[PATCH] D49042: [LangRef] Clarify alloca of zero bytes.

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

In https://reviews.llvm.org/D49042#1155256, @sanjoy wrote:

> Does this mean we can "construct" `undef` as:
>
>   x = alloca 0
>   y = alloca 0
>   undef = x == y  // Non-deterministically 0 or 1
>
>
> ?  If so, this is a problem since it means even if we spec un-initialized memory as `poison` we still have `undef` in the IR (with all the problems it brings).


It's complicated...
x, y are still different objects. Comparing pointers of different objects yields false, except for the +n case, since p+n == q may be true at execution time if p,q are contiguous objects.
If x,y are zero-sized, then x == y is the +n case.

We either define p+n == q as undef or as non-deterministic value (freeze(poison) essentially). Both options have prons and cons (hard to hoist vs hard to copy), but we have to live with them, unfortunately. So alloc 0 doesn't bring any new difficulty.


Repository:
  rL LLVM

https://reviews.llvm.org/D49042