[LV] Use vscale for tuning to improve branch weight estimates (#144733)

In addBranchWeightToMiddleTerminator we attempt to add branch weights to
the middle block terminator. We pessimistically assume vscale=1, whereas
we can improve the estimate by using the value of vscale used for
tuning.

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7327,9 +7327,11 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
                            OrigLoop->getHeader()->getContext());
   VPlanTransforms::runPass(VPlanTransforms::replicateByVF, BestVPlan, BestVF);
   VPlanTransforms::runPass(VPlanTransforms::materializeBroadcasts, BestVPlan);
-  if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator()))
+  if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator())) {
+    std::optional<unsigned> VScale = CM.getVScaleForTuning();
     VPlanTransforms::runPass(VPlanTransforms::addBranchWeightToMiddleTerminator,
-                             BestVPlan, BestVF);
+                             BestVPlan, BestVF, VScale);
+  }
   VPlanTransforms::optimizeForVFAndUF(BestVPlan, BestVF, BestUF, PSE);
   VPlanTransforms::simplifyRecipes(BestVPlan, *Legal->getWidestInductionType());
   VPlanTransforms::narrowInterleaveGroups(

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -3330,8 +3330,8 @@ void VPlanTransforms::narrowInterleaveGroups(VPlan &Plan, ElementCount VF,
 
 /// Add branch weight metadata, if the \p Plan's middle block is terminated by a
 /// BranchOnCond recipe.
-void VPlanTransforms::addBranchWeightToMiddleTerminator(VPlan &Plan,
-                                                        ElementCount VF) {
+void VPlanTransforms::addBranchWeightToMiddleTerminator(
+    VPlan &Plan, ElementCount VF, std::optional<unsigned> VScaleForTuning) {
   VPBasicBlock *MiddleVPBB = Plan.getMiddleBlock();
   auto *MiddleTerm =
       dyn_cast_or_null<VPInstruction>(MiddleVPBB->getTerminator());
@@ -3343,6 +3343,8 @@ void VPlanTransforms::addBranchWeightToMiddleTerminator(VPlan &Plan,
          "must have a BranchOnCond");
   // Assume that `TripCount % VectorStep ` is equally distributed.
   unsigned VectorStep = Plan.getUF() * VF.getKnownMinValue();
+  if (VF.isScalable() && VScaleForTuning.has_value())
+    VectorStep *= *VScaleForTuning;
   assert(VectorStep > 0 && "trip count should not be zero");
   MDBuilder MDB(Plan.getScalarHeader()->getIRBasicBlock()->getContext());
   MDNode *BranchWeights =

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -238,7 +238,9 @@ struct VPlanTransforms {
 
   /// Add branch weight metadata, if the \p Plan's middle block is terminated by
   /// a BranchOnCond recipe.
-  static void addBranchWeightToMiddleTerminator(VPlan &Plan, ElementCount VF);
+  static void
+  addBranchWeightToMiddleTerminator(VPlan &Plan, ElementCount VF,
+                                    std::optional<unsigned> VScaleForTuning);
 };
 
 } // namespace llvm

llvm/test/Transforms/LoopVectorize/AArch64/check-prof-info.ll

@@ -92,7 +92,7 @@ for.cond.cleanup:                                 ; preds = %for.body
 ; CHECK-V1-IC1: [[LOOP1]] = distinct !{[[LOOP1]], [[META2:![0-9]+]], [[META3:![0-9]+]]}
 ; CHECK-V1-IC1: [[META2]] = !{!"llvm.loop.isvectorized", i32 1}
 ; CHECK-V1-IC1: [[META3]] = !{!"llvm.loop.unroll.runtime.disable"}
-; CHECK-V1-IC1: [[PROF4]] = !{!"branch_weights", i32 1, i32 3}
+; CHECK-V1-IC1: [[PROF4]] = !{!"branch_weights", i32 1, i32 7}
 ; CHECK-V1-IC1: [[PROF5]] = !{!"branch_weights", i32 0, i32 0}
 ; CHECK-V1-IC1: [[LOOP6]] = distinct !{[[LOOP6]], [[META3]], [[META2]]}
 ;.