Add publish/subscribe topics and channel: pass_events

This patch adds a new "struct compiler_channels" to hold channels
relating to the compiler that plugins (or diagnostic sinks) might want
to subscribe to events for, accessed from the global gcc::context
object, along with a new gcc/topics/ source subdirectory to hold
strongly-typed publish/subscribe topics relating to the compiler.

For now, there is just one: pass_events_channel, which, if there are any
subscribers, issues notifications about passes starting/stopping on
particular functions, using topics::pass_events, declared in
topics/pass-events.h, but followup patches add more kinds of
notification channel.

A toy plugin in the testsuite shows how this could be used to build a
progress notification UI for the compiler, and a followup patch uses the
channel to (optionally) capture CFG information at each stage of
optimization in machine-readable form into a SARIF sink.

gcc/ChangeLog:
	* channels.h: New file.
	* context.cc: Define INCLUDE_LIST.  Include "channels.h".
	(gcc::context::context): Create m_channels.
	(gcc::context::~context): Delete it.
	* context.h (struct compiler_channels): New forward decl.
	(gcc::context::get_channels): New accessor.
	(gcc::context::m_channels): New field.
	* passes.cc: Define INCLUDE_LIST.  Include "topics/pass-events.h"
	and "channels.h".
	(execute_one_pass): If the global context's pass_events_channel
	has subscribers, publish before_pass and after_pass events to it.
	* topics/pass-events.h: New file.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/plugin.exp: Add progress_notifications_plugin.cc.
	* gcc.dg/plugin/progress_notifications_plugin.cc: New test plugin.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

gcc/channels.h

@@ -0,0 +1,43 @@
+/* channels.h - Publish/Subscribe channels on compiler-specific topics.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   Contributed by David Malcolm <dmalcolm@redhat.com>.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_CHANNELS_H
+#define GCC_CHANNELS_H
+
+#include "pub-sub.h"
+
+namespace gcc {
+
+/* Forward decls of subscribers for the various topics we have
+   publish/subscribe channels for.  */
+namespace topics {
+  namespace pass_events { struct subscriber; }
+} // namespace gcc::topics
+
+/* Publish/subscribe channels on various compiler-specific topics.  */
+
+struct compiler_channels
+{
+  pub_sub::channel<topics::pass_events::subscriber> pass_events_channel;
+};
+
+} // namespace gcc
+
+#endif /* ! GCC_CHANNELS_H */

gcc/context.cc

@@ -17,6 +17,7 @@ You should have received a copy of the GNU General Public License
 along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
+#define INCLUDE_LIST
 #include "config.h"
 #include "system.h"
 #include "coretypes.h"
@@ -24,12 +25,15 @@ along with GCC; see the file COPYING3.  If not see
 #include "pass_manager.h"
 #include "dumpfile.h"
 #include "realmpfr.h"
+#include "channels.h"
 
 /* The singleton holder of global state: */
 gcc::context *g;
 
 gcc::context::context ()
-  : m_passes (NULL), m_dumps (new gcc::dump_manager ())
+: m_passes (NULL),
+  m_dumps (new gcc::dump_manager ()),
+  m_channels (new gcc::compiler_channels ())
 {
   have_offload = false;
 }
@@ -38,6 +42,7 @@ gcc::context::~context ()
 {
   delete m_passes;
   delete m_dumps;
+  delete m_channels;
 
   /* Release MPFR caches to avoid Valgrind leak reports.  */
   mpfr_free_cache ();

gcc/context.h

@@ -24,6 +24,7 @@ namespace gcc {
 
 class pass_manager;
 class dump_manager;
+struct compiler_channels;
 
 /* GCC's internal state can be divided into zero or more
    "parallel universe" of state; an instance of this class is one such
@@ -51,6 +52,15 @@ public:
 
   dump_manager *get_dumps () {gcc_assert (m_dumps); return m_dumps; }
 
+  /* Publish/subscribe channels for events
+     on various compiler-specific topics.  */
+  compiler_channels &
+  get_channels () const
+  {
+    gcc_assert (m_channels);
+    return *m_channels;
+  }
+
 private:
   /* Pass-management.  */
   pass_manager *m_passes;
@@ -58,6 +68,8 @@ private:
   /* Dump files.  */
   dump_manager *m_dumps;
 
+  compiler_channels *m_channels;
+
 }; // class context
 
 } // namespace gcc

gcc/passes.cc

@@ -22,6 +22,7 @@ along with GCC; see the file COPYING3.  If not see
    in the proper order, and counts the time used by each.
    Error messages and low-level interface to malloc also handled here.  */
 
+#define INCLUDE_LIST
 #include "config.h"
 #include "system.h"
 #include "coretypes.h"
@@ -63,6 +64,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "diagnostic-core.h" /* for fnotice */
 #include "stringpool.h"
 #include "attribs.h"
+#include "topics/pass-events.h"
+#include "channels.h"
 
 /* Reserved TODOs */
 #define TODO_verify_il			(1u << 31)
@@ -2575,10 +2578,15 @@ skip_pass (opt_pass *pass)
 bool
 execute_one_pass (opt_pass *pass)
 {
+  namespace pass_events = gcc::topics::pass_events;
+
   unsigned int todo_after = 0;
 
   bool gate_status;
 
+  if (auto channel = g->get_channels ().pass_events_channel.get_if_active ())
+    channel->publish (pass_events::before_pass {pass, cfun});
+
   /* IPA passes are executed on whole program, so cfun should be NULL.
      Other passes need function context set.  */
   if (pass->type == SIMPLE_IPA_PASS || pass->type == IPA_PASS)
@@ -2741,6 +2749,10 @@ execute_one_pass (opt_pass *pass)
 
   if (pass->type == SIMPLE_IPA_PASS || pass->type == IPA_PASS)
     report_heap_memory_use ();
+
+  if (auto channel = g->get_channels ().pass_events_channel.get_if_active ())
+    channel->publish (pass_events::after_pass {pass, cfun});
+
   return true;
 }
 

gcc/testsuite/gcc.dg/plugin/plugin.exp

@@ -200,6 +200,7 @@ set plugin_test_list [list \
 	  cpython-plugin-test-PyList_Append.c \
 	  cpython-plugin-test-PyList_New.c \
 	  cpython-plugin-test-PyLong_FromLong.c } \
+    { progress_notifications_plugin.cc } \
 ]
 
 foreach plugin_test $plugin_test_list {

gcc/testsuite/gcc.dg/plugin/progress_notifications_plugin.cc

@@ -0,0 +1,51 @@
+/* Toy implementation of progress notifications about the compiler,
+   using gcc::topics::pass_events.  */
+
+#define INCLUDE_LIST
+#include "gcc-plugin.h"
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tree-pass.h"
+#include "diagnostic-core.h"
+#include "context.h"
+#include "channels.h"
+#include "topics/pass-events.h"
+
+int plugin_is_GPL_compatible;
+
+namespace pass_events = gcc::topics::pass_events;
+
+namespace {
+
+class notification_event_subscriber : public pass_events::subscriber
+{
+public:
+  void on_message (const pass_events::before_pass &m) final override
+  {
+    if (m.fun)
+      inform (m.fun->function_start_locus, "starting pass %qs on %qD",
+	      m.pass->name, m.fun->decl);
+    else
+      inform (UNKNOWN_LOCATION, "starting pass %qs", m.pass->name);
+  }
+  void on_message (const pass_events::after_pass &m) final override
+  {
+    if (m.fun)
+      inform (m.fun->function_end_locus, "finished pass %qs on %qD",
+	      m.pass->name, m.fun->decl);
+    else
+      inform (UNKNOWN_LOCATION, "finished pass %qs", m.pass->name);
+  }
+} my_event_subscriber;
+
+} // anonymous namespace
+
+int
+plugin_init (struct plugin_name_args *,
+	     struct plugin_gcc_version *)
+{
+  g->get_channels ().pass_events_channel.add_subscriber (my_event_subscriber);
+
+  return 0;
+}

gcc/topics/pass-events.h

@@ -0,0 +1,59 @@
+/* pass-events.h - pub/sub messages about GCC optimization passes.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_TOPICS_PASS_EVENTS_H
+#define GCC_TOPICS_PASS_EVENTS_H
+
+#include "pub-sub.h"
+
+namespace gcc {
+namespace topics {
+
+/* A topic for messages relating to GCC optimization passes.  */
+
+namespace pass_events {
+
+struct before_pass
+{
+  opt_pass *pass;
+  function *fun;
+};
+
+struct after_pass
+{
+  opt_pass *pass;
+  function *fun;
+};
+
+/* Abstract base class for a subscriber to messages about
+   GCC optimization passes.  */
+
+struct subscriber
+{
+  virtual ~subscriber () = default;
+
+  virtual void on_message (const before_pass &) = 0;
+  virtual void on_message (const after_pass &) = 0;
+};
+
+} // namespace gcc::topics::pass_events
+} // namespace gcc::topics
+} // namespace gcc
+
+#endif /* ! GCC_TOPICS_PASS_EVENTS_H */