[llvm] [VPlan] Materialize VectorTripCount in narrowInterleaveGroups. (PR #182146)

[Ramkumar Ramachandra via llvm-commits]

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@@ -9363,12 +9363,28 @@ static void fixScalarResumeValuesFromBypass(BasicBlock *BypassBlock, Loop *L,
   // Fix induction resume values from the additional bypass block.
   IRBuilder<> BypassBuilder(BypassBlock, BypassBlock->getFirstInsertionPt());
   for (const auto &[IVPhi, II] : LVL.getInductionVars()) {
-    auto *Inc = cast<PHINode>(IVPhi->getIncomingValueForBlock(PH));
     Value *V = createInductionAdditionalBypassValues(
         IVPhi, II, BypassBuilder, ExpandedSCEVs, MainVectorTripCount,
         LVL.getPrimaryInduction());
     // TODO: Directly add as extra operand to the VPResumePHI recipe.
-    Inc->setIncomingValueForBlock(BypassBlock, V);
+    if (auto *Inc = dyn_cast<PHINode>(IVPhi->getIncomingValueForBlock(PH))) {
+      Inc->setIncomingValueForBlock(BypassBlock, V);
+    } else {
+      // If the resume value in the scalar preheader was simplified (e.g., when
+      // narrowInterleaveGroups optimized away the resume PHIs), create a new
+      // PHI to merge the bypass value with the original value.
+      Value *OrigVal = IVPhi->getIncomingValueForBlock(PH);
+      PHINode *NewPhi =
+          PHINode::Create(IVPhi->getType(), pred_size(PH), "bc.resume.val",
+                          PH->getFirstNonPHIIt());
+      for (auto *Pred : predecessors(PH)) {
+        if (Pred == BypassBlock)
+          NewPhi->addIncoming(V, Pred);
+        else
+          NewPhi->addIncoming(OrigVal, Pred);
+      }
+      IVPhi->setIncomingValueForBlock(PH, NewPhi);
+    }
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artagnon wrote:

This is an unfortunate hack that is compounded by the existing fixScalarResumeValuesFromBypass hack?

https://github.com/llvm/llvm-project/pull/182146