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

[Johannes Doerfert via llvm-dev]

On 8/30/21 1:36 PM, Arthur Eubanks wrote:
> 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.

Yeah, maybe. I think it's clear undef is often not the best choice.


> 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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