For programs that don't use RTTI, the rtti component is just replaced with a zero. This way, vtables that don't use RTTI can still cooperate with vtables that use RTTI since offset calculations on the ABI level would still work. However, if throughout your whole program you don't use RTTI at all (such as the embedded case), then this is just an unused pointer-sized component that's wasting space. This adds an experimental option for removing the RTTI component from the vtable.
- This is only allowed when RTTI is disabled, so we don't have to worry about things like typeid or dynamic_cast.
- This is a "use at your own risk" since, similar to relative vtables, everything must be compiled with this since it's an ABI breakage. That is, a program compiled with this is not guaranteed to work with a program compiled without this, even if RTTI is disabled for both programs.
Note that this is a completely different ABI flavor orthogonal to the relative-vtables ABI. That is, they can be enabled/disabled independently.