[PATCHv8 4/4] gdb: cache program space entry point information

After the previous two patches, there are now two places where we
check if a frame is an entry point frame, these are in get_prev_frame
and frame_unwind_caller_frame.  Both of these locations call
inside_entry_func, which then calls program_space::get_entry_point_info.

The calls to program_space::get_entry_point_info are not crazy
expensive, but they are not free either, there are reads from target
memory to read the auxv vector and the entry address offset, so on
remote targets this could introduce a small delay.

However, the result from program_space::get_entry_point_info is not
expected to change from one call to the next.  This information should
be a property of the executable and libraries, so we really only need
to figure it out once.

This commit caches the entry point information within program_space,
clearing it whenever an inferior starts or exits, or whenever the
executable is updated.  After clearing GDB will compute, and cache the
updated information the next time it is needed, which will be whenever
GDB needs to unwind a frame.

It will be possible to observe this change by, for example, monitoring
the remote target packets, but as far as the normal GDB output is
concerned, there should be no user visible changes after this commit.
---
 gdb/progspace.c | 45 ++++++++++++++++++++++++++++++++++++++++++---
 gdb/progspace.h |  9 ++++++++-
 2 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/gdb/progspace.c b/gdb/progspace.c
index 4de32bc522f..b1e313f97ff 100644
--- a/gdb/progspace.c
+++ b/gdb/progspace.c
@@ -289,9 +289,12 @@ program_space::exec_entry_point_address () const
 
 /* See progspace.h.  */
 
-entry_point_info
+const entry_point_info &
 program_space::get_entry_point_info () const
 {
+  if (m_entry_point_info.has_value ())
+    return m_entry_point_info.value ();
+
   std::optional<CORE_ADDR> exec_entry_address
     = this->exec_entry_point_address_if_available ();
 
@@ -299,8 +302,9 @@ program_space::get_entry_point_info () const
   if (m_solib_ops != nullptr)
     inferior_entry_address = m_solib_ops->inferior_entry_point_address ();
 
-  return entry_point_info (std::move (inferior_entry_address),
-			   std::move (exec_entry_address));
+  m_entry_point_info.emplace (std::move (inferior_entry_address),
+			      std::move (exec_entry_address));
+  return m_entry_point_info.value ();
 }
 
 /* Implement the 'maint info entry-address' command.  */
@@ -538,11 +542,46 @@ program_space::clear_solib_cache ()
   deleted_solibs.clear ();
 }
 
+/* Clear cached entry point information in the program space of INF.  */
+
+static void
+clear_cached_entry_point_info_for_inferior (inferior *inf)
+{
+  inf->pspace->clear_cached_entry_point_info ();
+}
+
+/* Clear cached entry point information in the program space PSPACE.  */
+
+static void
+clear_cached_entry_point_info_for_pspace (program_space *pspace,
+					  bool /* reload */)
+{
+  pspace->clear_cached_entry_point_info ();
+}
+
+/* Clear cached entry point information in the program space of EXEC_INF.  */
+
+static void
+clear_cached_entry_point_info_after_exec (inferior *exec_inf,
+					  inferior */* follow_inf */)
+{
+  exec_inf->pspace->clear_cached_entry_point_info ();
+}
+
 /* See progspace.h.  */
 
 void
 initialize_progspace ()
 {
+  gdb::observers::inferior_created.attach
+    (clear_cached_entry_point_info_for_inferior, "program-space");
+  gdb::observers::inferior_exit.attach
+    (clear_cached_entry_point_info_for_inferior, "program-space");
+  gdb::observers::executable_changed.attach
+    (clear_cached_entry_point_info_for_pspace, "program-space");
+  gdb::observers::inferior_execd.attach
+    (clear_cached_entry_point_info_after_exec, "program-space");
+
   add_cmd ("program-spaces", class_maintenance,
 	   maintenance_info_program_spaces_command,
 	   _("Info about currently known program spaces."),
diff --git a/gdb/progspace.h b/gdb/progspace.h
index be2d3f7e88c..bfe0e5ec36f 100644
--- a/gdb/progspace.h
+++ b/gdb/progspace.h
@@ -370,7 +370,11 @@ struct program_space
   /* Return information about the entry point in the main executable, and
      the entry point for the inferior, which might be different from the
      main executable.  */
-  entry_point_info get_entry_point_info () const;
+  const entry_point_info &get_entry_point_info () const;
+
+  /* Clear any cached entry point information.  */
+  void clear_cached_entry_point_info ()
+  { m_entry_point_info.reset (); }
 
   /* If there is a valid and known entry point in the main executable of
      this program space, return it.  Otherwise return an empty optional.  */
@@ -463,6 +467,9 @@ struct program_space
 
   /* See `exec_filename`.  */
   gdb::unique_xmalloc_ptr<char> m_exec_filename;
+
+  /* Cached entry point information.  See get_entry_point_info.  */
+  mutable std::optional<entry_point_info> m_entry_point_info;
 };
 
 /* The list of all program spaces.  There's always at least one.  */
-- 
2.25.4