B must be a non-virtual base class of D.

Returns the offset from the beginning of a D object to the start of the B base subobject within it. The result might be zero. It is the offset (in bytes) that must be added to a D* p to implement the equivalent of static_cast<B*>(p). For example:

__asm B* my_static_base_cast(D* /*p*/) {
    if __offsetof_base(D, B) <> 0 // optimise zero offset case
        ADD r0, r0, #__offsetof_base(D, B)
    endif
    MOV pc, lr
}