> While you would expect a comparison to another pointer to be legal, I don't believe it has to be consistent. That's the whole point of undefined behavior. In one sense, it's like the question of whether NaN == NaN. In many implementations, that expression is not necessarily true--even though based on your pointer example logic it should be.

Right, this is a question of whether alloca(0) is undefined behavior, an unspecified return value, or implementation-defined behavior. And I argue that alloca's behavior is implementation-defined in the first place, so it LLVM-gcc wants alloca(0) to be UB it needs to define that :)