Document CMake Changes Needed to Generate Attribute Code from TableGen

[andrej]

Summary

I've been struggling with generating the C++ class declarations and definitions for custom attributes from TableGen, as described on this documentation page: https://mlir.llvm.org/docs/DefiningDialects/AttributesAndTypes/#adding-a-new-attribute-or-type-definition

The code for custom types is automatically generated when the MLIR Dialect is added with add_mlir_dialect() in the CMake file.

The same is not the case for custom attributes. I think people could benefit from learning how to adjsut their CMakeLists.txt to automatically generate the classes as described on that documentation page. This change adds the necessary information for this.

makslevental on Discord was so kind to help me figure this out myself

[makslevental]

mlir/docs/DefiningDialects/AttributesAndTypes.md

51

I would phrase this as something like

One difference is that triggering C++ codegen for Attributes will require...

192

nit: wherever

mlir/docs/DefiningDialects/AttributesAndTypes.md

 within an instance of an `MLIRContext`.
 
 The structure for defining Attributes and Types is nearly identical, with only a
 few differences depending on the context. As such, a majority of this document
 describes the process for defining both Attributes and Types side-by-side with
 examples for both. If necessary, a section will explicitly call out any
 distinct differences.
 
+One difference is that generating C++ classes from declarative TableGen 
+definitions will require adding additional targets to your `CMakeLists.txt`.
+This is not necessary for custom types. The details are outlined further below.
+
 ### Adding a new Attribute or Type definition
 
 As described above, C++ Attribute and Type objects in MLIR are value-typed and
 essentially function as helpful wrappers around an internal storage object that
 holds the actual data for the type. Similarly to Operations, Attributes and Types
 are defined declaratively via [TableGen](https://llvm.org/docs/TableGen/index.html);
 a generic language with tooling to maintain records of domain-specific information.
 It is highly recommended that users review the

 
 The name of the C++ class which gets generated defaults to
 `<classParamName>Attr` or `<classParamName>Type` for attributes and types
 respectively. In the examples above, this was the `name` template parameter that
 was provided to `MyDialect_Attr` and `MyDialect_Type`. For the definitions we
 added above, we would get C++ classes named `IntegerType` and `IntegerAttr`
 respectively. This can be explicitly overridden via the `cppClassName` field.
 
+### CMake Targets
+
+If you added your dialect using `add_mlir_dialect()` in your `CMakeLists.txt`,
+the above mentioned classes will automatically get generated for custom 
+_types_. They will be output in a file named `<Your Dialect>Types.h.inc`. 
+
+To also generate the classes for custom _attributes_, you will need to add
+two additional TableGen targets to your `CMakeLists.txt`: 
+
+```cmake
+mlir_tablegen(<Your Dialect>AttrDefs.h.inc -gen-attrdef-decls 
+              -attrdefs-dialect=<Your Dialect>)
+mlir_tablegen(<Your Dialect>AttrDefs.cpp.inc -gen-attrdef-defs 
+              -attrdefs-dialect=<Your Dialect>)
+add_public_tablegen_target(<Your Dialect>AttrDefsIncGen)
+```
+
+The generated `<Your Dialect>AttrDefs.h.inc` will need to be included whereever
+you are referencing the custom attribute types.
+
 ### Documentation
 
 The `summary` and `description` fields allow for providing user documentation
 for the attribute or type. The `summary` field expects a simple single-line
 string, with the `description` field used for long and extensive documentation.
 This documentation can be used to generate markdown documentation for the
 dialect and is used by upstream
 [MLIR dialects](https://mlir.llvm.org/docs/Dialects/).