[hctaw]

Doesn't the referenced data have to be guaranteed to outlive the callee, which would only be true if the callee is guaranteed to return to the calling scope?

You can get around the immutability of the reference if your compiler implements the ABI with copy on write semantics, which I think is a reasonable compromise. But I'm still not certain how you would handle arbitrary control flow that the compiler may not be able to reason about.

If for example your arguments may be behind const references, how would you implement getcontext/swapcontext for your ABI? If everything is an integral value in registers or on the stack then it's really easy, but i would think it would have to be a compiler intrinsic if it depends on the function signature of the calling context, in order to perform the required copies.