[llvm] [SCEV] Preserve nuw on scaled recurrence start values. (PR #227458)

[Nikita Popov via llvm-commits]

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@@ -3344,6 +3348,10 @@ SCEVUse ScalarEvolution::getMulExpr(SmallVectorImpl<SCEVUse> &Ops,
       }
 
       const SCEV *NewRec = getAddRecExpr(NewOps, AddRec->getLoop(), Flags);
+      // Flags also hold for the canonical recurrence in the same loop.
+      if (auto *CanonicalRec = dyn_cast<SCEVAddRecExpr>(NewRec->getCanonical());
+          CanonicalRec && CanonicalRec->getLoop() == AddRec->getLoop())
+        setNoWrapFlags(const_cast<SCEVAddRecExpr *>(CanonicalRec), Flags);
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nikic wrote:

Hm, I'm not entirely sure if this is always safe in this form (maybe the canonical addrec happens to fold in some different way?)

Where do we end up needing this?

https://github.com/llvm/llvm-project/pull/227458