> 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.
Aren't you just describing multiprecision integer math, as seen in e.g.
(note: four years ago, and I haven't revisited it in two years; I don't know how Abseil, Boost.Multiprecision, or Clang's _BitInt stack up these days)
? You don't need any particular core-language support for that — although it doesn't hurt. You certainly don't need standard library support. There are plenty of third-party libraries that implement wide math already.
–Arthur