> Who cares about the specifics of how that operation is implemented. It is a good way to EXPRESS the operation in the C++ language, and in principle, the codegen can be optimal. If it isn't, that's a quality-of-implementation issue and not relevant to the C++ standard.

Ok, here's a challenge. Try implementing the standard set of 512bit operations in terms of 128bit casts, and let's see how fun that is.
Let's see if it is the "better" interface.