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

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

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[LV] Use vscale for tuning to improve branch weight estimates
david-arm Jun 18, 2025
00ed355
Fix review comments
david-arm Jun 27, 2025
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6 changes: 4 additions & 2 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7326,9 +7326,11 @@ 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();
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NOTE: I'm forced to create a temporary variable here because runPass takes values by reference.

VPlanTransforms::runPass(VPlanTransforms::addBranchWeightToMiddleTerminator,
BestVPlan, BestVF);
BestVPlan, BestVF, VScale);
}
VPlanTransforms::optimizeForVFAndUF(BestVPlan, BestVF, BestUF, PSE);
VPlanTransforms::simplifyRecipes(BestVPlan, *Legal->getWidestInductionType());
VPlanTransforms::narrowInterleaveGroups(
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6 changes: 4 additions & 2 deletions llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
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Expand Up @@ -3328,8 +3328,8 @@ 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());
Expand All @@ -3341,6 +3341,8 @@ 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 =
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4 changes: 3 additions & 1 deletion llvm/lib/Transforms/Vectorize/VPlanTransforms.h
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion llvm/test/Transforms/LoopVectorize/AArch64/check-prof-info.ll
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Expand Up @@ -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]]}
;.
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