[llvm] [LV] Avoid bailing on SCEVUnknown in uniformity-rew (PR #160144)

[Florian Hahn via llvm-commits]

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@@ -528,14 +528,6 @@ class SCEVAddRecForUniformityRewriter
     return SCEVRewriteVisitor<SCEVAddRecForUniformityRewriter>::visit(S);
   }
 
-  const SCEV *visitUnknown(const SCEVUnknown *S) {
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fhahn wrote:

Hmm, I don't think removing all checks for SCEVUnknown is safe.

The rewriter is used to rewrite a SCEV expression for each lane, and checks if the result is the same SCEV for all lanes.

I think with this change, SCEVUnknown would always be treated as uniform, but they could be any load instruction for example, which may or may not read a different value from memory on every iteration? (needs a test case I think).

We might be able to refine the checks to handle additional cases, e.g. read-only loops?

https://github.com/llvm/llvm-project/pull/160144