gcc/doc/plugins.texi

@c Copyright (C) 2009-2021 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

GCC plugins are loadable modules that provide extra features to the
compiler.  Like GCC itself they can be distributed in source and
binary forms.

GCC plugins provide developers with a rich subset of
the GCC API to allow them to extend GCC as they see fit.
Whether it is writing an additional optimization pass,
transforming code, or analyzing information, plugins
can be quite useful.

@menu
* Plugins loading::      How can we load plugins.
* Plugin API::           The APIs for plugins.
* Plugins pass::         How a plugin interact with the pass manager.
* Plugins GC::           How a plugin Interact with GCC Garbage Collector.
* Plugins description::  Giving information about a plugin itself.
* Plugins attr::         Registering custom attributes or pragmas.
* Plugins recording::    Recording information about pass execution.
* Plugins gate::         Controlling which passes are being run.
* Plugins tracking::     Keeping track of available passes.
* Plugins building::     How can we build a plugin.
@end menu

@node Plugins loading
@section Loading Plugins

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

Plugins are loaded with

@option{-fplugin=/path/to/@var{name}.@var{ext}} @option{-fplugin-arg-@var{name}-@var{key1}[=@var{value1}]}

Where @var{name} is the plugin name and @var{ext} is the platform-specific
dynamic library extension. It should be @code{dll} on Windows/MinGW,
@code{dylib} on Darwin/Mac OS X, and @code{so} on all other platforms.
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.

A plugin can be simply given by its short name (no dots or
slashes). When simply passing @option{-fplugin=@var{name}}, the plugin is
loaded from the @file{plugin} directory, so @option{-fplugin=@var{name}} is
the same as @option{-fplugin=`gcc -print-file-name=plugin`/@var{name}.@var{ext}},
using backquote shell syntax to query the @file{plugin} directory.

@node Plugin API
@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 @var{name} is not licensed under a GPL-compatible license
@var{name}: undefined symbol: plugin_is_GPL_compatible
compilation terminated
@end smallexample

The declared type of the symbol should be int, to match a forward declaration
in @file{gcc-plugin.h} that suppresses C++ mangling.  It does not need to be in
any allocated section, though.  The compiler merely asserts that
the symbol 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.

The version of GCC used to compile the plugin can be found in the symbol
@code{gcc_version} defined in the header @file{plugin-version.h}. The
recommended version check to perform looks like

@smallexample
#include "plugin-version.h"
...

int
plugin_init (struct plugin_name_args *plugin_info,
             struct plugin_gcc_version *version)
@{
  if (!plugin_default_version_check (version, &gcc_version))
    return 1;

@}
@end smallexample

but you can also check the individual fields if you want a less strict check.

@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_START_PARSE_FUNCTION,  /* Called before parsing the body of a function. */
  PLUGIN_FINISH_PARSE_FUNCTION, /* After finishing parsing a function. */
  PLUGIN_PASS_MANAGER_SETUP,    /* To hook into pass manager.  */
  PLUGIN_FINISH_TYPE,           /* After finishing parsing a type.  */
  PLUGIN_FINISH_DECL,           /* After finishing parsing a declaration. */
  PLUGIN_FINISH_UNIT,           /* Useful for summary processing.  */
  PLUGIN_PRE_GENERICIZE,        /* Allows to see low level AST in C and C++ frontends.  */
  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_PRAGMAS,               /* Called during pragma registration. */
  /* Called before first pass from all_passes.  */
  PLUGIN_ALL_PASSES_START,
  /* Called after last pass from all_passes.  */
  PLUGIN_ALL_PASSES_END,
  /* Called before first ipa pass.  */
  PLUGIN_ALL_IPA_PASSES_START,
  /* Called after last ipa pass.  */
  PLUGIN_ALL_IPA_PASSES_END,
  /* Allows to override pass gate decision for current_pass.  */
  PLUGIN_OVERRIDE_GATE,
  /* Called before executing a pass.  */
  PLUGIN_PASS_EXECUTION,
  /* Called before executing subpasses of a GIMPLE_PASS in
     execute_ipa_pass_list.  */
  PLUGIN_EARLY_GIMPLE_PASSES_START,
  /* Called after executing subpasses of a GIMPLE_PASS in
     execute_ipa_pass_list.  */
  PLUGIN_EARLY_GIMPLE_PASSES_END,
  /* Called when a pass is first instantiated.  */
  PLUGIN_NEW_PASS,
/* Called when a file is #include-d or given via the #line directive.
   This could happen many times.  The event data is the included file path,
   as a const char* pointer.  */
  PLUGIN_INCLUDE_FILE,

  /* Called when -fanalyzer starts. The event data is an
     ana::plugin_analyzer_init_iface *.  */
  PLUGIN_ANALYZER_INIT,

  PLUGIN_EVENT_FIRST_DYNAMIC    /* Dummy event used for indexing callback
                                   array.  */
@};
@end smallexample

In addition, plugins can also look up the enumerator of a named event,
and / or generate new events dynamically, by calling the function
@code{get_named_event_id}.

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

@itemize
@item @code{char *name}: Plugin name.
@item @code{int 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 @i{PLUGIN_PASS_MANAGER_SETUP}, @i{PLUGIN_INFO}, and
@i{PLUGIN_REGISTER_GGC_ROOTS} pseudo-events the @code{callback} should be null,
and the @code{user_data} is specific.

When the @i{PLUGIN_PRAGMAS} event is triggered (with a null pointer as
data from GCC), plugins may register their own pragmas.  Notice that
pragmas are not available from @file{lto1}, so plugins used with
@code{-flto} option to GCC during link-time optimization cannot use
pragmas and do not even see functions like @code{c_register_pragma} or
@code{pragma_lex}.

The @i{PLUGIN_INCLUDE_FILE} event, with a @code{const char*} file path as
GCC data, is triggered for processing of @code{#include} or
@code{#line} directives.

The @i{PLUGIN_FINISH} event is the last time that plugins can call GCC
functions, notably emit diagnostics with @code{warning}, @code{error}
etc.


@node Plugins pass
@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 register_pass_info} 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 register_pass_info
@{
  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 register_pass_info 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


@node Plugins GC
@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 through the @code{ggc_mark} macro
(and conversely, these routines should usually not be used in plugins
outside of the @code{PLUGIN_GGC_MARKING} event).  Plugins that wish to hold
weak references to gc data may also use this event to drop weak references when
the object is about to be collected.  The @code{ggc_marked_p} function can be
used to tell if an object is marked, or is about to  be collected.  The
@code{gt_clear_cache} overloads which some types define may also be of use in
managing weak references.

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 (of type @code{struct
ggc_root_tab*}) as @code{user_data}.  Running the
 @code{gengtype -p @var{source-dir} @var{file-list} @var{plugin*.c} ...}
utility generates these extra root tables.

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


@node Plugins description
@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.

@node Plugins attr
@section Registering custom attributes or pragmas

For analysis (or other) purposes it is useful to be able to add custom
attributes or pragmas.

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, false, handle_user_attribute, NULL @};

/* 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


The @i{PLUGIN_PRAGMAS} callback is called once during pragmas
registration. Use the @code{c_register_pragma},
@code{c_register_pragma_with_data},
@code{c_register_pragma_with_expansion},
@code{c_register_pragma_with_expansion_and_data} functions to register
custom pragmas and their handlers (which often want to call
@code{pragma_lex}) from @file{c-family/c-pragma.h}.

@smallexample
/* Plugin callback called during pragmas registration. Registered with
     register_callback (plugin_name, PLUGIN_PRAGMAS,
                        register_my_pragma, NULL);
*/
static void
register_my_pragma (void *event_data, void *data)
@{
  warning (0, G_("Callback to register pragmas"));
  c_register_pragma ("GCCPLUGIN", "sayhello", handle_pragma_sayhello);
@}
@end smallexample

It is suggested to pass @code{"GCCPLUGIN"} (or a short name identifying
your plugin) as the ``space'' argument of your pragma.

Pragmas registered with @code{c_register_pragma_with_expansion} or
@code{c_register_pragma_with_expansion_and_data} support
preprocessor expansions. For example:

@smallexample
#define NUMBER 10
#pragma GCCPLUGIN foothreshold (NUMBER)
@end smallexample

@node Plugins recording
@section Recording information about pass execution

The event PLUGIN_PASS_EXECUTION passes the pointer to the executed pass
(the same as current_pass) as @code{gcc_data} to the callback.  You can also
inspect cfun to find out about which function this pass is executed for.
Note that this event will only be invoked if the gate check (if
applicable, modified by PLUGIN_OVERRIDE_GATE) succeeds.
You can use other hooks, like @code{PLUGIN_ALL_PASSES_START},
@code{PLUGIN_ALL_PASSES_END}, @code{PLUGIN_ALL_IPA_PASSES_START},
@code{PLUGIN_ALL_IPA_PASSES_END}, @code{PLUGIN_EARLY_GIMPLE_PASSES_START},
and/or @code{PLUGIN_EARLY_GIMPLE_PASSES_END} to manipulate global state
in your plugin(s) in order to get context for the pass execution.


@node Plugins gate
@section Controlling which passes are being run

After the original gate function for a pass is called, its result
- the gate status - is stored as an integer.
Then the event @code{PLUGIN_OVERRIDE_GATE} is invoked, with a pointer
to the gate status in the @code{gcc_data} parameter to the callback function.
A nonzero value of the gate status means that the pass is to be executed.
You can both read and write the gate status via the passed pointer.


@node Plugins tracking
@section Keeping track of available passes

When your plugin is loaded, you can inspect the various
pass lists to determine what passes are available.  However, other
plugins might add new passes.  Also, future changes to GCC might cause
generic passes to be added after plugin loading.
When a pass is first added to one of the pass lists, the event
@code{PLUGIN_NEW_PASS} is invoked, with the callback parameter
@code{gcc_data} pointing to the new pass.


@node Plugins building
@section Building GCC plugins

If plugins are enabled, GCC installs the headers needed to build a
plugin (somewhere 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).

Inside plugins, this @code{plugin} directory name can be queried by
calling @code{default_plugin_dir_name ()}.

Plugins may know, when they are compiled, the GCC version for which
@file{plugin-version.h} is provided.  The constant macros
@code{GCCPLUGIN_VERSION_MAJOR}, @code{GCCPLUGIN_VERSION_MINOR},
@code{GCCPLUGIN_VERSION_PATCHLEVEL}, @code{GCCPLUGIN_VERSION} are
integer numbers, so a plugin could ensure it is built for GCC 4.7 with 
@smallexample
#if GCCPLUGIN_VERSION != 4007
#error this GCC plugin is for GCC 4.7
#endif
@end smallexample

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

@smallexample
HOST_GCC=g++
TARGET_GCC=gcc
PLUGIN_SOURCE_FILES= plugin1.c plugin2.cc
GCCPLUGINS_DIR:= $(shell $(TARGET_GCC) -print-file-name=plugin)
CXXFLAGS+= -I$(GCCPLUGINS_DIR)/include -fPIC -fno-rtti -O2

plugin.so: $(PLUGIN_SOURCE_FILES)
   $(HOST_GCC) -shared $(CXXFLAGS) $^ -o $@@
@end smallexample

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

Plugin support on Windows/MinGW has a number of limitations and
additional requirements. When building a plugin on Windows we have to
link an import library for the corresponding backend executable, for
example, @file{cc1.exe}, @file{cc1plus.exe}, etc., in order to gain
access to the symbols provided by GCC. This means that on Windows a
plugin is language-specific, for example, for C, C++, etc. If you wish
to use your plugin with multiple languages, then you will need to
build multiple plugin libraries and either instruct your users on how
to load the correct version or provide a compiler wrapper that does
this automatically.

Additionally, on Windows the plugin library has to export the
@code{plugin_is_GPL_compatible} and @code{plugin_init} symbols. If you
do not wish to modify the source code of your plugin, then you can use
the @option{-Wl,--export-all-symbols} option or provide a suitable DEF
file. Alternatively, you can export just these two symbols by decorating
them with @code{__declspec(dllexport)}, for example:

@smallexample
#ifdef _WIN32
__declspec(dllexport)
#endif
int plugin_is_GPL_compatible;

#ifdef _WIN32
__declspec(dllexport)
#endif
int plugin_init (plugin_name_args *, plugin_gcc_version *)
@end smallexample

The import libraries are installed into the @code{plugin} directory
and their names are derived by appending the @code{.a} extension to
the backend executable names, for example, @file{cc1.exe.a},
@file{cc1plus.exe.a}, etc. The following command line shows how to
build the single source file plugin on Windows to be used with the C++
compiler:

@smallexample
g++ -I`gcc -print-file-name=plugin`/include -shared -Wl,--export-all-symbols \
-o plugin.dll plugin.c `gcc -print-file-name=plugin`/cc1plus.exe.a
@end smallexample

When a plugin needs to use @command{gengtype}, be sure that both
@file{gengtype} and @file{gtype.state} have the same version as the
GCC for which the plugin is built.