[mlir] Move PyConcreteAttribute to header. NFC.

[ftynse]

Summary

This allows out-of-tree users to derive PyConcreteAttribute to bind custom
attributes.

[stellaraccident]

Noted in the other thread: can you include this in the discourse discussion? (This was part of an ongoing discussion on the circt side and would be good to join)

[jdd]

This is great, though IRModule.h isn't in the mlir include directory, so it's kinda awkward for out-of-tree users to #include it. Could it be moved? (Along with a few other header files in the python directory.)

[mikeurbach]

I saw this, and agree this is great. I had the same question as John when I saw it. I've tested a branch of circt that included headers from this directory before, but I wasn't sure if they should move to the normal header directory eventually.

mlir/lib/Bindings/Python/IRAttributes.cpp

 using llvm::Twine;
 
 namespace {
 
 static MlirStringRef toMlirStringRef(const std::string &s) {
   return mlirStringRefCreate(s.data(), s.size());
 }
 
-/// CRTP base classes for Python attributes that subclass Attribute and should
-/// be castable from it (i.e. via something like StringAttr(attr)).
-/// By default, attribute class hierarchies are one level deep (i.e. a
-/// concrete attribute class extends PyAttribute); however, intermediate
-/// python-visible base classes can be modeled by specifying a BaseTy.
-template <typename DerivedTy, typename BaseTy = PyAttribute>
-class PyConcreteAttribute : public BaseTy {
-public:
-  // Derived classes must define statics for:
-  //   IsAFunctionTy isaFunction
-  //   const char *pyClassName
-  using ClassTy = py::class_<DerivedTy, BaseTy>;
-  using IsAFunctionTy = bool (*)(MlirAttribute);
-
-  PyConcreteAttribute() = default;
-  PyConcreteAttribute(PyMlirContextRef contextRef, MlirAttribute attr)
-      : BaseTy(std::move(contextRef), attr) {}
-  PyConcreteAttribute(PyAttribute &orig)
-      : PyConcreteAttribute(orig.getContext(), castFrom(orig)) {}
-
-  static MlirAttribute castFrom(PyAttribute &orig) {
-    if (!DerivedTy::isaFunction(orig)) {
-      auto origRepr = py::repr(py::cast(orig)).cast<std::string>();
-      throw SetPyError(PyExc_ValueError, Twine("Cannot cast attribute to ") +
-                                             DerivedTy::pyClassName +
-                                             " (from " + origRepr + ")");
-    }
-    return orig;
-  }
-
-  static void bind(py::module &m) {
-    auto cls = ClassTy(m, DerivedTy::pyClassName, py::buffer_protocol());
-    cls.def(py::init<PyAttribute &>(), py::keep_alive<0, 1>());
-    DerivedTy::bindDerived(cls);
-  }
-
-  /// Implemented by derived classes to add methods to the Python subclass.
-  static void bindDerived(ClassTy &m) {}
-};
-
 class PyAffineMapAttribute : public PyConcreteAttribute<PyAffineMapAttribute> {
 public:
   static constexpr IsAFunctionTy isaFunction = mlirAttributeIsAAffineMap;
   static constexpr const char *pyClassName = "AffineMapAttr";
   using PyConcreteAttribute::PyConcreteAttribute;
 
   static void bindDerived(ClassTy &c) {
     c.def_static(

mlir/lib/Bindings/Python/IRModule.h

private:
   // Since the MlirNamedAttr contains an internal pointer to the actual
   // memory of the owned string, it must be heap allocated to remain valid.
   // Otherwise, strings that fit within the small object optimization threshold
   // will have their memory address change as the containing object is moved,
   // resulting in an invalid aliased pointer.
   std::unique_ptr<std::string> ownedName;
 };
 
+/// CRTP base classes for Python attributes that subclass Attribute and should
+/// be castable from it (i.e. via something like StringAttr(attr)).
+/// By default, attribute class hierarchies are one level deep (i.e. a
+/// concrete attribute class extends PyAttribute); however, intermediate
+/// python-visible base classes can be modeled by specifying a BaseTy.
+template <typename DerivedTy, typename BaseTy = PyAttribute>
+class PyConcreteAttribute : public BaseTy {
+public:
+  // Derived classes must define statics for:
+  //   IsAFunctionTy isaFunction
+  //   const char *pyClassName
+  using ClassTy = pybind11::class_<DerivedTy, BaseTy>;
+  using IsAFunctionTy = bool (*)(MlirAttribute);
+
+  PyConcreteAttribute() = default;
+  PyConcreteAttribute(PyMlirContextRef contextRef, MlirAttribute attr)
+      : BaseTy(std::move(contextRef), attr) {}
+  PyConcreteAttribute(PyAttribute &orig)
+      : PyConcreteAttribute(orig.getContext(), castFrom(orig)) {}
+
+  static MlirAttribute castFrom(PyAttribute &orig) {
+    if (!DerivedTy::isaFunction(orig)) {
+      auto origRepr = pybind11::repr(pybind11::cast(orig)).cast<std::string>();
+      throw SetPyError(PyExc_ValueError,
+                       llvm::Twine("Cannot cast attribute to ") +
+                           DerivedTy::pyClassName + " (from " + origRepr + ")");
+    }
+    return orig;
+  }
+
+  static void bind(pybind11::module &m) {
+    auto cls = ClassTy(m, DerivedTy::pyClassName, pybind11::buffer_protocol());
+    cls.def(pybind11::init<PyAttribute &>(), pybind11::keep_alive<0, 1>());
+    DerivedTy::bindDerived(cls);
+  }
+
+  /// Implemented by derived classes to add methods to the Python subclass.
+  static void bindDerived(ClassTy &m) {}
+};
+
 /// Wrapper around the generic MlirType.
 /// The lifetime of a type is bound by the PyContext that created it.
 class PyType : public BaseContextObject {
 public:
   PyType(PyMlirContextRef contextRef, MlirType type)
       : BaseContextObject(std::move(contextRef)), type(type) {}
   bool operator==(const PyType &other);
   operator MlirType() const { return type; }