offset of extra function argument


I am instrumenting certain calls, and want to add an extra argument.
original: foo(int x, int y)
changed into
modified: foo(int x, int y, int EXTRA)

This is in opt, before lowering.
Given the list of original arguments, is it possible to tell the stack offset of the EXTRA argument?

Thank you,

On many architectures, like x86-64, EXTRA isn't on the stack at all.
In general, LLVM doesn't calculate the locations of arguments until
the instruction selection (i.e. deep in CodeGen).