That's not a reason to implement a standard ABI, that's a reason to not use the standard ABI, a bad one at that. Remember my point about making classes with no standard ABI inherit from ones that do? There's no reason std::string could not inherit from say std::stable:: linked_list_string on said mythical cpu. You would still get your performance gains without sacrificing a standard ABI in the process. The point of a standard ABI is to ensure functions and data in said ABI follow the same size definitions. For example intmax_t related functions just need to be inlines that call the correct functions matching the current size of intmax_t. A standard ABI does not mean optimisations cannot be applied to the implementation of the ABI, only that the data and calls will be of a fixed size.