[PATCH] D136097: InstSimplify: Fold fdiv nnan nsz x, 0 -> inf

Tue Oct 18 01:28:42 PDT 2022

nlopes added a comment.

In D136097#3863230 <https://reviews.llvm.org/D136097#3863230>, @spatel wrote:

> In D136097#3863158 <https://reviews.llvm.org/D136097#3863158>, @nlopes wrote:
>
>> In D136097#3862946 <https://reviews.llvm.org/D136097#3862946>, @spatel wrote:
>>
>>> In D136097#3862928 <https://reviews.llvm.org/D136097#3862928>, @kpn wrote:
>>>
>>>> It looks like by 754 sec 7.3 a (-x)/(+0) == -Inf, but the nsz is documented as saying the sign of the result is "insignificant", so I think we're good here.
>>>>
>>>> LGTM
>>>
>>> "nsz" means the sign of a **zero** result is insignificant, not any result. -42.0 / 0.0 should be -Inf, not +Inf?
>>
>> It's documented as ignoring the sign of a zero argument or a zero result.
>> Thus, when nsw is involved, we should consider the result of -42.0 / 0.0 and -42.0 / -0.0. This case the 0.0 is a constant, but that doesn't matter.
>>
>> Alive2 is correct here AFAICT.
>
> Ah, thanks for clearing that up. The "nsz" wording has always been a less solid than I'd like, but I'm not sure how to change it. 
> I suspect this transform would cause some fallout with users because it's a stretch to go from -0.0 isn't different than 0.0, therefore, -Inf isn't different than +Inf. There was a suggestion in a previous discussion that we really need entirely new FP types to use with fast-math-flags, so it's clear that -0.0, Inf, or NaN values simply do not exist when the corresponding compile flag is used, but I don't think anyone has looked seriously at implementing that. There's more discussion about the general problem here:
> https://github.com/llvm/llvm-project/issues/51601

That bug report seems to be a bit orthogonal. The issue there is that the fmath flags are attached to some instructions, but not all. Since values can't have fmath flags themselves, things like function arguments, when flowing directly to instructions without fmath flags make clang lose information when generating IR. Hence the proposal to add fmath to loads (I don't think you need fmath in store).

Regarding the semantics of nsz, I don't have any opinion as I'm clueless about FP. I'm just reading the current semantics to the letter, but I'm happy to implement whatever semantics you want in Alive2.

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