[MLIR][Doc] Add documentation for OpAsmAttr/TypeInterface

mlir/docs/DefiningDialects/Assembly.md

@@ -4,29 +4,42 @@
 
 ## Generating Aliases
 
-To reduce verbosity in the resulting assembly, `AsmPrinter` can generate aliases for frequently used types and attributes.
+`AsmPrinter` can generate aliases for frequently used types and attributes when not printing them in generic form. For example, `!my_dialect.type<a=3,b=4,c=5,d=tuple,e=another_type>` and `#my_dialect.attr<a=3>` can be aliased to `!my_dialect_type` and `#my_dialect_attr`.
 
-For example, `!my_dialect.type<a=3,b=4,c=5,d=tuple,e=another_type>` and `#my_dialect.attr<a=3>` can be aliased to `!my_dialect_type` and `#my_dialect_attr`, simplifying further references.
+There are mainly two ways to hook into the `AsmPrinter`. One is the attribute/type interface and the other is the dialect interface.
 
-To enable this, the owning dialect of these types/attributes can define an interface to hook into the `AsmPrinter`. This is effective only when the assembly is not printed in generic form.
+The attribute/type interface is the first hook to check. If no such hook is found, or the hook returns `OverridableAlias` (see definition below), then dialect interfaces are involved.
+
+The dialect interface for one specific dialect could generate alias for all types/attributes, even when it does not "own" them. The `AsmPrinter` checks all dialect interfaces based on their order of registration. For example, the default alias `map` for `builtin` attribute `AffineMapAttr` could be overriden by the dialect interface for `my_dialect` as custom dialect is often registered after the `builtin` dialect.
 
 ```cpp
-// OpAsmDialectInterface is defined in
-// https://github.com/llvm/llvm-project/blob/91ab10e8d6c256d841da1a1a1b47c334e08d95b9/mlir/include/mlir/IR/OpImplementation.h#L1738
+/// Holds the result of `OpAsm{Dialect,Attr,Type}Interface::getAlias` hook call.
+enum class OpAsmAliasResult {
+  /// The object (type or attribute) is not supported by the hook
+  /// and an alias was not provided.
+  NoAlias,
+  /// An alias was provided, but it might be overriden by other hook.
+  OverridableAlias,
+  /// An alias was provided and it should be used
+  /// (no other hooks will be checked).
+  FinalAlias
+};
+```
+
+If multiple types/attributes have the same alias from `getAlias` hooks, a number is appended to the alias to avoid conflicts.
+
+### `OpAsmDialectInterface`
+
+```cpp
+#include "mlir/IR/OpImplementation.h"
+
 struct MyDialectOpAsmDialectInterface : public OpAsmDialectInterface {
  public:
   using OpAsmDialectInterface::OpAsmDialectInterface;
 
   AliasResult getAlias(Type type, raw_ostream& os) const override {
     if (mlir::isa<MyType>(type)) {
       os << "my_dialect_type";
-      // Could return OverridableAlias when
-      // allowing other dialect to override the alias.
-      //
-      // Other dialects are allowed to provide alias for
-      // type/attribute not owned by them
-      // but the final result would depend on the registration order
-      // of these dialects in the MLIRContext
       return AliasResult::FinalAlias;
     }
     return AliasResult::NoAlias;
@@ -47,8 +60,31 @@ void MyDialect::initialize() {
 }
 ```
 
-* If `getAlias` provides an alias with a trailing digit, `AsmPrinter` appends an underscore to avoid conflicts with autogenerated IDs.
-* If multiple types/attributes have the same alias from `getAlias`, a number is appended to the alias to avoid conflicts.
+### `OpAsmAttrInterface` and `OpAsmTypeInterface`
+
+The easiest way to use these interfaces is toggling `genMnemonicAlias` in the tablegen file of the attribute/alias. It directly uses the mnemonic as alias. See [Defining Dialect Attributes and Types](/docs/DefiningDialects/AttributesAndTypes) for details.
+
+If a more custom behavior is wanted, the following modification to the attribute/type should be made
+
+1. Add `OpAsmAttrInterface` or `OpAsmTypeInterface` into its trait list.
+2. Implement the `getAlias` method, either in tablegen or its cpp file.
+
+```tablegen
+include "mlir/IR/OpAsmInterface.td"
+
+// Add OpAsmAttrInterface trait
+def MyAttr : MyDialect_Attr<"MyAttr",
+         [ OpAsmAttrInterface ] > {
+
+  // This method could be put in the cpp file.
+  let extraClassDeclaration = [{
+    ::mlir::OpAsmAliasResult getAlias(::llvm::raw_ostream &os) const {
+      os << "alias_name";
+      return ::mlir::OpAsmAliasResult::OverridableAlias;
+    }
+  }];
+}
+```
 
 ## Suggesting SSA/Block Names
 
@@ -98,6 +134,8 @@ Similarly, an `Operation` can suggest the name for its block arguments using `ge
 For custom block names, `OpAsmOpInterface` has a method `getAsmBlockNames` so that
 the operation can suggest a custom prefix instead of a generic `^bb0`.
 
+Alternatively, `OpAsmTypeInterface` provides a `getAsmName` method for scenarios where the name could be inferred from its type.
+
 ## Defining Default Dialect
 
 An `Operation` can indicate that the nested region in it has a default dialect prefix, and the operations in the region could elide the dialect prefix.