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

[Arthur Eubanks via llvm-dev]

I frequently use llvm-reduce just to minimize a crash caused by some change
and present that to the author of a change to look at. I don't think that
having tons of freeze poisons in a repro file is nice to work with. If a
crash repros with a `0` as opposed to a `freeze poison` the `0` seems much
more appealing to present.

We could add a flag to reduce to the various options here if people have
different needs.

On Mon, Aug 30, 2021 at 11:31 AM Johannes Doerfert via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
> >>
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>
> --
> ──────────────────────
> ∽ Johannes Doerfert ∽
> ∽ Pronouns: he/him  ∽
> ∽ Researcher @ ANL  ∽
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