@c Copyright (c) 2009 Free Software Foundation, Inc.
@c Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Plugins
@chapter Plugins
@cindex Plugins

@section Loading Plugins

Plugins are supported on platforms that support @option{-ldl
-rdynamic}.  They are loaded by the compiler using @code{dlopen}
and invoked at pre-determined locations in the compilation
process.

Plugins are loaded with 

@option{-fplugin=/path/to/NAME.so} @option{-fplugin-arg-NAME-<key1>[=<value1>]}

The plugin arguments are parsed by GCC and passed to respective
plugins as key-value pairs. Multiple plugins can be invoked by
specifying multiple @option{-fplugin} arguments.


@section Plugin API

Plugins are activated by the compiler at specific events as defined in
@file{gcc-plugin.h}.  For each event of interest, the plugin should
call @code{register_callback} specifying the name of the event and
address of the callback function that will handle that event.

The header @file{gcc-plugin.h} must be the first gcc header to be included.

@subsection Plugin license check

Every plugin should define the global symbol @code{plugin_is_GPL_compatible}
to assert that it has been licensed under a GPL-compatible license.
If this symbol does not exist, the compiler will emit a fatal error
and exit with the error message:

@smallexample
fatal error: plugin <name> is not licensed under a GPL-compatible license
<name>: undefined symbol: plugin_is_GPL_compatible
compilation terminated
@end smallexample

The type of the symbol is irrelevant.  The compiler merely asserts that
it exists in the global scope.  Something like this is enough:

@smallexample
int plugin_is_GPL_compatible;
@end smallexample

@subsection Plugin initialization

Every plugin should export a function called @code{plugin_init} that
is called right after the plugin is loaded. This function is
responsible for registering all the callbacks required by the plugin
and do any other required initialization.

This function is called from @code{compile_file} right before invoking
the parser.  The arguments to @code{plugin_init} are:

@itemize @bullet
@item @code{plugin_info}: Plugin invocation information.
@item @code{version}: GCC version.
@end itemize

The @code{plugin_info} struct is defined as follows:

@smallexample
struct plugin_name_args
@{
  char *base_name;              /* Short name of the plugin
                                   (filename without .so suffix). */
  const char *full_name;        /* Path to the plugin as specified with
                                   -fplugin=. */
  int argc;                     /* Number of arguments specified with
                                   -fplugin-arg-.... */
  struct plugin_argument *argv; /* Array of ARGC key-value pairs. */
  const char *version;          /* Version string provided by plugin. */
  const char *help;             /* Help string provided by plugin. */
@}
@end smallexample

If initialization fails, @code{plugin_init} must return a non-zero
value.  Otherwise, it should return 0.

The version of the GCC compiler loading the plugin is described by the
following structure:

@smallexample
struct plugin_gcc_version
@{
  const char *basever;
  const char *datestamp;
  const char *devphase;
  const char *revision;
  const char *configuration_arguments;
@};
@end smallexample

The function @code{plugin_default_version_check} takes two pointers to
such structure and compare them field by field. It can be used by the
plugin's @code{plugin_init} function.


@subsection Plugin callbacks

Callback functions have the following prototype:

@smallexample
/* The prototype for a plugin callback function.
     gcc_data  - event-specific data provided by GCC
     user_data - plugin-specific data provided by the plug-in.  */
typedef void (*plugin_callback_func)(void *gcc_data, void *user_data);
@end smallexample

Callbacks can be invoked at the following pre-determined events:


@smallexample
enum plugin_event
@{
  PLUGIN_PASS_MANAGER_SETUP,    /* To hook into pass manager.  */
  PLUGIN_FINISH_TYPE,           /* After finishing parsing a type.  */
  PLUGIN_FINISH_UNIT,           /* Useful for summary processing.  */
  PLUGIN_CXX_CP_PRE_GENERICIZE, /* Allows to see low level AST in C++ FE.  */
  PLUGIN_FINISH,                /* Called before GCC exits.  */
  PLUGIN_INFO,                  /* Information about the plugin. */
  PLUGIN_GGC_START,		/* Called at start of GCC Garbage Collection. */
  PLUGIN_GGC_MARKING,		/* Extend the GGC marking. */
  PLUGIN_GGC_END,		/* Called at end of GGC. */
  PLUGIN_REGISTER_GGC_ROOTS,	/* Register an extra GGC root table. */
  PLUGIN_ATTRIBUTES,            /* Called during attribute registration */
  PLUGIN_START_UNIT,            /* Called before processing a translation unit.  */
  PLUGIN_EVENT_LAST             /* Dummy event used for indexing callback
                                   array.  */
@};
@end smallexample


To register a callback, the plugin calls @code{register_callback} with
the arguments:

@itemize
@item @code{char *name}: Plugin name.
@item @code{enum plugin_event event}: The event code.
@item @code{plugin_callback_func callback}: The function that handles @code{event}.
@item @code{void *user_data}: Pointer to plugin-specific data.
@end itemize

For the PLUGIN_PASS_MANAGER_SETUP, PLUGIN_INFO, and
PLUGIN_REGISTER_GGC_ROOTS pseudo-events the @code{callback} should be
null, and the @code{user_data} is specific.

@section Interacting with the pass manager

There needs to be a way to add/reorder/remove passes dynamically. This
is useful for both analysis plugins (plugging in after a certain pass
such as CFG or an IPA pass) and optimization plugins.

Basic support for inserting new passes or replacing existing passes is
provided. A plugin registers a new pass with GCC by calling
@code{register_callback} with the @code{PLUGIN_PASS_MANAGER_SETUP}
event and a pointer to a @code{struct plugin_pass} object defined as follows

@smallexample
enum pass_positioning_ops
@{
  PASS_POS_INSERT_AFTER,  // Insert after the reference pass.
  PASS_POS_INSERT_BEFORE, // Insert before the reference pass.
  PASS_POS_REPLACE        // Replace the reference pass.
@};

struct plugin_pass
@{
  struct opt_pass *pass;            /* New pass provided by the plugin.  */
  const char *reference_pass_name;  /* Name of the reference pass for hooking
                                       up the new pass.  */
  int ref_pass_instance_number;     /* Insert the pass at the specified
                                       instance number of the reference pass.  */
                                    /* Do it for every instance if it is 0.  */
  enum pass_positioning_ops pos_op; /* how to insert the new pass.  */
@};


/* Sample plugin code that registers a new pass.  */
int
plugin_init (struct plugin_name_args *plugin_info,
             struct plugin_gcc_version *version)
@{
  struct plugin_pass pass_info;

  ...

  /* Code to fill in the pass_info object with new pass information.  */

  ...

  /* Register the new pass.  */
  register_callback (plugin_info->base_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &pass_info);

  ...
@}
@end smallexample


@section Interacting with the GCC Garbage Collector 

Some plugins may want to be informed when GGC (the GCC Garbage
Collector) is running. They can register callbacks for the
@code{PLUGIN_GGC_START} and @code{PLUGIN_GGC_END} events (for which
the callback is called with a null @code{gcc_data}) to be notified of
the start or end of the GCC garbage collection.

Some plugins may need to have GGC mark additional data. This can be
done by registering a callback (called with a null @code{gcc_data})
for the @code{PLUGIN_GGC_MARKING} event. Such callbacks can call the
@code{ggc_set_mark} routine, preferably thru the @code{ggc_mark} macro
(and conversely, these routines should usually not be used in plugins
outside of the @code{PLUGIN_GGC_MARKING} event).  

Some plugins may need to add extra GGC root tables, e.g. to handle
their own @code{GTY}-ed data. This can be done with the
@code{PLUGIN_REGISTER_GGC_ROOTS} pseudo-event with a null callback and
the extra root table as @code{user_data}.  Running the @code{gengtype
-p @var{source-dir} @var{file-list} @var{plugin*.c} ...} utility
generates this extra root table.

You should understand the details of memory management inside GCC
before using @code{PLUGIN_GGC_MARKING} or
@code{PLUGIN_REGISTER_GGC_ROOTS}.


@section Giving information about a plugin

A plugin should give some information to the user about itself. This
uses the following structure:

@smallexample
struct plugin_info
@{
  const char *version;
  const char *help;
@};
@end smallexample

Such a structure is passed as the @code{user_data} by the plugin's
init routine using @code{register_callback} with the
@code{PLUGIN_INFO} pseudo-event and a null callback.

@section Registering custom attributes

For analysis purposes it is useful to be able to add custom attributes.

The @code{PLUGIN_ATTRIBUTES} callback is called during attribute
registration. Use the @code{register_attribute} function to register
custom attributes.

@smallexample
/* Attribute handler callback */
static tree
handle_user_attribute (tree *node, tree name, tree args,
			int flags, bool *no_add_attrs)
@{
  return NULL_TREE;
@}

/* Attribute definition */
static struct attribute_spec user_attr =
  @{ "user", 1, 1, false,  false, false, handle_user_attribute @};

/* Plugin callback called during attribute registration.
Registered with register_callback (plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL)
*/
static void 
register_attributes (void *event_data, void *data)
@{
  warning (0, G_("Callback to register attributes"));
  register_attribute (&user_attr);
@}

@end smallexample


@section Building GCC plugins

If plugins are enabled, GCC installs the headers needed to build a
plugin (somehwere in the installation tree, e.g. under
@file{/usr/local}).  In particular a @file{plugin/include} directory
is installed, containing all the header files needed to build plugins.

On most systems, you can query this @code{plugin} directory by
invoking @command{gcc -print-file-name=plugin} (replace if needed
@command{gcc} with the appropriate program path).

The following GNU Makefile excerpt shows how to build a simple plugin:

@smallexample
GCC=gcc
PLUGIN_SOURCE_FILES= plugin1.c plugin2.c
PLUGIN_OBJECT_FILES= $(patsubst %.c,%.o,$(PLUGIN_SOURCE_FILES))
GCCPLUGINS_DIR:= $(shell $(GCC) -print-file-name=plugin)
CFLAGS+= -I$(GCCPLUGINS_DIR)/include -fPIC -O2

plugin.so: $(PLUGIN_OBJECT_FILES)
   $(GCC) -shared $^ -o $@@
@end smallexample

A single source file plugin may be built with @code{gcc -I`gcc
-print-file-name=plugin`/include -fPIC -shared -O2 plugin.c -o
plugin.so}, using backquote shell syntax to query the @file{plugin}
directory.

Plugins needing to use @command{gengtype} require a GCC build
directory for the same version of GCC that they will be linked
against.
