MLIR

Multi-Level IR Compiler Framework

Chapter 7: Adding a Composite Type to Toy

In the previous chapter , we demonstrated an end-to-end compilation flow from our Toy front-end to LLVM IR. In this chapter, we will extend the Toy language to support a new composite struct type.

Defining a struct in Toy

The first thing we need to define is the interface of this type in our toy source language. The general syntax of a struct type in Toy is as follows:

# A struct is defined by using the `struct` keyword followed by a name.
struct MyStruct {
  # Inside of the struct is a list of variable declarations without initializers
  # or shapes, which may also be other previously defined structs.
  var a;
  var b;
}

Structs may now be used in functions as variables or parameters by using the name of the struct instead of var. The members of the struct are accessed via a . access operator. Values of struct type may be initialized with a composite initializer, or a comma-separated list of other initializers surrounded by {}. An example is shown below:

struct Struct {
  var a;
  var b;
}

# User defined generic function may operate on struct types as well.
def multiply_transpose(Struct value) {
  # We can access the elements of a struct via the '.' operator.
  return transpose(value.a) * transpose(value.b);
}

def main() {
  # We initialize struct values using a composite initializer.
  Struct value = {[[1, 2, 3], [4, 5, 6]], [[1, 2, 3], [4, 5, 6]]};

  # We pass these arguments to functions like we do with variables.
  var c = multiply_transpose(value);
  print(c);
}

Defining a struct in MLIR

In MLIR, we will also need a representation for our struct types. MLIR does not provide a type that does exactly what we need, so we will need to define our own. We will simply define our struct as an unnamed container of a set of element types. The name of the struct and its elements are only useful for the AST of our toy compiler, so we don’t need to encode it in the MLIR representation.

Defining the Type Class

Reserving a Range of Type Kinds

Types in MLIR rely on having a unique kind value to ensure that casting checks remain extremely efficient ( rationale ). For toy, this means we need to explicitly reserve a static range of type kind values in the symbol registry file DialectSymbolRegistry .

DEFINE_SYM_KIND_RANGE(LINALG) // Linear Algebra Dialect
DEFINE_SYM_KIND_RANGE(TOY)    // Toy language (tutorial) Dialect

// The following ranges are reserved for experimenting with MLIR dialects in a
// private context.
DEFINE_SYM_KIND_RANGE(PRIVATE_EXPERIMENTAL_0)

These definitions will provide a range in the Type::Kind enum to use when defining the derived types.

/// Create a local enumeration with all of the types that are defined by Toy.
namespace ToyTypes {
enum Types {
  Struct = mlir::Type::FIRST_TOY_TYPE,
};
} // end namespace ToyTypes

Defining the Type Class

As mentioned in chapter 2 , Type objects in MLIR are value-typed and rely on having an internal storage object that holds the actual data for the type. The Type class in itself acts as a simple wrapper around an internal TypeStorage object that is uniqued within an instance of an MLIRContext. When constructing a Type, we are internally just constructing and uniquing an instance of a storage class.

When defining a new Type that requires additional information beyond just the kind (e.g. the struct type, which requires additional information to hold the element types), we will need to provide a derived storage class. The primitive types that don’t have any additional data (e.g. the index type ) don’t require a storage class.

Defining the Storage Class

Type storage objects contain all of the data necessary to construct and unique a type instance. Derived storage classes must inherit from the base mlir::TypeStorage and provide a set of aliases and hooks that will be used by the MLIRContext for uniquing. Below is the definition of the storage instance for our struct type, with each of the necessary requirements detailed inline:

/// This class represents the internal storage of the Toy `StructType`.
struct StructTypeStorage : public mlir::TypeStorage {
  /// The `KeyTy` is a required type that provides an interface for the storage
  /// instance. This type will be used when uniquing an instance of the type
  /// storage. For our struct type, we will unique each instance structurally on
  /// the elements that it contains.
  using KeyTy = llvm::ArrayRef<mlir::Type>;

  /// A constructor for the type storage instance.
  StructTypeStorage(llvm::ArrayRef<mlir::Type> elementTypes)
      : elementTypes(elementTypes) {}

  /// Define the comparison function for the key type with the current storage
  /// instance. This is used when constructing a new instance to ensure that we
  /// haven't already uniqued an instance of the given key.
  bool operator==(const KeyTy &key) const { return key == elementTypes; }

  /// Define a hash function for the key type. This is used when uniquing
  /// instances of the storage.
  /// Note: This method isn't necessary as both llvm::ArrayRef and mlir::Type
  /// have hash functions available, so we could just omit this entirely.
  static llvm::hash_code hashKey(const KeyTy &key) {
    return llvm::hash_value(key);
  }

  /// Define a construction function for the key type from a set of parameters.
  /// These parameters will be provided when constructing the storage instance
  /// itself, see the `StructType::get` method further below.
  /// Note: This method isn't necessary because KeyTy can be directly
  /// constructed with the given parameters.
  static KeyTy getKey(llvm::ArrayRef<mlir::Type> elementTypes) {
    return KeyTy(elementTypes);
  }

  /// Define a construction method for creating a new instance of this storage.
  /// This method takes an instance of a storage allocator, and an instance of a
  /// `KeyTy`. The given allocator must be used for *all* necessary dynamic
  /// allocations used to create the type storage and its internal.
  static StructTypeStorage *construct(mlir::TypeStorageAllocator &allocator,
                                      const KeyTy &key) {
    // Copy the elements from the provided `KeyTy` into the allocator.
    llvm::ArrayRef<mlir::Type> elementTypes = allocator.copyInto(key);

    // Allocate the storage instance and construct it.
    return new (allocator.allocate<StructTypeStorage>())
        StructTypeStorage(elementTypes);
  }

  /// The following field contains the element types of the struct.
  llvm::ArrayRef<mlir::Type> elementTypes;
};
Defining the Type Class

With the storage class defined, we can add the definition for the user-visible StructType class. This is the class that we will actually interface with.

/// This class defines the Toy struct type. It represents a collection of
/// element types. All derived types in MLIR must inherit from the CRTP class
/// 'Type::TypeBase'. It takes as template parameters the concrete type
/// (StructType), the base class to use (Type), and the storage class
/// (StructTypeStorage).
class StructType : public mlir::Type::TypeBase<StructType, mlir::Type,
                                               StructTypeStorage> {
public:
  /// Inherit some necessary constructors from 'TypeBase'.
  using Base::Base;

  /// This static method is used to support type inquiry through isa, cast,
  /// and dyn_cast.
  static bool kindof(unsigned kind) { return kind == ToyTypes::Struct; }

  /// Create an instance of a `StructType` with the given element types. There
  /// *must* be at least one element type.
  static StructType get(llvm::ArrayRef<mlir::Type> elementTypes) {
    assert(!elementTypes.empty() && "expected at least 1 element type");

    // Call into a helper 'get' method in 'TypeBase' to get a uniqued instance
    // of this type. The first two parameters are the context to unique in and
    // the kind of the type. The parameters after the type kind are forwarded to
    // the storage instance.
    mlir::MLIRContext *ctx = elementTypes.front().getContext();
    return Base::get(ctx, ToyTypes::Struct, elementTypes);
  }

  /// Returns the element types of this struct type.
  llvm::ArrayRef<mlir::Type> getElementTypes() {
    // 'getImpl' returns a pointer to the internal storage instance.
    return getImpl()->elementTypes;
  }

  /// Returns the number of element type held by this struct.
  size_t getNumElementTypes() { return getElementTypes().size(); }
};

We register this type in the ToyDialect constructor in a similar way to how we did with operations:

ToyDialect::ToyDialect(mlir::MLIRContext *ctx)
    : mlir::Dialect(getDialectNamespace(), ctx) {
  addTypes<StructType>();
}

With this we can now use our StructType when generating MLIR from Toy. See examples/toy/Ch7/mlir/MLIRGen.cpp for more details.

Parsing and Printing

At this point we can use our StructType during MLIR generation and transformation, but we can’t output or parse .mlir. For this we need to add support for parsing and printing instances of the StructType. This can be done by overriding the parseType and printType methods on the ToyDialect.

class ToyDialect : public mlir::Dialect {
public:
  /// Parse an instance of a type registered to the toy dialect.
  mlir::Type parseType(mlir::DialectAsmParser &parser) const override;

  /// Print an instance of a type registered to the toy dialect.
  void printType(mlir::Type type,
                 mlir::DialectAsmPrinter &printer) const override;
};

These methods take an instance of a high-level parser or printer that allows for easily implementing the necessary functionality. Before going into the implementation, let’s think about the syntax that we want for the struct type in the printed IR. As described in the MLIR language reference , dialect types are generally represented as: ! dialect-namespace < type-data >, with a pretty form available under certain circumstances. The responsibility of our Toy parser and printer is to provide the type-data bits. We will define our StructType as having the following form:

  struct-type ::= `struct` `<` type (`,` type)* `>`

Parsing

An implementation of the parser is shown below:

/// Parse an instance of a type registered to the toy dialect.
mlir::Type ToyDialect::parseType(mlir::DialectAsmParser &parser) const {
  // Parse a struct type in the following form:
  //   struct-type ::= `struct` `<` type (`,` type)* `>`

  // NOTE: All MLIR parser function return a ParseResult. This is a
  // specialization of LogicalResult that auto-converts to a `true` boolean
  // value on failure to allow for chaining, but may be used with explicit
  // `mlir::failed/mlir::succeeded` as desired.

  // Parse: `struct` `<`
  if (parser.parseKeyword("struct") || parser.parseLess())
    return Type();

  // Parse the element types of the struct.
  SmallVector<mlir::Type, 1> elementTypes;
  do {
    // Parse the current element type.
    llvm::SMLoc typeLoc = parser.getCurrentLocation();
    mlir::Type elementType;
    if (parser.parseType(elementType))
      return nullptr;

    // Check that the type is either a TensorType or another StructType.
    if (!elementType.isa<mlir::TensorType>() &&
        !elementType.isa<StructType>()) {
      parser.emitError(typeLoc, "element type for a struct must either "
                                "be a TensorType or a StructType, got: ")
          << elementType;
      return Type();
    }
    elementTypes.push_back(elementType);

    // Parse the optional: `,`
  } while (succeeded(parser.parseOptionalComma()));

  // Parse: `>`
  if (parser.parseGreater())
    return Type();
  return StructType::get(elementTypes);
}

Printing

An implementation of the printer is shown below:

/// Print an instance of a type registered to the toy dialect.
void ToyDialect::printType(mlir::Type type,
                           mlir::DialectAsmPrinter &printer) const {
  // Currently the only toy type is a struct type.
  StructType structType = type.cast<StructType>();

  // Print the struct type according to the parser format.
  printer << "struct<";
  mlir::interleaveComma(structType.getElementTypes(), printer);
  printer << '>';
}

Before moving on, let’s look at a quick of example showcasing the functionality we have now:

struct Struct {
  var a;
  var b;
}

def multiply_transpose(Struct value) {
}

Which generates the following:

module {
  func @multiply_transpose(%arg0: !toy.struct<tensor<*xf64>, tensor<*xf64>>) {
    toy.return
  }
}

Operating on StructType

Now that the struct type has been defined, and we can round-trip it through the IR. The next step is to add support for using it within our operations.

Updating Existing Operations

A few of our existing operations will need to be updated to handle StructType. The first step is to make the ODS framework aware of our Type so that we can use it in the operation definitions. A simple example is shown below:

// Provide a definition for the Toy StructType for use in ODS. This allows for
// using StructType in a similar way to Tensor or MemRef.
def Toy_StructType :
    Type<CPred<"$_self.isa<StructType>()">, "Toy struct type">;

// Provide a definition of the types that are used within the Toy dialect.
def Toy_Type : AnyTypeOf<[F64Tensor, Toy_StructType]>;

We can then update our operations, e.g. ReturnOp, to also accept the Toy_StructType:

def ReturnOp : Toy_Op<"return", [Terminator, HasParent<"FuncOp">]> {
  ...
  let arguments = (ins Variadic<Toy_Type>:$input);
  ...
}

Adding New Toy Operations

In addition to the existing operations, we will be adding a few new operations that will provide more specific handling of structs.

toy.struct_constant

This new operation materializes a constant value for a struct. In our current modeling, we just use an array attribute that contains a set of constant values for each of the struct elements.

  %0 = toy.struct_constant [
    dense<[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]]> : tensor<2x3xf64>
  ] : !toy.struct<tensor<*xf64>>
toy.struct_access

This new operation materializes the Nth element of a struct value.

  // Using %0 from above
  %1 = toy.struct_access %0[0] : !toy.struct<tensor<*xf64>> -> tensor<*xf64>

With these operations, we can revisit our original example:

struct Struct {
  var a;
  var b;
}

# User defined generic function may operate on struct types as well.
def multiply_transpose(Struct value) {
  # We can access the elements of a struct via the '.' operator.
  return transpose(value.a) * transpose(value.b);
}

def main() {
  # We initialize struct values using a composite initializer.
  Struct value = {[[1, 2, 3], [4, 5, 6]], [[1, 2, 3], [4, 5, 6]]};

  # We pass these arguments to functions like we do with variables.
  var c = multiply_transpose(value);
  print(c);
}

and finally get a full MLIR module:

module {
  func @multiply_transpose(%arg0: !toy.struct<tensor<*xf64>, tensor<*xf64>>) -> tensor<*xf64> {
    %0 = toy.struct_access %arg0[0] : !toy.struct<tensor<*xf64>, tensor<*xf64>> -> tensor<*xf64>
    %1 = toy.transpose(%0 : tensor<*xf64>) to tensor<*xf64>
    %2 = toy.struct_access %arg0[1] : !toy.struct<tensor<*xf64>, tensor<*xf64>> -> tensor<*xf64>
    %3 = toy.transpose(%2 : tensor<*xf64>) to tensor<*xf64>
    %4 = toy.mul %1, %3 : tensor<*xf64>
    toy.return %4 : tensor<*xf64>
  }
  func @main() {
    %0 = toy.struct_constant [
      dense<[[1.000000e+00, 2.000000e+00, 3.000000e+00], [4.000000e+00, 5.000000e+00, 6.000000e+00]]> : tensor<2x3xf64>,
      dense<[[1.000000e+00, 2.000000e+00, 3.000000e+00], [4.000000e+00, 5.000000e+00, 6.000000e+00]]> : tensor<2x3xf64>
    ] : !toy.struct<tensor<*xf64>, tensor<*xf64>>
    %1 = toy.generic_call @multiply_transpose(%0) : (!toy.struct<tensor<*xf64>, tensor<*xf64>>) -> tensor<*xf64>
    toy.print %1 : tensor<*xf64>
    toy.return
  }
}

Optimizing Operations on StructType

Now that we have a few operations operating on StructType, we also have many new constant folding opportunities.

After inlining, the MLIR module in the previous section looks something like:

module {
  func @main() {
    %0 = toy.struct_constant [
      dense<[[1.000000e+00, 2.000000e+00, 3.000000e+00], [4.000000e+00, 5.000000e+00, 6.000000e+00]]> : tensor<2x3xf64>,
      dense<[[1.000000e+00, 2.000000e+00, 3.000000e+00], [4.000000e+00, 5.000000e+00, 6.000000e+00]]> : tensor<2x3xf64>
    ] : !toy.struct<tensor<*xf64>, tensor<*xf64>>
    %1 = toy.struct_access %0[0] : !toy.struct<tensor<*xf64>, tensor<*xf64>> -> tensor<*xf64>
    %2 = toy.transpose(%1 : tensor<*xf64>) to tensor<*xf64>
    %3 = toy.struct_access %0[1] : !toy.struct<tensor<*xf64>, tensor<*xf64>> -> tensor<*xf64>
    %4 = toy.transpose(%3 : tensor<*xf64>) to tensor<*xf64>
    %5 = toy.mul %2, %4 : tensor<*xf64>
    toy.print %5 : tensor<*xf64>
    toy.return
  }
}

We have several toy.struct_access operations that access into a toy.struct_constant. As detailed in chapter 3 (FoldConstantReshape), we can add folders for these toy operations by setting the hasFolder bit on the operation definition and providing a definition of the *Op::fold method.

/// Fold constants.
OpFoldResult ConstantOp::fold(ArrayRef<Attribute> operands) { return value(); }

/// Fold struct constants.
OpFoldResult StructConstantOp::fold(ArrayRef<Attribute> operands) {
  return value();
}

/// Fold simple struct access operations that access into a constant.
OpFoldResult StructAccessOp::fold(ArrayRef<Attribute> operands) {
  auto structAttr = operands.front().dyn_cast_or_null<mlir::ArrayAttr>();
  if (!structAttr)
    return nullptr;

  size_t elementIndex = index().getZExtValue();
  return structAttr[elementIndex];
}

To ensure that MLIR generates the proper constant operations when folding our Toy operations, i.e. ConstantOp for TensorType and StructConstant for StructType, we will need to provide an override for the dialect hook materializeConstant. This allows for generic MLIR operations to create constants for the Toy dialect when necessary.

mlir::Operation *ToyDialect::materializeConstant(mlir::OpBuilder &builder,
                                                 mlir::Attribute value,
                                                 mlir::Type type,
                                                 mlir::Location loc) {
  if (type.isa<StructType>())
    return builder.create<StructConstantOp>(loc, type,
                                            value.cast<mlir::ArrayAttr>());
  return builder.create<ConstantOp>(loc, type,
                                    value.cast<mlir::DenseElementsAttr>());
}

With this, we can now generate code that can be generated to LLVM without any changes to our pipeline.

module {
  func @main() {
    %0 = toy.constant dense<[[1.000000e+00, 2.000000e+00, 3.000000e+00], [4.000000e+00, 5.000000e+00, 6.000000e+00]]> : tensor<2x3xf64>
    %1 = toy.transpose(%0 : tensor<2x3xf64>) to tensor<3x2xf64>
    %2 = toy.mul %1, %1 : tensor<3x2xf64>
    toy.print %2 : tensor<3x2xf64>
    toy.return
  }
}

You can build toyc-ch7 and try yourself: toyc-ch7 test/Examples/Toy/Ch7/struct-codegen.toy -emit=mlir. More details on defining custom types can be found in DefiningAttributesAndTypes .