[LV] Retrieve reduction resume values directly for epilogue vec. (NFC)

Use the reduction resume values from the phis in the scalar header,
instead of collecting them in a map. This removes some complexity from
the general executePlan code paths and pushes it to only the epilogue
vectorization part.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h

@@ -435,11 +435,10 @@ class LoopVectorizationPlanner {
   /// \p ExpandedSCEVs is passed during execution of the plan for epilogue loop
   /// to re-use expansion results generated during main plan execution.
   ///
-  /// Returns a mapping of SCEVs to their expanded IR values and a mapping for
-  /// the reduction resume values. Note that this is a temporary workaround
-  /// needed due to the current epilogue handling.
-  std::pair<DenseMap<const SCEV *, Value *>,
-            DenseMap<const RecurrenceDescriptor *, Value *>>
+  /// Returns a mapping of SCEVs to their expanded IR values.
+  /// Note that this is a temporary workaround needed due to the current
+  /// epilogue handling.
+  DenseMap<const SCEV *, Value *>
   executePlan(ElementCount VF, unsigned UF, VPlan &BestPlan,
               InnerLoopVectorizer &LB, DominatorTree *DT,
               bool IsEpilogueVectorization,

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -676,11 +676,6 @@ class InnerLoopVectorizer {
   /// Structure to hold information about generated runtime checks, responsible
   /// for cleaning the checks, if vectorization turns out unprofitable.
   GeneratedRTChecks &RTChecks;
-
-  // Holds the resume values for reductions in the loops, used to set the
-  // correct start value of reduction PHIs when vectorizing the epilogue.
-  SmallMapVector<const RecurrenceDescriptor *, PHINode *, 4>
-      ReductionResumeValues;
 };
 
 /// Encapsulate information regarding vectorization of a loop and its epilogue.
@@ -7426,10 +7421,9 @@ static void addRuntimeUnrollDisableMetaData(Loop *L) {
 }
 
 // Check if \p RedResult is a ComputeReductionResult instruction, and if it is
-// create a merge phi node for it and add it to \p ReductionResumeValues.
+// create a merge phi node for it.
 static void createAndCollectMergePhiForReduction(
     VPInstruction *RedResult,
-    DenseMap<const RecurrenceDescriptor *, Value *> &ReductionResumeValues,
     VPTransformState &State, Loop *OrigLoop, BasicBlock *LoopMiddleBlock,
     bool VectorizingEpilogue) {
   if (!RedResult ||
@@ -7487,13 +7481,9 @@ static void createAndCollectMergePhiForReduction(
   OrigPhi->setIncomingValue(SelfEdgeBlockIdx, BCBlockPhi);
   Instruction *LoopExitInst = RdxDesc.getLoopExitInstr();
   OrigPhi->setIncomingValue(IncomingEdgeBlockIdx, LoopExitInst);
-
-  ReductionResumeValues[&RdxDesc] = BCBlockPhi;
 }
 
-std::pair<DenseMap<const SCEV *, Value *>,
-          DenseMap<const RecurrenceDescriptor *, Value *>>
-LoopVectorizationPlanner::executePlan(
+DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
     ElementCount BestVF, unsigned BestUF, VPlan &BestVPlan,
     InnerLoopVectorizer &ILV, DominatorTree *DT, bool IsEpilogueVectorization,
     const DenseMap<const SCEV *, Value *> *ExpandedSCEVs) {
@@ -7579,12 +7569,11 @@ LoopVectorizationPlanner::executePlan(
   BestVPlan.execute(&State);
 
   // 2.5 Collect reduction resume values.
-  DenseMap<const RecurrenceDescriptor *, Value *> ReductionResumeValues;
   auto *ExitVPBB =
       cast<VPBasicBlock>(BestVPlan.getVectorLoopRegion()->getSingleSuccessor());
   for (VPRecipeBase &R : *ExitVPBB) {
     createAndCollectMergePhiForReduction(
-        dyn_cast<VPInstruction>(&R), ReductionResumeValues, State, OrigLoop,
+        dyn_cast<VPInstruction>(&R), State, OrigLoop,
         State.CFG.VPBB2IRBB[ExitVPBB], ExpandedSCEVs);
   }
 
@@ -7634,7 +7623,7 @@ LoopVectorizationPlanner::executePlan(
     setBranchWeights(*MiddleTerm, Weights, /*IsExpected=*/false);
   }
 
-  return {State.ExpandedSCEVs, ReductionResumeValues};
+  return State.ExpandedSCEVs;
 }
 
 //===--------------------------------------------------------------------===//
@@ -10121,8 +10110,8 @@ bool LoopVectorizePass::processLoop(Loop *L) {
                                            EPI, &LVL, &CM, BFI, PSI, Checks);
 
         std::unique_ptr<VPlan> BestMainPlan(BestPlan.duplicate());
-        const auto &[ExpandedSCEVs, ReductionResumeValues] = LVP.executePlan(
-            EPI.MainLoopVF, EPI.MainLoopUF, *BestMainPlan, MainILV, DT, true);
+        auto ExpandedSCEVs = LVP.executePlan(EPI.MainLoopVF, EPI.MainLoopUF,
+                                             *BestMainPlan, MainILV, DT, true);
         ++LoopsVectorized;
 
         // Second pass vectorizes the epilogue and adjusts the control flow
@@ -10167,10 +10156,11 @@ bool LoopVectorizePass::processLoop(Loop *L) {
           Value *ResumeV = nullptr;
           // TODO: Move setting of resume values to prepareToExecute.
           if (auto *ReductionPhi = dyn_cast<VPReductionPHIRecipe>(&R)) {
+            ResumeV = cast<PHINode>(ReductionPhi->getUnderlyingInstr())
+                          ->getIncomingValueForBlock(L->getLoopPreheader());
             const RecurrenceDescriptor &RdxDesc =
                 ReductionPhi->getRecurrenceDescriptor();
             RecurKind RK = RdxDesc.getRecurrenceKind();
-            ResumeV = ReductionResumeValues.find(&RdxDesc)->second;
             if (RecurrenceDescriptor::isAnyOfRecurrenceKind(RK)) {
               // VPReductionPHIRecipes for AnyOf reductions expect a boolean as
               // start value; compare the final value from the main vector loop