[PATCH] D90529: Allow nonnull attribute to accept poison

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

> In D90529#2449703 <https://reviews.llvm.org/D90529#2449703>, @nlopes wrote:
>
>> For the partial undef memory access example that Juneyoung gave.. Well, maybe we need to make it UB to dereference a non-deterministic value. Doesn't seem like it's a very useful thing to do, and this non-determinism comes from some previous undefined behavior, so it seems fine to just make dereference of partial undef UB. Simplifies things.
>
> There was a discussion for this: https://groups.google.com/g/llvm-dev/c/2Qk4fOHUoAE/m/OxZa3bIhAgAJ
> This partially undef thing is a bit painful.. :/

If we allow partial undef pointers to be dereferenced, then it's nearly impossible to derive `noundef` for pointers. So we will have to rely on frontends to annotate. So the question is whether there's any benefit of allowing such accesses and whether that breaks any frontend if not. Can't think of any reason to allow them and that thread doesn't really provide any either AFAICT.


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