[llvm] LSV: forbid load-cycles when vectorizing; fix bug (PR #104815)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1499,6 +1499,12 @@ std::optional<APInt> Vectorizer::getConstantOffset(Value *PtrA, Value *PtrB,
 
   // Try to compute B - A.
   const SCEV *DistScev = SE.getMinusSCEV(SE.getSCEV(PtrB), SE.getSCEV(PtrA));
+  if (DistScev->isZero()) {
+    // A load in the chain is dependent on another load in the chain, and
+    // attempting to vectorize this chain would create a cycle.
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artagnon wrote:

The SCEV expression corresponding to `%gep.x.2` is computed as the SCEVUnknown `%x`, which I think is correct. Did you expect an AddExpr?

It took me some time to figure out the fix, although it's quite a simple one in its final form: I think the patch correctly identifies the root cause.

https://github.com/llvm/llvm-project/pull/104815