[MLIR][Tensor] Add canonicalization patterns for `tensor.pack`

[chelini]

Summary

• Fold an unpack(pack(x)) to x.

• Rewrite a tensor.pack to an tensor.expand_shape if only one dimension is packed.

[chelini]

Take into account padding semantics.

[chelini]

Add test for padding

[rengolin]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

3440

Not clear why you outlined this code. Seems simple enough to be inside matchAndRewrite.

[chelini]

Rename

[mravishankar]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

3424

Does this need to check for padding semantics? Maybe not, but I am not sure.

3437

This doesnt look like a canonicalization to me. I'd rather add a separate "populate*" method to pull this in.

[chelini]

Address comments:

• Add a populate* method to simplify tensor.pack.

[rengolin]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

3424

I think we do. Both padding and inner tiles.

If the pack has padding, than the packed tensor may be "unpackable" unless we define the operations on the padding irrelevant. But if that's true, then we need to let unpack know that this is what we've done, so that it can match its output shape to be the same as the original (not sure how, since pack doesn't have an amount to pad, just a constant to use as padding).

If the unpack has different tiles as the pack, then the order of elements will be different and will create a different output tensor, so we can't collapse the pair.

The size of the output tensor has to validate from the shape of the type. In both pad and uneven tiling cases, we have to make sure that the pre-packed shape is identical to the post-unpacked shape. We can do that via use-def chains or by writing a strict shape policy and carry padding sizes around.

None of those things are simple, so I'd just avoid them for now. Just make sure there's no padding and the inner tiles are the same, too.

[hanchung]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

3037–3038

I think using RankedTensorType is a bit more accurate.

3053

I looked into other file and think this should be // namespace.

mlir/test/lib/Dialect/Tensor/TestTensorTransforms.cpp

297–299

llvm style nit: we don't use braces for single statements.

https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

[chelini]

• Bail out if tensor.pack has padding

• Check tiles for pack and unpack to be the same when canonicalize chain

• Use single-braces in TestTensorTransforms.cpp

• Fix namespace: end namespace -> namespace

• Add more tests with dynamic tiles/dims

• Use RankedTensorType instead if ShapedType in SimplifyPackToExandShape

• Improve code reuse between pack::canonicalize and unpack::canonicalize

[rengolin]

Thanks @chelini, LGTM now, but I'll let others make the final approval.

[hanchung]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

3028–3030

[optional] The operandType is only used once; it does not improve readability. The method itself describes it. I think we can save one line here.

3422

My experience tells me that we seldom add braces to it (though I could not find style guide about it).

[tyb0807]

Thank you, LGTM

[chelini]

• Remove braces

• Address comment on operandType

[chelini]

Fix compile error

[mravishankar]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

3424

Agreed. I didnt see that the padding semantics is checked in the pattern. If the pack has padding, then we cant simply fold the pack and unpack away. Cause pack is doing more than what unpack is doing. THats what my concern was. I dont fully follow everything stated above, but the code itself looks fine.

mlir/include/mlir/Dialect/Tensor/IR/Tensor.h

 FailureOr<Value> getOrCreateDestination(OpBuilder &b, Location loc,
                                         OpResult opResult);
 
 /// This is a helper function for DestinationStyleOpInterface. Get or create
 /// destinations for every tensor OpResult of the given op.
 LogicalResult getOrCreateDestinations(OpBuilder &b, Location loc, Operation *op,
                                       SmallVector<Value> &result);
 
-/// Function to control the folding of constant and extract slice
+/// Function to control the folding of constant and extract slice.
 using ControlConstantExtractSliceFusionFn = std::function<bool(ExtractSliceOp)>;
 
-/// Patterns to fold the extract slice op with its constant operand
+/// Patterns to fold the extract slice op with its constant operand.
 void populateFoldConstantExtractSlicePatterns(
     RewritePatternSet &patterns,
     const ControlConstantExtractSliceFusionFn &controlFn =
         [](ExtractSliceOp op) {
           // Disable by default because the folding can generate a large
           // constant tensor, which would affect the compile time and storage.
           return false;
         });
 
+/// Patterns to simplify tensor.pack.
+void populateSimplifyTensorPack(RewritePatternSet &patterns);
+
 } // namespace tensor
 } // namespace mlir
 
 #endif // MLIR_DIALECT_TENSOR_IR_TENSOR_H_

mlir/include/mlir/Dialect/Tensor/IR/TensorOps.td

let extraClassDeclaration = commonExtraClassDeclaration # [{
     static ShapedType inferPackedType(ShapedType sourceType,
         ArrayRef<int64_t> innerTileSizes, ArrayRef<int64_t> innerDimsPos,
         ArrayRef<int64_t> outerDimsPerm = {});
 
     static Value createDestinationTensor(OpBuilder &b, Location loc,
         Value source, ArrayRef<OpFoldResult> innerTileSizes,
         ArrayRef<int64_t> innerDimsPos, ArrayRef<int64_t> outerDimsPerm);
   }];
+
+  let hasCanonicalizeMethod = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // UnPackOp
 //===----------------------------------------------------------------------===//
 
 def Tensor_UnPackOp : Tensor_RelayoutOp<"unpack"> {
   let summary = "tensor unpack operation";

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

OpFoldResult SplatOp::fold(ArrayRef<Attribute> operands) {
   // splat.
   return SplatElementsAttr::get(getType(), {constOperand});
 }
 
 //===----------------------------------------------------------------------===//
 // PackOp/UnPackOp Common
 //===----------------------------------------------------------------------===//
 
+namespace {
+
+/// Packing one-dimensional tensor can be expressed as an expand shape op.
+struct SimplifyPackToExandShape : public OpRewritePattern<PackOp> {
+  using OpRewritePattern<PackOp>::OpRewritePattern;
+
+  Value insertExpand(RewriterBase &rewriter, Location loc, Value operand,
+                     Type newOperandType, ArrayAttr reassociation) const {
+    if (operand.getType() == newOperandType)
+      return operand;
+    return rewriter.create<tensor::ExpandShapeOp>(loc, newOperandType, operand,
+                                                  reassociation);
+  }
+
+  LogicalResult matchAndRewrite(PackOp packOp,
+                                PatternRewriter &rewriter) const override {
+    RankedTensorType sourceType = packOp.getSourceType();
+    RankedTensorType destType = packOp.getDestType();
+    if (sourceType.getRank() != 1 || packOp.getPaddingValue())
+      return failure();
+    auto reassociation =
+        getReassociationIndicesForReshape(sourceType, destType);
+    if (!reassociation)
+      return failure();
+    Value expanded = insertExpand(
+        rewriter, packOp.getLoc(), packOp.getSource(), destType,
+        getReassociationIndicesAttribute(rewriter, *reassociation));
+    rewriter.replaceOp(packOp, expanded);
+    return success();
+  }
+};
+
+} // namespace
+
+void mlir::tensor::populateSimplifyTensorPack(RewritePatternSet &patterns) {
+  patterns.add<SimplifyPackToExandShape>(patterns.getContext());
+}
+
 template <typename OpTy>
 static LogicalResult
 reifyResultShapesImpl(OpTy op, OpBuilder &builder,
                       ReifiedRankedShapedTypeDims &reifiedReturnShapes) {
   static_assert(llvm::is_one_of<OpTy, PackOp, UnPackOp>::value,
                 "applies to only pack or unpack operations");
   int64_t destRank = op.getDestRank();
   reifiedReturnShapes.resize(1, SmallVector<Value>(destRank));
Speculation::Speculatability PackOp::getSpeculatability() {
   // sizes of the tiles do not divide perfectly the dimension; thus, check only
   // to have constant tiles and tiled inner dimensions.
   if (!areTilesAndTiledDimsAllConstant(*this))
     return Speculation::NotSpeculatable;
 
   return Speculation::Speculatable;
 }
 
+// Return true if `inner_dims_pos` and `outer_dims_perm` target the same
+// dimensions for pack and unpack.
+static bool hasSameInnerOuterAttribute(PackOp packOp, UnPackOp unPackOp) {
+  if (packOp.getInnerDimsPos() != unPackOp.getInnerDimsPos())
+    return false;
+  return packOp.getOuterDimsPerm() == unPackOp.getOuterDimsPerm();
+}
+
+// Return true if pack and unpack have the same tiles.
+// Same SSA values or same integer constants.
+static bool haveSameTiles(PackOp packOp, UnPackOp unPackOp) {
+  auto packTiles = packOp.getMixedTiles();
+  auto unPackTiles = unPackOp.getMixedTiles();
+  if (packTiles.size() != unPackTiles.size())
+    return false;
+  for (size_t i = 0, e = packTiles.size(); i < e; i++) {
+    if (!isEqualConstantIntOrValue(packTiles[i], unPackTiles[i]))
+      return false;
+  }
+  return true;
+}
+
+/// Fold an unpack(pack(x)) to x.
+LogicalResult PackOp::canonicalize(PackOp packOp, PatternRewriter &rewriter) {
+  UnPackOp unPackOp = packOp.getSource().getDefiningOp<UnPackOp>();
+  if (!unPackOp || unPackOp.getSourceType() != packOp.getDestType())
+    return failure();
+  if (packOp.getPaddingValue() ||
+      !hasSameInnerOuterAttribute(packOp, unPackOp) ||
+      !haveSameTiles(packOp, unPackOp))
+    return failure();
+  rewriter.replaceOp(packOp, unPackOp.getSource());
+  return success();
+}
+
 //===----------------------------------------------------------------------===//
 // UnPackOp
 //===----------------------------------------------------------------------===//
 
 void UnPackOp::getAsmResultNames(
     function_ref<void(Value, StringRef)> setNameFn) {
   setNameFn(getResult(), "unpack");
 }
void UnPackOp::build(OpBuilder &builder, OperationState &state, Value source,
   build(builder, state, dest.getType(), source, dest,
         outerDimsPerm.empty() ? nullptr
                               : builder.getDenseI64ArrayAttr(outerDimsPerm),
         builder.getDenseI64ArrayAttr(innerDimsPos), dynamicTileSizes,
         builder.getDenseI64ArrayAttr(staticTileSizes));
 }
 
 /// pack(unpack(x)) -> x
-LogicalResult UnPackOp::canonicalize(UnPackOp unpackOp,
+LogicalResult UnPackOp::canonicalize(UnPackOp unPackOp,
                                      PatternRewriter &rewriter) {
-  PackOp packOp = unpackOp.getSource().getDefiningOp<tensor::PackOp>();
-  if (!packOp || packOp.getDestType() != unpackOp.getSourceType())
-    return failure();
-  if (packOp.getInnerDimsPos() != unpackOp.getInnerDimsPos())
+  PackOp packOp = unPackOp.getSource().getDefiningOp<tensor::PackOp>();
+  if (!packOp || packOp.getDestType() != unPackOp.getSourceType())
     return failure();
-  if (packOp.getOuterDimsPerm() != unpackOp.getOuterDimsPerm())
+  if (packOp.getPaddingValue() ||
+      !hasSameInnerOuterAttribute(packOp, unPackOp) ||
+      !haveSameTiles(packOp, unPackOp))
     return failure();
-  rewriter.replaceOp(unpackOp, packOp.getSource());
+  rewriter.replaceOp(unPackOp, packOp.getSource());
   return success();
 }
 
 //===----------------------------------------------------------------------===//
 // Common Canonicalizers and Folders.
 //===----------------------------------------------------------------------===//
 
 /// Folds a tensor.cast op into a consuming DestinationStyleOpInterface op if

mlir/test/Dialect/Tensor/canonicalize.mlir

 func.func @unpack_pack(%t: tensor<128x128xf32>, %tile1: index, %tile2: index) -> tensor<128x128xf32> {
   %tensor_empty = tensor.empty(%tile1, %tile2) : tensor<16x16x?x?xf32>
   %packed = tensor.pack %t inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty : tensor<128x128xf32> -> tensor<16x16x?x?xf32>
   %tensor_empty1 = tensor.empty() : tensor<128x128xf32>
   %unpacked = tensor.unpack %packed inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty1 : tensor<16x16x?x?xf32> -> tensor
 <128x128xf32>
   return %unpacked : tensor<128x128xf32>
 }
+
+// -----
+
+// Chain NCnc -> NC -> NC -> NCnc
+// CHECK: func.func @pack_unpack(
+// CHECK-SAME: %[[T:.+]]: tensor<16x16x?x?xf32>,
+// CHECK: return %[[T]] : tensor<16x16x?x?xf32>
+func.func @pack_unpack(%t: tensor<16x16x?x?xf32>, %tile1: index, %tile2: index) -> tensor<16x16x?x?xf32> {
+  %tensor_empty = tensor.empty() : tensor<128x128xf32>
+  %unpacked = tensor.unpack %t inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty : tensor<16x16x?x?xf32> -> tensor<128x128xf32>
+  %tensor_empty1 = tensor.empty(%tile1, %tile2) : tensor<16x16x?x?xf32>
+  %packed = tensor.pack %unpacked inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty1 : tensor<128x128xf32> -> tensor<16x16x?x?xf32>
+  return %packed : tensor<16x16x?x?xf32>
+}
+
+// -----
+
+// Chain NCnc -> NC -> NC -> NCnc
+// CHECK: func.func @pack_unpack(
+// CHECK-SAME: %[[T:.+]]: tensor<16x16x8x8xf32>
+// CHECK: return %[[T]] : tensor<16x16x8x8xf32>
+func.func @pack_unpack(%t: tensor<16x16x8x8xf32>) -> tensor<16x16x8x8xf32> {
+  %tensor_empty = tensor.empty() : tensor<128x128xf32>
+  %unpacked = tensor.unpack %t inner_dims_pos = [0, 1] inner_tiles = [8, 8] into %tensor_empty : tensor<16x16x8x8xf32> -> tensor<128x128xf32>
+  %tensor_empty1 = tensor.empty() : tensor<16x16x8x8xf32>
+  %packed = tensor.pack %unpacked inner_dims_pos = [0, 1] inner_tiles = [8, 8] into %tensor_empty1 : tensor<128x128xf32> -> tensor<16x16x8x8xf32>
+  return %packed : tensor<16x16x8x8xf32>
+}
+
+// -----
+
+// CHECK: func.func @pack_unpack_same_tiles(
+// CHECK-SAME:  %[[T:.+]]: tensor<?x?x?x?xf32>,
+// CHECK: return %[[T]] : tensor<?x?x?x?xf32>
+func.func @pack_unpack_same_tiles(%t: tensor<?x?x?x?xf32>, %dim1: index, %dim2: index, %dim3: index, %dim4: index, %dim5: index, %dim6: index,
+                       %tile1: index, %tile2: index) -> tensor<?x?x?x?xf32> {
+  %tensor_empty = tensor.empty(%dim1, %dim2) : tensor<?x?xf32>
+  %unpacked = tensor.unpack %t inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty : tensor<?x?x?x?xf32> -> tensor<?x?xf32>
+  %tensor_empty1 = tensor.empty(%dim3, %dim4, %dim5, %dim6) : tensor<?x?x?x?xf32>
+  %packed = tensor.pack %unpacked inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty1 : tensor<?x?xf32> -> tensor<?x?x?x?xf32>
+  return %packed : tensor<?x?x?x?xf32>
+}
+
+// -----
+
+// CHECK: func.func @pack_unpack_different_tiles(
+// CHECK-SAME:  %[[T:.+]]: tensor<?x?x?x?xf32>,
+// CHECK-NOT: return %[[T]] : tensor<?x?x?x?xf32>
+func.func @pack_unpack_different_tiles(%t: tensor<?x?x?x?xf32>, %dim1: index, %dim2: index, %dim3: index, %dim4: index, %dim5: index, %dim6: index,
+                       %tile1: index, %tile2: index) -> tensor<?x?x?x?xf32> {
+  %tensor_empty = tensor.empty(%dim1, %dim2) : tensor<?x?xf32>
+  %unpacked = tensor.unpack %t inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty : tensor<?x?x?x?xf32> -> tensor<?x?xf32>
+  %tensor_empty1 = tensor.empty(%dim3, %dim4, %dim5, %dim6) : tensor<?x?x?x?xf32>
+  %packed = tensor.pack %unpacked inner_dims_pos = [0, 1] inner_tiles = [%tile2, %tile1] into %tensor_empty1 : tensor<?x?xf32> -> tensor<?x?x?x?xf32>
+  return %packed : tensor<?x?x?x?xf32>
+}
+
+// -----
+
+// CHECK: func.func @pack_unpack_dynamic_with_padding(
+// CHECK-SAME:  %[[T:.+]]: tensor<?x?x?x?xf32>,
+// CHECK-NOT: return %[[T]] : tensor<?x?x?x?xf32>
+func.func @pack_unpack_dynamic_with_padding(%t: tensor<?x?x?x?xf32>, %dim1: index, %dim2: index, %dim3: index, %dim4: index, %dim5: index, %dim6: index,
+                       %tile1: index, %tile2: index, %pad: f32) -> tensor<?x?x?x?xf32> {
+  %tensor_empty = tensor.empty(%dim1, %dim2) : tensor<?x?xf32>
+  %unpacked = tensor.unpack %t inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty : tensor<?x?x?x?xf32> -> tensor<?x?xf32>
+  %tensor_empty1 = tensor.empty(%dim3, %dim4, %dim5, %dim6) : tensor<?x?x?x?xf32>
+  %packed = tensor.pack %unpacked padding_value(%pad: f32) inner_dims_pos = [0, 1] inner_tiles = [%tile1, %tile2] into %tensor_empty1 : tensor<?x?xf32> -> tensor<?x?x?x?xf32>
+  return %packed : tensor<?x?x?x?xf32>
+}

mlir/test/Dialect/Tensor/simplify-tensor-pack.mlir

+// RUN: mlir-opt -split-input-file -test-tensor-transform-patterns="test-simplify-pack-patterns" %s | FileCheck %s
+
+// CHECK: func.func @single_dim_packing(
+// CHECK-SAME: %[[ARG0:.+]]: tensor<256xf32>)
+// CHECK: %[[EXPANDED:.+]] = tensor.expand_shape %[[ARG0]] {{\[}}[0, 1]] : tensor<256xf32> into tensor<8x32xf32>
+// CHECK: return %[[EXPANDED]] : tensor<8x32xf32>
+func.func @single_dim_packing(%arg0: tensor<256xf32>) -> tensor<8x32xf32> {
+  %empty = tensor.empty() : tensor<8x32xf32>
+  %0 = tensor.pack %arg0 inner_dims_pos = [0] inner_tiles = [32] into %empty : tensor<256xf32> -> tensor<8x32xf32>
+  return %0 : tensor<8x32xf32>
+}
+
+// -----
+
+// CHECK: func.func @single_dim_packing_with_padding(
+// CHECK-SAME: %[[ARG0:.+]]: tensor<255xf32>)
+// CHECK-NOT: tensor.expand_shape
+// CHECK: tensor.pack
+func.func @single_dim_packing_with_padding(%arg0: tensor<255xf32>) -> tensor<8x32xf32> {
+  %empty = tensor.empty() : tensor<8x32xf32>
+  %cst = arith.constant 0.000000e+00 : f32
+  %0 = tensor.pack %arg0 padding_value(%cst : f32) inner_dims_pos = [0] inner_tiles = [32] into %empty : tensor<255xf32> -> tensor<8x32xf32>
+  return %0 : tensor<8x32xf32>
+}

mlir/test/lib/Dialect/Tensor/TestTensorTransforms.cpp

Option<bool> testEmptyOpFolding{
       llvm::cl::desc("Test folding of tensor.empty"), llvm::cl::init(false)};
 
   Option<bool> useForeach{
       *this, "use-foreach",
       llvm::cl::desc(
           "Use the scf.foreach_thread operation when generating loop nests for "
           "the extract_slice of collapse_shape pattern"),
       llvm::cl::init(false)};
+
+  Option<bool> testSimplifyPackPatterns{
+      *this, "test-simplify-pack-patterns",
+      llvm::cl::desc("Test patterns to simplify tensor.pack"),
+      llvm::cl::init(false)};
 };
 } // namespace
 
 static void applyReassociativeReshapeFoldingPatterns(Operation *rootOp) {
   RewritePatternSet patterns(rootOp->getContext());
   tensor::populateReassociativeReshapeFoldingPatterns(patterns);
   (void)applyPatternsAndFoldGreedily(rootOp, std::move(patterns));
 }

 }
 
 static void applyFoldConsecutiveInsertExtractSlicePatterns(Operation *rootOp) {
   RewritePatternSet patterns(rootOp->getContext());
   tensor::populateMergeConsecutiveInsertExtractSlicePatterns(patterns);
   (void)applyPatternsAndFoldGreedily(rootOp, std::move(patterns));
 }
 
+static void applySimplifyPackPatterns(Operation *rootOp) {
+  RewritePatternSet patterns(rootOp->getContext());
+  tensor::populateSimplifyTensorPack(patterns);
+  (void)applyPatternsAndFoldGreedily(rootOp, std::move(patterns));
+}
+
 namespace {
 /// Base pattern to rewrite  a `tensor.collapse_shape -> tensor.extract_slice`.
 /// The `tensor.extract_slice` is replaced by a loop or gather operation that
 /// stitches together the desired tile from slices of the source of the collapse
 /// shape op.
 struct RewriteExtractSliceFromCollapseShapeBase
     : public OpRewritePattern<tensor::ExtractSliceOp> {
   RewriteExtractSliceFromCollapseShapeBase(MLIRContext *context)
applyRewriteExtractFromCollapseShapePatterns(Operation *rootOp,
   else
     patterns.add<RewriteExtractSliceFromCollapseShapeUsingScfFor>(
         rootOp->getContext());
   return applyPatternsAndFoldGreedily(rootOp, std::move(patterns));
 }
 
 void TestTensorTransforms::runOnOperation() {
   Operation *rootOp = getOperation();
+  if (testSimplifyPackPatterns)
+    applySimplifyPackPatterns(rootOp);
   if (testSplitPaddingPatterns)
     applySplitPaddingPatterns(rootOp);
   if (testFoldConstantExtractSlice)
     applyFoldConstantExtractSlicePatterns(rootOp);
   if (testFoldConsecutiveInsertExtractSlice)
     applyFoldConsecutiveInsertExtractSlicePatterns(rootOp);
   if (testReassociativeReshapeFolding)
     applyReassociativeReshapeFoldingPatterns(rootOp);
   if (testEmptyOpFolding)
     applyEmptyOpFoldingPatterns(rootOp);
   if (testRewriteExtractSliceWithTiledCollapseShape) {
     if (failed(
             applyRewriteExtractFromCollapseShapePatterns(rootOp, useForeach)))
       return signalPassFailure();
   }
 }
 
 namespace mlir {
 namespace test {
 void registerTestTensorTransforms() {
   PassRegistration<TestTensorTransforms>();
 }
 } // namespace test
 } // namespace mlir