[PATCH] D98509: [LV] Calculate max feasible scalable VF.

Tue May 4 15:25:34 PDT 2021

hubert.reinterpretcast added a comment.

In D98509#2736317 <https://reviews.llvm.org/D98509#2736317>, @fhahn wrote:

> In D98509#2732628 <https://reviews.llvm.org/D98509#2732628>, @hubert.reinterpretcast wrote:
>
>> In D98509#2725892 <https://reviews.llvm.org/D98509#2725892>, @hubert.reinterpretcast wrote:
>>
>>> In D98509#2725672 <https://reviews.llvm.org/D98509#2725672>, @sdesmalen wrote:
>>>
>>>> Reverting the patch for a few days is fine, I've pushed rG51d648c119d7773ce6fb809353bd6bd14bca8818 <https://reviews.llvm.org/rG51d648c119d7773ce6fb809353bd6bd14bca8818> for now.
>>>
>>> Thank you! Our internal CI is green again. I'll update with the investigation progress once it gets to a meaningful point.
>>
>> I haven't reached the root cause yet, but it appears that the issue we're hitting can be avoided if the `MaxSafeVF` parameter is made pass-by-reference instead of pass-by-value.
>> @sdesmalen, I hope the following is not too intrusive a change to your patch.
>
> @hubert.reinterpretcast can you provide an IR test case that triggers the failure with sanitizers?

The build compiler on the specific bot (due to what is currently shipping on the platform) is a proprietary compiler that does not use LLVM IR. We have not reached that point where we can confirm that this is a miscompile issue in the build compiler, but I think there is every chance that would be the conclusion (now that the reduction has reached the point where it is -- we have also encountered latent UB in LLVM source in the past). Prior issues of this sort have taken around two weeks to fix after it is reported to the appropriate team (which is delayed while I'm putting together a reduction). I had spent my whole weekend narrowing this down. I am not sure how much time I will have during the week to make further progress. I think we will be able to deploy a fix to the buildbot in early June.

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https://reviews.llvm.org/D98509