[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[via llvm-commits]

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@@ -593,13 +601,35 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
     Value *Addend = State.get(getOperand(1), Part, /* IsScalar */ true);
     return Builder.CreatePtrAdd(Ptr, Addend, Name);
   }
+  case VPInstruction::ResumePhi: {
+    if (Part != 0)
+      return State.get(this, 0, /*IsScalar*/ true);
+    Value *IncomingFromVPlanPred =
+        State.get(getOperand(0), Part, /* IsScalar */ true);
+    Value *IncomingFromOtherPreds =
+        State.get(getOperand(1), Part, /* IsScalar */ true);
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ayalz wrote:

Is it conceivable for the scalar preheader to have a single predecessor, namely, VPlanPred/middle-block? I.e., when trip count is known to be larger that VFxUF and no (other) runtime checks are needed to bypass the vector loop?

https://github.com/llvm/llvm-project/pull/94760