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

[Roman Lebedev via llvm-dev]

I've been thinking we should be using `freeze poison`,
but i don't think this question matters for the patch at hand,
it should just stick to the current practice of using undef.

Roman.

On Mon, Aug 30, 2021 at 9:14 PM Florian Hahn via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
>
>
> > 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
>
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