llvm dev - Apr 2017 - How to get the arguments of a function call in LLVM?

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 `164` and `512`, why does it
produce `i64` and `%0`?

-- 

Thanks & Regards,
Dipanjan
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On 19 April 2017 at 09:05, Dipanjan Das via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> If I run the pass through `opt`, it produces the following
>
>     malloc : 0x3df3f40
>     i64 %0
>
> What does `0x3df3f40` represent?
getArgOperand returns a pointer to a Value object. You're printing
that pointer directly rather than calling its print method to get a
sensible representation.
> Instead of `164` and `512`, why does it produce `i64` and `%0`?
That's because you're iterating through the argument as it exists at
the function's definition/declaration rather than the callsite. You
should be using the CallInst's accessors rather than the Function's if
you want the arguments used for this particular call.

Cheers.

Tim.