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# Defining Dialect Attributes and Types
This document is a quickstart to defining dialect specific extensions to the
[attribute](LangRef.md#attributes) and [type system](LangRef.md#type-system).
The main part of the tutorial focuses on defining types, but the instructions
are nearly identical for defining attributes.
See [MLIR specification](LangRef.md) 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](../Rationale/Rationale.md#reserving-dialect-type-kinds)). 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](https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/IR/DialectSymbolRegistry.def).
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.
```c++
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.
```c++
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](LangRef.md#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](LangRef.md#integer-type) has a
bitwidth, making `i8` and `i16` be different instances of
[`integer` type](LangRef.md#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.
```c++
/// 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), the base class to use (Type),
/// using the default storage class (TypeStorage) as the storage type.
/// 'Type::TypeBase' also provides several utility methods to simplify type
/// construction.
class SimpleType : public Type::TypeBase<SimpleType, Type, TypeStorage> {
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:
```c++
/// 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](#simple-non-parametric-types), except for a bit more of the
functionality of `Type::TypeBase` is put to use.
```c++
/// 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(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(type.getContext(), 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(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, 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(
Location loc, unsigned param, Type type) {
// Our type only allows non-zero parameters.
if (param == 0)
return emitError(loc) << "non-zero parameter passed to 'ComplexType'";
// Our type also expects an integer type.
if (!type.isa<IntegerType>())
return emitError(loc) << "non integer-type passed to 'ComplexType'";
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](LangRef.md#operations), `addTypes`.
```c++
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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