one exception allowing function passes to get data from module passes.

With the new pass manager, you'd create a LoopAnalysisManager and add the analysis pass you want and run it. See https://llvm.org/docs/NewPassManager.html#using-analyses .

Note that most of LLVM is presently written to support both pass managers at once. If you do this, you'll need to write your pass differently from most of LLVM's passes, you can't use the types with names like "WrapperPass" that exist to support both legacy and new pass managers.

Answer 2

In the new pass manager, the PassBuilder class can be used. In a Module pass, a FunctionAnalysisManager instance can be populated with analyses data using registerFunctionAnalyses().

// Inside the Module Pass

llvm::PassBuilder pb;
llvm::FunctionAnalysisManager fam;
pb.registerFunctionAnalyses(fam);

for (auto &f : m) {
  llvm::LoopInfo &li = fam.getResult<llvm::LoopAnalysis>(f);
  ...
}