Considering that `volatile register _Atomic int x;` is a totally legal statement, and two [1] of those keywords have semantics that are only relevant for memory operations, I'd say that the "register" keyword isn't sufficient to distinguish register from memory.

[1] Okay, the official semantics of "volatile" strictly speaking are completely orthogonal to memory, but the practical effect that everyone agrees on (don't optimize this memory access) is inherently oriented towards memory, and the fact that the semantics are so vague about what it actually does is partially a reflection of the fact the semantics themselves don't distinguish between register and memory.