[llvm] [VPlan] Introduce reduction selects for tail folding in foldTailByMasking. NFCI (PR #192987)

[Luke Lau via llvm-commits]

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@@ -1165,18 +1165,24 @@ void VPlanTransforms::foldTailByMasking(VPlan &Plan) {
     if (match(&R, m_ExtractLastPart(m_VPValue(V))))
       NeedsPhi[V].push_back(&R);
 
-  // Insert phis with a poison incoming value for past the end of the tail.
+  // Insert phis for values coming past the end of the tail.
   Builder.setInsertPoint(Latch, Latch->begin());
   VPTypeAnalysis TypeInfo(Plan);
   for (const auto &[V, Users] : NeedsPhi) {
     if (isa<VPIRValue>(V))
       continue;
-    // TODO: For reduction phis, use phi value instead of poison so we can
-    // remove the special casing for tail folding in
-    // LoopVectorizationPlanner::addReductionResultComputation
-    VPValue *Poison =
+    VPValue *Backedge =
         Plan.getOrAddLiveIn(PoisonValue::get(TypeInfo.inferScalarType(V)));
-    VPInstruction *Phi = Builder.createScalarPhi({V, Poison});
+    VPIRFlags Flags;
+    auto *RedIt = find_if(V->users(), IsaPred<VPReductionPHIRecipe>);
+    auto *RdxPhi =
+        RedIt != V->user_end() ? cast<VPReductionPHIRecipe>(*RedIt) : nullptr;
+    if (RdxPhi && !RdxPhi->isInLoop()) {
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lukel97 wrote:

Asserted in 64d477ebe8d200d22341ecddcb852a1fe9e278ad

https://github.com/llvm/llvm-project/pull/192987