mlir/docs/OpDefinitions.md

# Operation Definition Specification (ODS)

In addition to specializing the `mlir::Op` C++ template, MLIR also supports
defining operations and data types in a table-driven manner. This is achieved
via [TableGen][TableGen], which is both a generic language and its tooling to
maintain records of domain-specific information. Facts regarding an operation
are specified concisely into a TableGen record, which will be expanded into
an equivalent `mlir::Op` C++ template specialization at compiler build time.

This manual explains in detail all the available mechanisms for defining
operations in such a table-driven manner. It aims to be a specification instead
of a tutorial. Please refer to [Quickstart tutorial to adding MLIR graph
rewrite](Tutorials/QuickstartRewrites.md) for the latter.

In addition to detailing each mechanism, this manual also tries to capture
best practices. They are rendered as quoted bullet points.

## Motivation

MLIR allows pluggable dialects, and dialects contain, among others, a list of
operations. This open and extensible ecosystem leads to the "stringly" type IR
problem, e.g., repetitive string comparisons during optimization and analysis
passes, unintuitive accessor methods (e.g., generic/error prone `getOperand(3)`
vs self-documenting `getStride()`) with more generic return types, verbose and
generic constructors without default arguments, verbose textual IR dump, and
so on. Furthermore, operation verification is:

1. best case: a central string-to-verification-function map,
1. middle case: duplication of verification across the code base, or
1. worst case: no verification functions.

The fix is to support defining ops in a table-driven manner. Then for each
dialect, we can have a central place that contains everything you need to know
about each op, including its constraints, custom assembly form, etc. This
description is also used to generate helper functions and classes to allow
building, verification, parsing, printing, analysis, and many more.

## Benefits

Compared to the C++ template, this table-driven approach has several benefits
including but not limited to:

* **Single source of truth**: We strive to encode all facts regarding an
  operation into the record, so that readers don't need to jump among code
  snippets to fully understand an operation.
* **Removing boilerplate**: We can automatically generate
  operand/attribute/result getter methods, operation build methods, operation
  verify methods, and many more utilities from the record. This greatly reduces
  the boilerplate needed for defining a new op.
* **Facilitating auto-generation**: The usage of these operation information
  records are by no means limited to op definition itself. We can use them to
  drive the auto-generation of many other components, like computation graph
  serialization.

## TableGen Syntax

We use TableGen as the language for specifying operation information. TableGen
itself just provides syntax for writing records; the syntax and constructs
allowed in a TableGen file (typically with filename suffix `.td`) can be found
[here][TableGenProgRef].

*   TableGen `class` is similar to C++ class; it can be templated and
    subclassed.
*   TableGen `def` is similar to C++ object; it can be declared by specializing
    a TableGen `class` (e.g., `def MyDef : MyClass<...>;`) or completely
    independently (e.g., `def MyDef;`). It cannot be further templated or
    subclassed.
*   TableGen `dag` is a dedicated type for directed acyclic graph of elements. A
    `dag` has one operator and zero or more arguments. Its syntax is `(operator
    arg0, arg1, argN)`. The operator can be any TableGen `def`; an argument can
    be anything, including `dag` itself. We can have names attached to both the
    operator and the arguments like `(MyOp:$op_name MyArg:$arg_name)`.

Please see the [language reference][TableGenProgRef] to learn about all the
types and expressions supported by TableGen.

## Operation Definition

MLIR defines several common constructs to help operation definition and provide
their semantics via a special [TableGen backend][TableGenBackend]:
[`OpDefinitionsGen`][OpDefinitionsGen]. These constructs are defined in
[`OpBase.td`][OpBase]. The main ones are

*   The `Op` class: It is the main construct for defining operations. All facts
    regarding the operation are specified when specializing this class, with the
    help of the following constructs.
*   The `Dialect` class: Operations belonging to one logical group are placed in
    the same dialect. The `Dialect` class contains dialect-level information.
*   The `OpTrait` class hierarchy: They are used to specify special properties
    and constraints of the operation, including whether the operation has side
    effect or whether its output has the same shape as the input.
*   The `ins`/`outs` marker: These are two special makers builtin to the
    `OpDefinitionsGen` backend. They lead the definitions of operands/attributes
    and results respectively.
*   The `TypeConstraint` class hierarchy: They are used to specify the
    constraints over operands or results. A notable subclass hierarchy is
    `Type`, which stands for constraints for common C++ types.
*   The `AttrConstraint` class hierarchy: They are used to specify the
    constraints over attributes. A notable subclass hierarchy is `Attr`, which
    stands for constraints for attributes whose values are of common types.

An operation is defined by specializing the `Op` class with concrete contents
for all the fields it requires. For example, `tf.AvgPool` is defined as

```tablegen
def TF_AvgPoolOp : TF_Op<"AvgPool", [NoSideEffect]> {
  let summary = "Performs average pooling on the input.";

  let description = [{
Each entry in `output` is the mean of the corresponding size `ksize`
window in `value`.
  }];

  let arguments = (ins
    TF_FpTensor:$value,

    Confined<I64ArrayAttr, [ArrayMinCount<4>]>:$ksize,
    Confined<I64ArrayAttr, [ArrayMinCount<4>]>:$strides,
    TF_AnyStrAttrOf<["SAME", "VALID"]>:$padding,
    DefaultValuedAttr<TF_ConvertDataFormatAttr, "NHWC">:$data_format
  );

  let results = (outs
    TF_FpTensor:$output
  );

  TF_DerivedOperandTypeAttr T = TF_DerivedOperandTypeAttr<0>;
}
```

In the following we describe all the fields needed. Please see the definition
of the `Op` class for the complete list of fields supported.

### Operation name

The operation name is a unique identifier of the operation within MLIR, e.g.,
`tf.Add` for addition operation in the TensorFlow dialect. This is the
equivalent of the mnemonic in assembly language. It is used for parsing and
printing in the textual format. It is also used for pattern matching in graph
rewrites.

The full operation name is composed of the dialect name and the op name, with
the former provided via the dialect and the latter provided as the second
template parameter to the `Op` class.

### Operation documentation

This includes both a one-line `summary` and a longer human-readable
`description`. They will be used to drive automatic generation of dialect
documentation. They need to be provided in the operation's definition body:

```tablegen
let summary = "...";

let description = [{
...
}];
```

`description` should be written in Markdown syntax.

Placing the documentation at the beginning is recommended since
it helps in understanding the operation.

> * Place documentation at the beginning of the operation definition
> * The summary should be short and concise. It should be a one-liner without
>   trailing punctuation. Put expanded explanation in description.

### Operation arguments

There are two kinds of arguments: operands and attributes. Operands are runtime
values produced by other ops; while attributes are compile-time known constant
values, including two categories:

1. Natural attributes: these attributes affect the behavior of the operations
   (e.g., padding for convolution);
1. Derived attributes: these attributes are not needed to define the operation
   but are instead derived from information of the operation. E.g., the output
   shape of type. This is mostly used for convenience interface generation or
   interaction with other frameworks/translation.

   All derived attributes should be materializable as an Attribute. That is,
   even though they are not materialized, it should be possible to store as
   an attribute.

Both operands and attributes are specified inside the `dag`-typed `arguments`,
led by `ins`:

```tablegen
let arguments = (ins
  <type-constraint>:$<operand-name>,
  ...
  <attr-constraint>:$<attr-name>,
  ...
);
```

Here `<type-constraint>` is a TableGen `def` from the `TypeConstraint` class
hierarchy. Similarly, `<attr-constraint>` is a TableGen `def` from the
`AttrConstraint` class hierarchy. See [Constraints](#constraints) for more
information.

There is no requirements on the relative order of operands and attributes; they
can mix freely. The relative order of operands themselves matters. From each
named argument a named getter will be generated that returns the argument with
the return type (in the case of attributes the return type will be
constructed from the storage type, while for operands it will be `Value`). Each
attribute's raw value (e.g., as stored) can also be accessed via generated
`<name>Attr` getters for use in transformation passes where the more user
friendly return type is less suitable.

All the arguments should be named to 1) provide documentation, 2) drive
auto-generation of getter methods, 3) provide a handle to reference for other
places like constraints.

#### Variadic operands

To declare a variadic operand, wrap the `TypeConstraint` for the operand with
`Variadic<...>`.

Normally operations have no variadic operands or just one variadic operand. For
the latter case, it is easy to deduce which dynamic operands are for the static
variadic operand definition. Though, if an operation has more than one variable
length operands (either optional or variadic), it would be impossible to
attribute dynamic operands to the corresponding static variadic operand
definitions without further information from the operation. Therefore, either
the `SameVariadicOperandSize` or `AttrSizedOperandSegments` trait is needed to
indicate that all variable length operands have the same number of dynamic
values.

#### Optional operands

To declare an optional operand, wrap the `TypeConstraint` for the operand with
`Optional<...>`.

Normally operations have no optional operands or just one optional operand. For
the latter case, it is easy to deduce which dynamic operands are for the static
operand definition. Though, if an operation has more than one variable length
operands (either optional or variadic), it would be impossible to attribute
dynamic operands to the corresponding static variadic operand definitions
without further information from the operation. Therefore, either the
`SameVariadicOperandSize` or `AttrSizedOperandSegments` trait is needed to
indicate that all variable length operands have the same number of dynamic
values.

#### Optional attributes

To declare an optional attribute, wrap the `AttrConstraint` for the attribute
with `OptionalAttr<...>`.

#### Attributes with default values

To declare an attribute with a default value, wrap the `AttrConstraint` for the
attribute with `DefaultValuedAttr<..., "...">`.

The second parameter to `DefaultValuedAttr` should be a string containing the
C++ default value. For example, a float default value should be specified as
like `"0.5f"`, and an integer array default value should be specified as like
`"{1, 2, 3}"`.

#### Confining attributes

`Confined` is provided as a general mechanism to help modelling further
constraints on attributes beyond the ones brought by value types. You can use
`Confined` to compose complex constraints out of more primitive ones. For
example, a 32-bit integer attribute whose minimum value must be 10 can be
expressed as `Confined<I32Attr, [IntMinValue<10>]>`.

Right now, the following primitive constraints are supported:

*   `IntMinValue<N>`: Specifying an integer attribute to be greater than or
    equal to `N`
*   `IntMaxValue<N>`: Specifying an integer attribute to be less than or equal
    to `N`
*   `ArrayMinCount<N>`: Specifying an array attribute to have at least `N`
    elements
*   `IntArrayNthElemEq<I, N>`: Specifying an integer array attribute's `I`-th
    element to be equal to `N`
*   `IntArrayNthElemMinValue<I, N>`: Specifying an integer array attribute's
    `I`-th element to be greater than or equal to `N`

TODO: Design and implement more primitive constraints

### Operation regions

The regions of an operation are specified inside of the `dag`-typed `regions`,
led by `region`:

```tablegen
let regions = (region
  <region-constraint>:$<region-name>,
  ...
);
```

#### Variadic regions

Similar to the `Variadic` class used for variadic operands and results,
`VariadicRegion<...>` can be used for regions. Variadic regions can currently
only be specified as the last region in the regions list.

### Operation results

Similar to operands, results are specified inside the `dag`-typed `results`, led
by `outs`:

```tablegen
let results = (outs
  <type-constraint>:$<result-name>,
  ...
);
```

#### Variadic results

Similar to variadic operands, `Variadic<...>` can also be used for results.
And similarly, `SameVariadicResultSize` for multiple variadic results in the
same operation.

### Operation successors

For terminator operations, the successors are specified inside of the
`dag`-typed `successors`, led by `successor`:

```tablegen
let successors = (successor
  <successor-constraint>:$<successor-name>,
  ...
);
```

#### Variadic successors

Similar to the `Variadic` class used for variadic operands and results,
`VariadicSuccessor<...>` can be used for successors. Variadic successors can
currently only be specified as the last successor in the successor list.

### Operation traits and constraints

Traits are operation properties that affect syntax or semantics. MLIR C++
models various traits in the `mlir::OpTrait` namespace.

Both operation traits, [interfaces](#operation-interfaces), and constraints
involving multiple operands/attributes/results are provided as the second
template parameter to the `Op` class. They should be deriving from the `OpTrait`
class. See [Constraints](#constraints) for more information.

### Interfaces

[Interfaces](Interfaces.md#attribute-operation-type-interfaces) allow for
attributes, operations, and types to expose method calls without the caller
needing to know the derived type. Operation interfaces defined in C++ can be
accessed in the ODS framework via the `OpInterfaceTrait` class. Aside from using
pre-existing interfaces in the C++ API, the ODS framework also provides a
simplified mechanism for defining such interfaces which removes much of the
boilerplate necessary.

Providing a definition of the `AttrInterface`, `OpInterface`, or `TypeInterface`
class will auto-generate the C++ classes for the interface. An interface
includes a name, for the C++ class, a description, and a list of interface
methods.

```tablegen
def MyInterface : OpInterface<"MyInterface"> {
  let description = ...;
  let methods = [...];
}
```

There are two types of methods that can be used with an interface,
`InterfaceMethod` and `StaticInterfaceMethod`. They are both comprised of the
same core components, with the distinction that `StaticInterfaceMethod` models a
static method on the derived operation.

An `InterfaceMethod` is comprised of the following components:

*   Description
    -   A string description of what this method does and its invariants.
*   ReturnType
    -   A string corresponding to the C++ return type of the method.
*   MethodName
    -   A string corresponding to the desired name of the method.
*   Arguments (Optional)
    -   A dag of strings that correspond to a C++ type and variable name
        respectively.
*   MethodBody (Optional)
    -   An optional explicit implementation of the interface method.
    -   `ConcreteOp` is an implicitly defined typename that can be used to refer
        to the type of the derived operation currently being operated on.
    -   In non-static methods, a variable 'ConcreteOp op' is defined and may be
        used to refer to an instance of the derived operation.
*   DefaultImplementation (Optional)
    -   An optional explicit default implementation of the interface method.
    -   This method is placed within the `Trait` class that is attached to the
        operation. As such, this method has the same characteristics as any
        other [`Trait`](Traits.md) method.
    -   `ConcreteOp` is an implicitly defined typename that can be used to refer
        to the type of the derived operation currently being operated on.

ODS also allows generating the declarations for the `InterfaceMethod` of the op
if one specifies the interface with `DeclareOpInterfaceMethods` (see example
below).

Examples:

```tablegen
def MyInterface : OpInterface<"MyInterface"> {
  let description = [{
    My interface is very interesting. ...
  }];

  let methods = [
    // A simple non-static method with no inputs.
    InterfaceMethod<"'foo' is a non-static method with no inputs.",
      "unsigned", "foo"
    >,

    // A new non-static method accepting an input argument.
    InterfaceMethod<"/*insert doc here*/",
      "Value ", "bar", (ins "unsigned":$i)
    >,

    // Query a static property of the derived operation.
    StaticInterfaceMethod<"'fooStatic' is a static method with no inputs.",
      "unsigned", "fooStatic"
    >,

    // Provide the definition of a static interface method.
    // Note: `ConcreteOp` corresponds to the derived operation typename.
    StaticInterfaceMethod<"/*insert doc here*/",
      "Operation *", "create", (ins "OpBuilder &":$builder, "Location":$loc), [{
        return builder.create<ConcreteOp>(loc);
    }]>,

    // Provide a definition of the non-static method.
    // Note: `op` corresponds to the derived operation variable.
    InterfaceMethod<"/*insert doc here*/",
      "unsigned", "getNumInputsAndOutputs", (ins), [{
        return op.getNumInputs() + op.getNumOutputs();
    }]>,

    // Provide only a default definition of the method.
    // Note: `ConcreteOp` corresponds to the derived operation typename.
    InterfaceMethod<"/*insert doc here*/",
      "unsigned", "getNumWithDefault", (ins), /*methodBody=*/[{}], [{
        ConcreteOp op = cast<ConcreteOp>(this->getOperation());
        return op.getNumInputs() + op.getNumOutputs();
    }]>,
  ];
}

// Operation interfaces can optionally be wrapped inside
// DeclareOpInterfaceMethods. This would result in autogenerating declarations
// for members `foo`, `bar` and `fooStatic`. Methods with bodies are not
// declared inside the op declaration but instead handled by the op interface
// trait directly.
def OpWithInferTypeInterfaceOp : Op<...
    [DeclareOpInterfaceMethods<MyInterface>]> { ... }

// Methods that have a default implementation do not have declarations
// generated. If an operation wishes to override the default behavior, it can
// explicitly specify the method that it wishes to override. This will force
// the generation of a declaration for those methods.
def OpWithOverrideInferTypeInterfaceOp : Op<...
    [DeclareOpInterfaceMethods<MyInterface, ["getNumWithDefault"]>]> { ... }
```

Operation interfaces may also provide a verification method on `OpInterface` by
setting `verify`. Setting `verify` results in the generated trait having a
`verifyTrait` method that is applied to all operations implementing the trait.

### Builder methods

For each operation, there are a few builders automatically generated based on
the arguments and returns types. For example, given the following op definition:

```tablegen
def MyOp : ... {
  let arguments = (ins
    I32:$i32_operand,
    F32:$f32_operand,
    ...,

    I32Attr:$i32_attr,
    F32Attr:$f32_attr,
    ...
  );

  let results = (outs
    I32:$i32_result,
    F32:$f32_result,
    ...
  );
}
```

The following builders are generated:

```c++
// All result-types/operands/attributes have one aggregate parameter.
static void build(OpBuilder &odsBuilder, OperationState &odsState,
                  ArrayRef<Type> resultTypes,
                  ValueRange operands,
                  ArrayRef<NamedAttribute> attributes);

// Each result-type/operand/attribute has a separate parameter. The parameters
// for attributes are of mlir::Attribute types.
static void build(OpBuilder &odsBuilder, OperationState &odsState,
                  Type i32_result, Type f32_result, ...,
                  Value i32_operand, Value f32_operand, ...,
                  IntegerAttr i32_attr, FloatAttr f32_attr, ...);

// Each result-type/operand/attribute has a separate parameter. The parameters
// for attributes are raw values unwrapped with mlir::Attribute instances.
// (Note that this builder will not always be generated. See the following
// explanation for more details.)
static void build(OpBuilder &odsBuilder, OperationState &odsState,
                  Type i32_result, Type f32_result, ...,
                  Value i32_operand, Value f32_operand, ...,
                  APInt i32_attr, StringRef f32_attr, ...);

// Each operand/attribute has a separate parameter but result type is aggregate.
static void build(OpBuilder &odsBuilder, OperationState &odsState,
                  ArrayRef<Type> resultTypes,
                  Value i32_operand, Value f32_operand, ...,
                  IntegerAttr i32_attr, FloatAttr f32_attr, ...);

// All operands/attributes have aggregate parameters.
// Generated if return type can be inferred.
static void build(OpBuilder &odsBuilder, OperationState &odsState,
                  ValueRange operands, ArrayRef<NamedAttribute> attributes);

// (And manually specified builders depending on the specific op.)
```

The first form provides basic uniformity so that we can create ops using the
same form regardless of the exact op. This is particularly useful for
implementing declarative pattern rewrites.

The second and third forms are good for use in manually written code given that
they provide better guarantee via signatures.

The third form will be generated if any of the op's attribute has different
`Attr.returnType` from `Attr.storageType` and we know how to build an attribute
from an unwrapped value (i.e., `Attr.constBuilderCall` is defined.)
Additionally, for the third form, if an attribute appearing later in the
`arguments` list has a default value, the default value will be supplied in the
declaration. This works for `BoolAttr`, `StrAttr`, `EnumAttr` for now and the
list can grow in the future. So if possible, default valued attribute should be
placed at the end of the `arguments` list to leverage this feature. (This
behavior is essentially due to C++ function parameter default value placement
restrictions.) Otherwise, the builder of the third form will still be generated
but default values for the attributes not at the end of the `arguments` list
will not be supplied in the builder's signature.

ODS will generate a builder that doesn't require return type specified if

*   Op implements InferTypeOpInterface interface;
*   All return types are either buildable types or are the same as a given
    operand (e.g., `AllTypesMatch` constraint between operand and result);

And there may potentially exist other builders depending on the specific op;
please refer to the
[generated C++ file](#run-mlir-tblgen-to-see-the-generated-content) for the
complete list.

#### Custom builder methods

However, if the above cases cannot satisfy all needs, you can define additional
convenience build methods with `OpBuilder`.

`OpBuilder` is a class that takes the parameter list and the optional `build()`
method body. They are separated because we need to generate op declaration and
definition into separate files. The parameter list should not include `OpBuilder
&builder, OperationState &state` as they will be inserted automatically and the
placeholders `$_builder` and `$_state` used. For legacy/to be deprecated reason
if the `OpBuilder` parameter starts with `OpBuilder` param, then the parameter
is used. If the `body` is not provided, only the builder declaration will be
generated; this provides a way to define complicated builders entirely in C++
files.

For example, for the following op:

```tablegen
def MyOp : Op<"my_op", []> {
  let arguments = (ins F32Attr:$attr);

  let results = (outs);
}
```

If we want to define a builder with a default value for the only attribute, we
can add into `MyOp`:

```tablegen
def MyOp : ... {
  ...

  let builders = [
    OpBuilder<"float val = 0.5f", [{
      $_state.addAttribute("attr", $_builder.getF32FloatAttr(val));
    }]>
  ];
}
```

The generated builder will look like:

```c++
static void build(OpBuilder &builder, OperationState &state, float val = 0.5f) {
  state.addAttribute("attr", builder.getF32FloatAttr(val));
}
```

### Custom parser and printer methods

Functions to parse and print the operation's custom assembly form.

### Custom verifier code

Verification code will be automatically generated for
[constraints](#constraints) specified on various entities of the op. To
perform _additional_ verification, you can use

```tablegen
let verifier = [{
  ...
}];
```

Code placed in `verifier` will be called after the auto-generated verification
code. The order of trait verification excluding those of `verifier` should not
be relied upon.

### Declarative Assembly Format

The custom assembly form of the operation may be specified in a declarative
string that matches the operations operands, attributes, etc. With the ability
to express additional information that needs to be parsed to build the
operation:

```tablegen
def CallOp : Std_Op<"call", ...> {
  let arguments = (ins FlatSymbolRefAttr:$callee, Variadic<AnyType>:$args);
  let results = (outs Variadic<AnyType>);

  let assemblyFormat = [{
    $callee `(` $args `)` attr-dict `:` functional-type($args, results)
  }];
}
```

The format is comprised of three components:

#### Directives

A directive is a type of builtin function, with an optional set of arguments.
The available directives are as follows:

*   `attr-dict`

    -   Represents the attribute dictionary of the operation.

*   `attr-dict-with-keyword`

    -   Represents the attribute dictionary of the operation, but prefixes the
        dictionary with an `attributes` keyword.

*   `custom` < UserDirective > ( Params )

    -   Represents a custom directive implemented by the user in C++.
    -   See the [Custom Directives](#custom-directives) section below for more
        details.

*   `functional-type` ( inputs , results )

    -   Formats the `inputs` and `results` arguments as a
        [function type](LangRef.md#function-type).
    -   The constraints on `inputs` and `results` are the same as the `input` of
        the `type` directive.

*   `operands`

    -   Represents all of the operands of an operation.

*   `regions`

    -   Represents all of the regions of an operation.

*   `results`

    -   Represents all of the results of an operation.

*   `successors`

    -   Represents all of the successors of an operation.

*   `type` ( input )

    -   Represents the type of the given input.
    -   `input` must be either an operand or result [variable](#variables), the
        `operands` directive, or the `results` directive.

*   `type_ref` ( input )

    -   Represents a reference to the type of the given input that must have
        already been resolved.
    -   `input` must be either an operand or result [variable](#variables), the
        `operands` directive, or the `results` directive.
    -   Used to pass previously parsed types to custom directives.

#### Literals

A literal is either a keyword or punctuation surrounded by \`\`.

The following are the set of valid punctuation:

`:`, `,`, `=`, `<`, `>`, `(`, `)`, `{`, `}`, `[`, `]`, `->`

#### Variables

A variable is an entity that has been registered on the operation itself, i.e.
an argument(attribute or operand), region, result, successor, etc. In the
`CallOp` example above, the variables would be `$callee` and `$args`.

Attribute variables are printed with their respective value type, unless that
value type is buildable. In those cases, the type of the attribute is elided.

#### Custom Directives

The declarative assembly format specification allows for handling a large
majority of the common cases when formatting an operation. For the operations
that require or desire specifying parts of the operation in a form not supported
by the declarative syntax, custom directives may be specified. A custom
directive essentially allows for users to use C++ for printing and parsing
subsections of an otherwise declaratively specified format. Looking at the
specification of a custom directive above:

```
custom-directive ::= `custom` `<` UserDirective `>` `(` Params `)`
```

A custom directive has two main parts: The `UserDirective` and the `Params`. A
custom directive is transformed into a call to a `print*` and a `parse*` method
when generating the C++ code for the format. The `UserDirective` is an
identifier used as a suffix to these two calls, i.e., `custom<MyDirective>(...)`
would result in calls to `parseMyDirective` and `printMyDirective` wihtin the
parser and printer respectively. `Params` may be any combination of variables
(i.e. Attribute, Operand, Successor, etc.) and type directives. The type
directives must refer to a variable, but that variable need not also be a
parameter to the custom directive.

The arguments to the `parse<UserDirective>` method is firstly a reference to the
`OpAsmParser`(`OpAsmParser &`), and secondly a set of output parameters
corresponding to the parameters specified in the format. The mapping of
declarative parameter to `parse` method argument is detailed below:

*   Attribute Variables
    -   Single: `<Attribute-Storage-Type>(e.g. Attribute) &`
    -   Optional: `<Attribute-Storage-Type>(e.g. Attribute) &`
*   Operand Variables
    -   Single: `OpAsmParser::OperandType &`
    -   Optional: `Optional<OpAsmParser::OperandType> &`
    -   Variadic: `SmallVectorImpl<OpAsmParser::OperandType> &`
*   Region Variables
    -   Single: `Region &`
    -   Variadic: `SmallVectorImpl<std::unique_ptr<Region>> &`
*   Successor Variables
    -   Single: `Block *&`
    -   Variadic: `SmallVectorImpl<Block *> &`
*   Type Directives
    -   Single: `Type &`
    -   Optional: `Type &`
    -   Variadic: `SmallVectorImpl<Type> &`
*   TypeRef Directives
    -   Single: `Type`
    -   Optional: `Type`
    -   Variadic: `const SmallVectorImpl<Type> &`

When a variable is optional, the value should only be specified if the variable
is present. Otherwise, the value should remain `None` or null.

The arguments to the `print<UserDirective>` method is firstly a reference to the
`OpAsmPrinter`(`OpAsmPrinter &`), and secondly a set of output parameters
corresponding to the parameters specified in the format. The mapping of
declarative parameter to `print` method argument is detailed below:

*   Attribute Variables
    -   Single: `<Attribute-Storage-Type>(e.g. Attribute)`
    -   Optional: `<Attribute-Storage-Type>(e.g. Attribute)`
*   Operand Variables
    -   Single: `Value`
    -   Optional: `Value`
    -   Variadic: `OperandRange`
*   Region Variables
    -   Single: `Region &`
    -   Variadic: `MutableArrayRef<Region>`
*   Successor Variables
    -   Single: `Block *`
    -   Variadic: `SuccessorRange`
*   Type Directives
    -   Single: `Type`
    -   Optional: `Type`
    -   Variadic: `TypeRange`
*   TypeRef Directives
    -   Single: `Type`
    -   Optional: `Type`
    -   Variadic: `TypeRange`

When a variable is optional, the provided value may be null.

#### Optional Groups

In certain situations operations may have "optional" information, e.g.
attributes or an empty set of variadic operands. In these situations a section
of the assembly format can be marked as `optional` based on the presence of this
information. An optional group is defined by wrapping a set of elements within
`()` followed by a `?` and has the following requirements:

*   The first element of the group must either be a attribute, literal, operand,
    or region.
    -   This is because the first element must be optionally parsable.
*   Exactly one argument variable within the group must be marked as the anchor
    of the group.
    -   The anchor is the element whose presence controls whether the group
        should be printed/parsed.
    -   An element is marked as the anchor by adding a trailing `^`.
    -   The first element is *not* required to be the anchor of the group.
    -   When a non-variadic region anchors a group, the detector for printing
        the group is if the region is empty.
*   Literals, variables, custom directives, and type directives are the only
    valid elements within the group.
    -   Any attribute variable may be used, but only optional attributes can be
        marked as the anchor.
    -   Only variadic or optional operand arguments can be used.
    -   All region variables can be used. When a non-variable length region is
        used, if the group is not present the region is empty.
    -   The operands to a type directive must be defined within the optional
        group.

An example of an operation with an optional group is `std.return`, which has a
variadic number of operands.

```tablegen
def ReturnOp : ... {
  let arguments = (ins Variadic<AnyType>:$operands);

  // We only print the operands and types if there are a non-zero number
  // of operands.
  let assemblyFormat = "attr-dict ($operands^ `:` type($operands))?";
}
```

##### Unit Attributes

In MLIR, the [`unit` Attribute](LangRef.md#unit-attribute) is special in that it
only has one possible value, i.e. it derives meaning from its existence. When a
unit attribute is used to anchor an optional group and is not the first element
of the group, the presence of the unit attribute can be directly correlated with
the presence of the optional group itself. As such, in these situations the unit
attribute will not be printed or present in the output and will be automatically
inferred when parsing by the presence of the optional group itself.

For example, the following operation:

```tablegen
def FooOp : ... {
  let arguments = (ins UnitAttr:$is_read_only);

  let assemblyFormat = "attr-dict (`is_read_only` $is_read_only^)?";
}
```

would be formatted as such:

```mlir
// When the unit attribute is present:
foo.op is_read_only

// When the unit attribute is not present:
foo.op
```

#### Requirements

The format specification has a certain set of requirements that must be adhered
to:

1.  The output and operation name are never shown as they are fixed and cannot
    be altered.
1.  All operands within the operation must appear within the format, either
    individually or with the `operands` directive.
1.  All regions within the operation must appear within the format, either
    individually or with the `regions` directive.
1.  All successors within the operation must appear within the format, either
    individually or with the `successors` directive.
1.  All operand and result types must appear within the format using the various
    `type` directives, either individually or with the `operands` or `results`
    directives.
1.  The `attr-dict` directive must always be present.
1.  Must not contain overlapping information; e.g. multiple instances of
    'attr-dict', types, operands, etc.
    -   Note that `attr-dict` does not overlap with individual attributes. These
        attributes will simply be elided when printing the attribute dictionary.

##### Type Inference

One requirement of the format is that the types of operands and results must
always be present. In certain instances, the type of a variable may be deduced
via type constraints or other information available. In these cases, the type of
that variable may be elided from the format.

* Buildable Types

Some type constraints may only have one representation, allowing for them to
be directly buildable; for example the `I32` or `Index` types. Types in `ODS`
may mark themselves as buildable by setting the `builderCall` field or
inheriting from the `BuildableType` class.

* Trait Equality Constraints

There are many operations that have known type equality constraints registered
as traits on the operation; for example the true, false, and result values of a
`select` operation often have the same type. The assembly format may inspect
these equal constraints to discern the types of missing variables. The currently
supported traits are: `AllTypesMatch`, `TypesMatchWith`, `SameTypeOperands`,
and `SameOperandsAndResultType`.

### `hasCanonicalizer`

This boolean field indicate whether canonicalization patterns have been defined
for this operation. If it is `1`, then `::getCanonicalizationPatterns()` should
be defined.

### `hasFolder`

This boolean field indicate whether general folding rules have been defined
for this operation. If it is `1`, then `::fold()` should be defined.

### Extra declarations

One of the goals of table-driven op definition is to auto-generate as much logic
and methods needed for each op as possible. With that said, there will always be
long-tail cases that won't be covered. For such cases, you can use
`extraClassDeclaration`. Code in `extraClassDeclaration` will be copied
literally to the generated C++ op class.

Note that `extraClassDeclaration` is a mechanism intended for long-tail cases
by power users; for not-yet-implemented widely-applicable cases, improving the
infrastructure is preferable.

### Generated C++ code

[OpDefinitionsGen][OpDefinitionsGen] processes the op definition spec file and
generates two files containing the corresponding C++ code: one for declarations,
the other for definitions. The former is generated via the `-gen-op-decls`
command-line option, while the latter is via the `-gen-op-defs` option.

The definition file contains all the op method definitions, which can be
included and enabled by defining `GET_OP_CLASSES`. For each operation,
OpDefinitionsGen generates an operation class and an
[operand adaptor](#operand-adaptors) class. Besides, it also contains a
comma-separated list of all defined ops, which can be included and enabled by
defining `GET_OP_LIST`.

#### Class name and namespaces

For each operation, its generated C++ class name is the symbol `def`ed with
TableGen with dialect prefix removed. The first `_` serves as the delimiter.
For example, for `def TF_AddOp`, the C++ class name would be `AddOp`.
We remove the `TF` prefix because it is for scoping ops; other dialects
may as well define their own `AddOp`s.

The namespaces of the generated C++ class will come from the dialect's
`cppNamespace` field. For example, if a dialect's `cppNamespace` is `A::B`,
then an op of that dialect will be placed in
`namespace A { namespace B { ... } }`. If a dialect does not specify a
`cppNamespace`, we then use the dialect's name as the namespace.

This means the qualified name of the generated C++ class does not necessarily
match exactly with the operation name as explained in
[Operation name](#operation-name). This is to allow flexible naming to satisfy
coding style requirements.

#### Operand adaptors

For each operation, we automatically generate an _operand adaptor_. This class
solves the problem of accessing operands provided as a list of `Value`s without
using "magic" constants. The operand adaptor takes a reference to an array of
`Value` and provides methods with the same names as those in the operation class
to access them. For example, for a binary arithmetic operation, it may provide
`.lhs()` to access the first operand and `.rhs()` to access the second operand.

The operand adaptor class lives in the same namespace as the operation class,
and has the name of the operation followed by `Adaptor` as well as an alias
`Adaptor` inside the op class.

Operand adaptors can be used in function templates that also process operations:

```c++
template <typename BinaryOpTy>
std::pair<Value, Value> zip(BinaryOpTy &&op) {
  return std::make_pair(op.lhs(), op.rhs());;
}

void process(AddOp op, ArrayRef<Value> newOperands) {
  zip(op);
  zip(Adaptor<AddOp>(newOperands));
  /*...*/
}
```

## Constraints

Constraint is a core concept in table-driven operation definition: operation
verification and graph operation matching are all based on satisfying
constraints. So both the operation definition and rewrite rules specification
significantly involve writing constraints. We have the `Constraint` class in
[`OpBase.td`][OpBase] has the common base class for all constraints.

An operation's constraint can cover different range; it may

* Only concern a single attribute (e.g. being a 32-bit integer greater than 5),
* Multiple operands and results (e.g., the 1st result's shape must be the same
  as the 1st operand), or
* Intrinsic to the operation itself (e.g., having no side effect).

We call them as single-entity constraint, multi-entity constraint, and traits,
respectively.

### Single-entity constraint

Constraints scoped to a single operand, attribute, or result are specified at
the entity's declaration place as described in
[Operation arguments](#operation-arguments) and
[Operation results](#operation-results).

To help modelling constraints of common types, a set of `TypeConstraint`s are
created; they are the `Type` subclass hierarchy. It includes `F32` for the
constraints of being a float, `TensorOf<[F32]>` for the constraints of being
a float tensor, and so on.

Similarly, a set of `AttrConstraint`s are created for helping modelling
constraints of common attribute kinds. They are the `Attr` subclass hierarchy.
It includes `F32Attr` for the constraints of being a float attribute,
`F32ArrayAttr` for the constraints of being a float array attribute, and so on.

### Multi-entity constraint

Constraints involving more than one operand/attribute/result are quite common
on operations, like the element type and shape relation between operands and
results. These constraints should be specified as the `Op` class template
parameter as described in
[Operation traits and constraints](#operation-traits-and-constraints).

Multi-entity constraints are modeled as `PredOpTrait` (a subclass of `OpTrait`)
in [`OpBase.td`][OpBase].A bunch of constraint primitives are provided to help
specification. See [`OpBase.td`][OpBase] for the complete list.

### Trait

Traits are intrinsic properties of the operation like having side effect or not,
commutative or not, whether is a terminator, etc. These constraints should be
specified as the `Op` class template parameter as described in
[Operation traits and constraints](#operation-traits-and-constraints).

Traits are modeled as `NativeOpTrait` (a subclass of `OpTrait`) in
[`OpBase.td`][OpBase]. They are backed and will be translated into the
corresponding C++ `mlir::OpTrait` classes.

### How to specify new constraint

To write a constraint, you need to provide its predicates and give it a
descriptive name. Predicates, modeled with the `Pred` class, are the workhorse
for composing constraints. The predicate for a constraint is typically built up
in a nested manner, using the two categories of predicates:

1.  `CPred`: the primitive leaf predicate.
2.  Compound predicate: a predicate composed from child predicates using
    predicate combiners (conjunction: `And`, disjunction: `Or`, negation: `Neg`,
    substitution: `SubstLeaves`, concatenation: `Concat`).

`CPred` is the basis for composing more complex predicates. It is the "atom"
predicate from the perspective of TableGen and the "interface" between
TableGen and C++. What is inside is already C++ code, which will be treated
as opaque strings with special placeholders to be substituted.

You can put any C++ code that returns a boolean value inside a `CPred`,
including evaluating expressions, calling functions, calling class methods,
and so on.

To help interaction with the C++ environment, there are a few special
placeholders provided to refer to entities in the context where this predicate
is used. They serve as "hooks" to the enclosing environment.  This includes
`$_builder`, `$_op`, and `$_self`:

* `$_builder` will be replaced by a `mlir::Builder` instance so that you can
  access common build methods.
* `$_op` will be replaced by the current operation so that you can access
  information of the current operation.
* `$_self` will be replaced with the entity this predicate is attached to.
  E.g., `BoolAttr` is an attribute constraint that wraps a
  `CPred<"$_self.isa<BoolAttr>()">`. Then for `F32:$attr`,`$_self` will be
  replaced by `$attr`. For type constraints, it's a little bit special since
  we want the constraints on each type definition reads naturally and we want
  to attach type constraints directly to an operand/result, `$_self` will be
  replaced by the operand/result's type. E.g., for `F32` in `F32:$operand`, its
  `$_self` will be expanded as `getOperand(...).getType()`.

TODO: Reconsider the leading symbol for special placeholders. Eventually we want
to allow referencing operand/result $-names; such $-names can start with
underscore.

For example, to write an attribute `attr` is an `IntegerAttr`, in C++ you can
just call `attr.isa<IntegerAttr>()`. The code can be wrapped in a `CPred` as
`$_self.isa<IntegerAttr>()`, with `$_self` as the special placeholder to be
replaced by the current attribute `attr` at expansion time.

For more complicated predicates, you can wrap it in a single `CPred`, or you
can use predicate combiners to combine them. For example, to write the
constraint that an attribute `attr` is a 32-bit or 64-bit integer, you can
write it as

```tablegen
And<[
  CPred<"$_self.isa<IntegerAttr>()">,
  Or<[
    CPred<"$_self.cast<IntegerAttr>().getType().isInteger(32)">,
    CPred<"$_self.cast<IntegerAttr>().getType().isInteger(64)">
  ]>
]>
```

(Note that the above is just to show with a familiar example how you can use
`CPred` and predicate combiners to write complicated predicates. For integer
attributes specifically, [`OpBase.td`][OpBase] already defines `I32Attr` and
`I64Attr`. So you can actually reuse them to write it as `Or<[I32Attr.predicate,
I64Attr.predicate]>`.)

TODO: Build up a library of reusable primitive constraints

If the predicate is very complex to write with `CPred` together with predicate
combiners, you can also write it as a normal C++ function and use the `CPred`
as a way to "invoke" the function. For example, to verify an attribute `attr`
has some property, you can write a C++ function like

```cpp
bool HasSomeProperty(Attribute attr) { ... }
```

and then define the op as:

```tablegen
def HasSomeProperty : AttrConstraint<CPred<"HasSomeProperty($_self)">,
                                     "has some property">;

def MyOp : Op<...> {
  let arguments = (ins
    ...
    HasSomeProperty:$attr
  );
}
```

As to whether we should define the predicate using a single `CPred` wrapping
the whole expression, multiple `CPred`s with predicate combiners, or a single
`CPred` "invoking" a function, there are no clear-cut criteria. Defining using
`CPred` and predicate combiners is preferable since it exposes more information
(instead hiding all the logic behind a C++ function) into the op definition spec
so that it can potentially drive more auto-generation cases. But it will
require a nice library of common predicates as the building blocks to avoid the
duplication, which is being worked on right now.

## Attribute Definition

An attribute is a compile-time known constant of an operation.

ODS provides attribute wrappers over C++ attribute classes. There are a few
common C++ [attribute classes][AttrClasses] defined in MLIR's core IR library
and one is free to define dialect-specific attribute classes. ODS allows one
to use these attributes in TableGen to define operations, potentially with
more fine-grained constraints. For example, `StrAttr` directly maps to
`StringAttr`; `F32Attr`/`F64Attr` requires the `FloatAttr` to additionally
be of a certain bitwidth.

ODS attributes are defined as having a storage type (corresponding to a backing
`mlir::Attribute` that _stores_ the attribute), a return type (corresponding to
the C++ _return_ type of the generated of the helper getters) as well as method
to convert between the internal storage and the helper method.

### Attribute decorators

There are a few important attribute adapters/decorators/modifiers that can be
applied to ODS attributes to specify common additional properties like
optionality, default values, etc.:

*   `DefaultValuedAttr`: specifies the
    [default value](#attributes-with-default-values) for an attribute.
*   `OptionalAttr`: specifies an attribute as [optional](#optional-attributes).
*   `Confined`: adapts an attribute with
    [further constraints](#confining-attributes).

### Enum attributes

Some attributes can only take values from a predefined enum, e.g., the
comparison kind of a comparison op. To define such attributes, ODS provides
several mechanisms: `StrEnumAttr`, `IntEnumAttr`, and `BitEnumAttr`.

*   `StrEnumAttr`: each enum case is a string, the attribute is stored as a
    [`StringAttr`][StringAttr] in the op.
*   `IntEnumAttr`: each enum case is an integer, the attribute is stored as a
    [`IntegerAttr`][IntegerAttr] in the op.
*   `BitEnumAttr`: each enum case is a bit, the attribute is stored as a
    [`IntegerAttr`][IntegerAttr] in the op.

All these `*EnumAttr` attributes require fully specifying all of the allowed
cases via their corresponding `*EnumAttrCase`. With this, ODS is able to
generate additional verification to only accept allowed cases. To facilitate the
interaction between `*EnumAttr`s and their C++ consumers, the
[`EnumsGen`][EnumsGen] TableGen backend can generate a few common utilities: a
C++ enum class, `llvm::DenseMapInfo` for the enum class, conversion functions
from/to strings. This is controlled via the `-gen-enum-decls` and
`-gen-enum-defs` command-line options of `mlir-tblgen`.

For example, given the following `EnumAttr`:

```tablegen
def Case15: I32EnumAttrCase<"Case15", 15>;
def Case20: I32EnumAttrCase<"Case20", 20>;

def MyIntEnum: I32EnumAttr<"MyIntEnum", "An example int enum",
                           [Case15, Case20]> {
  let cppNamespace = "Outer::Inner";
  let stringToSymbolFnName = "ConvertToEnum";
  let symbolToStringFnName = "ConvertToString";
}
```

The following will be generated via `mlir-tblgen -gen-enum-decls`:

```c++
namespace Outer {
namespace Inner {
// An example int enum
enum class MyIntEnum : uint32_t {
  Case15 = 15,
  Case20 = 20,
};

llvm::Optional<MyIntEnum> symbolizeMyIntEnum(uint32_t);
llvm::StringRef ConvertToString(MyIntEnum);
llvm::Optional<MyIntEnum> ConvertToEnum(llvm::StringRef);
inline constexpr unsigned getMaxEnumValForMyIntEnum() {
  return 20;
}

} // namespace Inner
} // namespace Outer

namespace llvm {
template<> struct DenseMapInfo<Outer::Inner::MyIntEnum> {
  using StorageInfo = llvm::DenseMapInfo<uint32_t>;

  static inline Outer::Inner::MyIntEnum getEmptyKey() {
    return static_cast<Outer::Inner::MyIntEnum>(StorageInfo::getEmptyKey());
  }

  static inline Outer::Inner::MyIntEnum getTombstoneKey() {
    return static_cast<Outer::Inner::MyIntEnum>(StorageInfo::getTombstoneKey());
  }

  static unsigned getHashValue(const Outer::Inner::MyIntEnum &val) {
    return StorageInfo::getHashValue(static_cast<uint32_t>(val));
  }

  static bool isEqual(const Outer::Inner::MyIntEnum &lhs, const Outer::Inner::MyIntEnum &rhs) {
    return lhs == rhs;
  }
};
}
```

The following will be generated via `mlir-tblgen -gen-enum-defs`:

```c++
namespace Outer {
namespace Inner {
llvm::StringRef ConvertToString(MyIntEnum val) {
  switch (val) {
    case MyIntEnum::Case15: return "Case15";
    case MyIntEnum::Case20: return "Case20";
  }
  return "";
}

llvm::Optional<MyIntEnum> ConvertToEnum(llvm::StringRef str) {
  return llvm::StringSwitch<llvm::Optional<MyIntEnum>>(str)
      .Case("Case15", MyIntEnum::Case15)
      .Case("Case20", MyIntEnum::Case20)
      .Default(llvm::None);
}
llvm::Optional<MyIntEnum> symbolizeMyIntEnum(uint32_t value) {
  switch (value) {
  case 15: return MyIntEnum::Case15;
  case 20: return MyIntEnum::Case20;
  default: return llvm::None;
  }
}

} // namespace Inner
} // namespace Outer
```

Similarly for the following `BitEnumAttr` definition:

```tablegen
def None: BitEnumAttrCase<"None", 0x0000>;
def Bit1: BitEnumAttrCase<"Bit1", 0x0001>;
def Bit2: BitEnumAttrCase<"Bit2", 0x0002>;
def Bit3: BitEnumAttrCase<"Bit3", 0x0004>;

def MyBitEnum: BitEnumAttr<"MyBitEnum", "An example bit enum",
                           [None, Bit1, Bit2, Bit3]>;
```

We can have:

```c++
// An example bit enum
enum class MyBitEnum : uint32_t {
  None = 0,
  Bit1 = 1,
  Bit2 = 2,
  Bit3 = 4,
};

llvm::Optional<MyBitEnum> symbolizeMyBitEnum(uint32_t);
std::string stringifyMyBitEnum(MyBitEnum);
llvm::Optional<MyBitEnum> symbolizeMyBitEnum(llvm::StringRef);
inline MyBitEnum operator|(MyBitEnum lhs, MyBitEnum rhs) {
  return static_cast<MyBitEnum>(static_cast<uint32_t>(lhs) | static_cast<uint32_t>(rhs));
}
inline MyBitEnum operator&(MyBitEnum lhs, MyBitEnum rhs) {
  return static_cast<MyBitEnum>(static_cast<uint32_t>(lhs) & static_cast<uint32_t>(rhs));
}
inline bool bitEnumContains(MyBitEnum bits, MyBitEnum bit) {
  return (static_cast<uint32_t>(bits) & static_cast<uint32_t>(bit)) != 0;
}

namespace llvm {
template<> struct DenseMapInfo<::MyBitEnum> {
  using StorageInfo = llvm::DenseMapInfo<uint32_t>;

  static inline ::MyBitEnum getEmptyKey() {
    return static_cast<::MyBitEnum>(StorageInfo::getEmptyKey());
  }

  static inline ::MyBitEnum getTombstoneKey() {
    return static_cast<::MyBitEnum>(StorageInfo::getTombstoneKey());
  }

  static unsigned getHashValue(const ::MyBitEnum &val) {
    return StorageInfo::getHashValue(static_cast<uint32_t>(val));
  }

  static bool isEqual(const ::MyBitEnum &lhs, const ::MyBitEnum &rhs) {
    return lhs == rhs;
  }
};
```

```c++
std::string stringifyMyBitEnum(MyBitEnum symbol) {
  auto val = static_cast<uint32_t>(symbol);
  // Special case for all bits unset.
  if (val == 0) return "None";

  llvm::SmallVector<llvm::StringRef, 2> strs;
  if (1u & val) { strs.push_back("Bit1"); val &= ~1u; }
  if (2u & val) { strs.push_back("Bit2"); val &= ~2u; }
  if (4u & val) { strs.push_back("Bit3"); val &= ~4u; }

  if (val) return "";
  return llvm::join(strs, "|");
}

llvm::Optional<MyBitEnum> symbolizeMyBitEnum(llvm::StringRef str) {
  // Special case for all bits unset.
  if (str == "None") return MyBitEnum::None;

  llvm::SmallVector<llvm::StringRef, 2> symbols;
  str.split(symbols, "|");

  uint32_t val = 0;
  for (auto symbol : symbols) {
    auto bit = llvm::StringSwitch<llvm::Optional<uint32_t>>(symbol)
      .Case("Bit1", 1)
      .Case("Bit2", 2)
      .Case("Bit3", 4)
      .Default(llvm::None);
    if (bit) { val |= *bit; } else { return llvm::None; }
  }
  return static_cast<MyBitEnum>(val);
}

llvm::Optional<MyBitEnum> symbolizeMyBitEnum(uint32_t value) {
  // Special case for all bits unset.
  if (value == 0) return MyBitEnum::None;

  if (value & ~(1u | 2u | 4u)) return llvm::None;
  return static_cast<MyBitEnum>(value);
}
```

## Type Definitions

MLIR defines the TypeDef class hierarchy to enable generation of data types
from their specifications. A type is defined by specializing the TypeDef
class with concrete contents for all the fields it requires. For example, an
integer type could be defined as:

```tablegen
// All of the types will extend this class.
class Test_Type<string name> : TypeDef<Test_Dialect, name> { }

// An alternate int type.
def IntegerType : Test_Type<"TestInteger"> {
  let mnemonic = "int";

  let summary = "An integer type with special semantics";

  let description = [{
    An alternate integer type. This type differentiates itself from the
    standard integer type by not having a SignednessSemantics parameter, just
    a width.
  }];

  let parameters = (ins "unsigned":$width);

  // We define the printer inline.
  let printer = [{
    $_printer << "int<" << getImpl()->width << ">";
  }];

  // The parser is defined here also.
  let parser = [{
    if (parser.parseLess())
      return Type();
    int width;
    if ($_parser.parseInteger(width))
      return Type();
    if ($_parser.parseGreater())
      return Type();
    return get(ctxt, width);
  }];
```

### Type name

The name of the C++ class which gets generated defaults to
`<classParamName>Type` (e.g. `TestIntegerType` in the above example). This
can be overridden via the the `cppClassName` field. The field `mnemonic` is
to specify the asm name for parsing. It is optional and not specifying it
will imply that no parser or printer methods are attached to this class.

### Type documentation

The `summary` and `description` fields exist and are to be used the same way
as in Operations. Namely, the summary should be a one-liner and `description`
should be a longer explanation.

### Type parameters

The `parameters` field is a list of the types parameters. If no parameters
are specified (the default), this type is considered a singleton type.
Parameters are in the `"c++Type":$paramName` format.
To use C++ types as parameters which need allocation in the storage
constructor, there are two options:

- Set `hasCustomStorageConstructor` to generate the TypeStorage class with
a constructor which is just declared -- no definition -- so you can write it
yourself.
- Use the `TypeParameter` tablegen class instead of the "c++Type" string.

### TypeParameter tablegen class

This is used to further specify attributes about each of the types
parameters. It includes documentation (`description` and `syntax`), the C++
type to use, and a custom allocator to use in the storage constructor method.

```tablegen
// DO NOT DO THIS!
let parameters = (ins
  "ArrayRef<int>":$dims);
```

The default storage constructor blindly copies fields by value. It does not
know anything about the types. In this case, the ArrayRef<int> requires
allocation with `dims = allocator.copyInto(dims)`.

You can specify the necessary constuctor by specializing the `TypeParameter`
tblgen class:

```tablegen
class ArrayRefIntParam :
    TypeParameter<"::llvm::ArrayRef<int>", "Array of ints"> {
  let allocator = [{$_dst = $_allocator.copyInto($_self);}];
}

...

let parameters = (ins
  ArrayRefIntParam:$dims);
```

The `allocator` code block has the following substitutions:
- `$_allocator` is the TypeStorageAllocator in which to allocate objects.
- `$_dst` is the variable in which to place the allocated data.

MLIR includes several specialized classes for common situations:
- `StringRefParameter<descriptionOfParam>` for StringRefs.
- `ArrayRefParameter<arrayOf, descriptionOfParam>` for ArrayRefs of value
types
- `SelfAllocationParameter<descriptionOfParam>` for C++ classes which contain
a method called `allocateInto(StorageAllocator &allocator)` to allocate
itself into `allocator`.
- `ArrayRefOfSelfAllocationParameter<arrayOf, descriptionOfParam>` for arrays
of objects which self-allocate as per the last specialization.

If we were to use one of these included specializations:

```tablegen
let parameters = (ins
  ArrayRefParameter<"int", "The dimensions">:$dims
);
```

### Parsing and printing

If a mnemonic is specified, the `printer` and `parser` code fields are active.
The rules for both are:
- If null, generate just the declaration.
- If non-null and non-empty, use the code in the definition. The `$_printer`
or `$_parser` substitutions are valid and should be used.
- It is an error to have an empty code block.

For each dialect, two "dispatch" functions will be created: one for parsing
and one for printing. You should add calls to these in your
`Dialect::printType` and `Dialect::parseType` methods. They are created in
the dialect's namespace and their function signatures are:
```c++
Type generatedTypeParser(MLIRContext* ctxt, DialectAsmParser& parser,
                         StringRef mnemonic);
LogicalResult generatedTypePrinter(Type type, DialectAsmPrinter& printer);
```

The mnemonic, parser, and printer fields are optional. If they're not
defined, the generated code will not include any parsing or printing code and
omit the type from the dispatch functions above. In this case, the dialect
author is responsible for parsing/printing the types in `Dialect::printType`
and `Dialect::parseType`.

### Other fields

- If the `genStorageClass` field is set to 1 (the default) a storage class is
generated with member variables corresponding to each of the specified
`parameters`.
- If the `genAccessors` field is 1 (the default) accessor methods will be
generated on the Type class (e.g. `int getWidth() const` in the example
above).
- If the `genVerifyInvariantsDecl` field is set, a declaration for a method
`static LogicalResult verifyConstructionInvariants(Location, parameters...)`
is added to the class as well as a `getChecked(Location, parameters...)`
method which gets the result of `verifyConstructionInvariants` before calling
`get`.
- The `storageClass` field can be used to set the name of the storage class.
- The `storageNamespace` field is used to set the namespace where the storage
class should sit. Defaults to "detail".
- The `extraClassDeclaration` field is used to include extra code in the
class declaration.

## Debugging Tips

### Run `mlir-tblgen` to see the generated content

TableGen syntax sometimes can be obscure; reading the generated content can be
a very helpful way to understand and debug issues. To build `mlir-tblgen`, run
`cmake --build . --target mlir-tblgen` in your build directory and find the
`mlir-tblgen` binary in the `bin/` subdirectory. All the supported generators
can be found via `mlir-tblgen --help`. For example, `--gen-op-decls` and
`--gen-op-defs` as explained in [Generated C++ code](#generated-c++-code).

To see the generated code, invoke `mlir-tblgen` with a specific generator by
providing include paths via `-I`. For example,

```sh
# To see op C++ class declaration
mlir-tblgen --gen-op-decls -I /path/to/mlir/include /path/to/input/td/file
# To see op C++ class definition
mlir-tblgen --gen-op-defs -I /path/to/mlir/include /path/to/input/td/file
# To see op documentation
mlir-tblgen --gen-dialect-doc -I /path/to/mlir/include /path/to/input/td/file

# To see op interface C++ class declaration
mlir-tblgen --gen-op-interface-decls -I /path/to/mlir/include /path/to/input/td/file
# To see op interface C++ class definition
mlir-tblgen --gen-op-interface-defs -I /path/to/mlir/include /path/to/input/td/file
# To see op interface documentation
mlir-tblgen --gen-op-interface-doc -I /path/to/mlir/include /path/to/input/td/file
```

## Appendix

### Requirements and existing mechanisms analysis

The op description should as declarative as possible to allow a wide range of
tools to work with them and query methods generated from them. In particular
this means specifying traits, constraints and shape inference information in
a way that is easily analyzable (e.g., avoid opaque calls to C++ functions where
possible).

We considered the approaches of several contemporary systems and focused on
requirements that were desirable:

*   Ops registered using a registry separate from C++ code.
    *   Unknown ops are allowed in MLIR, so ops need not be registered. The
        ability of the compiler to optimize those ops or graphs containing those
        ops is constrained but correct.
    *   The current proposal does not include a runtime op description, but it
        does not preclude such description, it can be added later.
    *   The op registry is essential for generating C++ classes that make
        manipulating ops, verifying correct construction etc. in C++ easier by
        providing a typed representation and accessors.
*   The op registry will be defined in
    [TableGen](https://llvm.org/docs/TableGen/index.html) and be used to
    generate C++ classes and utility functions
    (builder/verifier/parser/printer).
    *   TableGen is a modelling specification language used by LLVM's backends
        and fits in well with trait-based modelling. This is an implementation
        decision and there are alternative ways of doing this. But the
        specification language is good for the requirements of modelling the
        traits (as seen from usage in LLVM processor backend modelling) and easy
        to extend, so a practical choice. If another good option comes up, we
        will consider it.
*   MLIR allows both defined and undefined ops.
    *   Defined ops should have fixed semantics and could have a corresponding
        reference implementation defined using, for example, EDSC.
    *   Dialects are under full control of the dialect owner and normally live
        with the framework of the dialect.
*   The op's traits (e.g., commutative) are modelled along with the op in the
    registry.
*   The op's operand/return type constraints are modelled along with the op in
    the registry (see [Shape inference](ShapeInference.md) discussion below),
    this allows (e.g.) optimized concise syntax in textual dumps.
*   Behavior of the op is documented along with the op with a summary and a
    description. The description is written in markdown and extracted for
    inclusion in the generated LangRef section of the dialect.
*   The generic assembly form of printing and parsing is available as normal,
    but a custom parser and printer can either be specified or automatically
    generated from an optional string representation showing the mapping of the
    "assembly" string to operands/type.
    *   Parser-level remappings (e.g., `eq` to enum) will be supported as part
        of the parser generation.
*   Matching patterns are specified separately from the op description.
    *   Contrasted with LLVM there is no "base" set of ops that every backend
        needs to be aware of. Instead there are many different dialects and the
        transformations/legalizations between these dialects form a graph of
        transformations.
*   Reference implementation may be provided along with the op definition.

    *   The reference implementation may be in terms of either standard ops or
        other reference implementations.

    TODO: document expectation if the dependent op's definition changes.

[TableGen]: https://llvm.org/docs/TableGen/index.html
[TableGenProgRef]: https://llvm.org/docs/TableGen/ProgRef.html
[TableGenBackend]: https://llvm.org/docs/TableGen/BackEnds.html#introduction
[OpBase]: https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/IR/OpBase.td
[OpDefinitionsGen]: https://github.com/llvm/llvm-project/blob/master/mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp
[EnumsGen]: https://github.com/llvm/llvm-project/blob/master/mlir/tools/mlir-tblgen/EnumsGen.cpp
[StringAttr]: LangRef.md#string-attribute
[IntegerAttr]: LangRef.md#integer-attribute
[AttrClasses]: https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/IR/Attributes.h