[llvm-dev] [llvm-reduce] Reduction to undef/poison/null? (original) (raw)

Florian Hahn via llvm-dev llvm-dev at lists.llvm.org
Mon Aug 30 11:14:20 PDT 2021

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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