LLVM unable to get a required analysis - llvm

Question

I am writing a pass that needs information about loops. Therefore I am overriding getAnalysisUsage(AnalysisUsage&) to let the pass manager know that my pass depends on LoopInfoWrapperPass. However, when I try to get the result of that analysis, LLVM asserts that the analysis wasn't required by my pass. Here's a simple pass that I'm having trouble with:
#include <llvm/Pass.h>
#include <llvm/Support/raw_ostream.h>
#include <llvm/Analysis/LoopInfo.h>
struct Example : public ModulePass {
static char ID;
Example() : ModulePass(ID) {}
bool runOnModule(Module& M) override {
errs() << "what\n";
LoopInfo& loops = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
loops.print(errs());
return false;
}
virtual void getAnalysisUsage(AnalysisUsage& AU) const override {
errs() << "here\n";
AU.addRequired<LoopInfoWrapperPass>();
}
};
char Example::ID = 0;
static RegisterPass<Example> X("example", "an example", false, false);
When I run this pass, the two debug statements are printed in the correct order (here then what) but when getAnalysis<LoopInfoWrapperPass>() is called, I get this assertion error:
opt: /home/matt/llvm/llvm/include/llvm/PassAnalysisSupport.h:211: AnalysisType& llvm::Pass::getAnalysisID(llvm::AnalysisID) const [with AnalysisType = llvm::LoopInfoWrapperPass; llvm::AnalysisID = const void*]: Assertion `ResultPass && "getAnalysis*() called on an analysis that was not " "'required' by pass!"' failed.
This is the same method that is given in LLVM's documentation on writing passes, so I'm not quite sure what's going wrong here. Could anyone point me in the right direction?

Answer 1

LoopInfoWrapperPass is derived from FunctionPass. Your Example class, however, derives from ModulePass. It works on the module level, so you'll need to tell LoopInfoWrapperPass which function you want to analyze. Basically, you might want to loop every function f in the module, and use getAnalysis<LoopInfoWrapperPass>(f).
Alternatively, the easiest way to fix the code above is to replace ModulePass with FunctionPass and runOnModule(Module& M) with runOnFunction(Function& F). Then, getAnalysis<LoopInfoWrapperPass>() should work just fine.