Constraints

Attribute / Type Constraints

When defining the arguments of an operation in TableGen, users can specify either plain attributes/types or use attribute/type constraints to levy additional requirements on the attribute value or operand type.

def My_Type1 : MyDialect_Type<"Type1", "type1"> { ... }
def My_Type2 : MyDialect_Type<"Type2", "type2"> { ... }

// Plain type
let arguments = (ins MyType1:$val);
// Type constraint
let arguments = (ins AnyTypeOf<[MyType1, MyType2]>:$val);

AnyTypeOf is an example for a type constraints. Many useful type constraints can be found in mlir/IR/CommonTypeConstraints.td. Additional verification code is generated for type/attribute constraints. Type constraints can not only be used when defining operation arguments, but also when defining type parameters.

Optionally, C++ functions can be generated, so that type/attribute constraints can be checked from C++. The name of the C++ function must be specified in the cppFunctionName field. If no function name is specified, no C++ function is emitted.

// Example: Element type constraint for VectorType
def Builtin_VectorTypeElementType : AnyTypeOf<[AnyInteger, Index, AnyFloat]> {
  let cppFunctionName = "isValidVectorTypeElementType";
}

The above example tranlates into the following C++ code:

bool isValidVectorTypeElementType(::mlir::Type type) {
  return (((::llvm::isa<::mlir::IntegerType>(type))) || ((::llvm::isa<::mlir::IndexType>(type))) || ((::llvm::isa<::mlir::FloatType>(type))));
}

An extra TableGen rule is needed to emit C++ code for type/attribute constraints. This will generate only the declarations/definitions of the type/attribute constaraints that are defined in the specified .td file, but not those that are in included .td files.

mlir_tablegen(<Your Dialect>TypeConstraints.h.inc -gen-type-constraint-decls)
mlir_tablegen(<Your Dialect>TypeConstraints.cpp.inc -gen-type-constraint-defs)
mlir_tablegen(<Your Dialect>AttrConstraints.h.inc -gen-attr-constraint-decls)
mlir_tablegen(<Your Dialect>AttrConstraints.cpp.inc -gen-attr-constraint-defs)

The generated <Your Dialect>TypeConstraints.h.inc respectivelly <Your Dialect>AttrConstraints.h.inc will need to be included whereever you are referencing the type/attributes constraint in C++. Note that no C++ namespace will be emitted by the code generator. The #include statements of the .h.inc/.cpp.inc files should be wrapped in C++ namespaces by the user.