This is the this adjustment necessary when making a call to f with a pointer to a D. It is either zero if f is found in D or the same as __offsetof_base(D,B), where B is a nonvirtual base class of D that contains f.

Where D is a class type and f is a nonstatic member function defined in D or a nonvirtual base class of D.

If __mcall_this_offset(D,f) is used when f is found in a virtual base class of D it returns an arbitrary value designed to cause an assembly error if used. This is so that such invalid uses of __mcall_this_offset can occur in sections of assembly code that are to be skipped.