[IshKebab]

I think you misunderstood my suggestion. It isn't that misaligned accesses must either all succeed or all fail. That's not possible in general because of MMIO devices.

The suggestion is that each individual access must either succeed or trap. Those are the only possible outcomes, but different accesses can result in different outcomes.

[1718627440]

And you misunderstand me. Your proposal means, that the compiler, must emit an individual access with the right values at the right time. If an access may succeed or fail, then the compiler can not just convert it into an aligned read, or not a read at all.

If your proposal includes, that a trapping access gets treated by the compiler as if it didn't and the compiler emits code that performs all kind of side-effects, that are logically independent if it would not trap, before the access, then you are back to undefined behaviour under a different name.