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> C does not distinguish between registers and memory Just nitpicking, but doesn't C have the register keyword? (I know it does nothing at all nowadays, but AF

• jcranmer 4y ago
  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.
• tialaramex 4y ago
  C's register keyword is a storage specifier, it actually does do something, what it does is announce "This might not have an address" (your CPU registers don't have addresses) and so you can't make a pointer to it with the unary & address operator. Your C compiler might conclude - even if it doesn't decide to put the variable in a register - that since the variable notionally doesn't have an address some optimisations are available which it can't otherwise be sure are safe.