[llvm] r333106 - [InstCombine] Fold unfolded masked merge pattern with variable mask!

[Sanjoy Das via llvm-commits]

On Wed, May 30, 2018 at 10:34 PM, Chandler Carruth <chandlerc at gmail.com> wrote:
> This is technically UB according to C++, but it isn't clear at all this is
> UB in C. More practically speaking, real code really does assume that
> "failing to observe" uninitialized memory is sufficient.

That's ... scary!  Unless we have a precise definition of what
"failing to observe" means I think it will be difficult to model this
in LLVM.  E.g. is (x - x) failing to observe x?

-- Sanjoy