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Quickstart tutorial to defining custom dialect attributes and types

This document is a quickstart to defining dialect specific extensions to the attribute and type system. The main part of the tutorial focuses on defining types, but the instructions are nearly identical for defining attributes.

See MLIR specification for more information about MLIR, the structure of the IR, operations, etc.

Types

Types in MLIR (like attributes, locations, and many other things) are value-typed. This means that instances of Type should be passed around by-value, as opposed to by-pointer or by-reference. The Type class in itself acts as a wrapper around an internal storage object that is uniqued within an instance of an MLIRContext.

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 a dialect author, this means that a range of type kind values must be explicitly, and statically, reserved. A dialect can reserve a range of values by adding a new entry to the DialectSymbolRegistry. To support out-of-tree and experimental dialects, the registry predefines a set of privates ranges, PRIVATE_EXPERIMENTAL_[0-9], that are free for immediate use.

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 without having to register them here.
DEFINE_SYM_KIND_RANGE(PRIVATE_EXPERIMENTAL_0)

For the sake of this tutorial, we will use the predefined PRIVATE_EXPERIMENTAL_0 range. These definitions will provide a range in the Type::Kind enum to use when defining the derived types.

namespace MyTypes {
enum Kinds {
  // These kinds will be used in the examples below.
  Simple = Type::Kind::FIRST_PRIVATE_EXPERIMENTAL_0_TYPE,
  Complex
};
}

Defining the type class

As described above, Type objects in MLIR are value-typed and rely on having an implicitly internal storage object that holds the actual data for the type. When defining a new Type it isn't always necessary to define a new storage class. So before defining the derived Type, it's important to know which of the two classes of Type we are defining. Some types are primitives meaning they do not have any parameters and are singletons uniqued by kind, like the index type. Parametric types on the other hand, have additional information that differentiates different instances of the same Type kind. For example the integer type has a bitwidth, making i8 and i16 be different instances of integer type.

Simple non-parametric types

For simple parameterless types, we can jump straight into defining the derived type class. Given that these types are uniqued solely on kind, we don't need to provide our own storage class.

/// This class defines a simple parameterless type. All derived types must
/// inherit from the CRTP class 'Type::TypeBase'. It takes as template
/// parameters the concrete type (SimpleType), and the base class to use (Type).
/// 'Type::TypeBase' also provides several utility methods to simplify type
/// construction.
class SimpleType : public Type::TypeBase<SimpleType, Type> {
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 == MyTypes::Simple; }

  /// This method is used to get an instance of the 'SimpleType'. Given that
  /// this is a parameterless type, it just needs to take the context for
  /// uniquing purposes.
  static SimpleType get(MLIRContext *context) {
    // Call into a helper 'get' method in 'TypeBase' to get a uniqued instance
    // of this type.
    return Base::get(context, MyTypes::Simple);
  }
};

Parametric types

Parametric types are those that have additional construction or uniquing constraints outside of the type kind. As such, these types require defining a type storage class.

Defining a type storage

Type storage objects contain all of the data necessary to construct and unique a parametric type instance. The storage classes must obey the following:

Inherit from the base type storage class TypeStorage.
Define a type alias, KeyTy, that maps to a type that uniquely identifies an instance of the parent type.
Provide a construction method that is used to allocate a new instance of the storage class.
- Storage *construct(TypeStorageAllocator &, const KeyTy &key)
Provide a comparison method between the storage and KeyTy.
- bool operator==(const KeyTy &) const
Provide a method to generate the KeyTy from a list of arguments passed to the uniquer. (Note: This is only necessary if the KeyTy cannot be default constructed from these arguments).
- static KeyTy getKey(Args...&& args)
Provide a method to hash an instance of the KeyTy. (Note: This is not necessary if an llvm::DenseMapInfo<KeyTy> specialization exists)
- static llvm::hash_code hashKey(const KeyTy &)

Let's look at an example:

/// Here we define a storage class for a ComplexType, that holds a non-zero
/// integer and an integer type.
struct ComplexTypeStorage : public TypeStorage {
  ComplexTypeStorage(unsigned nonZeroParam, Type integerType)
      : nonZeroParam(nonZeroParam), integerType(integerType) {}

  /// The hash key for this storage is a pair of the integer and type params.
  using KeyTy = std::pair<unsigned, Type>;

  /// Define the comparison function for the key type.
  bool operator==(const KeyTy &key) const {
    return key == KeyTy(nonZeroParam, integerType);
  }

  /// Define a hash function for the key type.
  /// Note: This isn't necessary because std::pair, unsigned, and Type all have
  /// hash functions already available.
  static llvm::hash_code hashKey(const KeyTy &key) {
    return llvm::hash_combine(key.first, key.second);
  }

  /// Define a construction function for the key type.
  /// Note: This isn't necessary because KeyTy can be directly constructed with
  /// the given parameters.
  static KeyTy getKey(unsigned nonZeroParam, Type integerType) {
    return KeyTy(nonZeroParam, integerType);
  }

  /// Define a construction method for creating a new instance of this storage.
  static ComplexTypeStorage *construct(TypeStorageAllocator &allocator,
                                       const KeyTy &key) {
    return new (allocator.allocate<ComplexTypeStorage>())
        ComplexTypeStorage(key.first, key.second);
  }

  unsigned nonZeroParam;
  Type integerType;
};

Type class definition

Now that the storage class has been created, the derived type class can be defined. This structure is similar to the simple type, except for a bit more of the functionality of Type::TypeBase is put to use.

/// This class defines a parametric type. All derived types must inherit from
/// the CRTP class 'Type::TypeBase'. It takes as template parameters the
/// concrete type (ComplexType), the base class to use (Type), and the storage
/// class (ComplexTypeStorage). 'Type::TypeBase' also provides several utility
/// methods to simplify type construction and verification.
class ComplexType : public Type::TypeBase<ComplexType, Type,
                                          ComplexTypeStorage> {
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 == MyTypes::Complex; }

  /// This method is used to get an instance of the 'ComplexType'. This method
  /// asserts that all of the construction invariants were satisfied. To
  /// gracefully handle failed construction, getChecked should be used instead.
  static ComplexType get(MLIRContext *context, unsigned param, Type type) {
    // Call into a helper 'get' method in 'TypeBase' to get a uniqued instance
    // of this type. All parameters to the storage class are passed after the
    // type kind.
    return Base::get(context, MyTypes::Complex, param, type);
  }

  /// This method is used to get an instance of the 'ComplexType', defined at
  /// the given location. If any of the construction invariants are invalid,
  /// errors are emitted with the provided location and a null type is returned.
  /// Note: This method is completely optional.
  static ComplexType getChecked(MLIRContext *context, unsigned param, Type type,
                                Location location) {
    // Call into a helper 'getChecked' method in 'TypeBase' to get a uniqued
    // instance of this type. All parameters to the storage class are passed
    // after the type kind.
    return Base::getChecked(location, context, MyTypes::Complex, param, type);
  }

  /// This method is used to verify the construction invariants passed into the
  /// 'get' and 'getChecked' methods. Note: This method is completely optional.
  static LogicalResult verifyConstructionInvariants(
      llvm::Optional<Location> loc, MLIRContext *context, unsigned param,
      Type type) {
    // Our type only allows non-zero parameters.
    if (param == 0) {
      if (loc)
        context->emitError(loc) << "non-zero parameter passed to 'ComplexType'";
      return failure();
    }
    // Our type also expects an integer type.
    if (!type.isa<IntegerType>()) {
      if (loc)
        context->emitError(loc) << "non integer-type passed to 'ComplexType'";
      return failure();
    }
    return success();
  }

  /// Return the parameter value.
  unsigned getParameter() {
    // 'getImpl' returns a pointer to our internal storage instance.
    return getImpl()->nonZeroParam;
  }

  /// Return the integer parameter type.
  IntegerType getParameterType() {
    // 'getImpl' returns a pointer to our internal storage instance.
    return getImpl()->integerType;
  }
};

Registering types with a Dialect

Once the dialect types have been defined, they must then be registered with a Dialect. This is done via similar mechanism to operations, addTypes.

struct MyDialect : public Dialect {
  MyDialect(MLIRContext *context) : Dialect(/*name=*/"mydialect", context) {
    /// Add these types to the dialect.
    addTypes<SimpleType, ComplexType>();
  }
};

Parsing and Printing

As a final step after registration, a dialect must override the printType and parseType hooks. These enable native support for roundtripping the type in the textual IR.

Attributes

As stated in the introduction, the process for defining dialect attributes is nearly identical to that of defining dialect types. That key difference is that the things named *Type are generally now named *Attr.

Type::TypeBase -> Attribute::AttrBase
TypeStorageAllocator -> AttributeStorageAllocator
addTypes -> addAttributes

Aside from that, all of the interfaces for uniquing and storage construction are all the same.

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