llvm dev - Nov 2020 - How to identify a library function call in IR?

Hi all,

Recently I've been working on LLVM IR. I need to identify function calls 
to a dynamic library, like glibc. I found isDeclaration() of Function 
class might be useful. So could it identify external function calls? Is 
there a more graceful way of identifying such calls?

Moreover, when used in a link-time optimization pass, will it give out 
different results from those when used in normal optimization pass?

Thanks and Regards.