[mlir][linalg] Add pattern to split reduction dimension in a linalg op

[ThomasRaoux]

Summary

This transformation allow to break up a reduction dimension in a
parallel and a reduction dimension. This is followed by a separate
reduction op. This allows to generate tree reduction which is beneficial
on target allowing to take advantage parallelism.

[ThomasRaoux]

typo

[mravishankar]

Just did a skim for now. Will come back for a more detailed review.

mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp

59

Could probably replace uses of this with this method https://github.com/llvm/llvm-project/blob/251d062e4e2737d88ba53e65969b210f2830c4df/mlir/include/mlir/Dialect/Linalg/IR/LinalgInterfaces.td#L1006

76

Have seen this come up many places. Maybe worth making this an interface function.

[nirvedhmeshram]

mlir/test/lib/Dialect/Linalg/TestLinalgTransforms.cpp

627

Is this the current controlSplitReductionFn? If so dont we need something better than returning 4?

[ThomasRaoux]

mlir/test/lib/Dialect/Linalg/TestLinalgTransforms.cpp

627

this is just a dummy heuristic for the lit tests above :)
The transformation will eventually be controlled by an heuristic or a codegen strategy.

[ThomasRaoux]

address review comments

[ThomasRaoux]

mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp

76

Added a function to the interface. I can replace other usages of such pattern in a follow up patch if everybody is fine with the new interface.

[ThomasRaoux]

Fine tune the control

[ThomasRaoux]

I addressed the style comments and changed a bit the control to allow deciding the shape of the intermediate tensor. Please take a look.

[mravishankar]

Mostly looks fine. Some of the output handling code , im not able to fully understand. Maybe worth taking a look again?

mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp

27

Could we make this Optional<Attribute> or FailureOr<Attribute> . Returning nullptr can just lead to segfaults if not handled correctly everywhere.

59

Nit: General not, better practice to return rewriter.notifyMatchFailure() instead of failure(). Makes debug log easier to read.

62

Do we also want to check that the reduction is innermost, or does that not matter?

101

Nit: use llvm::seq<unsigned>(0, map.getNumResults())

130

Not sure I understand the comment. AFAICS, the ControlSplitReductionFn returned the place to add the new dimension. That was account for in the reshape above. Why is the same not done for the outputs. I would expect both to be handled the same way.

137

You already checked the num outputs to be 1. So this loop is dead?

148

Actually the code looks fine, the comment is misleading to me. What is exactly the "most inner dimension" in the comment marked above?

[ThomasRaoux]

Address review comments

[ThomasRaoux]

mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp

62

It shouldn't matter, I have a test where it isn't

130

Fixed the comment

137

this is the number of results of the output map which is the rank of the destination

148

yes the comment was wrong :( thanks for catching this

mlir/include/mlir/Dialect/Linalg/IR/LinalgInterfaces.td

StaticInterfaceMethod<
         Returns the region builder for constructing the body for linalg.generic.
         Returns a null function if this named op does not define a region
         builder.
       }],
       /*retTy=*/"std::function<void(ImplicitLocOpBuilder &, Block &, ArrayRef<NamedAttribute>)>",
       /*methodName=*/"getRegionBuilder",
       (ins),
       [{ return ConcreteOp::getRegionBuilder(); }]
+    >,
+    InterfaceMethod<
+      /*desc=*/[{
+        Return true if all the indexing maps are projected permutations.
+        Otherwise return false.
+      }],
+      /*retTy=*/"bool",
+      /*methodName=*/"hasOnlyProjectedPermutations",
+      (ins),
+      [{
+        return llvm::all_of($_op.getIndexingMaps(),
+                            [](AffineMap map) { return map.isProjectedPermutation(); });
+      }]
     >
   ];
 
   let extraClassDeclaration = [{
     /// Return the flat list of all operand dimension sizes in the order they
     /// appear in the operands.
     SmallVector<Value, 4> createFlatListOfOperandDims(OpBuilder &, Location);
 

mlir/include/mlir/Dialect/Linalg/Transforms/Transforms.h

static void insert(RewritePatternSet &patterns,
                      const LinalgTilingOptions &options,
                      const LinalgTransformationFilter &f) {
     patterns.add<LinalgTilingPattern>(OpTy::getOperationName(),
                                       patterns.getContext(), options, f);
     TilingPatterns<OpTypes...>::insert(patterns, options, f);
   }
 };
 
+/// Function signature to control reduction splitting. This returns a pair
+/// containing a ratio and a dimension index. The ratio is used to split the
+/// reduction dimension. The dimension index is used to control where the extra
+/// dimension is added to the intermediate tensor shape. If the ratio value is
+/// less or equal to 1 then nothing will be done.
+using ControlSplitReductionFn =
+    std::function<std::pair<int64_t, unsigned>(LinalgOp op)>;
+
+/// Patterns to apply `splitReduction` below.
+void populateSplitReductionPattern(
+    RewritePatternSet &patterns,
+    ControlSplitReductionFn controlSplitReductionFn,
+    LinalgTransformationFilter f = LinalgTransformationFilter());
+
+/// Apply transformation to split the single linalg op reduction into a parallel
+/// and reduction dimension. Then create a new linalg.generic op doing the rest
+/// of the reduction. Return the new linalg op with an extra parallel dimension
+/// or failure if the transformation didn't happen.
+/// Example:
+/// ```
+///  %r = linalg.generic {indexing_maps = [affine_map<(d0) -> (d0)>,
+///                                        affine_map<(d0) -> ()>],
+///       iterator_types = ["reduction"]}
+///  ins(%in : tensor<32xf32>)
+///  outs(%out : tensor<f32>) {
+///  ^bb0(%arg1: f32, %arg2: f32):
+///    %y = arith.addf %arg1, %arg2 : f32
+///    linalg.yield %y : f32
+///  } -> tensor<f32>
+/// ```
+/// To:
+/// ```
+///  %cst = arith.constant 0.000000e+00 : f32
+///  %0 = tensor.expand_shape %in [[0, 1]] : tensor<32xf32> into tensor<4x8xf32>
+///  %1 = linalg.init_tensor [4] : tensor<4xf32>
+///  %2 = linalg.fill ins(%cst : f32) outs(%1 : tensor<4xf32>) -> tensor<4xf32>
+///  %3 = linalg.generic {indexing_maps = [affine_map<(d0, d1) -> (d0, d1)>,
+///                                        affine_map<(d0, d1) -> (d0)>],
+///    iterator_types = ["parallel", "reduction"]}
+///    ins(%0 : tensor<4x8xf32>) outs(%2 : tensor<4xf32>) {
+///    ^bb0(%arg3: f32, %arg5: f32):
+///    %5 = arith.addf %arg3, %arg4 : f32
+///    linalg.yield %5 : f32
+///  } -> tensor<4xf32>
+/// %r = linalg.generic {indexing_maps = [affine_map<(d0) -> (d0)>,
+///                                       affine_map<(d0) -> ()>],
+///   iterator_types = ["reduction"]}
+///   ins(%3 : tensor<4xf32>) outs(%out : tensor<f32>) {
+///   ^bb0(%arg3: f32, %arg4: f32):
+///   %5 = arith.addf %arg3, %arg4 : f32
+///   linalg.yield %5 : f32
+/// } -> tensor<f32>
+/// ```
+FailureOr<LinalgOp>
+splitReduction(PatternRewriter &b, LinalgOp op,
+               ControlSplitReductionFn controlSplitReductionFn,
+               LinalgTransformationFilter f);
+
 } // namespace linalg
 } // namespace mlir
 
 #endif // MLIR_DIALECT_LINALG_TRANSFORMS_TRANSFORMS_H

mlir/lib/Dialect/Linalg/Transforms/CMakeLists.txt

add_mlir_dialect_library(MLIRLinalgTransforms
   InlineScalarOperands.cpp
   Interchange.cpp
   Loops.cpp
   LinalgStrategyPasses.cpp
   NamedOpConversions.cpp
   PadOpInterchange.cpp
   Promotion.cpp
   SparseTensorRewriting.cpp
+  SplitReduction.cpp
   Tiling.cpp
   Transforms.cpp
   Vectorization.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/Linalg
 
   DEPENDS

mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp

+//===-------- SplitReduction.cpp - Split reduction dimesion ---------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements linalg transformation to break a reduction dimension
+// between a parallel and a reduction dimension.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Analysis/SliceAnalysis.h"
+#include "mlir/Dialect/Arithmetic/IR/Arithmetic.h"
+#include "mlir/Dialect/Linalg/IR/Linalg.h"
+#include "mlir/Dialect/Linalg/Transforms/Transforms.h"
+#include "mlir/Dialect/Linalg/Utils/Utils.h"
+#include "mlir/Dialect/Tensor/IR/Tensor.h"
+#include "mlir/IR/PatternMatch.h"
+
+using namespace mlir;
+using namespace mlir::linalg;
+
+/// Return the identity numeric value associated to the give op.
+static Optional<Attribute> getIdentity(Operation *op) {
+  // Builder only used as helper for attribute creation.
+  OpBuilder b(op->getContext());
+  Type resultType = op->getResult(0).getType();
+  if (auto floatType = resultType.dyn_cast<FloatType>()) {
+    const llvm::fltSemantics &semantic = floatType.getFloatSemantics();
+    if (isa<arith::AddFOp>(op))
+      return b.getFloatAttr(resultType, llvm::APFloat::getZero(semantic));
+    if (isa<arith::MulFOp>(op))
+      return b.getFloatAttr(resultType, llvm::APFloat(semantic, 1));
+    if (isa<arith::MaxFOp>(op))
+      return b.getFloatAttr(resultType,
+                            llvm::APFloat::getLargest(semantic, true));
+    if (isa<arith::MinFOp>(op))
+      return b.getFloatAttr(resultType,
+                            llvm::APFloat::getLargest(semantic, true));
+    return llvm::None;
+  }
+  if (isa<arith::AddIOp, arith::OrIOp, arith::XOrIOp>(op))
+    return b.getIntegerAttr(resultType, 0);
+  if (isa<arith::AndIOp>(op))
+    return b.getIntegerAttr(resultType, -1);
+  if (isa<arith::MaxSIOp>(op))
+    return b.getIntegerAttr(resultType, std::numeric_limits<int64_t>::min());
+  if (isa<arith::MinSIOp>(op))
+    return b.getIntegerAttr(resultType, std::numeric_limits<int64_t>::max());
+  if (isa<arith::MulIOp>(op))
+    return b.getIntegerAttr(resultType, 1);
+  return llvm::None;
+}
+
+FailureOr<LinalgOp>
+mlir::linalg::splitReduction(PatternRewriter &b, LinalgOp op,
+                             ControlSplitReductionFn controlSplitReductionFn,
+                             LinalgTransformationFilter filter) {
+  if (failed(filter.checkAndNotify(b, op)) || !op.hasTensorSemantics() ||
+      op.getNumReductionLoops() != 1 || op.getNumOutputs() != 1 ||
+      !op.hasOnlyProjectedPermutations())
+    return b.notifyMatchFailure(op, "precondition not met");
+  std::pair<int64_t, unsigned> control = controlSplitReductionFn(op);
+  int64_t ratio = control.first;
+  unsigned insertDimIndex = control.second;
+  if (ratio <= 1)
+    return b.notifyMatchFailure(op, "split ratio needs to be greater than 1");
+  SmallVector<unsigned> dims;
+  op.getReductionDims(dims);
+  assert(dims.size() == 1);
+  unsigned reductionDim = dims[0];
+  Optional<SmallVector<int64_t, 4>> loopRanges = op.getStaticLoopRanges();
+  if (!loopRanges)
+    return b.notifyMatchFailure(op, "Cannot analyze loops");
+  int64_t reductionDimSize = (*loopRanges)[reductionDim];
+  if (reductionDimSize == ShapedType::kDynamicSize ||
+      reductionDimSize % ratio != 0 || insertDimIndex >= loopRanges->size())
+    return b.notifyMatchFailure(
+        op, "Reduction dimension not divisible by split ratio");
+  SmallVector<Operation *, 4> combinerOps;
+  if (!matchReduction(op.getRegionOutputArgs(), 0, combinerOps) ||
+      combinerOps.size() != 1)
+    return b.notifyMatchFailure(op, "Cannot match the reduction pattern");
+  Operation *reductionOp = combinerOps[0];
+  Optional<Attribute> identity = getIdentity(reductionOp);
+  if (!identity)
+    return b.notifyMatchFailure(op, "Unknown identity value for the redution");
+
+  Location loc = op->getLoc();
+  SmallVector<Value> newInputs;
+  SmallVector<AffineMap> newMaps;
+  // Calculate the new shapes and indexing maps of the input operands.
+  for (OpOperand *operand : op.getInputOperands()) {
+    AffineMap map = op.getTiedIndexingMap(operand);
+    SmallVector<int64_t> newShape;
+    SmallVector<AffineExpr> exprs;
+    SmallVector<ReassociationIndices> reassociation;
+    unsigned index = 0;
+    for (unsigned idx : llvm::seq<unsigned>(0, map.getNumResults())) {
+      unsigned dim = map.getDimPosition(idx);
+      if (reductionDim == dim) {
+        newShape.push_back(ratio);
+        newShape.push_back(op.getShape(operand)[idx] / ratio);
+        reassociation.push_back({index++, index++});
+        exprs.push_back(b.getAffineDimExpr(insertDimIndex));
+        exprs.push_back(
+            b.getAffineDimExpr(dim < insertDimIndex ? dim : dim + 1));
+        continue;
+      }
+      newShape.push_back(op.getShape(operand)[idx]);
+      exprs.push_back(b.getAffineDimExpr(dim < insertDimIndex ? dim : dim + 1));
+      reassociation.push_back({index++});
+    }
+    newMaps.push_back(
+        AffineMap::get(map.getNumDims() + 1, 0, exprs, op.getContext()));
+    // If the shape is unchanged the input doesn't change.
+    if (newShape == op.getShape(operand)) {
+      newInputs.push_back(operand->get());
+      continue;
+    }
+    Type newType = RankedTensorType::get(
+        newShape,
+        operand->get().getType().cast<RankedTensorType>().getElementType());
+    Value newInput = b.create<tensor::ExpandShapeOp>(
+        loc, newType, operand->get(), reassociation);
+    newInputs.push_back(newInput);
+  }
+  // Calculate the new output map and shape, we insert the new dimension based
+  // on the index returned by `controlSplitReductionFn`.
+  SmallVector<int64_t> newOutputShape;
+  AffineMap oldOutputMap = op.getTiedIndexingMap(op.getOutputOperand(0));
+  ArrayRef<int64_t> oldShape = op.getShape(op.getOutputOperand(0));
+  SmallVector<AffineExpr> outputExpr;
+  for (unsigned idx :
+       llvm::seq<unsigned>(0, oldOutputMap.getNumResults() + 1)) {
+    if (idx == insertDimIndex) {
+      newOutputShape.push_back(ratio);
+      outputExpr.push_back(b.getAffineDimExpr(insertDimIndex));
+      continue;
+    }
+    unsigned oldDim = idx < insertDimIndex ? idx : idx - 1;
+    newOutputShape.push_back(oldShape[oldDim]);
+    unsigned dim = oldOutputMap.getDimPosition(oldDim);
+    outputExpr.push_back(
+        b.getAffineDimExpr(dim < insertDimIndex ? dim : dim + 1));
+  }
+  Value initTensor = b.create<linalg::InitTensorOp>(
+      loc, newOutputShape, op.getRegionOutputArgs()[0].getType());
+  Value constantOp = b.create<arith::ConstantOp>(loc, *identity);
+  Value identityTensor =
+      b.create<linalg::FillOp>(op->getLoc(), constantOp, initTensor)
+          .getResult(0);
+
+  newMaps.push_back(AffineMap::get(oldOutputMap.getNumDims() + 1, 0, outputExpr,
+                                   op.getContext()));
+  SmallVector<StringRef> newIteratorTypes;
+  for (auto &it : llvm::enumerate(op.iterator_types())) {
+    if (insertDimIndex == it.index())
+      newIteratorTypes.push_back(getParallelIteratorTypeName());
+    newIteratorTypes.push_back(it.value().cast<StringAttr>().getValue());
+  }
+  // Create the new op matching the original op with an extra parallel
+  // dimension.
+  GenericOp genericOp = b.create<GenericOp>(
+      loc, TypeRange({initTensor.getType()}), newInputs,
+      ValueRange({identityTensor}), newMaps, newIteratorTypes);
+  b.inlineRegionBefore(op->getRegion(0), genericOp.region(),
+                       genericOp.region().begin());
+
+  // Then create a new reduction that only reduce the newly added dimension from
+  // the previous op.
+  unsigned intermRank = newOutputShape.size();
+  AffineMap inputMap = b.getMultiDimIdentityMap(intermRank);
+  SmallVector<Value> outputOperands = op.getOutputOperands();
+  SmallVector<StringRef> reductionIteratorTypes;
+  SmallVector<AffineExpr> exprs;
+  for (unsigned i : llvm::seq<unsigned>(0, intermRank)) {
+    if (insertDimIndex == i) {
+      reductionIteratorTypes.push_back(getReductionIteratorTypeName());
+    } else {
+      exprs.push_back(b.getAffineDimExpr(i));
+      reductionIteratorTypes.push_back(getParallelIteratorTypeName());
+    }
+  }
+  AffineMap outputMap = AffineMap::get(intermRank, 0, exprs, op.getContext());
+  SmallVector<AffineMap> reductionMaps = {inputMap, outputMap};
+
+  auto reduction = b.create<GenericOp>(
+      loc, op->getResultTypes(), ValueRange({genericOp.getResult(0)}),
+      outputOperands, reductionMaps, reductionIteratorTypes,
+      [reductionOp](OpBuilder &b, Location loc, ValueRange inputs) {
+        Operation *clonedReductionOp = b.clone(*reductionOp);
+        clonedReductionOp->setOperand(0, inputs[0]);
+        clonedReductionOp->setOperand(1, inputs[1]);
+        b.create<linalg::YieldOp>(loc, clonedReductionOp->getResult(0));
+      });
+  b.replaceOp(op, reduction.getResults());
+  filter.replaceLinalgTransformationFilter(b, genericOp);
+  filter.replaceLinalgTransformationFilter(b, reduction);
+  return cast<LinalgOp>(genericOp.getOperation());
+}
+
+namespace {
+
+struct LinalgSplitReduction : public OpInterfaceRewritePattern<LinalgOp> {
+  /// Construct a generic pattern applied to all LinalgOp that verify `filter`.
+  LinalgSplitReduction(MLIRContext *context,
+                       ControlSplitReductionFn controlSplitReductionFn,
+                       LinalgTransformationFilter f, PatternBenefit benefit = 1)
+      : OpInterfaceRewritePattern<LinalgOp>(context, benefit),
+        controlSplitReductionFn(controlSplitReductionFn), filter(std::move(f)) {
+  }
+
+  LogicalResult matchAndRewrite(LinalgOp op,
+                                PatternRewriter &rewriter) const override {
+    return splitReduction(rewriter, op, controlSplitReductionFn, filter);
+  }
+
+private:
+  ControlSplitReductionFn controlSplitReductionFn;
+  LinalgTransformationFilter filter;
+};
+
+} // namespace
+
+void linalg::populateSplitReductionPattern(
+    RewritePatternSet &patterns,
+    ControlSplitReductionFn controlSplitReductionFn,
+    LinalgTransformationFilter f) {
+  patterns.add<LinalgSplitReduction>(patterns.getContext(),
+                                     controlSplitReductionFn, f);
+}

mlir/test/Dialect/Linalg/split_reduction.mlir

+// RUN: mlir-opt %s -test-linalg-transform-patterns=test-split-reduction  -split-input-file  | FileCheck %s
+
+func @matmul_split(%A : tensor<16x256xf32>, %B: tensor<256x32xf32>, %C: tensor<16x32xf32>) -> tensor<16x32xf32> {
+  %0 = linalg.matmul ins(%A, %B: tensor<16x256xf32>, tensor<256x32xf32>)
+                    outs(%C: tensor<16x32xf32>) -> tensor<16x32xf32>
+  return %0: tensor<16x32xf32>
+}
+
+//  CHECK-DAG: #[[$MAP0:.*]] = affine_map<(d0, d1, d2, d3) -> (d0, d2, d3)>
+//  CHECK-DAG: #[[$MAP1:.*]] = affine_map<(d0, d1, d2, d3) -> (d2, d3, d1)>
+//  CHECK-DAG: #[[$MAP2:.*]] = affine_map<(d0, d1, d2, d3) -> (d0, d1, d2)>
+//  CHECK-DAG: #[[$MAP3:.*]] = affine_map<(d0, d1, d2) -> (d0, d1, d2)>
+//  CHECK-DAG: #[[$MAP4:.*]] = affine_map<(d0, d1, d2) -> (d0, d1)>
+//  CHECK-LABEL: @matmul_split
+//  CHECK-DAG: %[[ID:.*]] = arith.constant 0.000000e+00 : f32
+//  CHECK-DAG: %[[I1:.*]] = tensor.expand_shape %{{.*}}[0], [1, 2]] : tensor<16x256xf32> into tensor<16x4x64xf32>
+//  CHECK-DAG: %[[I2:.*]] = tensor.expand_shape %{{.*}}[0, 1], [2]] : tensor<256x32xf32> into tensor<4x64x32xf32>
+//  CHECK-DAG: %[[INI:.*]] = linalg.init_tensor [16, 32, 4] : tensor<16x32x4xf32>
+//      CHECK: %[[F:.*]] = linalg.fill ins(%[[ID]] : f32) outs(%[[INI]] : tensor<16x32x4xf32>) -> tensor<16x32x4xf32>
+//      CHECK: %[[G:.*]] = linalg.generic {indexing_maps = [#[[$MAP0]], #[[$MAP1]], #[[$MAP2]]]
+// CHECK-SAME:   , iterator_types = ["parallel", "parallel", "parallel", "reduction"]}
+// CHECK-SAME:   ins(%[[I1]], %[[I2]] : tensor<16x4x64xf32>, tensor<4x64x32xf32>) outs(%[[F]] : tensor<16x32x4xf32>) {
+//      CHECK:   arith.mulf
+//      CHECK:   arith.addf
+//      CHECK:   linalg.yield
+//      CHECK: } -> tensor<16x32x4xf32>
+//      CHECK: %[[R:.*]] = linalg.generic {indexing_maps = [#[[$MAP3]], #[[$MAP4]]],
+// CHECK-SAME:   iterator_types = ["parallel", "parallel", "reduction"]} ins(%[[G]] : tensor<16x32x4xf32>) outs(%{{.*}} : tensor<16x32xf32>) {
+//      CHECK:   arith.addf
+//      CHECK:   linalg.yield %{{.*}} : f32
+//      CHECK: } -> tensor<16x32xf32>
+//      CHECK: return %[[R]] : tensor<16x32xf32>
+
+// -----
+
+func @generic_split_1d(%arg0: tensor<32xf32>, %arg1: tensor<f32>, %out: tensor<f32>) -> tensor<f32> {
+  %red = linalg.generic {indexing_maps = [affine_map<(d0) -> (d0)>,
+                                          affine_map<(d0) -> ()>,
+                                          affine_map<(d0) -> ()>],
+   iterator_types = ["reduction"]}
+   ins(%arg0, %arg1 : tensor<32xf32>, tensor<f32>)
+   outs(%out : tensor<f32>) {
+    ^bb0(%arg7: f32, %arg8: f32, %arg9: f32):
+      %40 = arith.subf %arg7, %arg8 : f32
+      %41 = math.exp %40 : f32
+      %42 = arith.mulf %41, %arg9 : f32
+      linalg.yield %42 : f32
+    } -> tensor<f32>
+  return %red : tensor<f32>
+}
+
+//  CHECK-DAG: #[[$MAP0:.*]] = affine_map<(d0, d1) -> (d0, d1)>
+//  CHECK-DAG: #[[$MAP1:.*]] = affine_map<(d0, d1) -> ()>
+//  CHECK-DAG: #[[$MAP2:.*]] = affine_map<(d0, d1) -> (d0)>
+//  CHECK-DAG: #[[$MAP3:.*]] = affine_map<(d0) -> (d0)>
+//  CHECK-DAG: #[[$MAP4:.*]] = affine_map<(d0) -> ()>
+//CHECK-LABEL: @generic_split_1d
+//      CHECK: %[[ID:.*]] = arith.constant 1.000000e+00 : f32
+//      CHECK: %[[I1:.*]] = tensor.expand_shape %{{.*}}[0, 1]] : tensor<32xf32> into tensor<4x8xf32>
+//      CHECK: %[[INI:.*]] = linalg.init_tensor [4] : tensor<4xf32>
+//      CHECK: %[[F:.*]] = linalg.fill ins(%[[ID]] : f32) outs(%[[INI]] : tensor<4xf32>) -> tensor<4xf32>
+//      CHECK: %[[G:.*]] = linalg.generic
+//      CHECK:   {indexing_maps = [#[[$MAP0]], #[[$MAP1]], #[[$MAP2]]],
+//      CHECK:   iterator_types = ["parallel", "reduction"]} ins(%[[I1]], %{{.*}} : tensor<4x8xf32>, tensor<f32>) outs(%[[F]] : tensor<4xf32>) {
+//      CHECK:   arith.subf
+//      CHECK:   math.exp
+//      CHECK:   arith.mulf
+//      CHECK:   linalg.yield
+//      CHECK: } -> tensor<4xf32>
+//      CHECK: %[[R:.*]] = linalg.generic {indexing_maps = [#[[$MAP3]], #[[$MAP4]]], iterator_types = ["reduction"]} ins(%[[G]] : tensor<4xf32>) outs(%{{.*}} : tensor<f32>) {
+//      CHECK:   arith.mulf
+//      CHECK:   linalg.yield
+//      CHECK: } -> tensor<f32>
+//      CHECK: return %[[R]] : tensor<f32>
+
+// -----
+
+func @generic_split_3d(%input: tensor<32x2xf32>, %input_2: tensor<5x32xf32>, %output: tensor<5x2xf32>)
+  -> tensor<5x2xf32>
+{
+  %0 = linalg.generic {
+      indexing_maps = [
+        affine_map<(d0, d1, d2) -> (d1, d0)>,
+        affine_map<(d0, d1, d2) -> (d2, d1)>,
+        affine_map<(d0, d1, d2) -> (d2, d0)>
+      ],
+      iterator_types = ["parallel", "reduction", "parallel"]
+    } ins(%input, %input_2 : tensor<32x2xf32>, tensor<5x32xf32>) outs(%output : tensor<5x2xf32>) {
+    ^bb0(%arg0: f32, %arg1: f32, %arg2: f32):
+      %3 = arith.addf %arg0, %arg1 : f32
+      %4 = arith.maxf %3, %arg2 : f32
+      linalg.yield %4 : f32
+    } -> tensor<5x2xf32>
+  return %0 : tensor<5x2xf32>
+}
+
+//  CHECK-DAG: #[[$MAP0:.*]] = affine_map<(d0, d1, d2, d3) -> (d2, d1, d0)>
+//  CHECK-DAG: #[[$MAP1:.*]] = affine_map<(d0, d1, d2, d3) -> (d3, d2, d1)>
+//  CHECK-DAG: #[[$MAP2:.*]] = affine_map<(d0, d1, d2, d3) -> (d3, d0, d2)>
+//  CHECK-DAG: #[[$MAP3:.*]] = affine_map<(d0, d1, d2) -> (d0, d1, d2)>
+//  CHECK-DAG: #[[$MAP4:.*]] = affine_map<(d0, d1, d2) -> (d0, d1)>
+// CHECK-LABEL:  func @generic_split_3d
+//      CHECK: %[[ID:.*]] = arith.constant -3.40282347E+38 : f32
+//  CHECK-DAG: %[[I1:.*]] = tensor.expand_shape %{{.*}}[0, 1], [2]] : tensor<32x2xf32> into tensor<4x8x2xf32>
+//  CHECK-DAG: %[[I2:.*]] = tensor.expand_shape %{{.*}}[0], [1, 2]] : tensor<5x32xf32> into tensor<5x4x8xf32>
+//      CHECK: %[[INI:.*]] = linalg.init_tensor [5, 2, 4] : tensor<5x2x4xf32>
+//      CHECK: %[[F:.*]] = linalg.fill ins(%[[ID]] : f32) outs(%[[INI]] : tensor<5x2x4xf32>) -> tensor<5x2x4xf32>
+//      CHECK: %[[G:.*]] = linalg.generic {indexing_maps = [#[[$MAP0]], #[[$MAP1]], #[[$MAP2]]], iterator_types = ["parallel", "reduction", "parallel", "parallel"]}
+// CHECK-SAME:   ins(%[[I1]], %[[I2]] : tensor<4x8x2xf32>, tensor<5x4x8xf32>) outs(%[[F]] : tensor<5x2x4xf32>) {
+//      CHECK:   arith.addf
+//      CHECK:   arith.maxf
+//      CHECK:   linalg.yield
+//      CHECK: } -> tensor<5x2x4xf32>
+//      CHECK: %[[R:.*]] = linalg.generic {indexing_maps = [#[[$MAP3]], #[[$MAP4]]], iterator_types = ["parallel", "parallel", "reduction"]}
+// CHECK-SAME:   ins(%[[G]] : tensor<5x2x4xf32>) outs(%{{.*}} : tensor<5x2xf32>) {
+//      CHECK:   arith.maxf
+//      CHECK:   linalg.yield
+//      CHECK:  } -> tensor<5x2xf32>
+//      CHECK: return %[[R]] : tensor<5x2xf32>

mlir/test/lib/Dialect/Linalg/TestLinalgTransforms.cpp

Option<bool> testGeneralizePadTensor{
       *this, "test-generalize-pad-tensor",
       llvm::cl::desc("Test transform pad tensor by copying with generic ops"),
       llvm::cl::init(false)};
   Option<bool> testSwapSubTensorPadTensor{
       *this, "test-swap-subtensor-padtensor",
       llvm::cl::desc("Test rewrite of subtensor(pad_tensor) into "
                      "pad_tensor(subtensor)"),
       llvm::cl::init(false)};
+  Option<bool> testSplitReduction{
+      *this, "test-split-reduction",
+      llvm::cl::desc("Test split reduction transformation"),
+      llvm::cl::init(false)};
   ListOption<int64_t> peeledLoops{
       *this, "peeled-loops",
       llvm::cl::desc("Loops to be peeled when test-tile-pattern"),
       llvm::cl::ZeroOrMore, llvm::cl::MiscFlags::CommaSeparated};
   ListOption<int64_t> tileSizes{
       *this, "tile-sizes",
       llvm::cl::desc("Linalg tile sizes for test-tile-pattern"),
       llvm::cl::ZeroOrMore, llvm::cl::MiscFlags::CommaSeparated};
if (scalarizeDynamicDims) {
     linalgTilingOptions.setTileSizes(tileSizes);
   }
   linalg::LinalgTransformationFilter f(StringAttr::get(context, "tile"));
   TilingPatterns<linalg::MatmulOp, linalg::GenericOp>::insert(
       tilingPattern, linalgTilingOptions, f);
   (void)applyPatternsAndFoldGreedily(funcOp, std::move(tilingPattern));
 }
 
+static void applySplitReduction(FuncOp funcOp) {
+  RewritePatternSet patterns(funcOp.getContext());
+  linalg::populateSplitReductionPattern(
+      patterns,
+      [](LinalgOp op) {
+        unsigned insertDimIndex = op.getNumLoops() - 1;
+        return std::make_pair(4, insertDimIndex);
+      },
+      LinalgTransformationFilter(
+          ArrayRef<StringAttr>{},
+          StringAttr::get(funcOp.getContext(), "SPLIT")));
+  (void)applyPatternsAndFoldGreedily(funcOp, std::move(patterns));
+}
+
 /// Apply transformations specified as patterns.
 void TestLinalgTransforms::runOnOperation() {
   auto lambda = [&](void *) {
     getOperation().walk([](LinalgOp op) {
       op->removeAttr(LinalgTransforms::kLinalgTransformMarker);
     });
   };
   std::unique_ptr<void, decltype(lambda)> cleanupGuard{(void *)1, lambda};
void TestLinalgTransforms::runOnOperation() {
   if (testSwapSubTensorPadTensor)
     return applyExtractSliceOfPadTensorSwapPattern(getOperation());
   if (testTilePattern)
     return applyTilePattern(getOperation(), loopType, tileSizes, peeledLoops,
                             /*scalarizeDynamicDims=*/false);
   if (testTileScalarizeDynamicDims)
     return applyTilePattern(getOperation(), loopType, tileSizes,
                             /*peeledLoops=*/{}, /*scalarizeDynamicDims=*/true);
+  if (testSplitReduction)
+    return applySplitReduction(getOperation());
 }
 
 namespace mlir {
 namespace test {
 void registerTestLinalgTransforms() {
   PassRegistration<TestLinalgTransforms>();
 }
 } // namespace test
 } // namespace mlir