[llvm-dev] [llvm-reduce] Reduction to undef/poison/null?

[Florian Hahn via llvm-dev]

> On 30 Aug 2021, at 19:59, David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Nicer because it's less likely to introduce new UB? Or some other reason?
> 

Using undef/poison is problematic, because there are multiple ways this could cause new UB (e.g. branch on undef, passing poison/undef to a function with a noundef argument).

I’m not sure if using zero will work well in certain cases, because it can introduce UB as well (e.g. load from null, passing as nonnull argument). 

I think ideally we would have a way to materialise values we know nothing about, but are not undef. Perhaps we could add some oracle function, but that would come with its own drawbacks.

Cheers,
Florian