[LV] Move selecting vectorization factor logic to LVP (NFC).

Split off from D143938. This moves the planning logic to select the
vectorization factor to LoopVectorizationPlanner as a step towards only
computing costs for individual VFs in LoopVectorizationCostModel and do
planning in LVP.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D150197

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h

@@ -25,6 +25,7 @@
 #define LLVM_TRANSFORMS_VECTORIZE_LOOPVECTORIZATIONPLANNER_H
 
 #include "VPlan.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/Support/InstructionCost.h"
 
 namespace llvm {
@@ -217,6 +218,16 @@ struct VectorizationFactor {
   }
 };
 
+/// ElementCountComparator creates a total ordering for ElementCount
+/// for the purposes of using it in a set structure.
+struct ElementCountComparator {
+  bool operator()(const ElementCount &LHS, const ElementCount &RHS) const {
+    return std::make_tuple(LHS.isScalable(), LHS.getKnownMinValue()) <
+           std::make_tuple(RHS.isScalable(), RHS.getKnownMinValue());
+  }
+};
+using ElementCountSet = SmallSet<ElementCount, 16, ElementCountComparator>;
+
 /// A class that represents two vectorization factors (initialized with 0 by
 /// default). One for fixed-width vectorization and one for scalable
 /// vectorization. This can be used by the vectorizer to choose from a range of
@@ -280,6 +291,9 @@ class LoopVectorizationPlanner {
 
   SmallVector<VPlanPtr, 4> VPlans;
 
+  /// Profitable vector factors.
+  SmallVector<VectorizationFactor, 8> ProfitableVFs;
+
   /// A builder used to construct the current plan.
   VPBuilder Builder;
 
@@ -342,6 +356,12 @@ class LoopVectorizationPlanner {
   /// Check if the number of runtime checks exceeds the threshold.
   bool requiresTooManyRuntimeChecks() const;
 
+  /// \return The most profitable vectorization factor and the cost of that VF
+  /// for vectorizing the epilogue. Returns VectorizationFactor::Disabled if
+  /// epilogue vectorization is not supported for the loop.
+  VectorizationFactor
+  selectEpilogueVectorizationFactor(const ElementCount MaxVF);
+
 protected:
   /// Build VPlans for power-of-2 VF's between \p MinVF and \p MaxVF inclusive,
   /// according to the information gathered by Legal when it checked if it is
@@ -376,6 +396,20 @@ class LoopVectorizationPlanner {
   void adjustRecipesForReductions(VPBasicBlock *LatchVPBB, VPlanPtr &Plan,
                                   VPRecipeBuilder &RecipeBuilder,
                                   ElementCount MinVF);
+
+  /// \return The most profitable vectorization factor and the cost of that VF.
+  /// This method checks every VF in \p CandidateVFs.
+  VectorizationFactor
+  selectVectorizationFactor(const ElementCountSet &CandidateVFs);
+
+  /// Returns true if the per-lane cost of VectorizationFactor A is lower than
+  /// that of B.
+  bool isMoreProfitable(const VectorizationFactor &A,
+                        const VectorizationFactor &B) const;
+
+  /// Determines if we have the infrastructure to vectorize the loop and its
+  /// epilogue, assuming the main loop is vectorized by \p VF.
+  bool isCandidateForEpilogueVectorization(const ElementCount VF) const;
 };
 
 } // namespace llvm