How to get the arguments of a function call in LLVM?

Question

I want to write an LLVM pass that'll extract the arguments of function calls. If the argument is a constant one, my objective is to recover what that constant is.

The IR looks like

%2 = call noalias i8* @malloc(i64 512) #3

The LLVM pass looks like

bool runOnFunction(Function &F) override {
    for (auto& B : F) {
        for (auto& I : B) {
            if(CallInst* call_inst = dyn_cast<CallInst>(&I)) {
                Function* fn = call_inst->getCalledFunction();
                StringRef fn_name = fn->getName();
                errs() << fn_name << " : " << call_inst->getArgOperand(0) << "\n";
                for(auto arg = fn->arg_begin(); arg != fn->arg_end(); ++arg) {
                    errs() << *arg << "\n";
                }
            }
        }
    }

    return false;
} 

If I run the pass through opt, it produces the following

malloc : 0x3df3f40
i64 %0

What does 0x3df3f40 represent? Instead of i64 and 512, why does it produce i64 and %0?

Answer 1

It is a pointer to Value. Try cast<>ing it to ConstantInt and then calling getValue():

for(auto arg = fn->arg_begin(); arg != fn->arg_end(); ++arg) {
  if(auto* ci = dyn_cast<ConstantInt>(arg))
    errs() << ci->getValue() << "\n";
  errs() << *arg << "\n";
}