=============
Clang Plugins
=============

Clang Plugins make it possible to run extra user defined actions during a
compilation. This document will provide a basic walkthrough of how to write and
run a Clang Plugin.

Introduction
============

Clang Plugins run FrontendActions over code. See the :doc:`FrontendAction
tutorial <RAVFrontendAction>` on how to write a ``FrontendAction`` using the
``RecursiveASTVisitor``. In this tutorial, we'll demonstrate how to write a
simple clang plugin.

Writing a ``PluginASTAction``
=============================

The main difference from writing normal ``FrontendActions`` is that you can
handle plugin command line options. The ``PluginASTAction`` base class declares
a ``ParseArgs`` method which you have to implement in your plugin.

.. code-block:: c++

  bool ParseArgs(const CompilerInstance &CI,
                 const std::vector<std::string>& args) {
    for (unsigned i = 0, e = args.size(); i != e; ++i) {
      if (args[i] == "-some-arg") {
        // Handle the command line argument.
      }
    }
    return true;
  }

Registering a plugin
====================

A plugin is loaded from a dynamic library at runtime by the compiler. To
register a plugin in a library, use ``FrontendPluginRegistry::Add<>``:

.. code-block:: c++

  static FrontendPluginRegistry::Add<MyPlugin> X("my-plugin-name", "my plugin description");

Defining pragmas
================

Plugins can also define pragmas by declaring a ``PragmaHandler`` and
registering it using ``PragmaHandlerRegistry::Add<>``:

.. code-block:: c++

  // Define a pragma handler for #pragma example_pragma
  class ExamplePragmaHandler : public PragmaHandler {
  public:
    ExamplePragmaHandler() : PragmaHandler("example_pragma") { }
    void HandlePragma(Preprocessor &PP, PragmaIntroducer Introducer,
                      Token &PragmaTok) {
      // Handle the pragma
    }
  };

  static PragmaHandlerRegistry::Add<ExamplePragmaHandler> Y("example_pragma","example pragma description");

Defining attributes
===================

Plugins can define attributes by declaring a ``ParsedAttrInfo`` and registering
it using ``ParsedAttrInfoRegister::Add<>``:

.. code-block:: c++

  class ExampleAttrInfo : public ParsedAttrInfo {
  public:
    ExampleAttrInfo() {
      Spellings.push_back({ParsedAttr::AS_GNU,"example"});
    }
    AttrHandling handleDeclAttribute(Sema &S, Decl *D,
                                     const ParsedAttr &Attr) const override {
      // Handle the attribute
      return AttributeApplied;
    }
  };

  static ParsedAttrInfoRegistry::Add<ExampleAttrInfo> Z("example_attr","example attribute description");

The members of ``ParsedAttrInfo`` that a plugin attribute must define are:

 * ``Spellings``, which must be populated with every `Spelling
   </doxygen/structclang_1_1ParsedAttrInfo_1_1Spelling.html>`_ of the
   attribute, each of which consists of an attribute syntax and how the
   attribute name is spelled for that syntax. If the syntax allows a scope then
   the spelling must be "scope::attr" if a scope is present or "::attr" if not.
 * ``handleDeclAttribute``, which is the function that applies the attribute to
   a declaration. It is responsible for checking that the attribute's arguments
   are valid, and typically applies the attribute by adding an ``Attr`` to the
   ``Decl``. It returns either ``AttributeApplied``, to indicate that the
   attribute was successfully applied, or ``AttributeNotApplied`` if it wasn't.

The members of ``ParsedAttrInfo`` that may need to be defined, depending on the
attribute, are:

 * ``NumArgs`` and ``OptArgs``, which set the number of required and optional
   arguments to the attribute.
 * ``diagAppertainsToDecl``, which checks if the attribute has been used on the
   right kind of declaration and issues a diagnostic if not.
 * ``diagLangOpts``, which checks if the attribute is permitted for the current
   language mode and issues a diagnostic if not.
 * ``existsInTarget``, which checks if the attribute is permitted for the given
   target.

To see a working example of an attribute plugin, see `the Attribute.cpp example
<https://github.com/llvm/llvm-project/blob/main/clang/examples/Attribute/Attribute.cpp>`_.

Putting it all together
=======================

Let's look at an example plugin that prints top-level function names.  This
example is checked into the clang repository; please take a look at
the `latest version of PrintFunctionNames.cpp
<https://github.com/llvm/llvm-project/blob/main/clang/examples/PrintFunctionNames/PrintFunctionNames.cpp>`_.

Running the plugin
==================


Using the compiler driver
--------------------------

The Clang driver accepts the `-fplugin` option to load a plugin.
Clang plugins can receive arguments from the compiler driver command
line via the `fplugin-arg-<plugin name>-<argument>` option. Using this
method, the plugin name cannot contain dashes itself, but the argument
passed to the plugin can.


.. code-block:: console

  $ export BD=/path/to/build/directory
  $ make -C $BD CallSuperAttr
  $ clang++ -fplugin=$BD/lib/CallSuperAttr.so \
            -fplugin-arg-call_super_plugin-help \
            test.cpp

If your plugin name contains dashes, either rename the plugin or used the
cc1 command line options listed below.


Using the cc1 command line
--------------------------

To run a plugin, the dynamic library containing the plugin registry must be
loaded via the `-load` command line option. This will load all plugins
that are registered, and you can select the plugins to run by specifying the
`-plugin` option. Additional parameters for the plugins can be passed with
`-plugin-arg-<plugin-name>`.

Note that those options must reach clang's cc1 process. There are two
ways to do so:

* Directly call the parsing process by using the `-cc1` option; this
  has the downside of not configuring the default header search paths, so
  you'll need to specify the full system path configuration on the command
  line.
* Use clang as usual, but prefix all arguments to the cc1 process with
  `-Xclang`.

For example, to run the ``print-function-names`` plugin over a source file in
clang, first build the plugin, and then call clang with the plugin from the
source tree:

.. code-block:: console

  $ export BD=/path/to/build/directory
  $ (cd $BD && make PrintFunctionNames )
  $ clang++ -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS \
            -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -D_GNU_SOURCE \
            -I$BD/tools/clang/include -Itools/clang/include -I$BD/include -Iinclude \
            tools/clang/tools/clang-check/ClangCheck.cpp -fsyntax-only \
            -Xclang -load -Xclang $BD/lib/PrintFunctionNames.so -Xclang \
            -plugin -Xclang print-fns

Also see the print-function-name plugin example's
`README <https://github.com/llvm/llvm-project/blob/main/clang/examples/PrintFunctionNames/README.txt>`_


Using the clang command line
----------------------------

Using `-fplugin=plugin` on the clang command line passes the plugin
through as an argument to `-load` on the cc1 command line. If the plugin
class implements the ``getActionType`` method then the plugin is run
automatically. For example, to run the plugin automatically after the main AST
action (i.e. the same as using `-add-plugin`):

.. code-block:: c++

  // Automatically run the plugin after the main AST action
  PluginASTAction::ActionType getActionType() override {
    return AddAfterMainAction;
  }

Interaction with ``-clear-ast-before-backend``
----------------------------------------------

To reduce peak memory usage of the compiler, plugins are recommended to run
*before* the main action, which is usually code generation. This is because
having any plugins that run after the codegen action automatically turns off
``-clear-ast-before-backend``.  ``-clear-ast-before-backend`` reduces peak
memory by clearing the Clang AST after generating IR and before running IR
optimizations. Use ``CmdlineBeforeMainAction`` or ``AddBeforeMainAction`` as
``getActionType`` to run plugins while still benefitting from
``-clear-ast-before-backend``. Plugins must make sure not to modify the AST,
otherwise they should run after the main action.