LLVM LoopInfo in FunctionPass doesn't compile

Why your code could not be build

AU.addRequired<typename passclass>() need a type of LLMV::Pass, however what you pass in is LoopInfo, which is just an LLVM internal class for loop information maintenance. It does not has a field ID.

LoopInfoWrapperPass should be used instead

If you want to get the loop information. Try to change it to AU.addRequired<LoopInfoWrapperPass> as shown in the LLVM Write a new Pass document. LoopInfoWrapperPass is used to generate LoopInfo.

LoopInfo should be get from the pass

There is also a problem in your code about how to get the LoopInfo, you are trying to use new to create a LoopInfo, what you get will be an empty LoopInfo.

The "should work" code for your question

Following is a modified version of your code, which could printout expected informations.

#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "iostream"
#include "llvm/Pass.h"
#include "llvm/IR/InstIterator.h"
#include <llvm/IR/Instructions.h>
#include <llvm/Analysis/LoopInfo.h>


using namespace llvm;


namespace {
  struct CFG : public FunctionPass {
    static char ID; // Pass identification, replacement for typeid
    CFG() : FunctionPass(ID) {}

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.setPreservesCFG();
      AU.addRequired<LoopInfoWrapperPass>();
    }
    bool runOnFunction(Function &F) override {
      LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
      errs().write_escaped(F.getName());
      errs() << " : ";
      for( Function::iterator b = F.begin() , be = F.end(); b != be; ++b){
        errs() << "\n\t BB : ";
        bool isLoop = LI.getLoopFor(b);
        if(isLoop){ 
          errs() << "loop{";
        }
        for(BasicBlock::iterator i = b->begin() , ie = b->end(); i!=ie; ++i){
          if( isa<CallInst>(&(*i)) || isa<InvokeInst>(&(*i))){
            errs() << cast<CallInst>(&(*i))->getCalledFunction()->getName() << "\t";
          }
        }
        if(isLoop){ 
          errs() << "}";
        }
     }
     errs() << '\n';
     return false;
   }
  };
}

char CFG::ID = 0;
static RegisterPass<CFG> X("CFG", "Gen CFG",true ,true);

For the following code feed to LLVM opt:

#include <stdio.h>

#define ARRAY_SIZE 100

int foo(int* a , int n) {
  int i;
  int sum = 0;
  for (; i < n; i++) {
    sum += a[i];
  }
  return sum;
}


int main() {
  int a[ARRAY_SIZE] = {1};

  int sum = foo(a, ARRAY_SIZE);

  printf("sum:0x%x\n", sum);
  return 0;
}

The output will be:

foo : 
     BB : 
     BB : loop{}
     BB : loop{}
     BB : loop{}
     BB : 
main : 
     BB : llvm.memset.p0i8.i64  foo printf