Roman Lebedev via llvm-dev
2021-Aug-30 18:22 UTC
[llvm-dev] [llvm-reduce] Reduction to undef/poison/null?
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 > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Johannes Doerfert via llvm-dev
2021-Aug-30 18:31 UTC
[llvm-dev] [llvm-reduce] Reduction to undef/poison/null?
On 8/30/21 1:22 PM, Roman Lebedev via llvm-dev wrote:> 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.I like freeze poison. It conveys the idea w/o making things UB all the time. It basically is an oracle w/o the side effects. FWIW, when I ported tests to the Attributor, e.g., from ArgumentPromotion or IPSCCP, I had to manually remove all the UB that made the test meaningless first. In general, tests that contain statically provable UB are less likely to be meaningful over time and/or be reusable. ~ Johannes> 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 >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-- ────────────────────── ∽ Johannes Doerfert ∽ ∽ Pronouns: he/him ∽ ∽ Researcher @ ANL ∽