referencing call signature in llvm for k&r style function calls

i'm looking at IR in one of my passes.

if I have something like:

float p();

the IR "call instruction" shows the calling function as being null.

what is the correct way to determine the signature?

in this case

float p();

int main() {
float x1 = p();
float x2 = p(x1);

I need to know the parameters and return type.