[llvm] [LAA] Consider unknown SCEVs defined in the loop as IndirectUnsafe. (PR #99239)

[Florian Hahn via llvm-commits]

fhahn wrote:

> I think we may need a deeper change here. Consider the case where an index is not a unknown, but some complicated non-affine AddRec (or other complicated computable SCEV expression involving wrapping). Without proving that the expression can't produce the same value twice, don't we have the same basic problem?
> 
> Actually, given the reasoning above, wouldn't a simple (urem (%base,+,1), 2) be enough to hit this problem? How do we avoid miscompiling that case? Looking at the generated checks, it looks like we bail if we're not dealing with an AddRec. I do notice that we're not checking for NoSelfWrap which I think we probably need to when computing bounds.
> 
> p.s. I'm totally fine with this as a workaround. If you want to land this as a workaround, and then continue the discussion, I'm fine with that.

That's an excellent point, I had a closer look at how we try to detect  non-wrapping add-recs and where the current gaps are; I put up https://github.com/llvm/llvm-project/pull/99577 as an alternative which simply refines the checks for strides accesses

https://github.com/llvm/llvm-project/pull/99239