[llvm] [VPlan] Recognize disjoint 'or' in getSCEVExprForVPValue (PR #218405)

[Mel Chen via llvm-commits]

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@@ -164,6 +164,11 @@ const SCEV *vputils::getSCEVExprForVPValue(const VPValue *V,
     return CreateSCEV({LHSVal, RHSVal}, [&](ArrayRef<SCEVUse> Ops) {
       return SE.getAddExpr(Ops[0], Ops[1], SCEV::FlagAnyWrap, 0);
     });
+  if (match(V, m_BinaryOr(m_VPValue(LHSVal), m_VPValue(RHSVal))))
+    if (cast<VPRecipeWithIRFlags>(V->getDefiningRecipe())->isDisjoint())
+      return CreateSCEV({LHSVal, RHSVal}, [&](ArrayRef<SCEVUse> Ops) {
+        return SE.getAddExpr(Ops[0], Ops[1], SCEV::FlagNSW | SCEV::FlagNUW, 0);
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Mel-Chen wrote:

As I understand it, SCEV treats 'or disjoint' and regular 'add' as the same expression during analysis. Unconditionally attaching 'nuw'/'nsw' flags to an 'add' expression derived from an 'or disjoint' could cause otherwise valid computations to be poisoned.

https://github.com/llvm/llvm-project/pull/218405