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

[Johannes Doerfert via llvm-dev]

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 ∽
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