[VPlan] Address comments missed in #142309.

Address additional comments from
#142309.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -10321,8 +10321,9 @@ bool LoopVectorizePass::processLoop(Loop *L) {
 
       // TODO: Move to general VPlan pipeline once epilogue loops are also
       // supported.
-      VPlanTransforms::runPass(VPlanTransforms::materializeVectorTripCount,
-                               BestPlan, VF.Width, IC, PSE);
+      VPlanTransforms::runPass(
+          VPlanTransforms::materializeConstantVectorTripCount, BestPlan,
+          VF.Width, IC, PSE);
 
       LVP.executePlan(VF.Width, IC, BestPlan, Unroller, DT, false);
 
@@ -10393,8 +10394,9 @@ bool LoopVectorizePass::processLoop(Loop *L) {
                                Checks, BestPlan);
         // TODO: Move to general VPlan pipeline once epilogue loops are also
         // supported.
-        VPlanTransforms::runPass(VPlanTransforms::materializeVectorTripCount,
-                                 BestPlan, VF.Width, IC, PSE);
+        VPlanTransforms::runPass(
+            VPlanTransforms::materializeConstantVectorTripCount, BestPlan,
+            VF.Width, IC, PSE);
 
         LVP.executePlan(VF.Width, IC, BestPlan, LB, DT, false);
         ++LoopsVectorized;

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -3222,27 +3222,34 @@ void VPlanTransforms::materializeBroadcasts(VPlan &Plan) {
   }
 }
 
-void VPlanTransforms::materializeVectorTripCount(
+void VPlanTransforms::materializeConstantVectorTripCount(
     VPlan &Plan, ElementCount BestVF, unsigned BestUF,
     PredicatedScalarEvolution &PSE) {
   assert(Plan.hasVF(BestVF) && "BestVF is not available in Plan");
   assert(Plan.hasUF(BestUF) && "BestUF is not available in Plan");
 
   VPValue *TC = Plan.getTripCount();
   // Skip cases for which the trip count may be non-trivial to materialize.
+  // I.e., when a scalar tail is absent - due to tail folding, or when a scalar
+  // tail is required.
   if (!Plan.hasScalarTail() ||
       Plan.getMiddleBlock()->getSingleSuccessor() ==
           Plan.getScalarPreheader() ||
       !TC->isLiveIn())
     return;
+
   // Materialize vector trip counts for constants early if it can simply
   // be computed as (Original TC / VF * UF) * VF * UF.
+  // TODO: Compute vector trip counts for loops requiring a scalar epilogue and
+  // tail-folded loops.
   ScalarEvolution &SE = *PSE.getSE();
   auto *TCScev = SE.getSCEV(TC->getLiveInIRValue());
+  if (!isa<SCEVConstant>(TCScev))
+    return;
   const SCEV *VFxUF = SE.getElementCount(TCScev->getType(), BestVF * BestUF);
   auto VecTCScev = SE.getMulExpr(SE.getUDivExpr(TCScev, VFxUF), VFxUF);
-  if (auto *NewC = dyn_cast<SCEVConstant>(VecTCScev))
-    Plan.getVectorTripCount().setUnderlyingValue(NewC->getValue());
+  if (auto *ConstVecTC = dyn_cast<SCEVConstant>(VecTCScev))
+    Plan.getVectorTripCount().setUnderlyingValue(ConstVecTC->getValue());
 }
 
 void VPlanTransforms::materializeBackedgeTakenCount(VPlan &Plan,

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -252,9 +252,10 @@ struct VPlanTransforms {
 
   // Materialize vector trip counts for constants early if it can simply be
   // computed as (Original TC / VF * UF) * VF * UF.
-  static void materializeVectorTripCount(VPlan &Plan, ElementCount BestVF,
-                                         unsigned BestUF,
-                                         PredicatedScalarEvolution &PSE);
+  static void
+  materializeConstantVectorTripCount(VPlan &Plan, ElementCount BestVF,
+                                     unsigned BestUF,
+                                     PredicatedScalarEvolution &PSE);
 
   /// Materialize the backedge-taken count to be computed explicitly using
   /// VPInstructions.