[mlir][ods] Optional Attribute or Type Parameters

[Mogball]

Summary

Implements optional attribute or type parameters, including support for such parameters in the assembly format struct directive. Also implements optional groups.

Depends on D117971

[mehdi_amini]

mlir/docs/Tutorials/DefiningAttributesAndTypes.md

1

Seeing this file path again, that reminds me: wasn't it supposed to be converted to a doc like the other instead of being under "tutorial"?

[rriddle]

mlir/docs/Tutorials/DefiningAttributesAndTypes.md

1

Oooh, never pushed that for review. Let me send that out tomorrow.

[rriddle]

mlir/docs/Tutorials/DefiningAttributesAndTypes.md

537

?

mlir/include/mlir/IR/OpBase.td

3133–3136

mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp

257–261

326–330

Avoid floating statements.

372

This seems quadratic, why not cache this computation off somewhere.

409

nit: Why not use ods as prefix instead of _?

588

Is the *_result_{0} here presenting a parse failure?

608–609

743–744

[Mogball]

mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp

588

This condition guards on "no value". E.g. FailureOr<Attribute> = failure() is parse failure but FailureOr<Attribute> = nullptr is no error but no value either.

[Mogball]

review comments

[Mogball]

review comments

[Mogball]

@rriddle ping .-.

[rriddle]

The windows build is failing BTW, might want to look into that.

[Mogball]

fix windows build

[Mogball]

another one

mlir/docs/Tutorials/DefiningAttributesAndTypes.md

 # Defining Dialect Attributes and Types
 
 This document is a quickstart to defining dialect specific extensions to the
-[attribute](../LangRef.md/#attributes) and [type](../LangRef.md/#type-system) systems in
-MLIR. The main part of this tutorial focuses on defining types, but the
-instructions are nearly identical for defining attributes.
+[attribute](../LangRef.md/#attributes) and [type](../LangRef.md/#type-system)
+systems in MLIR. The main part of this 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` are 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`.
 
 ### Defining the type class
 
 As described above, `Type` objects in MLIR are value-typed and rely on having an
 implicit 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 _singleton_ in nature, meaning they have no parameters and only
-ever have one instance, like the [`index` type](../Dialects/Builtin.md/#indextype).
+Some types are *singleton* in nature, meaning they have no parameters and only
+ever have one instance, like the
+[`index` type](../Dialects/Builtin.md/#indextype).
 
-Other types are _parametric_, and contain additional information that
+Other types are *parametric*, and contain additional information that
 differentiates different instances of the same `Type`. For example the
-[`integer` type](../Dialects/Builtin.md/#integertype) contains a bitwidth, with `i8` and
-`i16` representing different instances of
-[`integer` type](../Dialects/Builtin.md/#integertype). _Parametric_ may also contain a
-mutable component, which can be used, for example, to construct self-referring
-recursive types. The mutable component _cannot_ be used to differentiate
-instances of a type class, so usually such types contain other parametric
-components that serve to identify them.
+[`integer` type](../Dialects/Builtin.md/#integertype) contains a bitwidth, with
+`i8` and `i16` representing different instances of
+[`integer` type](../Dialects/Builtin.md/#integertype). *Parametric* may also
+contain a mutable component, which can be used, for example, to construct
+self-referring recursive types. The mutable component *cannot* be used to
+differentiate instances of a type class, so usually such types contain other
+parametric components that serve to identify them.
 
 #### Singleton types
 
 For singleton types, we can jump straight into defining the derived type class.
 Given that only one instance of such types may exist, there is no need to
 provide our own storage class.
 
 ```c++

 all the same.
 
 ## Defining Custom Parsers and Printers using Assembly Formats
 
 Attributes and types defined in ODS with a mnemonic can define an
 `assemblyFormat` to declaratively describe custom parsers and printers. The
 assembly format consists of literals, variables, and directives.
 
-* A literal is a keyword or valid punctuation enclosed in backticks, e.g.
-  `` `keyword` `` or `` `<` ``.
-* A variable is a parameter name preceeded by a dollar sign, e.g. `$param0`,
+*   A literal is a keyword or valid punctuation enclosed in backticks, e.g. ``
+    `keyword` `` or `` `<` ``.
+*   A variable is a parameter name preceeded by a dollar sign, e.g. `$param0`,
     which captures one attribute or type parameter.
-* A directive is a keyword followed by an optional argument list that defines
+*   A directive is a keyword followed by an optional argument list that defines
     special parser and printer behaviour.
 
 ```tablegen
 // An example type with an assembly format.
 def MyType : TypeDef<My_Dialect, "MyType"> {
   // Define a mnemonic to allow the dialect's parser hook to call into the
   // generated parser.
   let mnemonic = "my_type";
 
   // Define two parameters whose C++ types are indicated in string literals.
   let parameters = (ins "int":$count, "AffineMap":$map);
 
   // Define the assembly format. Surround the format with less `<` and greater
   // `>` so that MLIR's printers use the pretty format.
   let assemblyFormat = "`<` $count `,` `map` `=` $map `>`";
 }
 ```
 
-The declarative assembly format for `MyType` results in the following format
-in the IR:
+The declarative assembly format for `MyType` results in the following format in
+the IR:
 
 ```mlir
 !my_dialect.my_type<42, map = affine_map<(i, j) -> (j, i)>
 ```
 
 ### Parameter Parsing and Printing
 
-For many basic parameter types, no additional work is needed to define how
-these parameters are parsed or printed.
+For many basic parameter types, no additional work is needed to define how these
+parameters are parsed or printed.
 
-* The default printer for any parameter is `$_printer << $_self`,
-  where `$_self` is the C++ value of the parameter and `$_printer` is an
+*   The default printer for any parameter is `$_printer << $_self`, where
+    `$_self` is the C++ value of the parameter and `$_printer` is an
     `AsmPrinter`.
-* The default parser for a parameter is
+*   The default parser for a parameter is
     `FieldParser<$cppClass>::parse($_parser)`, where `$cppClass` is the C++ type
     of the parameter and `$_parser` is an `AsmParser`.
 
 Printing and parsing behaviour can be added to additional C++ types by
 overloading these functions or by defining a `parser` and `printer` in an ODS
 parameter class.
 
 Example of overloading:
 
 ```c++
let parser = [{ [&] -> FailureOr<std::pair<int, int>> {
     if ($_parser.parseInteger(a) || $_parser.parseStar() ||
         $_parser.parseInteger(b))
       return failure();
     return std::make_pair(a, b);
   }() }];
 }
 ```
 
-A type using this parameter with the assembly format `` `<` $myParam `>` ``
-will look as follows in the IR:
+A type using this parameter with the assembly format `` `<` $myParam `>` `` will
+look as follows in the IR:
 
 ```mlir
 !my_dialect.my_type<42 * 24>
 ```
 
 #### Non-POD Parameters
 
 Parameters that aren't plain-old-data (e.g. references) may need to define a
-`cppStorageType` to contain the data until it is copied into the allocator.
-For example, `StringRefParameter` uses `std::string` as its storage type,
-whereas `ArrayRefParameter` uses `SmallVector` as its storage type. The parsers
-for these parameters are expected to return `FailureOr<$cppStorageType>`.
+`cppStorageType` to contain the data until it is copied into the allocator. For
+example, `StringRefParameter` uses `std::string` as its storage type, whereas
+`ArrayRefParameter` uses `SmallVector` as its storage type. The parsers for
+these parameters are expected to return `FailureOr<$cppStorageType>`.
+
+#### Optional Parameters
+
+Optional parameters in the assembly format can be indicated by setting
+`isOptional`. The C++ type of an optional parameter is required to satisfy the
+following requirements:
+
+*   is default-constructible
+*   is contextually convertible to `bool`
+*   only the default-constructed value is `false`
+
+The parameter parser should return the default-constructed value to indicate "no
+value present". The printer will guard on the presence of a value to print the
+parameter.
+
+If a value was not parsed for an optional parameter, then the parameter will be
+set to its default-constructed C++ value. For example, `Optional<int>` will be
+set to `llvm::None` and `Attribute` will be set to `nullptr`.
+
+Only optional parameters or directives that only capture optional parameters can
+be used in optional groups. An optional group is a set of elements optionally
+printed based on the presence of an anchor. Suppose parameter `a` is an
+`IntegerAttr`.
+
+```
+( `(` $a^ `)` ) : (`x`)?
+```
+
+In the above assembly format, if `a` is present (non-null), then it will be
+printed as `(5 : i32)`. If it is not present, it will be `x`. Directives that
+are used inside optional groups are allowed only if all captured parameters are
+also optional.
 
 ### Assembly Format Directives
 
 Attribute and type assembly formats have the following directives:
 
 *   `params`: capture all parameters of an attribute or type.
 *   `qualified`: mark a parameter to be printed with its leading dialect and
     mnemonic.
 *   `struct`: generate a "struct-like" parser and printer for a list of
     key-value pairs.
 
 #### `params` Directive
 
-This directive is used to refer to all parameters of an attribute or type.
-When used as a top-level directive, `params` generates a parser and printer for
-a comma-separated list of the parameters. For example:
+This directive is used to refer to all parameters of an attribute or type. When
+used as a top-level directive, `params` generates a parser and printer for a
+comma-separated list of the parameters. For example:
 
 ```tablegen
 def MyPairType : TypeDef<My_Dialect, "MyPairType"> {
   let parameters = (ins "int":$a, "int":$b);
   let mnemonic = "pair";
   let assemblyFormat = "`<` params `>`";
 }
 ```
 
 In the IR, this type will appear as:

 
 In the IR, the types will appear as:
 
 ```mlir
 !my_dialect.outer<pair : <42, 24>>
 !my_dialect.outer_qual<pair : !mydialect.pair<42, 24>>
 ```
 
+If optional parameters are present, they are not printed in the parameter list
+if they are not present.
+
 #### `struct` Directive
 
 The `struct` directive accepts a list of variables to capture and will generate
-a parser and printer for a comma-separated list of key-value pairs. The
-variables are printed in the order they are specified in the argument list **but
-can be parsed in any order**. For example:
+a parser and printer for a comma-separated list of key-value pairs. If an
+optional parameter is included in the `struct`, it can be elided. The variables
+are printed in the order they are specified in the argument list **but can be
+parsed in any order**. For example:
 
 ```tablegen
 def MyStructType : TypeDef<My_Dialect, "MyStructType"> {
   let parameters = (ins StringRefParameter<>:$sym_name,
                         "int":$a, "int":$b, "int":$c);
   let mnemonic = "struct";
   let assemblyFormat = "`<` $sym_name `->` struct($a, $b, $c) `>`";
 }

mlir/include/mlir/IR/OpBase.td

class AttrOrTypeParameter<string type, string desc, string accessorType = ""> {
   string cppAccessorType = !if(!empty(accessorType), type, accessorType);
   // The C++ storage type of of this parameter if it is a reference, e.g.
   // `std::string` for `StringRef` or `SmallVector` for `ArrayRef`.
   string cppStorageType = ?;
   // One-line human-readable description of the argument.
   string summary = desc;
   // The format string for the asm syntax (documentation only).
   string syntax = ?;
-  // The default parameter parser is `::mlir::parseField<T>($_parser)`, which
-  // returns `FailureOr<T>`. Overload `parseField` to support parsing for your
-  // type. Or you can provide a customer printer. For attributes, "$_type" will
-  // be replaced with the required attribute type.
+  // The default parameter parser is `::mlir::FieldParser<T>::parse($_parser)`,
+  // which returns `FailureOr<T>`. Specialize `FieldParser` to support parsing
+  // for your type. Or you can provide a customer printer. For attributes,
+  // "$_type" will be replaced with the required attribute type.
   string parser = ?;
   // The default parameter printer is `$_printer << $_self`. Overload the stream
   // operator of `AsmPrinter` as necessary to print your type. Or you can
   // provide a custom printer.
   string printer = ?;
+  // Mark a parameter as optional. The C++ type of parameters marked as optional
+  // must be default constructible and be contextually convertible to `bool`.
+  // Any `Optional<T>` and any attribute type satisfies these requirements.
+  bit isOptional = 0;
 }
 class AttrParameter<string type, string desc, string accessorType = "">
  : AttrOrTypeParameter<type, desc, accessorType>;
 class TypeParameter<string type, string desc, string accessorType = "">
  : AttrOrTypeParameter<type, desc, accessorType>;
 
 // For StringRefs, which require allocation.
 class StringRefParameter<string desc = ""> :
class ArrayRefOfSelfAllocationParameter<string arrayOf, string desc> :
   let allocator = [{
     llvm::SmallVector<}] # arrayOf # [{, 4> tmpFields;
     for (size_t i = 0, e = $_self.size(); i < e; ++i)
       tmpFields.push_back($_self[i].allocateInto($_allocator));
     $_dst = $_allocator.copyInto(ArrayRef<}] # arrayOf # [{>(tmpFields));
   }];
 }
 
+// An optional parameter.
+class OptionalParameter<string type, string desc = ""> :
+    AttrOrTypeParameter<type, desc> {
+  let isOptional = 1;
+}
+
 // This is a special parameter used for AttrDefs that represents a `mlir::Type`
 // that is also used as the value `Type` of the attribute. Only one parameter
 // of the attribute may be of this type.
 class AttributeSelfTypeParameter<string desc,
                                  string derivedType = "::mlir::Type"> :
     AttrOrTypeParameter<derivedType, desc> {}
 
 #endif // OP_BASE

mlir/include/mlir/TableGen/AttrOrTypeDef.h

public:
   /// Returns true if this builder is able to infer the MLIRContext parameter.
   bool hasInferredContextParameter() const;
 };
 
 //===----------------------------------------------------------------------===//
 // AttrOrTypeParameter
 //===----------------------------------------------------------------------===//
 
-// A wrapper class for tblgen AttrOrTypeParameter, arrays of which belong to
-// AttrOrTypeDefs to parameterize them.
+/// A wrapper class for tblgen AttrOrTypeParameter, arrays of which belong to
+/// AttrOrTypeDefs to parameterize them.
 class AttrOrTypeParameter {
 public:
   explicit AttrOrTypeParameter(const llvm::DagInit *def, unsigned index)
       : def(def), index(index) {}
 
-  // Get the parameter name.
+  /// Get the parameter name.
   StringRef getName() const;
 
-  // If specified, get the custom allocator code for this parameter.
+  /// If specified, get the custom allocator code for this parameter.
   Optional<StringRef> getAllocator() const;
 
-  // If specified, get the custom comparator code for this parameter.
+  /// If specified, get the custom comparator code for this parameter.
   Optional<StringRef> getComparator() const;
 
-  // Get the C++ type of this parameter.
+  /// Get the C++ type of this parameter.
   StringRef getCppType() const;
 
-  // Get the C++ accessor type of this parameter.
+  /// Get the C++ accessor type of this parameter.
   StringRef getCppAccessorType() const;
 
-  // Get the C++ storage type of this parameter.
+  /// Get the C++ storage type of this parameter.
   StringRef getCppStorageType() const;
 
-  // Get an optional C++ parameter parser.
+  /// Get an optional C++ parameter parser.
   Optional<StringRef> getParser() const;
 
-  // Get an optional C++ parameter printer.
+  /// Get an optional C++ parameter printer.
   Optional<StringRef> getPrinter() const;
 
-  // Get a description of this parameter for documentation purposes.
+  /// Get a description of this parameter for documentation purposes.
   Optional<StringRef> getSummary() const;
 
-  // Get the assembly syntax documentation.
+  /// Get the assembly syntax documentation.
   StringRef getSyntax() const;
 
-  // Return the underlying def of this parameter.
-  const llvm::Init *getDef() const;
+  /// Returns true if the parameter is optional.
+  bool isOptional() const;
 
-  // The parameter is pointer-comparable.
+  /// Return the underlying def of this parameter.
+  llvm::Init *getDef() const;
+
+  /// The parameter is pointer-comparable.
   bool operator==(const AttrOrTypeParameter &other) const {
     return def == other.def && index == other.index;
   }
   bool operator!=(const AttrOrTypeParameter &other) const {
     return !(*this == other);
   }
 
 private:
+  /// A parameter can be either a string or a def. Get a potentially null value
+  /// from the def.
+  template <typename InitT>
+  auto getDefValue(StringRef name) const;
+
   /// The underlying tablegen parameter list this parameter is a part of.
   const llvm::DagInit *def;
   /// The index of the parameter within the parameter list (`def`).
   unsigned index;
 };
 
 //===----------------------------------------------------------------------===//
 // AttributeSelfTypeParameter

 //===----------------------------------------------------------------------===//
 
 /// Wrapper class that contains a TableGen AttrOrTypeDef's record and provides
 /// helper methods for accessing them.
 class AttrOrTypeDef {
 public:
   explicit AttrOrTypeDef(const llvm::Record *def);
 
-  // Get the dialect for which this def belongs.
+  /// Get the dialect for which this def belongs.
   Dialect getDialect() const;
 
-  // Returns the name of this AttrOrTypeDef record.
+  /// Returns the name of this AttrOrTypeDef record.
   StringRef getName() const;
 
-  // Query functions for the documentation of the def.
+  /// Query functions for the documentation of the def.
   bool hasDescription() const;
   StringRef getDescription() const;
   bool hasSummary() const;
   StringRef getSummary() const;
 
-  // Returns the name of the C++ class to generate.
+  /// Returns the name of the C++ class to generate.
   StringRef getCppClassName() const;
 
-  // Returns the name of the C++ base class to use when generating this def.
+  /// Returns the name of the C++ base class to use when generating this def.
   StringRef getCppBaseClassName() const;
 
-  // Returns the name of the storage class for this def.
+  /// Returns the name of the storage class for this def.
   StringRef getStorageClassName() const;
 
-  // Returns the C++ namespace for this def's storage class.
+  /// Returns the C++ namespace for this def's storage class.
   StringRef getStorageNamespace() const;
 
-  // Returns true if we should generate the storage class.
+  /// Returns true if we should generate the storage class.
   bool genStorageClass() const;
 
-  // Indicates whether or not to generate the storage class constructor.
+  /// Indicates whether or not to generate the storage class constructor.
   bool hasStorageCustomConstructor() const;
 
   /// Get the parameters of this attribute or type.
   ArrayRef<AttrOrTypeParameter> getParameters() const { return parameters; }
 
-  // Return the number of parameters
+  /// Return the number of parameters
   unsigned getNumParameters() const;
 
-  // Return the keyword/mnemonic to use in the printer/parser methods if we are
-  // supposed to auto-generate them.
+  /// Return the keyword/mnemonic to use in the printer/parser methods if we are
+  /// supposed to auto-generate them.
   Optional<StringRef> getMnemonic() const;
 
-  // Returns the code to use as the types printer method. If not specified,
-  // return a non-value. Otherwise, return the contents of that code block.
+  /// Returns the code to use as the types printer method. If not specified,
+  /// return a non-value. Otherwise, return the contents of that code block.
   Optional<StringRef> getPrinterCode() const;
 
-  // Returns the code to use as the parser method. If not specified, returns
-  // None. Otherwise, returns the contents of that code block.
+  /// Returns the code to use as the parser method. If not specified, returns
+  /// None. Otherwise, returns the contents of that code block.
   Optional<StringRef> getParserCode() const;
 
-  // Returns the custom assembly format, if one was specified.
+  /// Returns the custom assembly format, if one was specified.
   Optional<StringRef> getAssemblyFormat() const;
 
-  // An attribute or type with parameters needs a parser.
+  /// An attribute or type with parameters needs a parser.
   bool needsParserPrinter() const { return getNumParameters() != 0; }
 
-  // Returns true if this attribute or type has a generated parser.
+  /// Returns true if this attribute or type has a generated parser.
   bool hasGeneratedParser() const {
     return getParserCode() || getAssemblyFormat();
   }
 
-  // Returns true if this attribute or type has a generated printer.
+  /// Returns true if this attribute or type has a generated printer.
   bool hasGeneratedPrinter() const {
     return getPrinterCode() || getAssemblyFormat();
   }
 
-  // Returns true if the accessors based on the parameters should be generated.
+  /// Returns true if the accessors based on the parameters should be generated.
   bool genAccessors() const;
 
-  // Return true if we need to generate the verify declaration and getChecked
-  // method.
+  /// Return true if we need to generate the verify declaration and getChecked
+  /// method.
   bool genVerifyDecl() const;
 
-  // Returns the def's extra class declaration code.
+  /// Returns the def's extra class declaration code.
   Optional<StringRef> getExtraDecls() const;
 
-  // Get the code location (for error printing).
+  /// Get the code location (for error printing).
   ArrayRef<SMLoc> getLoc() const;
 
-  // Returns true if the default get/getChecked methods should be skipped during
-  // generation.
+  /// Returns true if the default get/getChecked methods should be skipped
+  /// during generation.
   bool skipDefaultBuilders() const;
 
-  // Returns the builders of this def.
+  /// Returns the builders of this def.
   ArrayRef<AttrOrTypeBuilder> getBuilders() const { return builders; }
 
-  // Returns the traits of this def.
+  /// Returns the traits of this def.
   ArrayRef<Trait> getTraits() const { return traits; }
 
-  // Returns whether two AttrOrTypeDefs are equal by checking the equality of
-  // the underlying record.
+  /// Returns whether two AttrOrTypeDefs are equal by checking the equality of
+  /// the underlying record.
   bool operator==(const AttrOrTypeDef &other) const;
 
-  // Compares two AttrOrTypeDefs by comparing the names of the dialects.
+  /// Compares two AttrOrTypeDefs by comparing the names of the dialects.
   bool operator<(const AttrOrTypeDef &other) const;
 
-  // Returns whether the AttrOrTypeDef is defined.
+  /// Returns whether the AttrOrTypeDef is defined.
   operator bool() const { return def != nullptr; }
 
-  // Return the underlying def.
+  /// Return the underlying def.
   const llvm::Record *getDef() const { return def; }
 
 protected:
   const llvm::Record *def;
 
-  // The builders of this definition.
+  /// The builders of this definition.
   SmallVector<AttrOrTypeBuilder> builders;
 
-  // The traits of this definition.
+  /// The traits of this definition.
   SmallVector<Trait> traits;
 
   /// The parameters of this attribute or type.
   SmallVector<AttrOrTypeParameter> parameters;
 };
 
 //===----------------------------------------------------------------------===//
 // AttrDef
 //===----------------------------------------------------------------------===//
 
 /// This class represents a wrapper around a tablegen AttrDef record.
 class AttrDef : public AttrOrTypeDef {
 public:
   using AttrOrTypeDef::AttrOrTypeDef;
 
-  // Returns the attributes value type builder code block, or None if it doesn't
-  // have one.
+  /// Returns the attributes value type builder code block, or None if it
+  /// doesn't have one.
   Optional<StringRef> getTypeBuilder() const;
 
   static bool classof(const AttrOrTypeDef *def);
 };
 
 //===----------------------------------------------------------------------===//
 // TypeDef
 //===----------------------------------------------------------------------===//

mlir/lib/TableGen/AttrOrTypeDef.cpp

 bool AttrDef::classof(const AttrOrTypeDef *def) {
   return def->getDef()->isSubClassOf("AttrDef");
 }
 
 //===----------------------------------------------------------------------===//
 // AttrOrTypeParameter
 //===----------------------------------------------------------------------===//
 
+template <typename InitT>
+auto AttrOrTypeParameter::getDefValue(StringRef name) const {
+  Optional<decltype(std::declval<InitT>().getValue())> result;
+  if (auto *param = dyn_cast<llvm::DefInit>(getDef()))
+    if (auto *init = param->getDef()->getValue(name))
+      if (auto *value = dyn_cast_or_null<InitT>(init->getValue()))
+        result = value->getValue();
+  return result;
+}
+
 StringRef AttrOrTypeParameter::getName() const {
   return def->getArgName(index)->getValue();
 }
 
 Optional<StringRef> AttrOrTypeParameter::getAllocator() const {
-  llvm::Init *parameterType = def->getArg(index);
-  if (isa<llvm::StringInit>(parameterType))
-    return Optional<StringRef>();
-  if (auto *param = dyn_cast<llvm::DefInit>(parameterType))
-    return param->getDef()->getValueAsOptionalString("allocator");
-  llvm::PrintFatalError("Parameters DAG arguments must be either strings or "
-                        "defs which inherit from AttrOrTypeParameter\n");
+  return getDefValue<llvm::StringInit>("allocator");
 }
 
 Optional<StringRef> AttrOrTypeParameter::getComparator() const {
-  llvm::Init *parameterType = def->getArg(index);
-  if (isa<llvm::StringInit>(parameterType))
-    return Optional<StringRef>();
-  if (auto *param = dyn_cast<llvm::DefInit>(parameterType))
-    return param->getDef()->getValueAsOptionalString("comparator");
-  llvm::PrintFatalError("Parameters DAG arguments must be either strings or "
-                        "defs which inherit from AttrOrTypeParameter\n");
+  return getDefValue<llvm::StringInit>("comparator");
 }
 
 StringRef AttrOrTypeParameter::getCppType() const {
-  auto *parameterType = def->getArg(index);
+  llvm::Init *parameterType = getDef();
   if (auto *stringType = dyn_cast<llvm::StringInit>(parameterType))
     return stringType->getValue();
   if (auto *param = dyn_cast<llvm::DefInit>(parameterType))
     return param->getDef()->getValueAsString("cppType");
   llvm::PrintFatalError(
       "Parameters DAG arguments must be either strings or defs "
       "which inherit from AttrOrTypeParameter\n");
 }
 
 StringRef AttrOrTypeParameter::getCppAccessorType() const {
-  if (auto *param = dyn_cast<llvm::DefInit>(def->getArg(index))) {
-    if (Optional<StringRef> type =
-            param->getDef()->getValueAsOptionalString("cppAccessorType"))
-      return *type;
-  }
-  return getCppType();
+  return getDefValue<llvm::StringInit>("cppAccessorType")
+      .getValueOr(getCppType());
 }
 
 StringRef AttrOrTypeParameter::getCppStorageType() const {
-  if (auto *param = dyn_cast<llvm::DefInit>(def->getArg(index))) {
-    if (auto type = param->getDef()->getValueAsOptionalString("cppStorageType"))
-      return *type;
-  }
-  return getCppType();
+  return getDefValue<llvm::StringInit>("cppStorageType")
+      .getValueOr(getCppType());
 }
 
 Optional<StringRef> AttrOrTypeParameter::getParser() const {
-  auto *parameterType = def->getArg(index);
-  if (auto *param = dyn_cast<llvm::DefInit>(parameterType)) {
-    if (auto parser = param->getDef()->getValueAsOptionalString("parser"))
-      return *parser;
-  }
-  return {};
+  return getDefValue<llvm::StringInit>("parser");
 }
 
 Optional<StringRef> AttrOrTypeParameter::getPrinter() const {
-  auto *parameterType = def->getArg(index);
-  if (auto *param = dyn_cast<llvm::DefInit>(parameterType)) {
-    if (auto printer = param->getDef()->getValueAsOptionalString("printer"))
-      return *printer;
-  }
-  return {};
+  return getDefValue<llvm::StringInit>("printer");
 }
 
 Optional<StringRef> AttrOrTypeParameter::getSummary() const {
-  auto *parameterType = def->getArg(index);
-  if (auto *param = dyn_cast<llvm::DefInit>(parameterType)) {
-    const auto *desc = param->getDef()->getValue("summary");
-    if (llvm::StringInit *ci = dyn_cast<llvm::StringInit>(desc->getValue()))
-      return ci->getValue();
-  }
-  return Optional<StringRef>();
+  return getDefValue<llvm::StringInit>("summary");
 }
 
 StringRef AttrOrTypeParameter::getSyntax() const {
-  auto *parameterType = def->getArg(index);
-  if (auto *stringType = dyn_cast<llvm::StringInit>(parameterType))
+  if (auto *stringType = dyn_cast<llvm::StringInit>(getDef()))
     return stringType->getValue();
-  if (auto *param = dyn_cast<llvm::DefInit>(parameterType)) {
-    const auto *syntax = param->getDef()->getValue("syntax");
-    if (syntax && isa<llvm::StringInit>(syntax->getValue()))
-      return cast<llvm::StringInit>(syntax->getValue())->getValue();
-    return getCppType();
-  }
-  llvm::PrintFatalError("Parameters DAG arguments must be either strings or "
-                        "defs which inherit from AttrOrTypeParameter");
+  return getDefValue<llvm::StringInit>("syntax").getValueOr(getCppType());
 }
 
-const llvm::Init *AttrOrTypeParameter::getDef() const {
-  return def->getArg(index);
+bool AttrOrTypeParameter::isOptional() const {
+  return getDefValue<llvm::BitInit>("isOptional").getValueOr(false);
 }
 
+llvm::Init *AttrOrTypeParameter::getDef() const { return def->getArg(index); }
+
 //===----------------------------------------------------------------------===//
 // AttributeSelfTypeParameter
 //===----------------------------------------------------------------------===//
 
 bool AttributeSelfTypeParameter::classof(const AttrOrTypeParameter *param) {
-  const llvm::Init *paramDef = param->getDef();
+  llvm::Init *paramDef = param->getDef();
   if (auto *paramDefInit = dyn_cast<llvm::DefInit>(paramDef))
     return paramDefInit->getDef()->isSubClassOf("AttributeSelfTypeParameter");
   return false;
 }

mlir/test/lib/Dialect/Test/TestTypeDefs.td

 include "TestDialect.td"
 include "TestAttrDefs.td"
 include "TestInterfaces.td"
 include "mlir/IR/BuiltinTypes.td"
 include "mlir/Interfaces/DataLayoutInterfaces.td"
 
 // All of the types will extend this class.
 class Test_Type<string name, list<Trait> traits = []>
     : TypeDef<Test_Dialect, name, traits>;
 
 def SimpleTypeA : Test_Type<"SimpleA"> {
   let mnemonic = "smpla";
 }
 
 // A more complex parameterized type.
 def CompoundTypeA : Test_Type<"CompoundA"> {
   let mnemonic = "cmpnd_a";
 
   // List of type parameters.
   let parameters = (
     ins
     "int":$widthOfSomething,
     "::mlir::Type":$oneType,
     // This is special syntax since ArrayRefs require allocation in the
     // constructor.
     ArrayRefParameter<
       "int", // The parameter C++ type.
       "An example of an array of ints" // Parameter description.
-      >: $arrayOfInts
+    >:$arrayOfInts
   );
 
   let extraClassDeclaration = [{
     struct SomeCppStruct {};
   }];
 }
 
 // A more complex and nested parameterized type.
TypeBuilder<(ins "unsigned":$width,
                      CArg<"SignednessSemantics", "Signless">:$signedness), [{
       return $_get($_ctxt, width, signedness);
     }]>
   ];
   let skipDefaultBuilders = 1;
 
   // The parser is defined here also.
   let parser = [{
-    if (parser.parseLess()) return Type();
+    if ($_parser.parseLess()) return Type();
     SignednessSemantics signedness;
-    if (parseSignedness($_parser, signedness)) return mlir::Type();
+    if (parseSignedness($_parser, signedness)) return Type();
     if ($_parser.parseComma()) return Type();
     int width;
     if ($_parser.parseInteger(width)) return Type();
     if ($_parser.parseGreater()) return Type();
-    ::mlir::Location loc = $_parser.getEncodedSourceLoc($_parser.getNameLoc());
+    Location loc = $_parser.getEncodedSourceLoc($_parser.getNameLoc());
     return getChecked(loc, loc.getContext(), width, signedness);
   }];
 
   // Any extra code one wants in the type's class declaration.
   let extraClassDeclaration = [{
     /// Signedness semantics.
     enum SignednessSemantics {
       Signless, /// No signedness semantics
let parameters = (
     "int":$v2,
     "int":$v3
   );
 
   let mnemonic = "struct_capture_all";
   let assemblyFormat = "`<` struct(params) `>`";
 }
 
+def TestTypeOptionalParam : Test_Type<"TestTypeOptionalParam"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a, "int":$b);
+  let mnemonic = "optional_param";
+  let assemblyFormat = "`<` $a `,` $b `>`";
+}
+
+def TestTypeOptionalParams : Test_Type<"TestTypeOptionalParams"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a,
+                        StringRefParameter<>:$b);
+  let mnemonic = "optional_params";
+  let assemblyFormat = "`<` params `>`";
+}
+
+def TestTypeOptionalParamsAfterRequired
+    : Test_Type<"TestTypeOptionalParamsAfterRequired"> {
+  let parameters = (ins StringRefParameter<>:$a,
+                        OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "optional_params_after";
+  let assemblyFormat = "`<` params `>`";
+}
+
+def TestTypeOptionalStruct : Test_Type<"TestTypeOptionalStruct"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a,
+                        StringRefParameter<>:$b);
+  let mnemonic = "optional_struct";
+  let assemblyFormat = "`<` struct(params) `>`";
+}
+
+def TestTypeAllOptionalParams : Test_Type<"TestTypeAllOptionalParams"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a,
+                        OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "all_optional_params";
+  let assemblyFormat = "`<` params `>`";
+}
+
+def TestTypeAllOptionalStruct : Test_Type<"TestTypeAllOptionalStruct"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a,
+                        OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "all_optional_struct";
+  let assemblyFormat = "`<` struct(params) `>`";
+}
+
+def TestTypeOptionalGroup : Test_Type<"TestTypeOptionalGroup"> {
+  let parameters = (ins "int":$a, OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "optional_group";
+  let assemblyFormat = "`<` (`(` $b^ `)`) : (`x`)? $a `>`";
+}
+
+def TestTypeOptionalGroupParams : Test_Type<"TestTypeOptionalGroupParams"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a,
+                        OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "optional_group_params";
+  let assemblyFormat = "`<` (`(` params^ `)`) : (`x`)? `>`";
+}
+
+def TestTypeOptionalGroupStruct : Test_Type<"TestTypeOptionalGroupStruct"> {
+  let parameters = (ins OptionalParameter<"mlir::Optional<int>">:$a,
+                        OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "optional_group_struct";
+  let assemblyFormat = "`<` (`(` struct(params)^ `)`) : (`x`)? `>`";
+}
+
 #endif // TEST_TYPEDEFS

mlir/test/lib/Dialect/Test/TestTypes.h

 struct FieldParser<test::CustomParam> {
   static FailureOr<test::CustomParam> parse(AsmParser &parser) {
     auto value = FieldParser<int>::parse(parser);
     if (failed(value))
       return failure();
     return test::CustomParam{value.getValue()};
   }
 };
+
 inline mlir::AsmPrinter &operator<<(mlir::AsmPrinter &printer,
                                     test::CustomParam param) {
   return printer << param.value;
 }
 
+/// Overload the attribute parameter parser for optional integers.
+template <>
+struct FieldParser<Optional<int>> {
+  static FailureOr<Optional<int>> parse(AsmParser &parser) {
+    Optional<int> value;
+    value.emplace();
+    OptionalParseResult result = parser.parseOptionalInteger(*value);
+    if (result.hasValue()) {
+      if (succeeded(*result))
+        return value;
+      return failure();
+    }
+    value.reset();
+    return value;
+  }
+};
 } // namespace mlir
 
 #include "TestTypeInterfaces.h.inc"
 
 #define GET_TYPEDEF_CLASSES
 #include "TestTypeDefs.h.inc"
 
 namespace test {

mlir/test/mlir-tblgen/attr-or-type-format-invalid.td

 // RUN: mlir-tblgen -gen-typedef-defs -I %S/../../include -asmformat-error-is-fatal=false %s 2>&1 | FileCheck %s
 
 include "mlir/IR/OpBase.td"
 
 def Test_Dialect : Dialect {
   let name = "TestDialect";
   let cppNamespace = "::test";
 }
 
 class InvalidType<string name, string asm> : TypeDef<Test_Dialect, name> {
   let mnemonic = asm;
 }
 
-/// Test format is missing a parameter capture.
+// Test format is missing a parameter capture.
 def InvalidTypeA : InvalidType<"InvalidTypeA", "invalid_a"> {
   let parameters = (ins "int":$v0, "int":$v1);
   // CHECK: format is missing reference to parameter: v1
   let assemblyFormat = "`<` $v0 `>`";
 }
 
-/// Test format has duplicate parameter captures.
+// Test format has duplicate parameter captures.
 def InvalidTypeB : InvalidType<"InvalidTypeB", "invalid_b"> {
   let parameters = (ins "int":$v0, "int":$v1);
   // CHECK: duplicate parameter 'v0'
   let assemblyFormat = "`<` $v0 `,` $v1 `,` $v0 `>`";
 }
 
-/// Test format has invalid syntax.
+// Test format has invalid syntax.
 def InvalidTypeC : InvalidType<"InvalidTypeC", "invalid_c"> {
   let parameters = (ins "int":$v0, "int":$v1);
   // CHECK: expected literal, variable, directive, or optional group
   let assemblyFormat = "`<` $v0, $v1 `>`";
 }
 
-/// Test struct directive has invalid syntax.
+// Test struct directive has invalid syntax.
 def InvalidTypeD : InvalidType<"InvalidTypeD", "invalid_d"> {
   let parameters = (ins "int":$v0);
   // CHECK: literals may only be used in the top-level section of the format
   // CHECK: expected a variable in `struct` argument list
   let assemblyFormat = "`<` struct($v0, `,`) `>`";
 }
 
-/// Test struct directive cannot capture zero parameters.
+// Test struct directive cannot capture zero parameters.
 def InvalidTypeE : InvalidType<"InvalidTypeE", "invalid_e"> {
   let parameters = (ins "int":$v0);
   // CHECK: `struct` argument list expected a variable or directive
   let assemblyFormat = "`<` struct() $v0 `>`";
 }
 
-/// Test capture parameter that does not exist.
+// Test capture parameter that does not exist.
 def InvalidTypeF : InvalidType<"InvalidTypeF", "invalid_f"> {
   let parameters = (ins "int":$v0);
   // CHECK: InvalidTypeF has no parameter named 'v1'
   let assemblyFormat = "`<` $v0 $v1 `>`";
 }
 
-/// Test duplicate capture of parameter in capture-all struct.
+// Test duplicate capture of parameter in capture-all struct.
 def InvalidTypeG : InvalidType<"InvalidTypeG", "invalid_g"> {
   let parameters = (ins "int":$v0, "int":$v1, "int":$v2);
   // CHECK: duplicate parameter 'v0'
   let assemblyFormat = "`<` struct(params) $v0 `>`";
 }
 
-/// Test capture-all struct duplicate capture.
+// Test capture-all struct duplicate capture.
 def InvalidTypeH : InvalidType<"InvalidTypeH", "invalid_h"> {
   let parameters = (ins "int":$v0, "int":$v1, "int":$v2);
   // CHECK: `params` captures duplicate parameter: v0
   let assemblyFormat = "`<` $v0 struct(params) `>`";
 }
 
-/// Test capture of parameter after `params` directive.
+// Test capture of parameter after `params` directive.
 def InvalidTypeI : InvalidType<"InvalidTypeI", "invalid_i"> {
   let parameters = (ins "int":$v0);
   // CHECK: duplicate parameter 'v0'
   let assemblyFormat = "`<` params $v0 `>`";
 }
+
+// Test `struct` with optional parameter followed by comma.
+def InvalidTypeJ : InvalidType<"InvalidTypeJ", "invalid_j"> {
+  let parameters = (ins OptionalParameter<"int">:$a, "int":$b);
+  // CHECK: directive with optional parameters cannot be followed by a comma literal
+  let assemblyFormat = "struct($a) `,` $b";
+}
+
+// Test `struct` in optional group must have all optional parameters.
+def InvalidTypeK : InvalidType<"InvalidTypeK", "invalid_k"> {
+  let parameters = (ins OptionalParameter<"int">:$a, "int":$b);
+  // CHECK: is only allowed in an optional group if all captured parameters are optional
+  let assemblyFormat = "(`(` struct(params)^ `)`)?";
+}
+
+// Test `struct` in optional group must have all optional parameters.
+def InvalidTypeL : InvalidType<"InvalidTypeL", "invalid_l"> {
+  let parameters = (ins OptionalParameter<"int">:$a, "int":$b);
+  // CHECK: directive allowed in optional group only if all parameters are optional
+  let assemblyFormat = "(`(` params^ `)`)?";
+}
+
+def InvalidTypeM : InvalidType<"InvalidTypeM", "invalid_m"> {
+  let parameters = (ins OptionalParameter<"int">:$a, "int":$b);
+  // CHECK: parameters in an optional group must be optional
+  let assemblyFormat = "(`(` $a^ `,` $b `)`)?";
+}
+
+def InvalidTypeN : InvalidType<"InvalidTypeN", "invalid_n"> {
+  let parameters = (ins OptionalParameter<"int">:$a);
+  // CHECK: optional group anchor must be a parameter or directive
+  let assemblyFormat = "(`(` $a `)`^)?";
+}

mlir/test/mlir-tblgen/attr-or-type-format-roundtrip.mlir

attributes {
   attr3 = #test.attr_params<42, 24>,
   // CHECK: #test.attr_with_type<i32, vector<4xi32>>
   attr4 = #test.attr_with_type<i32, vector<4xi32>>
 }
 
 // CHECK-LABEL: @test_roundtrip_default_parsers_struct
 // CHECK: !test.no_parser<255, [1, 2, 3, 4, 5], "foobar", 4>
 // CHECK: !test.struct_capture_all<v0 = 0, v1 = 1, v2 = 2, v3 = 3>
-func private @test_roundtrip_default_parsers_struct(!test.no_parser<255, [1, 2, 3, 4, 5], "foobar", 4>) -> !test.struct_capture_all<v3 = 3, v1 = 1, v2 = 2, v0 = 0>
+// CHECK: !test.optional_param<, 6>
+// CHECK: !test.optional_param<5, 6>
+// CHECK: !test.optional_params<"a">
+// CHECK: !test.optional_params<5, "a">
+// CHECK: !test.optional_struct<b = "a">
+// CHECK: !test.optional_struct<a = 5, b = "a">
+// CHECK: !test.optional_params_after<"a">
+// CHECK: !test.optional_params_after<"a", 5>
+// CHECK: !test.all_optional_params<>
+// CHECK: !test.all_optional_params<5>
+// CHECK: !test.all_optional_params<5, 6>
+// CHECK: !test.all_optional_struct<>
+// CHECK: !test.all_optional_struct<b = 5>
+// CHECK: !test.all_optional_struct<a = 5, b = 10>
+// CHECK: !test.optional_group<(5) 6>
+// CHECK: !test.optional_group<x 6>
+// CHECK: !test.optional_group_params<x>
+// CHECK: !test.optional_group_params<(5)>
+// CHECK: !test.optional_group_params<(5, 6)>
+// CHECK: !test.optional_group_struct<x>
+// CHECK: !test.optional_group_struct<(b = 5)>
+// CHECK: !test.optional_group_struct<(a = 10, b = 5)>
+func private @test_roundtrip_default_parsers_struct(
+  !test.no_parser<255, [1, 2, 3, 4, 5], "foobar", 4>
+) -> (
+  !test.struct_capture_all<v3 = 3, v1 = 1, v2 = 2, v0 = 0>,
+  !test.optional_param<, 6>,
+  !test.optional_param<5, 6>,
+  !test.optional_params<"a">,
+  !test.optional_params<5, "a">,
+  !test.optional_struct<b = "a">,
+  !test.optional_struct<b = "a", a = 5>,
+  !test.optional_params_after<"a">,
+  !test.optional_params_after<"a", 5>,
+  !test.all_optional_params<>,
+  !test.all_optional_params<5>,
+  !test.all_optional_params<5, 6>,
+  !test.all_optional_struct<>,
+  !test.all_optional_struct<b = 5>,
+  !test.all_optional_struct<b = 10, a = 5>,
+  !test.optional_group<(5) 6>,
+  !test.optional_group<x 6>,
+  !test.optional_group_params<x>,
+  !test.optional_group_params<(5)>,
+  !test.optional_group_params<(5, 6)>,
+  !test.optional_group_struct<x>,
+  !test.optional_group_struct<(b = 5)>,
+  !test.optional_group_struct<(b = 5, a = 10)>
+)

mlir/test/mlir-tblgen/attr-or-type-format.td

 
 def TypeParamB : TypeParameter<"TestParamD", "a type param D"> {
   let parser = "someFcnCall()";
   let printer = "myPrinter($_self)";
 }
 
 /// Check simple attribute parser and printer are generated correctly.
 
-// ATTR: ::mlir::Attribute TestAAttr::parse(::mlir::AsmParser &parser,
-// ATTR:                                    ::mlir::Type type) {
+// ATTR: ::mlir::Attribute TestAAttr::parse(::mlir::AsmParser &odsParser,
+// ATTR:                                    ::mlir::Type odsType) {
 // ATTR:   FailureOr<IntegerAttr> _result_value;
 // ATTR:   FailureOr<TestParamA> _result_complex;
-// ATTR:   if (parser.parseKeyword("hello"))
+// ATTR:   if (odsParser.parseKeyword("hello"))
 // ATTR:     return {};
-// ATTR:   if (parser.parseEqual())
+// ATTR:   if (odsParser.parseEqual())
 // ATTR:     return {};
-// ATTR:   _result_value = ::mlir::FieldParser<IntegerAttr>::parse(parser);
-// ATTR:   if (failed(_result_value))
+// ATTR:   _result_value = ::mlir::FieldParser<IntegerAttr>::parse(odsParser);
+// ATTR:   if (::mlir::failed(_result_value))
 // ATTR:     return {};
-// ATTR:   if (parser.parseComma())
+// ATTR:   if (odsParser.parseComma())
 // ATTR:     return {};
-// ATTR:   _result_complex = ::parseAttrParamA(parser, type);
-// ATTR:   if (failed(_result_complex))
+// ATTR:   _result_complex = ::parseAttrParamA(odsParser, odsType);
+// ATTR:   if (::mlir::failed(_result_complex))
 // ATTR:     return {};
-// ATTR:   if (parser.parseRParen())
+// ATTR:   if (odsParser.parseRParen())
 // ATTR:     return {};
-// ATTR:   return TestAAttr::get(parser.getContext(),
-// ATTR:                         _result_value.getValue(),
-// ATTR:                         _result_complex.getValue());
+// ATTR:   return TestAAttr::get(odsParser.getContext(),
+// ATTR:                         *_result_value,
+// ATTR:                         *_result_complex);
 // ATTR: }
 
-// ATTR: void TestAAttr::print(::mlir::AsmPrinter &printer) const {
-// ATTR:   printer << ' ' << "hello";
-// ATTR:   printer << ' ' << "=";
-// ATTR:   printer << ' ';
-// ATTR:   printer.printStrippedAttrOrType(getValue());
-// ATTR:   printer << ",";
-// ATTR:   printer << ' ';
-// ATTR:   ::printAttrParamA(printer, getComplex());
-// ATTR:   printer << ")";
+// ATTR: void TestAAttr::print(::mlir::AsmPrinter &odsPrinter) const {
+// ATTR:   odsPrinter << ' ' << "hello";
+// ATTR:   odsPrinter << ' ' << "=";
+// ATTR:   odsPrinter << ' ';
+// ATTR:   odsPrinter.printStrippedAttrOrType(getValue());
+// ATTR:   odsPrinter << ",";
+// ATTR:   odsPrinter << ' ';
+// ATTR:   ::printAttrParamA(odsPrinter, getComplex());
+// ATTR:   odsPrinter << ")";
 // ATTR: }
 
 def AttrA : TestAttr<"TestA"> {
   let parameters = (ins
       "IntegerAttr":$value,
       AttrParamA:$complex
   );
 
   let mnemonic = "attr_a";
   let assemblyFormat = "`hello` `=` $value `,` $complex `)`";
 }
 
 /// Test simple struct parser and printer are generated correctly.
 
-// ATTR: ::mlir::Attribute TestBAttr::parse(::mlir::AsmParser &parser,
-// ATTR:                                    ::mlir::Type type) {
+// ATTR: ::mlir::Attribute TestBAttr::parse(::mlir::AsmParser &odsParser,
+// ATTR:                                    ::mlir::Type odsType) {
 // ATTR:   bool _seen_v0 = false;
 // ATTR:   bool _seen_v1 = false;
-// ATTR:   for (unsigned _index = 0; _index < 2; ++_index) {
-// ATTR:     StringRef _paramKey;
-// ATTR:     if (parser.parseKeyword(&_paramKey))
-// ATTR:       return {};
-// ATTR:     if (parser.parseEqual())
+// ATTR:   const auto _loop_body = [&](::llvm::StringRef _paramKey) -> bool {
+// ATTR:     if (odsParser.parseEqual())
 // ATTR:       return {};
 // ATTR:     if (!_seen_v0 && _paramKey == "v0") {
 // ATTR:       _seen_v0 = true;
-// ATTR:       _result_v0 = ::parseAttrParamA(parser, type);
-// ATTR:       if (failed(_result_v0))
+// ATTR:       _result_v0 = ::parseAttrParamA(odsParser, odsType);
+// ATTR:       if (::mlir::failed(_result_v0))
 // ATTR:         return {};
 // ATTR:     } else if (!_seen_v1 && _paramKey == "v1") {
 // ATTR:       _seen_v1 = true;
-// ATTR:       _result_v1 = type ? ::parseAttrWithType(parser, type) : ::parseAttrWithout(parser);
-// ATTR:       if (failed(_result_v1))
+// ATTR:       _result_v1 = odsType ? ::parseAttrWithType(odsParser, odsType) :
+// ATTR-SAME:                         ::parseAttrWithout(odsParser);
+// ATTR:       if (::mlir::failed(_result_v1))
 // ATTR:         return {};
 // ATTR:     } else {
 // ATTR:       return {};
 // ATTR:     }
-// ATTR:     if ((_index != 2 - 1) && parser.parseComma())
+// ATTR:     return true;
+// ATTR:   }
+// ATTR:   for (unsigned odsStructIndex = 0; odsStructIndex < 2; ++odsStructIndex) {
+// ATTR:     StringRef _paramKey;
+// ATTR:     if (odsParser.parseKeyword(&_paramKey))
+// ATTR:       return {};
+// ATTR:     if (!_loop_body(_paramKey)) return {};
+// ATTR:     if ((odsStructIndex != 2 - 1) && odsParser.parseComma())
 // ATTR:       return {};
 // ATTR:   }
-// ATTR:   return TestBAttr::get(parser.getContext(),
-// ATTR:                         _result_v0.getValue(),
-// ATTR:                         _result_v1.getValue());
+// ATTR:   return TestBAttr::get(odsParser.getContext(),
+// ATTR:                         *_result_v0,
+// ATTR:                         *_result_v1);
 // ATTR: }
 
-// ATTR: void TestBAttr::print(::mlir::AsmPrinter &printer) const {
-// ATTR:   printer << "v0";
-// ATTR:   printer << ' ' << "=";
-// ATTR:   printer << ' ';
-// ATTR:   ::printAttrParamA(printer, getV0());
-// ATTR:   printer << ",";
-// ATTR:   printer << ' ' << "v1";
-// ATTR:   printer << ' ' << "=";
-// ATTR:   printer << ' ';
-// ATTR:   ::printAttrB(printer, getV1());
+// ATTR: void TestBAttr::print(::mlir::AsmPrinter &odsPrinter) const {
+// ATTR:   odsPrinter << "v0 = ";
+// ATTR:   ::printAttrParamA(odsPrinter, getV0());
+// ATTR:   odsPrinter << ", ";
+// ATTR:   odsPrinter << "v1 = ";
+// ATTR:   ::printAttrB(odsPrinter, getV1());
 // ATTR: }
 
 def AttrB : TestAttr<"TestB"> {
   let parameters = (ins
       AttrParamA:$v0,
       AttrParamB:$v1
   );
 
   let mnemonic = "attr_b";
   let assemblyFormat = "`{` struct($v0, $v1) `}`";
 }
 
 /// Test attribute with capture-all params has correct parser and printer.
 
-// ATTR: ::mlir::Attribute TestFAttr::parse(::mlir::AsmParser &parser,
-// ATTR:                                    ::mlir::Type type) {
+// ATTR: ::mlir::Attribute TestFAttr::parse(::mlir::AsmParser &odsParser,
+// ATTR:                                    ::mlir::Type odsType) {
 // ATTR:   ::mlir::FailureOr<int> _result_v0;
 // ATTR:   ::mlir::FailureOr<int> _result_v1;
-// ATTR:   _result_v0 = ::mlir::FieldParser<int>::parse(parser);
-// ATTR:   if (failed(_result_v0))
+// ATTR:   _result_v0 = ::mlir::FieldParser<int>::parse(odsParser);
+// ATTR:   if (::mlir::failed(_result_v0))
 // ATTR:     return {};
-// ATTR:   if (parser.parseComma())
+// ATTR:   if (odsParser.parseComma())
 // ATTR:     return {};
-// ATTR:   _result_v1 = ::mlir::FieldParser<int>::parse(parser);
-// ATTR:   if (failed(_result_v1))
+// ATTR:   _result_v1 = ::mlir::FieldParser<int>::parse(odsParser);
+// ATTR:   if (::mlir::failed(_result_v1))
 // ATTR:     return {};
-// ATTR:   return TestFAttr::get(parser.getContext(),
-// ATTR:     _result_v0.getValue(),
-// ATTR:     _result_v1.getValue());
-// ATTR: }
-
-// ATTR: void TestFAttr::print(::mlir::AsmPrinter &printer) const {
-// ATTR:   printer << ' ';
-// ATTR:   printer.printStrippedAttrOrType(getV0());
-// ATTR:   printer << ",";
-// ATTR:   printer << ' ';
-// ATTR:   printer.printStrippedAttrOrType(getV1());
+// ATTR:   return TestFAttr::get(odsParser.getContext(),
+// ATTR:     *_result_v0,
+// ATTR:     *_result_v1);
 // ATTR: }
 
 def AttrC : TestAttr<"TestF"> {
   let parameters = (ins "int":$v0, "int":$v1);
 
   let mnemonic = "attr_c";
   let assemblyFormat = "params";
 }
 
 /// Test type parser and printer that mix variables and struct are generated
 /// correctly.
 
-// TYPE: ::mlir::Type TestCType::parse(::mlir::AsmParser &parser) {
+// TYPE: ::mlir::Type TestCType::parse(::mlir::AsmParser &odsParser) {
 // TYPE:  FailureOr<IntegerAttr> _result_value;
 // TYPE:  FailureOr<TestParamC> _result_complex;
-// TYPE:  if (parser.parseKeyword("foo"))
+// TYPE:  if (odsParser.parseKeyword("foo"))
 // TYPE:    return {};
-// TYPE:  if (parser.parseComma())
+// TYPE:  if (odsParser.parseComma())
 // TYPE:    return {};
-// TYPE:  if (parser.parseColon())
+// TYPE:  if (odsParser.parseColon())
 // TYPE:    return {};
-// TYPE:  if (parser.parseKeyword("bob"))
+// TYPE:  if (odsParser.parseKeyword("bob"))
 // TYPE:    return {};
-// TYPE:  if (parser.parseKeyword("bar"))
+// TYPE:  if (odsParser.parseKeyword("bar"))
 // TYPE:    return {};
-// TYPE:  _result_value = ::mlir::FieldParser<IntegerAttr>::parse(parser);
-// TYPE:  if (failed(_result_value))
+// TYPE:  _result_value = ::mlir::FieldParser<IntegerAttr>::parse(odsParser);
+// TYPE:  if (::mlir::failed(_result_value))
 // TYPE:    return {};
 // TYPE:  bool _seen_complex = false;
-// TYPE:  for (unsigned _index = 0; _index < 1; ++_index) {
-// TYPE:    StringRef _paramKey;
-// TYPE:    if (parser.parseKeyword(&_paramKey))
-// TYPE:      return {};
+// TYPE:  const auto _loop_body = [&](::llvm::StringRef _paramKey) -> bool {
 // TYPE:    if (!_seen_complex && _paramKey == "complex") {
 // TYPE:      _seen_complex = true;
-// TYPE:      _result_complex = ::parseTypeParamC(parser);
-// TYPE:      if (failed(_result_complex))
+// TYPE:      _result_complex = ::parseTypeParamC(odsParser);
+// TYPE:      if (::mlir::failed(_result_complex))
 // TYPE:        return {};
 // TYPE:    } else {
 // TYPE:      return {};
 // TYPE:    }
-// TYPE:    if ((_index != 1 - 1) && parser.parseComma())
+// TYPE:    return true;
+// TYPE:  }
+// TYPE:  for (unsigned odsStructIndex = 0; odsStructIndex < 1; ++odsStructIndex) {
+// TYPE:    StringRef _paramKey;
+// TYPE:    if (odsParser.parseKeyword(&_paramKey))
+// TYPE:      return {};
+// TYPE:    if (!_loop_body(_paramKey)) return {};
+// TYPE:    if ((odsStructIndex != 1 - 1) && odsParser.parseComma())
 // TYPE:      return {};
 // TYPE:  }
-// TYPE:  if (parser.parseRParen())
+// TYPE:  if (odsParser.parseRParen())
 // TYPE:    return {};
 // TYPE:  }
 
-// TYPE: void TestCType::print(::mlir::AsmPrinter &printer) const {
-// TYPE:   printer << ' ' << "foo";
-// TYPE:   printer << ",";
-// TYPE:   printer << ' ' << ":";
-// TYPE:   printer << ' ' << "bob";
-// TYPE:   printer << ' ' << "bar";
-// TYPE:   printer << ' ';
-// TYPE:   printer.printStrippedAttrOrType(getValue());
-// TYPE:   printer << ' ' << "complex";
-// TYPE:   printer << ' ' << "=";
-// TYPE:   printer << ' ';
-// TYPE:   printer << getComplex();
-// TYPE:   printer << ")";
+// TYPE: void TestCType::print(::mlir::AsmPrinter &odsPrinter) const {
+// TYPE:   odsPrinter << ' ' << "foo";
+// TYPE:   odsPrinter << ",";
+// TYPE:   odsPrinter << ' ' << ":";
+// TYPE:   odsPrinter << ' ' << "bob";
+// TYPE:   odsPrinter << ' ' << "bar";
+// TYPE:   odsPrinter << ' ';
+// TYPE:   odsPrinter.printStrippedAttrOrType(getValue());
+// TYPE:   odsPrinter << "complex = ";
+// TYPE:   odsPrinter << getComplex();
+// TYPE:   odsPrinter << ")";
 // TYPE: }
 
 def TypeA : TestType<"TestC"> {
   let parameters = (ins
       "IntegerAttr":$value,
       TypeParamA:$complex
   );
 
   let mnemonic = "type_c";
   let assemblyFormat = "`foo` `,` `:` `bob` `bar` $value struct($complex) `)`";
 }
 
 /// Test type parser and printer with mix of variables and struct are generated
 /// correctly.
 
-// TYPE: ::mlir::Type TestDType::parse(::mlir::AsmParser &parser) {
-// TYPE:   _result_v0 = ::parseTypeParamC(parser);
-// TYPE:   if (failed(_result_v0))
+// TYPE: ::mlir::Type TestDType::parse(::mlir::AsmParser &odsParser) {
+// TYPE:   _result_v0 = ::parseTypeParamC(odsParser);
+// TYPE:   if (::mlir::failed(_result_v0))
 // TYPE:     return {};
 // TYPE:   bool _seen_v1 = false;
 // TYPE:   bool _seen_v2 = false;
-// TYPE:   for (unsigned _index = 0; _index < 2; ++_index) {
-// TYPE:     StringRef _paramKey;
-// TYPE:     if (parser.parseKeyword(&_paramKey))
-// TYPE:       return {};
-// TYPE:     if (parser.parseEqual())
+// TYPE:   const auto _loop_body = [&](::llvm::StringRef _paramKey) -> bool {
+// TYPE:     if (odsParser.parseEqual())
 // TYPE:       return {};
 // TYPE:     if (!_seen_v1 && _paramKey == "v1") {
 // TYPE:       _seen_v1 = true;
 // TYPE:       _result_v1 = someFcnCall();
-// TYPE:       if (failed(_result_v1))
+// TYPE:       if (::mlir::failed(_result_v1))
 // TYPE:         return {};
 // TYPE:     } else if (!_seen_v2 && _paramKey == "v2") {
 // TYPE:       _seen_v2 = true;
-// TYPE:       _result_v2 = ::parseTypeParamC(parser);
-// TYPE:       if (failed(_result_v2))
+// TYPE:       _result_v2 = ::parseTypeParamC(odsParser);
+// TYPE:       if (::mlir::failed(_result_v2))
 // TYPE:         return {};
 // TYPE:     } else  {
 // TYPE:       return {};
 // TYPE:     }
-// TYPE:     if ((_index != 2 - 1) && parser.parseComma())
+// TYPE:     return true;
+// TYPE:   }
+// TYPE:   for (unsigned odsStructIndex = 0; odsStructIndex < 2; ++odsStructIndex) {
+// TYPE:     StringRef _paramKey;
+// TYPE:     if (odsParser.parseKeyword(&_paramKey))
+// TYPE:       return {};
+// TYPE:     if (!_loop_body(_paramKey)) return {};
+// TYPE:     if ((odsStructIndex != 2 - 1) && odsParser.parseComma())
 // TYPE:       return {};
 // TYPE:   }
 // TYPE:   _result_v3 = someFcnCall();
-// TYPE:   if (failed(_result_v3))
+// TYPE:   if (::mlir::failed(_result_v3))
 // TYPE:     return {};
-// TYPE:   return TestDType::get(parser.getContext(),
-// TYPE:                         _result_v0.getValue(),
-// TYPE:                         _result_v1.getValue(),
-// TYPE:                         _result_v2.getValue(),
-// TYPE:                         _result_v3.getValue());
+// TYPE:   return TestDType::get(odsParser.getContext(),
+// TYPE:                         *_result_v0,
+// TYPE:                         *_result_v1,
+// TYPE:                         *_result_v2,
+// TYPE:                         *_result_v3);
 // TYPE: }
 
-// TYPE: void TestDType::print(::mlir::AsmPrinter &printer) const {
-// TYPE:   printer << getV0();
+// TYPE: void TestDType::print(::mlir::AsmPrinter &odsPrinter) const {
+// TYPE:   odsPrinter << getV0();
 // TYPE:   myPrinter(getV1());
-// TYPE:   printer << ' ' << "v2";
-// TYPE:   printer << ' ' << "=";
-// TYPE:   printer << ' ';
-// TYPE:   printer << getV2();
+// TYPE:   odsPrinter << "v2 = ";
+// TYPE:   odsPrinter << getV2();
 // TYPE:   myPrinter(getV3());
 // TYPE: }
 
 def TypeB : TestType<"TestD"> {
   let parameters = (ins
       TypeParamA:$v0,
       TypeParamB:$v1,
       TypeParamA:$v2,
       TypeParamB:$v3
   );
 
   let mnemonic = "type_d";
   let assemblyFormat = "`<` `foo` `:` $v0 `,` struct($v1, $v2) `,` $v3 `>`";
 }
 
 /// Type test with two struct directives has correctly generated parser and
 /// printer.
 
-// TYPE: ::mlir::Type TestEType::parse(::mlir::AsmParser &parser) {
+// TYPE: ::mlir::Type TestEType::parse(::mlir::AsmParser &odsParser) {
 // TYPE:   FailureOr<IntegerAttr> _result_v0;
 // TYPE:   FailureOr<IntegerAttr> _result_v1;
 // TYPE:   FailureOr<IntegerAttr> _result_v2;
 // TYPE:   FailureOr<IntegerAttr> _result_v3;
 // TYPE:   bool _seen_v0 = false;
 // TYPE:   bool _seen_v2 = false;
-// TYPE:   for (unsigned _index = 0; _index < 2; ++_index) {
-// TYPE:     StringRef _paramKey;
-// TYPE:     if (parser.parseKeyword(&_paramKey))
-// TYPE:       return {};
-// TYPE:     if (parser.parseEqual())
+// TYPE:   const auto _loop_body = [&](::llvm::StringRef _paramKey) -> bool {
+// TYPE:     if (odsParser.parseEqual())
 // TYPE:       return {};
 // TYPE:     if (!_seen_v0 && _paramKey == "v0") {
 // TYPE:       _seen_v0 = true;
-// TYPE:       _result_v0 = ::mlir::FieldParser<IntegerAttr>::parse(parser);
-// TYPE:       if (failed(_result_v0))
+// TYPE:       _result_v0 = ::mlir::FieldParser<IntegerAttr>::parse(odsParser);
+// TYPE:       if (::mlir::failed(_result_v0))
 // TYPE:         return {};
 // TYPE:     } else if (!_seen_v2 && _paramKey == "v2") {
 // TYPE:       _seen_v2 = true;
-// TYPE:       _result_v2 = ::mlir::FieldParser<IntegerAttr>::parse(parser);
-// TYPE:       if (failed(_result_v2))
+// TYPE:       _result_v2 = ::mlir::FieldParser<IntegerAttr>::parse(odsParser);
+// TYPE:       if (::mlir::failed(_result_v2))
 // TYPE:         return {};
 // TYPE:     } else  {
 // TYPE:       return {};
 // TYPE:     }
-// TYPE:     if ((_index != 2 - 1) && parser.parseComma())
+// TYPE:     return true;
+// TYPE:   }
+// TYPE:   for (unsigned odsStructIndex = 0; odsStructIndex < 2; ++odsStructIndex) {
+// TYPE:     StringRef _paramKey;
+// TYPE:     if (odsParser.parseKeyword(&_paramKey))
+// TYPE:       return {};
+// TYPE:     if (!_loop_body(_paramKey)) return {};
+// TYPE:     if ((odsStructIndex != 2 - 1) && odsParser.parseComma())
 // TYPE:       return {};
 // TYPE:   }
 // TYPE:   bool _seen_v1 = false;
 // TYPE:   bool _seen_v3 = false;
-// TYPE:   for (unsigned _index = 0; _index < 2; ++_index) {
-// TYPE:     StringRef _paramKey;
-// TYPE:     if (parser.parseKeyword(&_paramKey))
-// TYPE:       return {};
-// TYPE:     if (parser.parseEqual())
+// TYPE:   const auto _loop_body = [&](::llvm::StringRef _paramKey) -> bool {
+// TYPE:     if (odsParser.parseEqual())
 // TYPE:       return {};
 // TYPE:     if (!_seen_v1 && _paramKey == "v1") {
 // TYPE:       _seen_v1 = true;
-// TYPE:       _result_v1 = ::mlir::FieldParser<IntegerAttr>::parse(parser);
-// TYPE:       if (failed(_result_v1))
+// TYPE:       _result_v1 = ::mlir::FieldParser<IntegerAttr>::parse(odsParser);
+// TYPE:       if (::mlir::failed(_result_v1))
 // TYPE:         return {};
 // TYPE:     } else if (!_seen_v3 && _paramKey == "v3") {
 // TYPE:       _seen_v3 = true;
-// TYPE:       _result_v3 = ::mlir::FieldParser<IntegerAttr>::parse(parser);
-// TYPE:       if (failed(_result_v3))
+// TYPE:       _result_v3 = ::mlir::FieldParser<IntegerAttr>::parse(odsParser);
+// TYPE:       if (::mlir::failed(_result_v3))
 // TYPE:         return {};
 // TYPE:     } else  {
 // TYPE:       return {};
 // TYPE:     }
-// TYPE:     if ((_index != 2 - 1) && parser.parseComma())
+// TYPE:     return true;
+// TYPE:   }
+// TYPE:   for (unsigned odsStructIndex = 0; odsStructIndex < 2; ++odsStructIndex) {
+// TYPE:     StringRef _paramKey;
+// TYPE:     if (odsParser.parseKeyword(&_paramKey))
+// TYPE:       return {};
+// TYPE:     if (!_loop_body(_paramKey)) return {};
+// TYPE:     if ((odsStructIndex != 2 - 1) && odsParser.parseComma())
 // TYPE:       return {};
 // TYPE:   }
-// TYPE:   return TestEType::get(parser.getContext(),
-// TYPE:     _result_v0.getValue(),
-// TYPE:     _result_v1.getValue(),
-// TYPE:     _result_v2.getValue(),
-// TYPE:     _result_v3.getValue());
-// TYPE: }
-
-// TYPE: void TestEType::print(::mlir::AsmPrinter &printer) const {
-// TYPE:   printer << "v0";
-// TYPE:   printer << ' ' << "=";
-// TYPE:   printer << ' ';
-// TYPE:   printer.printStrippedAttrOrType(getV0());
-// TYPE:   printer << ",";
-// TYPE:   printer << ' ' << "v2";
-// TYPE:   printer << ' ' << "=";
-// TYPE:   printer << ' ';
-// TYPE:   printer.printStrippedAttrOrType(getV2());
-// TYPE:   printer << "v1";
-// TYPE:   printer << ' ' << "=";
-// TYPE:   printer << ' ';
-// TYPE:   printer.printStrippedAttrOrType(getV1());
-// TYPE:   printer << ",";
-// TYPE:   printer << ' ' << "v3";
-// TYPE:   printer << ' ' << "=";
-// TYPE:   printer << ' ';
-// TYPE:   printer.printStrippedAttrOrType(getV3());
+// TYPE:   return TestEType::get(odsParser.getContext(),
+// TYPE:     *_result_v0,
+// TYPE:     *_result_v1,
+// TYPE:     *_result_v2,
+// TYPE:     *_result_v3);
+// TYPE: }
+
+// TYPE: void TestEType::print(::mlir::AsmPrinter &odsPrinter) const {
+// TYPE:   odsPrinter << "v0 = ";
+// TYPE:   odsPrinter.printStrippedAttrOrType(getV0());
+// TYPE:   odsPrinter << ", ";
+// TYPE:   odsPrinter << "v2 = ";
+// TYPE:   odsPrinter.printStrippedAttrOrType(getV2());
+// TYPE:   odsPrinter << ", ";
+// TYPE:   odsPrinter << "v1 = ";
+// TYPE:   odsPrinter.printStrippedAttrOrType(getV1());
+// TYPE:   odsPrinter << ", ";
+// TYPE:   odsPrinter << "v3 = ";
+// TYPE:   odsPrinter.printStrippedAttrOrType(getV3());
 // TYPE: }
 
 def TypeC : TestType<"TestE"> {
   let parameters = (ins
       "IntegerAttr":$v0,
       "IntegerAttr":$v1,
       "IntegerAttr":$v2,
       "IntegerAttr":$v3
   );
 
   let mnemonic = "type_e";
   let assemblyFormat = "`{` struct($v0, $v2) `}` `{` struct($v1, $v3) `}`";
 }
+
+// TYPE: void TestFType::print(::mlir::AsmPrinter &odsPrinter) const {
+// TYPE if (getA()) {
+// TYPE   printer << ' ';
+// TYPE   printer.printStrippedAttrOrType(getA());
+def TypeD : TestType<"TestF"> {
+  let parameters = (ins OptionalParameter<"int">:$a);
+  let mnemonic = "type_f";
+  let assemblyFormat = "$a";
+}
+
+// TYPE: ::mlir::Type TestGType::parse(::mlir::AsmParser &odsParser) {
+// TYPE:   if (::mlir::failed(_result_a))
+// TYPE:     return {};
+// TYPE:   if (::mlir::succeeded(_result_a) && *_result_a)
+// TYPE:     if (odsParser.parseComma())
+// TYPE:       return {};
+
+// TYPE: if (getA())
+// TYPE:   odsPrinter.printStrippedAttrOrType(getA());
+// TYPE: odsPrinter << ", ";
+// TYPE: odsPrinter.printStrippedAttrOrType(getB());
+
+def TypeE : TestType<"TestG"> {
+  let parameters = (ins OptionalParameter<"int">:$a, "int":$b);
+  let mnemonic = "type_g";
+  let assemblyFormat = "params";
+}
+
+
+// TYPE: ::mlir::Type TestHType::parse(::mlir::AsmParser &odsParser) {
+// TYPE:   do {
+// TYPE:     if (!_loop_body(_paramKey)) return {};
+// TYPE:   } while(!odsParser.parseOptionalComma());
+// TYPE:   if (!_seen_b)
+// TYPE:     return {};
+
+// TYPE: void TestHType::print(::mlir::AsmPrinter &odsPrinter) const {
+// TYPE:   if (getA()) {
+// TYPE:     odsPrinter << "a = ";
+// TYPE:     odsPrinter.printStrippedAttrOrType(getA());
+// TYPE:     odsPrinter << ", ";
+// TYPE:   }
+
+def TypeF : TestType<"TestH"> {
+  let parameters = (ins OptionalParameter<"int">:$a, "int":$b);
+  let mnemonic = "type_h";
+  let assemblyFormat = "struct(params)";
+}
+
+
+// TYPE: do {
+// TYPE:   _result_a = ::mlir::FieldParser<int>::parse(odsParser);
+// TYPE:   if (::mlir::failed(_result_a))
+// TYPE:     return {};
+// TYPE:   if (odsParser.parseOptionalComma()) break;
+// TYPE:   _result_b = ::mlir::FieldParser<int>::parse(odsParser);
+// TYPE:   if (::mlir::failed(_result_b))
+// TYPE:     return {};
+// TYPE: } while(false);
+
+def TypeG : TestType<"TestI"> {
+  let parameters = (ins "int":$a, OptionalParameter<"int">:$b);
+  let mnemonic = "type_i";
+  let assemblyFormat = "params";
+}
+
+// TYPE: ::mlir::Type TestJType::parse(::mlir::AsmParser &odsParser) {
+// TYPE:   if (odsParser.parseOptionalLParen()) {
+// TYPE:     if (odsParser.parseKeyword("x")) return {};
+// TYPE:   } else {
+// TYPE:     _result_b = ::mlir::FieldParser<int>::parse(odsParser);
+// TYPE:     if (::mlir::failed(_result_b))
+// TYPE:       return {};
+// TYPE:     if (odsParser.parseRParen()) return {};
+// TYPE:   }
+// TYPE:   _result_a = ::mlir::FieldParser<int>::parse(odsParser);
+// TYPE:   if (::mlir::failed(_result_a))
+// TYPE:     return {};
+
+// TYPE: void TestJType::print(::mlir::AsmPrinter &odsPrinter) const {
+// TYPE:   if (getB()) {
+// TYPE:     odsPrinter << "(";
+// TYPE:     if (getB())
+// TYPE:       odsPrinter.printStrippedAttrOrType(getB());
+// TYPE:     odsPrinter << ")";
+// TYPE:   } else {
+// TYPE:     odsPrinter << ' ' << "x";
+// TYPE:   }
+// TYPE:   odsPrinter.printStrippedAttrOrType(getA());
+
+def TypeH : TestType<"TestJ"> {
+  let parameters = (ins "int":$a, OptionalParameter<"int">:$b);
+  let mnemonic = "type_j";
+  let assemblyFormat = "(`(` $b^ `)`) : (`x`)? $a";
+}

mlir/test/mlir-tblgen/attrdefs.td

 
 // DECL-LABEL: class CompoundAAttr : public ::mlir::Attribute
 // DECL: static CompoundAAttr getChecked(::llvm::function_ref<::mlir::InFlightDiagnostic()> emitError, ::mlir::MLIRContext *context, int widthOfSomething, ::test::SimpleTypeA exampleTdType, ::llvm::APFloat apFloat, ::llvm::ArrayRef<int> dims, ::mlir::Type inner);
 // DECL: static ::mlir::LogicalResult verify(::llvm::function_ref<::mlir::InFlightDiagnostic()> emitError, int widthOfSomething, ::test::SimpleTypeA exampleTdType, ::llvm::APFloat apFloat, ::llvm::ArrayRef<int> dims, ::mlir::Type inner);
 // DECL: static constexpr ::llvm::StringLiteral getMnemonic() {
 // DECL:   return {"cmpnd_a"};
 // DECL: }
 // DECL: static ::mlir::Attribute parse(
-// DECL-SAME: ::mlir::AsmParser &parser, ::mlir::Type type);
-// DECL: void print(::mlir::AsmPrinter &printer) const;
+// DECL-SAME: ::mlir::AsmParser &odsParser, ::mlir::Type odsType);
+// DECL: void print(::mlir::AsmPrinter &odsPrinter) const;
 // DECL: int getWidthOfSomething() const;
 // DECL: ::test::SimpleTypeA getExampleTdType() const;
 // DECL: ::llvm::APFloat getApFloat() const;
 
 // Check that AttributeSelfTypeParameter is handled properly.
 // DEF-LABEL: struct CompoundAAttrStorage
 // DEF: CompoundAAttrStorage(
 // DEF-SAME: : ::mlir::AttributeStorage(inner),
let parameters = (
       StringRefParameter<"Label for index">:$label
     );
 
 // DECL-LABEL: class IndexAttr : public ::mlir::Attribute
 // DECL: static constexpr ::llvm::StringLiteral getMnemonic() {
 // DECL:   return {"index"};
 // DECL: }
 // DECL: static ::mlir::Attribute parse(
-// DECL-SAME: ::mlir::AsmParser &parser, ::mlir::Type type);
-// DECL: void print(::mlir::AsmPrinter &printer) const;
+// DECL-SAME: ::mlir::AsmParser &odsParser, ::mlir::Type odsType);
+// DECL: void print(::mlir::AsmPrinter &odsPrinter) const;
 }
 
 def D_SingleParameterAttr : TestAttr<"SingleParameter"> {
   let parameters = (
     ins
     "int": $num
   );
 // DECL-LABEL: struct SingleParameterAttrStorage;

mlir/test/mlir-tblgen/typedefs.td

def B_CompoundTypeA : TestType<"CompoundA"> {
   let genVerifyDecl = 1;
 
 // DECL-LABEL: class CompoundAType : public ::mlir::Type
 // DECL: static CompoundAType getChecked(::llvm::function_ref<::mlir::InFlightDiagnostic()> emitError, ::mlir::MLIRContext *context, int widthOfSomething, ::test::SimpleTypeA exampleTdType, SomeCppStruct exampleCppType, ::llvm::ArrayRef<int> dims, ::mlir::Type inner);
 // DECL: static ::mlir::LogicalResult verify(::llvm::function_ref<::mlir::InFlightDiagnostic()> emitError, int widthOfSomething, ::test::SimpleTypeA exampleTdType, SomeCppStruct exampleCppType, ::llvm::ArrayRef<int> dims, ::mlir::Type inner);
 // DECL: static constexpr ::llvm::StringLiteral getMnemonic() {
 // DECL:   return {"cmpnd_a"};
 // DECL: }
-// DECL: static ::mlir::Type parse(::mlir::AsmParser &parser);
-// DECL: void print(::mlir::AsmPrinter &printer) const;
+// DECL: static ::mlir::Type parse(::mlir::AsmParser &odsParser);
+// DECL: void print(::mlir::AsmPrinter &odsPrinter) const;
 // DECL: int getWidthOfSomething() const;
 // DECL: ::test::SimpleTypeA getExampleTdType() const;
 // DECL: SomeCppStruct getExampleCppType() const;
 }
 
 def C_IndexType : TestType<"Index"> {
     let mnemonic = "index";
 
     let parameters = (
       ins
       StringRefParameter<"Label for index">:$label
     );
 
 // DECL-LABEL: class IndexType : public ::mlir::Type
 // DECL: static constexpr ::llvm::StringLiteral getMnemonic() {
 // DECL:   return {"index"};
 // DECL: }
-// DECL: static ::mlir::Type parse(::mlir::AsmParser &parser);
-// DECL: void print(::mlir::AsmPrinter &printer) const;
+// DECL: static ::mlir::Type parse(::mlir::AsmParser &odsParser);
+// DECL: void print(::mlir::AsmPrinter &odsPrinter) const;
 }
 
 def D_SingleParameterType : TestType<"SingleParameter"> {
   let parameters = (
     ins
     "int": $num
   );
 // DECL-LABEL: struct SingleParameterTypeStorage;

mlir/tools/mlir-tblgen/AttrOrTypeDefGen.cpp

void DefGen::emitParserPrinter() {
   mnemonic->body().indent() << strfmt("return {\"{0}\"};", *def.getMnemonic());
   // Declare the parser and printer, if needed.
   if (!def.needsParserPrinter() && !def.hasGeneratedParser() &&
       !def.hasGeneratedPrinter())
     return;
 
   // Declare the parser.
   SmallVector<MethodParameter> parserParams;
-  parserParams.emplace_back("::mlir::AsmParser &", "parser");
+  parserParams.emplace_back("::mlir::AsmParser &", "odsParser");
   if (isa<AttrDef>(&def))
-    parserParams.emplace_back("::mlir::Type", "type");
+    parserParams.emplace_back("::mlir::Type", "odsType");
   auto *parser = defCls.addMethod(
       strfmt("::mlir::{0}", valueType), "parse",
       def.hasGeneratedParser() ? Method::Static : Method::StaticDeclaration,
       std::move(parserParams));
   // Declare the printer.
   auto props =
       def.hasGeneratedPrinter() ? Method::Const : Method::ConstDeclaration;
   Method *printer =
       defCls.addMethod("void", "print", props,
-                       MethodParameter("::mlir::AsmPrinter &", "printer"));
+                       MethodParameter("::mlir::AsmPrinter &", "odsPrinter"));
   // Emit the bodies.
   emitParserPrinterBody(parser->body(), printer->body());
 }
 
 void DefGen::emitAccessors() {
   for (auto &param : params) {
     Method *m = defCls.addMethod(
         param.getCppAccessorType(), getParameterAccessorName(param.getName()),
PrintFatalError(def.getLoc(),
                         ": the generated printer from 'assemblyFormat' "
                         "requires 'genAccessors' to be true");
   }
 
   // Generate the parser and printer bodies.
   if (asmFormat)
     return generateAttrOrTypeFormat(def, parser, printer);
 
-  FmtContext ctx = FmtContext(
-      {{"_parser", "parser"}, {"_printer", "printer"}, {"_type", "type"}});
+  FmtContext ctx = FmtContext({{"_parser", "odsParser"},
+                               {"_printer", "odsPrinter"},
+                               {"_type", "odsType"}});
   if (parserCode) {
-    ctx.addSubst("_ctxt", "parser.getContext()");
+    ctx.addSubst("_ctxt", "odsParser.getContext()");
     parser.indent().getStream().printReindented(tgfmt(*parserCode, &ctx).str());
   }
   if (printerCode) {
-    ctx.addSubst("_ctxt", "printer.getContext()");
+    ctx.addSubst("_ctxt", "odsPrinter.getContext()");
     printer.indent().getStream().printReindented(
         tgfmt(*printerCode, &ctx).str());
   }
 }
 
 //===----------------------------------------------------------------------===//
 // Interface Method Emission
 

mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp

 #include "mlir/Support/LLVM.h"
 #include "mlir/Support/LogicalResult.h"
 #include "mlir/TableGen/AttrOrTypeDef.h"
 #include "mlir/TableGen/Format.h"
 #include "mlir/TableGen/GenInfo.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/ADT/TypeSwitch.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/TableGenBackend.h"
 
 using namespace mlir;
 using namespace mlir::tblgen;
 
 using llvm::formatv;
public:
 
   /// Indicate if this variable is printed "qualified" (that is it is
   /// prefixed with the `#dialect.mnemonic`).
   bool shouldBeQualified() { return shouldBeQualifiedFlag; }
   void setShouldBeQualified(bool qualified = true) {
     shouldBeQualifiedFlag = qualified;
   }
 
+  /// Returns true if the element contains an optional parameter.
+  bool isOptional() const { return param.isOptional(); }
+
+  /// Returns the name of the parameter.
+  StringRef getName() const { return param.getName(); }
+
 private:
   bool shouldBeQualifiedFlag = false;
   AttrOrTypeParameter param;
 };
 
+/// Shorthand functions that can be used with ranged-based conditions.
+static bool paramIsOptional(ParameterElement *el) { return el->isOptional(); }
+static bool paramNotOptional(ParameterElement *el) { return !el->isOptional(); }
+
 /// Base class for a directive that contains references to multiple variables.
 template <DirectiveElement::Kind DirectiveKind>
 class ParamsDirectiveBase : public DirectiveElementBase<DirectiveKind> {
 public:
   using Base = ParamsDirectiveBase<DirectiveKind>;
 
-  ParamsDirectiveBase(std::vector<FormatElement *> &&params)
+  ParamsDirectiveBase(std::vector<ParameterElement *> &&params)
       : params(std::move(params)) {}
 
   /// Get the parameters contained in this directive.
-  auto getParams() const {
-    return llvm::map_range(params, [](FormatElement *el) {
-      return cast<ParameterElement>(el)->getParam();
-    });
-  }
+  ArrayRef<ParameterElement *> getParams() const { return params; }
 
   /// Get the number of parameters.
   unsigned getNumParams() const { return params.size(); }
 
   /// Take all of the parameters from this directive.
-  std::vector<FormatElement *> takeParams() { return std::move(params); }
+  std::vector<ParameterElement *> takeParams() { return std::move(params); }
+
+  /// Returns true if there are optional parameters present.
+  bool hasOptionalParams() const {
+    return llvm::any_of(getParams(), paramIsOptional);
+  }
 
 private:
   /// The parameters captured by this directive.
-  std::vector<FormatElement *> params;
+  std::vector<ParameterElement *> params;
 };
 
 /// This class represents a `params` directive that refers to all parameters
 /// of an attribute or type. When used as a top-level directive, it generates
 /// a format of the form:
 ///
 ///   (param-value (`,` param-value)*)?
 ///

 
 /// Qualified printer for attribute or type parameters: it does not elide
 /// dialect and mnemonic.
 static const char *const qualifiedParameterPrinter = "$_printer << $_self";
 
 /// Print an error when failing to parse an element.
 ///
 /// $0: The parameter C++ class name.
-static const char *const parseErrorStr =
+static const char *const parserErrorStr =
     "$_parser.emitError($_parser.getCurrentLocation(), ";
 
-/// Loop declaration for struct parser.
-///
-/// $0: Number of expected parameters.
-static const char *const structParseLoopStart = R"(
-  for (unsigned _index = 0; _index < $0; ++_index) {
-    StringRef _paramKey;
-    if ($_parser.parseKeyword(&_paramKey)) {
-      $_parser.emitError($_parser.getCurrentLocation(),
-                         "expected a parameter name in struct");
-      return {};
-    }
-)";
-
-/// Terminator code segment for the struct parser loop. Check for duplicate or
-/// unknown parameters. Parse a comma except on the last element.
-///
-/// {0}: Code template for printing an error.
-/// {1}: Number of elements in the struct.
-static const char *const structParseLoopEnd = R"({{
-    {0}"duplicate or unknown struct parameter name: ") << _paramKey;
-    return {{};
-  }
-  if ((_index != {1} - 1) && parser.parseComma())
-    return {{};
-}
-)";
-
 /// Code format to parse a variable. Separate by lines because variable parsers
 /// may be generated inside other directives, which requires indentation.
 ///
 /// {0}: The parameter name.
 /// {1}: The parse code for the parameter.
 /// {2}: Code template for printing an error.
 /// {3}: Name of the attribute or type.
 /// {4}: C++ class of the parameter.
 static const char *const variableParser = R"(
 // Parse variable '{0}'
 _result_{0} = {1};
-if (failed(_result_{0})) {{
+if (::mlir::failed(_result_{0})) {{
   {2}"failed to parse {3} parameter '{0}' which is to be a `{4}`");
   return {{};
 }
 )";
 
 //===----------------------------------------------------------------------===//
-// AttrOrTypeFormat
+// DefFormat
 //===----------------------------------------------------------------------===//
 
 namespace {
-class AttrOrTypeFormat {
+class DefFormat {
 public:
-  AttrOrTypeFormat(const AttrOrTypeDef &def,
-                   std::vector<FormatElement *> &&elements)
+  DefFormat(const AttrOrTypeDef &def, std::vector<FormatElement *> &&elements)
       : def(def), elements(std::move(elements)) {}
 
   /// Generate the attribute or type parser.
   void genParser(MethodBody &os);
   /// Generate the attribute or type printer.
   void genPrinter(MethodBody &os);
 
 private:
   /// Generate the parser code for a specific format element.
   void genElementParser(FormatElement *el, FmtContext &ctx, MethodBody &os);
   /// Generate the parser code for a literal.
-  void genLiteralParser(StringRef value, FmtContext &ctx, MethodBody &os);
+  void genLiteralParser(StringRef value, FmtContext &ctx, MethodBody &os,
+                        bool isOptional = false);
   /// Generate the parser code for a variable.
-  void genVariableParser(const AttrOrTypeParameter &param, FmtContext &ctx,
-                         MethodBody &os);
+  void genVariableParser(ParameterElement *el, FmtContext &ctx, MethodBody &os);
   /// Generate the parser code for a `params` directive.
   void genParamsParser(ParamsDirective *el, FmtContext &ctx, MethodBody &os);
   /// Generate the parser code for a `struct` directive.
   void genStructParser(StructDirective *el, FmtContext &ctx, MethodBody &os);
+  /// Generate the parser code for an optional group.
+  void genOptionalGroupParser(OptionalElement *el, FmtContext &ctx,
+                              MethodBody &os);
 
   /// Generate the printer code for a specific format element.
   void genElementPrinter(FormatElement *el, FmtContext &ctx, MethodBody &os);
   /// Generate the printer code for a literal.
   void genLiteralPrinter(StringRef value, FmtContext &ctx, MethodBody &os);
   /// Generate the printer code for a variable.
-  void genVariablePrinter(const AttrOrTypeParameter &param, FmtContext &ctx,
-                          MethodBody &os, bool printQualified = false);
+  void genVariablePrinter(ParameterElement *el, FmtContext &ctx, MethodBody &os,
+                          bool skipGuard = false);
+  /// Generate a printer for comma-separated parameters.
+  void genCommaSeparatedPrinter(ArrayRef<ParameterElement *> params,
+                                FmtContext &ctx, MethodBody &os,
+                                function_ref<void(ParameterElement *)> extra);
   /// Generate the printer code for a `params` directive.
   void genParamsPrinter(ParamsDirective *el, FmtContext &ctx, MethodBody &os);
   /// Generate the printer code for a `struct` directive.
   void genStructPrinter(StructDirective *el, FmtContext &ctx, MethodBody &os);
+  /// Generate the printer code for an optional group.
+  void genOptionalGroupPrinter(OptionalElement *el, FmtContext &ctx,
+                               MethodBody &os);
 
   /// The ODS definition of the attribute or type whose format is being used to
   /// generate a parser and printer.
   const AttrOrTypeDef &def;
   /// The list of top-level format elements returned by the assembly format
   /// parser.
   std::vector<FormatElement *> elements;
 
   /// Flags for printing spaces.
   bool shouldEmitSpace = false;
   bool lastWasPunctuation = false;
 };
 } // namespace
 
 //===----------------------------------------------------------------------===//
 // ParserGen
 //===----------------------------------------------------------------------===//
 
-void AttrOrTypeFormat::genParser(MethodBody &os) {
+void DefFormat::genParser(MethodBody &os) {
   FmtContext ctx;
-  ctx.addSubst("_parser", "parser");
+  ctx.addSubst("_parser", "odsParser");
   if (isa<AttrDef>(def))
-    ctx.addSubst("_type", "type");
+    ctx.addSubst("_type", "odsType");
   os.indent();
 
-  /// Declare variables to store all of the parameters. Allocated parameters
-  /// such as `ArrayRef` and `StringRef` must provide a `storageType`. Store
-  /// FailureOr<T> to defer type construction for parameters that are parsed in
-  /// a loop (parsers return FailureOr anyways).
+  // Declare variables to store all of the parameters. Allocated parameters
+  // such as `ArrayRef` and `StringRef` must provide a `storageType`. Store
+  // FailureOr<T> to defer type construction for parameters that are parsed in
+  // a loop (parsers return FailureOr anyways).
   ArrayRef<AttrOrTypeParameter> params = def.getParameters();
   for (const AttrOrTypeParameter &param : params) {
-    os << formatv("  ::mlir::FailureOr<{0}> _result_{1};\n",
+    os << formatv("::mlir::FailureOr<{0}> _result_{1};\n",
                   param.getCppStorageType(), param.getName());
   }
 
-  /// Store the initial location of the parser.
-  ctx.addSubst("_loc", "loc");
+  // Store the initial location of the parser.
+  ctx.addSubst("_loc", "odsLoc");
   os << tgfmt("::llvm::SMLoc $_loc = $_parser.getCurrentLocation();\n"
               "(void) $_loc;\n",
               &ctx);
 
-  /// Generate call to each parameter parser.
+  // Generate call to each parameter parser.
   for (FormatElement *el : elements)
     genElementParser(el, ctx, os);
 
-  /// Generate call to the attribute or type builder. Use the checked getter
-  /// if one was generated.
+  // Emit an assert for each mandatory parameter. Triggering an assert means
+  // the generated parser is incorrect (i.e. there is a bug in this code).
+  for (const AttrOrTypeParameter &param : params) {
+    if (!param.isOptional()) {
+      os << formatv("assert(::mlir::succeeded(_result_{0}));\n",
+                    param.getName());
+    }
+  }
+
+  // Generate call to the attribute or type builder. Use the checked getter
+  // if one was generated.
   if (def.genVerifyDecl()) {
     os << tgfmt("return $_parser.getChecked<$0>($_loc, $_parser.getContext()",
                 &ctx, def.getCppClassName());
   } else {
     os << tgfmt("return $0::get($_parser.getContext()", &ctx,
                 def.getCppClassName());
   }
-  for (const AttrOrTypeParameter &param : params)
-    os << formatv(",\n    _result_{0}.getValue()", param.getName());
+  for (const AttrOrTypeParameter &param : params) {
+    if (param.isOptional())
+      os << formatv(",\n    _result_{0}.getValueOr({1}())", param.getName(),
+                    param.getCppStorageType());
+    else
+      os << formatv(",\n    *_result_{0}", param.getName());
+  }
   os << ");";
 }
 
-void AttrOrTypeFormat::genElementParser(FormatElement *el, FmtContext &ctx,
-                                        MethodBody &os) {
+void DefFormat::genElementParser(FormatElement *el, FmtContext &ctx,
   if (auto *literal = dyn_cast<LiteralElement>(el))
     return genLiteralParser(literal->getSpelling(), ctx, os);
   if (auto *var = dyn_cast<ParameterElement>(el))
-    return genVariableParser(var->getParam(), ctx, os);
+    return genVariableParser(var, ctx, os);
   if (auto *params = dyn_cast<ParamsDirective>(el))
     return genParamsParser(params, ctx, os);
   if (auto *strct = dyn_cast<StructDirective>(el))
     return genStructParser(strct, ctx, os);
+  if (auto *optional = dyn_cast<OptionalElement>(el))
+    return genOptionalGroupParser(optional, ctx, os);
 
   llvm_unreachable("unknown format element");
 }
 
-void AttrOrTypeFormat::genLiteralParser(StringRef value, FmtContext &ctx,
-                                        MethodBody &os) {
+void DefFormat::genLiteralParser(StringRef value, FmtContext &ctx,
+                                 MethodBody &os, bool isOptional) {
   os << "// Parse literal '" << value << "'\n";
   os << tgfmt("if ($_parser.parse", &ctx);
+  if (isOptional)
+    os << "Optional";
   if (value.front() == '_' || isalpha(value.front())) {
     os << "Keyword(\"" << value << "\")";
   } else {
     os << StringSwitch<StringRef>(value)
               .Case("->", "Arrow")
               .Case(":", "Colon")
               .Case(",", "Comma")
               .Case("=", "Equal")
               .Case("<", "Less")
               .Case(">", "Greater")
               .Case("{", "LBrace")
               .Case("}", "RBrace")
               .Case("(", "LParen")
               .Case(")", "RParen")
               .Case("[", "LSquare")
               .Case("]", "RSquare")
               .Case("?", "Question")
               .Case("+", "Plus")
               .Case("*", "Star")
        << "()";
   }
-  os << ")\n";
+  if (isOptional) {
+    // Leave the `if` unclosed to guard optional groups.
+    return;
+  }
   // Parser will emit an error
-  os << "  return {};\n";
+  os << ") return {};\n";
 }
 
-void AttrOrTypeFormat::genVariableParser(const AttrOrTypeParameter &param,
-                                         FmtContext &ctx, MethodBody &os) {
-  /// Check for a custom parser. Use the default attribute parser otherwise.
+void DefFormat::genVariableParser(ParameterElement *el, FmtContext &ctx,
+                                  MethodBody &os) {
+  // Check for a custom parser. Use the default attribute parser otherwise.
+  const AttrOrTypeParameter &param = el->getParam();
   auto customParser = param.getParser();
   auto parser =
       customParser ? *customParser : StringRef(defaultParameterParser);
   os << formatv(variableParser, param.getName(),
                 tgfmt(parser, &ctx, param.getCppStorageType()),
-                tgfmt(parseErrorStr, &ctx), def.getName(), param.getCppType());
+                tgfmt(parserErrorStr, &ctx), def.getName(), param.getCppType());
 }
 
-void AttrOrTypeFormat::genParamsParser(ParamsDirective *el, FmtContext &ctx,
-                                       MethodBody &os) {
+void DefFormat::genParamsParser(ParamsDirective *el, FmtContext &ctx,
   os << "// Parse parameter list\n";
-  llvm::interleave(
-      el->getParams(),
-      [&](auto param) { this->genVariableParser(param, ctx, os); },
-      [&]() { this->genLiteralParser(",", ctx, os); });
+
+  // If there are optional parameters, we need to switch to `parseOptionalComma`
+  // if there are no more required parameters after a certain point.
+  bool hasOptional = el->hasOptionalParams();
+  if (hasOptional) {
+    // Wrap everything in a do-while so that we can `break`.
+    os << "do {\n";
+    os.indent();
   }
 
-void AttrOrTypeFormat::genStructParser(StructDirective *el, FmtContext &ctx,
-                                       MethodBody &os) {
-  os << "// Parse parameter struct\n";
+  ArrayRef<ParameterElement *> params = el->getParams();
+  using IteratorT = ParameterElement *const *;
+  IteratorT it = params.begin();
+
+  // Find the last required parameter. Commas become optional aftewards.
+  // Note: IteratorT's copy assignment is deleted.
+  ParameterElement *lastReq = nullptr;
+  for (ParameterElement *param : params)
+    if (!param->isOptional())
+      lastReq = param;
+  IteratorT lastReqIt = lastReq ? llvm::find(params, lastReq) : params.begin();
+
+  auto eachFn = [&](ParameterElement *el) { genVariableParser(el, ctx, os); };
+  auto betweenFn = [&](IteratorT it) {
+    ParameterElement *el = *std::prev(it);
+    // Parse a comma if the last optional parameter had a value.
+    if (el->isOptional()) {
+      os << formatv("if (::mlir::succeeded(_result_{0}) && *_result_{0}) {{\n",
+                    el->getName());
+      os.indent();
+    }
+    if (it <= lastReqIt) {
+      genLiteralParser(",", ctx, os);
+    } else {
+      genLiteralParser(",", ctx, os, /*isOptional=*/true);
+      os << ") break;\n";
+    }
+    if (el->isOptional())
+      os.unindent() << "}\n";
+  };
+
+  // llvm::interleave
+  if (it != params.end()) {
+    eachFn(*it++);
+    for (IteratorT e = params.end(); it != e; ++it) {
+      betweenFn(it);
+      eachFn(*it);
+    }
+  }
+
+  if (hasOptional)
+    os.unindent() << "} while(false);\n";
+}
+
+void DefFormat::genStructParser(StructDirective *el, FmtContext &ctx,
+  // Loop declaration for struct parser with only required parameters.
+  //
+  // $0: Number of expected parameters.
+  const char *const loopHeader = R"(
+  for (unsigned odsStructIndex = 0; odsStructIndex < $0; ++odsStructIndex) {
+)";
 
-  /// Declare a "seen" variable for each key.
-  for (const AttrOrTypeParameter &param : el->getParams())
-    os << formatv("bool _seen_{0} = false;\n", param.getName());
+  // Loop body start for struct parser.
+  const char *const loopStart = R"(
+    ::llvm::StringRef _paramKey;
+    if ($_parser.parseKeyword(&_paramKey)) {
+      $_parser.emitError($_parser.getCurrentLocation(),
+                         "expected a parameter name in struct");
+      return {};
+    }
+    if (!_loop_body(_paramKey)) return {};
+)";
 
-  /// Generate the parsing loop.
-  os.getStream().printReindented(
-      tgfmt(structParseLoopStart, &ctx, el->getNumParams()).str());
-  os.indent();
-  genLiteralParser("=", ctx, os);
-  for (const AttrOrTypeParameter &param : el->getParams()) {
+  // Struct parser loop end. Check for duplicate or unknown struct parameters.
+  //
+  // {0}: Code template for printing an error.
+  const char *const loopEnd = R"({{
+  {0}"duplicate or unknown struct parameter name: ") << _paramKey;
+  return {{};
+}
+)";
+
+  // Struct parser loop terminator. Parse a comma except on the last element.
+  //
+  // {0}: Number of elements in the struct.
+  const char *const loopTerminator = R"(
+  if ((odsStructIndex != {0} - 1) && odsParser.parseComma())
+    return {{};
+}
+)";
+
+  // Check that a mandatory parameter was parse.
+  //
+  // {0}: Name of the parameter.
+  const char *const checkParam = R"(
+    if (!_seen_{0}) {
+      {1}"struct is missing required parameter: ") << "{0}";
+      return {{};
+    }
+)";
+
+  // Optional parameters in a struct must be parsed successfully if the
+  // keyword is present.
+  //
+  // {0}: Name of the parameter.
+  // {1}: Emit error string
+  const char *const checkOptionalParam = R"(
+    if (::mlir::succeeded(_result_{0}) && !*_result_{0}) {{
+      {1}"expected a value for parameter '{0}'");
+      return {{};
+    }
+)";
+
+  // First iteration of the loop parsing an optional struct.
+  const char *const optionalStructFirst = R"(
+  ::llvm::StringRef _paramKey;
+  if (!$_parser.parseOptionalKeyword(&_paramKey)) {
+    if (!_loop_body(_paramKey)) return {};
+    while (!$_parser.parseOptionalComma()) {
+)";
+
+  os << "// Parse parameter struct\n";
+
+  // Declare a "seen" variable for each key.
+  for (ParameterElement *param : el->getParams())
+    os << formatv("bool _seen_{0} = false;\n", param->getName());
+
+  // Generate the body of the parsing loop inside a lambda.
+  os << "{\n";
+  os.indent()
+      << "const auto _loop_body = [&](::llvm::StringRef _paramKey) -> bool {\n";
+  genLiteralParser("=", ctx, os.indent());
+  for (ParameterElement *param : el->getParams()) {
     os << formatv("if (!_seen_{0} && _paramKey == \"{0}\") {\n"
                   "  _seen_{0} = true;\n",
-                  param.getName());
+                  param->getName());
     genVariableParser(param, ctx, os.indent());
+    if (param->isOptional()) {
+      os.getStream().printReindented(strfmt(checkOptionalParam,
+                                            param->getName(),
+                                            tgfmt(parserErrorStr, &ctx).str()));
+    }
     os.unindent() << "} else ";
+    // Print the check for duplicate or unknown parameter.
   }
+  os.getStream().printReindented(strfmt(loopEnd, tgfmt(parserErrorStr, &ctx)));
+  os << "return true;\n";
+  os.unindent() << "};\n";
+
+  // Generate the parsing loop. If optional parameters are present, then the
+  // parse loop is guarded by commas.
+  unsigned numOptional = llvm::count_if(el->getParams(), paramIsOptional);
+  if (numOptional) {
+    // If the struct itself is optional, pull out the first iteration.
+    if (numOptional == el->getNumParams()) {
+      os.getStream().printReindented(tgfmt(optionalStructFirst, &ctx).str());
+      os.indent();
+    } else {
+      os << "do {\n";
+    }
+  } else {
+    os.getStream().printReindented(
+        tgfmt(loopHeader, &ctx, el->getNumParams()).str());
+  }
+  os.indent();
+  os.getStream().printReindented(tgfmt(loopStart, &ctx).str());
   os.unindent();
 
-  /// Duplicate or unknown parameter.
-  os.getStream().printReindented(strfmt(
-      structParseLoopEnd, tgfmt(parseErrorStr, &ctx), el->getNumParams()));
-
-  /// Because the loop loops N times and each non-failing iteration sets 1 of
-  /// N flags, successfully exiting the loop means that all parameters have been
-  /// seen. `parseOptionalComma` would cause issues with any formats that use
-  /// "struct(...) `,`" beacuse structs aren't sounded by braces.
+  // Print the loop terminator. For optional parameters, we have to check that
+  // all mandatory parameters have been parsed.
+  // The whole struct is optional if all its parameters are optional.
+  if (numOptional) {
+    if (numOptional == el->getNumParams()) {
+      os << "}\n";
+      os.unindent() << "}\n";
+    } else {
+      os << tgfmt("} while(!$_parser.parseOptionalComma());\n", &ctx);
+      for (ParameterElement *param : el->getParams()) {
+        if (param->isOptional())
+          continue;
+        os.getStream().printReindented(
+            strfmt(checkParam, param->getName(), tgfmt(parserErrorStr, &ctx)));
+      }
+    }
+  } else {
+    // Because the loop loops N times and each non-failing iteration sets 1 of
+    // N flags, successfully exiting the loop means that all parameters have
+    // been seen. `parseOptionalComma` would cause issues with any formats that
+    // use "struct(...) `,`" beacuse structs aren't sounded by braces.
+    os.getStream().printReindented(strfmt(loopTerminator, el->getNumParams()));
+  }
+  os.unindent() << "}\n";
+}
+
+void DefFormat::genOptionalGroupParser(OptionalElement *el, FmtContext &ctx,
+                                       MethodBody &os) {
+  ArrayRef<FormatElement *> elements =
+      el->getThenElements().drop_front(el->getParseStart());
+
+  FormatElement *first = elements.front();
+  const auto guardOn = [&](auto params) {
+    os << "if (!(";
+    llvm::interleave(
+        params, os,
+        [&](ParameterElement *el) {
+          os << formatv("(::mlir::succeeded(_result_{0}) && *_result_{0})",
+                        el->getName());
+        },
+        " || ");
+    os << ")) {\n";
+  };
+  if (auto *literal = dyn_cast<LiteralElement>(first)) {
+    genLiteralParser(literal->getSpelling(), ctx, os, /*isOptional=*/true);
+    os << ") {\n";
+  } else if (auto *param = dyn_cast<ParameterElement>(first)) {
+    genVariableParser(param, ctx, os);
+    guardOn(llvm::makeArrayRef(param));
+  } else if (auto *params = dyn_cast<ParamsDirective>(first)) {
+    genParamsParser(params, ctx, os);
+    guardOn(params->getParams());
+  } else {
+    auto *strct = cast<StructDirective>(first);
+    genStructParser(strct, ctx, os);
+    guardOn(params->getParams());
+  }
+  os.indent();
+
+  // Generate the parsers for the rest of the elements.
+  for (FormatElement *element : el->getElseElements())
+    genElementParser(element, ctx, os);
+  os.unindent() << "} else {\n";
+  os.indent();
+  for (FormatElement *element : elements.drop_front())
+    genElementParser(element, ctx, os);
+  os.unindent() << "}\n";
 }
 
 //===----------------------------------------------------------------------===//
 // PrinterGen
 //===----------------------------------------------------------------------===//
 
-void AttrOrTypeFormat::genPrinter(MethodBody &os) {
+void DefFormat::genPrinter(MethodBody &os) {
   FmtContext ctx;
-  ctx.addSubst("_printer", "printer");
+  ctx.addSubst("_printer", "odsPrinter");
+  os.indent();
 
   /// Generate printers.
   shouldEmitSpace = true;
   lastWasPunctuation = false;
   for (FormatElement *el : elements)
     genElementPrinter(el, ctx, os);
 }
 
-void AttrOrTypeFormat::genElementPrinter(FormatElement *el, FmtContext &ctx,
-                                         MethodBody &os) {
+void DefFormat::genElementPrinter(FormatElement *el, FmtContext &ctx,
   if (auto *literal = dyn_cast<LiteralElement>(el))
     return genLiteralPrinter(literal->getSpelling(), ctx, os);
   if (auto *params = dyn_cast<ParamsDirective>(el))
     return genParamsPrinter(params, ctx, os);
   if (auto *strct = dyn_cast<StructDirective>(el))
     return genStructPrinter(strct, ctx, os);
   if (auto *var = dyn_cast<ParameterElement>(el))
-    return genVariablePrinter(var->getParam(), ctx, os,
-                              var->shouldBeQualified());
+    return genVariablePrinter(var, ctx, os);
+  if (auto *optional = dyn_cast<OptionalElement>(el))
+    return genOptionalGroupPrinter(optional, ctx, os);
 
-  llvm_unreachable("unknown format element");
+  llvm::PrintFatalError("unsupported format element");
 }
 
-void AttrOrTypeFormat::genLiteralPrinter(StringRef value, FmtContext &ctx,
-                                         MethodBody &os) {
-  /// Don't insert a space before certain punctuation.
+void DefFormat::genLiteralPrinter(StringRef value, FmtContext &ctx,
+  // Don't insert a space before certain punctuation.
   bool needSpace =
       shouldEmitSpace && shouldEmitSpaceBefore(value, lastWasPunctuation);
-  os << tgfmt("  $_printer$0 << \"$1\";\n", &ctx, needSpace ? " << ' '" : "",
+  os << tgfmt("$_printer$0 << \"$1\";\n", &ctx, needSpace ? " << ' '" : "",
               value);
 
-  /// Update the flags.
+  // Update the flags.
   shouldEmitSpace =
       value.size() != 1 || !StringRef("<({[").contains(value.front());
   lastWasPunctuation = !(value.front() == '_' || isalpha(value.front()));
 }
 
-void AttrOrTypeFormat::genVariablePrinter(const AttrOrTypeParameter &param,
-                                          FmtContext &ctx, MethodBody &os,
-                                          bool printQualified) {
-  /// Insert a space before the next parameter, if necessary.
+void DefFormat::genVariablePrinter(ParameterElement *el, FmtContext &ctx,
+                                   MethodBody &os, bool skipGuard) {
+  const AttrOrTypeParameter &param = el->getParam();
+  ctx.withSelf(getParameterAccessorName(param.getName()) + "()");
+
+  // Guard the printer on the presence of optional parameters.
+  if (el->isOptional() && !skipGuard) {
+    os << tgfmt("if ($_self) {\n", &ctx);
+    os.indent();
+  }
+
+  // Insert a space before the next parameter, if necessary.
   if (shouldEmitSpace || !lastWasPunctuation)
-    os << tgfmt("  $_printer << ' ';\n", &ctx);
+    os << tgfmt("$_printer << ' ';\n", &ctx);
   shouldEmitSpace = true;
   lastWasPunctuation = false;
 
-  ctx.withSelf(getParameterAccessorName(param.getName()) + "()");
-  os << "  ";
-  if (printQualified)
+  if (el->shouldBeQualified())
     os << tgfmt(qualifiedParameterPrinter, &ctx) << ";\n";
   else if (auto printer = param.getPrinter())
     os << tgfmt(*printer, &ctx) << ";\n";
   else
     os << tgfmt(defaultParameterPrinter, &ctx) << ";\n";
+
+  if (el->isOptional() && !skipGuard)
+    os.unindent() << "}\n";
 }
 
-void AttrOrTypeFormat::genParamsPrinter(ParamsDirective *el, FmtContext &ctx,
-                                        MethodBody &os) {
+void DefFormat::genCommaSeparatedPrinter(
+    ArrayRef<ParameterElement *> params, FmtContext &ctx, MethodBody &os,
+    function_ref<void(ParameterElement *)> extra) {
+  // Emit a space if necessary, but only if the struct is present.
+  if (shouldEmitSpace || !lastWasPunctuation) {
+    bool allOptional = llvm::all_of(params, paramIsOptional);
+    if (allOptional) {
+      os << "if (";
       llvm::interleave(
-      el->getParams(),
-      [&](auto param) { this->genVariablePrinter(param, ctx, os); },
-      [&] { this->genLiteralPrinter(",", ctx, os); });
+          params, os,
+          [&](ParameterElement *param) {
+            os << getParameterAccessorName(param->getName()) << "()";
+          },
+          " || ");
+      os << ") {\n";
+      os.indent();
+    }
+    os << tgfmt("$_printer << ' ';\n", &ctx);
+    if (allOptional)
+      os.unindent() << "}\n";
   }
 
-void AttrOrTypeFormat::genStructPrinter(StructDirective *el, FmtContext &ctx,
-                                        MethodBody &os) {
+  // The first printed element does not need to emit a comma.
+  os << "{\n";
+  os.indent() << "bool _firstPrinted = true;\n";
+  for (ParameterElement *param : params) {
+    if (param->isOptional()) {
+      os << tgfmt("if ($_self()) {\n",
+                  &ctx.withSelf(getParameterAccessorName(param->getName())));
+      os.indent();
+    }
+    os << tgfmt("if (!_firstPrinted) $_printer << \", \";\n", &ctx);
+    os << "_firstPrinted = false;\n";
+    extra(param);
+    shouldEmitSpace = false;
+    lastWasPunctuation = true;
+    genVariablePrinter(param, ctx, os);
+    if (param->isOptional())
+      os.unindent() << "}\n";
+  }
+  os.unindent() << "}\n";
+}
+
+void DefFormat::genParamsPrinter(ParamsDirective *el, FmtContext &ctx,
+                                 MethodBody &os) {
+  genCommaSeparatedPrinter(llvm::to_vector(el->getParams()), ctx, os,
+                           [&](ParameterElement *param) {});
+}
+
+void DefFormat::genStructPrinter(StructDirective *el, FmtContext &ctx,
+  genCommaSeparatedPrinter(
+      llvm::to_vector(el->getParams()), ctx, os, [&](ParameterElement *param) {
+        os << tgfmt("$_printer << \"$0 = \";\n", &ctx, param->getName());
+      });
+}
+
+void DefFormat::genOptionalGroupPrinter(OptionalElement *el, FmtContext &ctx,
+                                        MethodBody &os) {
+  // Emit the check on whether the group should be printed.
+  const auto guardOn = [&](auto params) {
+    os << "if (";
     llvm::interleave(
-      el->getParams(),
-      [&](auto param) {
-        this->genLiteralPrinter(param.getName(), ctx, os);
-        this->genLiteralPrinter("=", ctx, os);
-        this->genVariablePrinter(param, ctx, os);
+        params, os,
+        [&](ParameterElement *el) {
+          os << getParameterAccessorName(el->getName()) << "()";
         },
-      [&] { this->genLiteralPrinter(",", ctx, os); });
+        " || ");
+    os << ") {\n";
+    os.indent();
+  };
+  FormatElement *anchor = el->getAnchor();
+  if (auto *param = dyn_cast<ParameterElement>(anchor)) {
+    guardOn(llvm::makeArrayRef(param));
+  } else if (auto *params = dyn_cast<ParamsDirective>(anchor)) {
+    guardOn(params->getParams());
+  } else {
+    auto *strct = dyn_cast<StructDirective>(anchor);
+    guardOn(strct->getParams());
+  }
+  // Generate the printer for the contained elements.
+  {
+    llvm::SaveAndRestore<bool> shouldEmitSpaceFlag(shouldEmitSpace);
+    llvm::SaveAndRestore<bool> lastWasPunctuationFlag(lastWasPunctuation);
+    for (FormatElement *element : el->getThenElements())
+      genElementPrinter(element, ctx, os);
+  }
+  os.unindent() << "} else {\n";
+  os.indent();
+  for (FormatElement *element : el->getElseElements())
+    genElementPrinter(element, ctx, os);
+  os.unindent() << "}\n";
 }
 
 //===----------------------------------------------------------------------===//
 // DefFormatParser
 //===----------------------------------------------------------------------===//
 
 namespace {
 class DefFormatParser : public FormatParser {
 public:
   DefFormatParser(llvm::SourceMgr &mgr, const AttrOrTypeDef &def)
       : FormatParser(mgr, def.getLoc()[0]), def(def),
         seenParams(def.getNumParameters()) {}
 
   /// Parse the attribute or type format and create the format elements.
-  FailureOr<AttrOrTypeFormat> parse();
+  FailureOr<DefFormat> parse();
 
 protected:
   /// Verify the parsed elements.
   LogicalResult verify(SMLoc loc, ArrayRef<FormatElement *> elements) override;
   /// Verify the elements of a custom directive.
   LogicalResult
   verifyCustomDirectiveArguments(SMLoc loc,
                                  ArrayRef<FormatElement *> arguments) override {
     return emitError(loc, "'custom' not supported (yet)");
   }
   /// Verify the elements of an optional group.
   LogicalResult
   verifyOptionalGroupElements(SMLoc loc, ArrayRef<FormatElement *> elements,
-                              Optional<unsigned> anchorIndex) override {
-    return emitError(loc, "optional groups not (yet) supported");
-  }
+                              Optional<unsigned> anchorIndex) override;
 
   /// Parse an attribute or type variable.
   FailureOr<FormatElement *> parseVariableImpl(SMLoc loc, StringRef name,
                                                Context ctx) override;
   /// Parse an attribute or type format directive.
   FailureOr<FormatElement *>
   parseDirectiveImpl(SMLoc loc, FormatToken::Kind kind, Context ctx) override;
 
private:
 
   /// Seen attribute or type parameters.
   BitVector seenParams;
 };
 } // namespace
 
 LogicalResult DefFormatParser::verify(SMLoc loc,
                                       ArrayRef<FormatElement *> elements) {
+  // Check that all parameters are referenced in the format.
   for (auto &it : llvm::enumerate(def.getParameters())) {
-    if (!seenParams.test(it.index())) {
+    if (!it.value().isOptional() && !seenParams.test(it.index())) {
       return emitError(loc, "format is missing reference to parameter: " +
                                 it.value().getName());
     }
   }
+  // A `struct` directive that contains optional parameters cannot be followed
+  // by a comma literal, which is ambiguous.
+  for (auto it : llvm::zip(elements.drop_back(), elements.drop_front())) {
+    auto *structEl = dyn_cast<StructDirective>(std::get<0>(it));
+    auto *literalEl = dyn_cast<LiteralElement>(std::get<1>(it));
+    if (!structEl || !literalEl)
+      continue;
+    if (literalEl->getSpelling() == "," && structEl->hasOptionalParams()) {
+      return emitError(loc, "`struct` directive with optional parameters "
+                            "cannot be followed by a comma literal");
+    }
+  }
+  return success();
+}
+
+LogicalResult
+DefFormatParser::verifyOptionalGroupElements(llvm::SMLoc loc,
+                                             ArrayRef<FormatElement *> elements,
+                                             Optional<unsigned> anchorIndex) {
+  // `params` and `struct` directives are allowed only if all the contained
+  // parameters are optional.
+  for (FormatElement *el : elements) {
+    if (auto *param = dyn_cast<ParameterElement>(el)) {
+      if (!param->isOptional()) {
+        return emitError(loc,
+                         "parameters in an optional group must be optional");
+      }
+    } else if (auto *params = dyn_cast<ParamsDirective>(el)) {
+      if (llvm::any_of(params->getParams(), paramNotOptional)) {
+        return emitError(loc, "`params` directive allowed in optional group "
+                              "only if all parameters are optional");
+      }
+    } else if (auto *strct = dyn_cast<StructDirective>(el)) {
+      if (llvm::any_of(strct->getParams(), paramNotOptional)) {
+        return emitError(loc, "`struct` is only allowed in an optional group "
+                              "if all captured parameters are optional");
+      }
+    }
+  }
+  // The anchor must be a parameter or one of the aforementioned directives.
+  if (anchorIndex && !isa<ParameterElement, ParamsDirective, StructDirective>(
+                         elements[*anchorIndex])) {
+    return emitError(loc,
+                     "optional group anchor must be a parameter or directive");
+  }
   return success();
 }
 
-FailureOr<AttrOrTypeFormat> DefFormatParser::parse() {
+FailureOr<DefFormat> DefFormatParser::parse() {
   FailureOr<std::vector<FormatElement *>> elements = FormatParser::parse();
   if (failed(elements))
     return failure();
-  return AttrOrTypeFormat(def, std::move(*elements));
+  return DefFormat(def, std::move(*elements));
 }
 
 FailureOr<FormatElement *>
 DefFormatParser::parseVariableImpl(SMLoc loc, StringRef name, Context ctx) {
-  /// Lookup the parameter.
+  // Lookup the parameter.
   ArrayRef<AttrOrTypeParameter> params = def.getParameters();
   auto *it = llvm::find_if(
       params, [&](auto &param) { return param.getName() == name; });
 
-  /// Check that the parameter reference is valid.
+  // Check that the parameter reference is valid.
   if (it == params.end()) {
     return emitError(loc,
                      def.getName() + " has no parameter named '" + name + "'");
   }
   auto idx = std::distance(params.begin(), it);
   if (seenParams.test(idx))
     return emitError(loc, "duplicate parameter '" + name + "'");
   seenParams.set(idx);
DefFormatParser::parseQualifiedDirective(SMLoc loc, Context ctx) {
   cast<ParameterElement>(*var)->setShouldBeQualified();
   if (failed(
           parseToken(FormatToken::r_paren, "expected ')' after argument list")))
     return failure();
   return var;
 }
 
 FailureOr<FormatElement *> DefFormatParser::parseParamsDirective(SMLoc loc) {
-  /// Collect all of the attribute's or type's parameters.
-  std::vector<FormatElement *> vars;
-  /// Ensure that none of the parameters have already been captured.
+  // Collect all of the attribute's or type's parameters.
+  std::vector<ParameterElement *> vars;
+  // Ensure that none of the parameters have already been captured.
   for (const auto &it : llvm::enumerate(def.getParameters())) {
     if (seenParams.test(it.index())) {
       return emitError(loc, "`params` captures duplicate parameter: " +
                                 it.value().getName());
     }
     seenParams.set(it.index());
     vars.push_back(create<ParameterElement>(it.value()));
   }
   return create<ParamsDirective>(std::move(vars));
 }
 
 FailureOr<FormatElement *> DefFormatParser::parseStructDirective(SMLoc loc) {
   if (failed(parseToken(FormatToken::l_paren,
                         "expected '(' before `struct` argument list")))
     return failure();
 
-  /// Parse variables captured by `struct`.
-  std::vector<FormatElement *> vars;
+  // Parse variables captured by `struct`.
+  std::vector<ParameterElement *> vars;
 
-  /// Parse first captured parameter or a `params` directive.
+  // Parse first captured parameter or a `params` directive.
   FailureOr<FormatElement *> var = parseElement(StructDirectiveContext);
   if (failed(var) || !isa<VariableElement, ParamsDirective>(*var)) {
     return emitError(loc,
                      "`struct` argument list expected a variable or directive");
   }
   if (isa<VariableElement>(*var)) {
-    /// Parse any other parameters.
-    vars.push_back(std::move(*var));
+    // Parse any other parameters.
+    vars.push_back(cast<ParameterElement>(*var));
     while (peekToken().is(FormatToken::comma)) {
       consumeToken();
       var = parseElement(StructDirectiveContext);
       if (failed(var) || !isa<VariableElement>(*var))
         return emitError(loc, "expected a variable in `struct` argument list");
-      vars.push_back(std::move(*var));
+      vars.push_back(cast<ParameterElement>(*var));
     }
   } else {
-    /// `struct(params)` captures all parameters in the attribute or type.
+    // `struct(params)` captures all parameters in the attribute or type.
     vars = cast<ParamsDirective>(*var)->takeParams();
   }
 
   if (failed(parseToken(FormatToken::r_paren,
                         "expected ')' at the end of an argument list")))
     return failure();
 
   return create<StructDirective>(std::move(vars));
 }
 
 //===----------------------------------------------------------------------===//
 // Interface
 //===----------------------------------------------------------------------===//
 
 void mlir::tblgen::generateAttrOrTypeFormat(const AttrOrTypeDef &def,
                                             MethodBody &parser,
                                             MethodBody &printer) {
   llvm::SourceMgr mgr;
   mgr.AddNewSourceBuffer(
       llvm::MemoryBuffer::getMemBuffer(*def.getAssemblyFormat()), SMLoc());
 
-  /// Parse the custom assembly format>
+  // Parse the custom assembly format>
   DefFormatParser fmtParser(mgr, def);
-  FailureOr<AttrOrTypeFormat> format = fmtParser.parse();
+  FailureOr<DefFormat> format = fmtParser.parse();
   if (failed(format)) {
     if (formatErrorIsFatal)
       PrintFatalError(def.getLoc(), "failed to parse assembly format");
     return;
   }
 
-  /// Generate the parser and printer.
+  // Generate the parser and printer.
   format->genParser(parser);
   format->genPrinter(printer);
 }