D is a class type and
a nonstatic member function defined in a virtual base class of
__mcall_offsetof_vbase() returns the
negative offset from the value of the vtable pointer of the vtable
slot that holds the base offset (from the beginning of a
to the start of the base that
f is defined in).
The base offset is the
necessary when making a call to
f with a pointer
The offset returns a positive number that then has to be subtracted from the value of the vtable pointer.