[llvm-dev] The semantics of nonnull attribute

[Nicolai Hähnle via llvm-dev]

On Wed, Feb 19, 2020 at 3:51 AM Juneyoung Lee via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I think not_poison (Johannes's used keyword) makes sense. We can simulate the original UB semantics by simply attaching it, as explained.
> For the attributes other than nonnull, we may need more discussion; align attribute seems to be okay with defining it as poison, dereferenceable may need UB even without nonnull (because it needs to be non-poison as shown Nuno's hoisting example).

For reference, the hoisting example was:

f(dereferenceable(4) %p) {
  loop() {
    %v = load %p
    use(%v)
  }
}
=>
f(dereferenceable(4) %p) {
  %v = load %p
  loop() {
    use(%v)
  }
}

Would it be correct to resolve this by saying that dereferenceable(N)
*implies* not_poison? This would be helpful as a clarification of how
it all fits together.

Cheers,
Nicolai