[llvm] r374535 - [SCEV] Add stricter verification option.

[Florian Hahn via llvm-commits]

> On Oct 11, 2019, at 17:35, Philip Reames via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> I may be missing the obvious, but why is a symbolic expression which *may* be zero a violation here?  It would seem to be a missed canonicalization, nothing more.
> 

Sorry for the late reply, I unfortunately missed your meal while I was traveling.

My intention here was primary to add a very strict option to surface more potential inconsistencies, as there seem to have surfaced a few issues recently that are related to SCEV expressions not being properly invalidated. It is overly strict, but I think it could be helpful in tracking down some cases we miss at the moment and we would have to evaluate the issues on a case-by-case basis.

> I agree that a SCEV which is known (via isKnownPredicate?) not to be zero is a bug.

I think using isKnownPredicate to check if the delta is zero is something we should do even for the normal -verify-scev option, as it slightly strengthens the check, closing the gap between the overly strict one. My initial builds of test-suite and LLVM bootstrapping seemed to pass when using isKnownPredicate. I would be happy to put up a patch in that direction.

Cheers,
Florian