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[LV] Vectorize Epilogues for loops with small VF but high IC (#108190)
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- Consider MainLoopVF * IC when determining whether Epilogue
Vectorization is profitable
- Allow the same VF for the Epilogue as for the main loop
- Use an upper bound for the trip count of the Epilogue when choosing
the Epilogue VF

PR: #108190
---------

Co-authored-by: Florian Hahn <flo@fhahn.com>
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@juliannagele @fhahn
juliannagele and fhahn authored Nov 17, 2024
1 parent feb9b37 commit a8538b9
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Showing 28 changed files with 2,575 additions and 1,051 deletions.
6 changes: 6 additions & 0 deletions llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
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Original file line number Diff line number Diff line change
Expand Up @@ -525,6 +525,12 @@ class LoopVectorizationPlanner {
bool isMoreProfitable(const VectorizationFactor &A,
const VectorizationFactor &B) const;

/// Returns true if the per-lane cost of VectorizationFactor A is lower than
/// that of B in the context of vectorizing a loop with known \p MaxTripCount.
bool isMoreProfitable(const VectorizationFactor &A,
const VectorizationFactor &B,
const unsigned MaxTripCount) 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;
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48 changes: 35 additions & 13 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1516,7 +1516,10 @@ class LoopVectorizationCostModel {
/// Returns true if epilogue vectorization is considered profitable, and
/// false otherwise.
/// \p VF is the vectorization factor chosen for the original loop.
bool isEpilogueVectorizationProfitable(const ElementCount VF) const;
/// \p Multiplier is an aditional scaling factor applied to VF before
/// comparing to EpilogueVectorizationMinVF.
bool isEpilogueVectorizationProfitable(const ElementCount VF,
const unsigned Multiplier) const;

/// Returns the execution time cost of an instruction for a given vector
/// width. Vector width of one means scalar.
Expand Down Expand Up @@ -4289,12 +4292,11 @@ getVScaleForTuning(const Loop *L, const TargetTransformInfo &TTI) {
}

bool LoopVectorizationPlanner::isMoreProfitable(
const VectorizationFactor &A, const VectorizationFactor &B) const {
const VectorizationFactor &A, const VectorizationFactor &B,
const unsigned MaxTripCount) const {
InstructionCost CostA = A.Cost;
InstructionCost CostB = B.Cost;

unsigned MaxTripCount = PSE.getSmallConstantMaxTripCount();

// Improve estimate for the vector width if it is scalable.
unsigned EstimatedWidthA = A.Width.getKnownMinValue();
unsigned EstimatedWidthB = B.Width.getKnownMinValue();
Expand Down Expand Up @@ -4343,6 +4345,12 @@ bool LoopVectorizationPlanner::isMoreProfitable(
return CmpFn(RTCostA, RTCostB);
}

bool LoopVectorizationPlanner::isMoreProfitable(
const VectorizationFactor &A, const VectorizationFactor &B) const {
const unsigned MaxTripCount = PSE.getSmallConstantMaxTripCount();
return LoopVectorizationPlanner::isMoreProfitable(A, B, MaxTripCount);
}

void LoopVectorizationPlanner::emitInvalidCostRemarks(
OptimizationRemarkEmitter *ORE) {
using RecipeVFPair = std::pair<VPRecipeBase *, ElementCount>;
Expand Down Expand Up @@ -4661,7 +4669,7 @@ bool LoopVectorizationPlanner::isCandidateForEpilogueVectorization(
}

bool LoopVectorizationCostModel::isEpilogueVectorizationProfitable(
const ElementCount VF) const {
const ElementCount VF, const unsigned Multiplier) const {
// FIXME: We need a much better cost-model to take different parameters such
// as register pressure, code size increase and cost of extra branches into
// account. For now we apply a very crude heuristic and only consider loops
Expand All @@ -4676,9 +4684,6 @@ bool LoopVectorizationCostModel::isEpilogueVectorizationProfitable(
if (TTI.getMaxInterleaveFactor(VF) <= 1)
return false;

unsigned Multiplier = 1;
if (VF.isScalable())
Multiplier = getVScaleForTuning(TheLoop, TTI).value_or(1);
if ((Multiplier * VF.getKnownMinValue()) >= EpilogueVectorizationMinVF)
return true;
return false;
Expand Down Expand Up @@ -4724,7 +4729,11 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
return Result;
}

if (!CM.isEpilogueVectorizationProfitable(MainLoopVF)) {
unsigned Multiplier = IC;
if (MainLoopVF.isScalable())
Multiplier = getVScaleForTuning(OrigLoop, TTI).value_or(1);

if (!CM.isEpilogueVectorizationProfitable(MainLoopVF, Multiplier)) {
LLVM_DEBUG(dbgs() << "LEV: Epilogue vectorization is not profitable for "
"this loop\n");
return Result;
Expand All @@ -4743,16 +4752,20 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
ScalarEvolution &SE = *PSE.getSE();
Type *TCType = Legal->getWidestInductionType();
const SCEV *RemainingIterations = nullptr;
unsigned MaxTripCount = 0;
for (auto &NextVF : ProfitableVFs) {
// Skip candidate VFs without a corresponding VPlan.
if (!hasPlanWithVF(NextVF.Width))
continue;

// Skip candidate VFs with widths >= the estimate runtime VF (scalable
// vectors) or the VF of the main loop (fixed vectors).
// Skip candidate VFs with widths >= the (estimated) runtime VF (scalable
// vectors) or > the VF of the main loop (fixed vectors).
if ((!NextVF.Width.isScalable() && MainLoopVF.isScalable() &&
ElementCount::isKnownGE(NextVF.Width, EstimatedRuntimeVF)) ||
ElementCount::isKnownGE(NextVF.Width, MainLoopVF))
(NextVF.Width.isScalable() &&
ElementCount::isKnownGE(NextVF.Width, MainLoopVF)) ||
(!NextVF.Width.isScalable() && !MainLoopVF.isScalable() &&
ElementCount::isKnownGT(NextVF.Width, MainLoopVF)))
continue;

// If NextVF is greater than the number of remaining iterations, the
Expand All @@ -4766,6 +4779,14 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
"Trip count SCEV must be computable");
RemainingIterations = SE.getURemExpr(
TC, SE.getConstant(TCType, MainLoopVF.getKnownMinValue() * IC));
MaxTripCount = MainLoopVF.getKnownMinValue() * IC - 1;
if (SE.isKnownPredicate(CmpInst::ICMP_ULT, RemainingIterations,
SE.getConstant(TCType, MaxTripCount))) {
MaxTripCount =
SE.getUnsignedRangeMax(RemainingIterations).getZExtValue();
}
LLVM_DEBUG(dbgs() << "LEV: Maximum Trip Count for Epilogue: "
<< MaxTripCount << "\n");
}
if (SE.isKnownPredicate(
CmpInst::ICMP_UGT,
Expand All @@ -4774,7 +4795,8 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
continue;
}

if (Result.Width.isScalar() || isMoreProfitable(NextVF, Result))
if (Result.Width.isScalar() ||
isMoreProfitable(NextVF, Result, MaxTripCount))
Result = NextVF;
}

Expand Down
40 changes: 20 additions & 20 deletions llvm/test/Transforms/LoopVectorize/AArch64/deterministic-type-shrinkage.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,7 @@ define void @test_pr25490(i32 %n, ptr noalias nocapture %a, ptr noalias nocaptur
; CHECK-NEXT: br i1 [[CMP_28]], label [[FOR_COND_CLEANUP:%.*]], label [[ITER_CHECK:%.*]]
; CHECK: iter.check:
; CHECK-NEXT: [[TMP0:%.*]] = zext i32 [[N]] to i64
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i32 [[N]], 8
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i32 [[N]], 4
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[VEC_EPILOG_SCALAR_PH:%.*]], label [[VECTOR_MAIN_LOOP_ITER_CHECK:%.*]]
; CHECK: vector.main.loop.iter.check:
; CHECK-NEXT: [[MIN_ITERS_CHECK1:%.*]] = icmp ult i32 [[N]], 16
Expand Down Expand Up @@ -50,33 +50,33 @@ define void @test_pr25490(i32 %n, ptr noalias nocapture %a, ptr noalias nocaptur
; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[N_VEC]], [[TMP0]]
; CHECK-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP_LOOPEXIT:%.*]], label [[VEC_EPILOG_ITER_CHECK:%.*]]
; CHECK: vec.epilog.iter.check:
; CHECK-NEXT: [[N_VEC_REMAINING:%.*]] = and i64 [[TMP0]], 8
; CHECK-NEXT: [[MIN_EPILOG_ITERS_CHECK_NOT_NOT:%.*]] = icmp eq i64 [[N_VEC_REMAINING]], 0
; CHECK-NEXT: br i1 [[MIN_EPILOG_ITERS_CHECK_NOT_NOT]], label [[VEC_EPILOG_SCALAR_PH]], label [[VEC_EPILOG_PH]]
; CHECK-NEXT: [[N_VEC_REMAINING:%.*]] = and i64 [[TMP0]], 12
; CHECK-NEXT: [[MIN_EPILOG_ITERS_CHECK:%.*]] = icmp eq i64 [[N_VEC_REMAINING]], 0
; CHECK-NEXT: br i1 [[MIN_EPILOG_ITERS_CHECK]], label [[VEC_EPILOG_SCALAR_PH]], label [[VEC_EPILOG_PH]]
; CHECK: vec.epilog.ph:
; CHECK-NEXT: [[VEC_EPILOG_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[VEC_EPILOG_ITER_CHECK]] ], [ 0, [[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
; CHECK-NEXT: [[N_VEC5:%.*]] = and i64 [[TMP0]], 4294967288
; CHECK-NEXT: [[N_VEC5:%.*]] = and i64 [[TMP0]], 4294967292
; CHECK-NEXT: br label [[VEC_EPILOG_VECTOR_BODY:%.*]]
; CHECK: vec.epilog.vector.body:
; CHECK-NEXT: [[INDEX6:%.*]] = phi i64 [ [[VEC_EPILOG_RESUME_VAL]], [[VEC_EPILOG_PH]] ], [ [[INDEX_NEXT10:%.*]], [[VEC_EPILOG_VECTOR_BODY]] ]
; CHECK-NEXT: [[TMP14:%.*]] = getelementptr inbounds i8, ptr [[C]], i64 [[INDEX6]]
; CHECK-NEXT: [[WIDE_LOAD7:%.*]] = load <8 x i8>, ptr [[TMP14]], align 1
; CHECK-NEXT: [[WIDE_LOAD7:%.*]] = load <4 x i8>, ptr [[TMP14]], align 1
; CHECK-NEXT: [[TMP15:%.*]] = getelementptr inbounds i8, ptr [[A]], i64 [[INDEX6]]
; CHECK-NEXT: [[WIDE_LOAD8:%.*]] = load <8 x i8>, ptr [[TMP15]], align 1
; CHECK-NEXT: [[TMP16:%.*]] = zext <8 x i8> [[WIDE_LOAD8]] to <8 x i16>
; CHECK-NEXT: [[TMP17:%.*]] = zext <8 x i8> [[WIDE_LOAD7]] to <8 x i16>
; CHECK-NEXT: [[TMP18:%.*]] = mul nuw <8 x i16> [[TMP16]], [[TMP17]]
; CHECK-NEXT: [[TMP19:%.*]] = lshr <8 x i16> [[TMP18]], splat (i16 8)
; CHECK-NEXT: [[TMP20:%.*]] = trunc nuw <8 x i16> [[TMP19]] to <8 x i8>
; CHECK-NEXT: store <8 x i8> [[TMP20]], ptr [[TMP15]], align 1
; CHECK-NEXT: [[WIDE_LOAD8:%.*]] = load <4 x i8>, ptr [[TMP15]], align 1
; CHECK-NEXT: [[TMP16:%.*]] = zext <4 x i8> [[WIDE_LOAD8]] to <4 x i16>
; CHECK-NEXT: [[TMP17:%.*]] = zext <4 x i8> [[WIDE_LOAD7]] to <4 x i16>
; CHECK-NEXT: [[TMP18:%.*]] = mul nuw <4 x i16> [[TMP16]], [[TMP17]]
; CHECK-NEXT: [[TMP19:%.*]] = lshr <4 x i16> [[TMP18]], splat (i16 8)
; CHECK-NEXT: [[TMP20:%.*]] = trunc nuw <4 x i16> [[TMP19]] to <4 x i8>
; CHECK-NEXT: store <4 x i8> [[TMP20]], ptr [[TMP15]], align 1
; CHECK-NEXT: [[TMP21:%.*]] = getelementptr inbounds i8, ptr [[B]], i64 [[INDEX6]]
; CHECK-NEXT: [[WIDE_LOAD9:%.*]] = load <8 x i8>, ptr [[TMP21]], align 1
; CHECK-NEXT: [[TMP22:%.*]] = zext <8 x i8> [[WIDE_LOAD9]] to <8 x i16>
; CHECK-NEXT: [[TMP23:%.*]] = mul nuw <8 x i16> [[TMP22]], [[TMP17]]
; CHECK-NEXT: [[TMP24:%.*]] = lshr <8 x i16> [[TMP23]], splat (i16 8)
; CHECK-NEXT: [[TMP25:%.*]] = trunc nuw <8 x i16> [[TMP24]] to <8 x i8>
; CHECK-NEXT: store <8 x i8> [[TMP25]], ptr [[TMP21]], align 1
; CHECK-NEXT: [[INDEX_NEXT10]] = add nuw i64 [[INDEX6]], 8
; CHECK-NEXT: [[WIDE_LOAD9:%.*]] = load <4 x i8>, ptr [[TMP21]], align 1
; CHECK-NEXT: [[TMP22:%.*]] = zext <4 x i8> [[WIDE_LOAD9]] to <4 x i16>
; CHECK-NEXT: [[TMP23:%.*]] = mul nuw <4 x i16> [[TMP22]], [[TMP17]]
; CHECK-NEXT: [[TMP24:%.*]] = lshr <4 x i16> [[TMP23]], splat (i16 8)
; CHECK-NEXT: [[TMP25:%.*]] = trunc nuw <4 x i16> [[TMP24]] to <4 x i8>
; CHECK-NEXT: store <4 x i8> [[TMP25]], ptr [[TMP21]], align 1
; CHECK-NEXT: [[INDEX_NEXT10]] = add nuw i64 [[INDEX6]], 4
; CHECK-NEXT: [[TMP26:%.*]] = icmp eq i64 [[INDEX_NEXT10]], [[N_VEC5]]
; CHECK-NEXT: br i1 [[TMP26]], label [[VEC_EPILOG_MIDDLE_BLOCK:%.*]], label [[VEC_EPILOG_VECTOR_BODY]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK: vec.epilog.middle.block:
Expand Down
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