gdb: pass frames as `const frame_info_ptr &`

We currently pass frames to function by value, as `frame_info_ptr`.
This is somewhat expensive:

 - the size of `frame_info_ptr` is 64 bytes, which is a bit big to pass
   by value
 - the constructors and destructor link/unlink the object in the global
   `frame_info_ptr::frame_list` list.  This is an `intrusive_list`, so
   it's not so bad: it's just assigning a few points, there's no memory
   allocation as if it was `std::list`, but still it's useless to do
   that over and over.

As suggested by Tom Tromey, change many function signatures to accept
`const frame_info_ptr &` instead of `frame_info_ptr`.

Some functions reassign their `frame_info_ptr` parameter, like:

  void
  the_func (frame_info_ptr frame)
  {
    for (; frame != nullptr; frame = get_prev_frame (frame))
      {
        ...
      }
  }

I wondered what to do about them, do I leave them as-is or change them
(and need to introduce a separate local variable that can be
re-assigned).  I opted for the later for consistency.  It might not be
clear why some functions take `const frame_info_ptr &` while others take
`frame_info_ptr`.  Also, if a function took a `frame_info_ptr` because
it did re-assign its parameter, I doubt that we would think to change it
to `const frame_info_ptr &` should the implementation change such that
it doesn't need to take `frame_info_ptr` anymore.  It seems better to
have a simple rule and apply it everywhere.

Change-Id: I59d10addef687d157f82ccf4d54f5dde9a963fd0
Approved-By: Andrew Burgess <aburgess@redhat.com>

2 files changed, 5 insertions, 5 deletions

diff --git a/gdb/mi/mi-cmd-stack.c b/gdb/mi/mi-cmd-stack.c
index d4202b9..09c31b6 100644
--- a/gdb/mi/mi-cmd-stack.c
+++ b/gdb/mi/mi-cmd-stack.c
@@ -44,7 +44,7 @@ enum what_to_list { locals, arguments, all };
 static void list_args_or_locals (const frame_print_options &fp_opts,
 				 enum what_to_list what,
 				 enum print_values values,
-				 frame_info_ptr fi,
+				 const frame_info_ptr &fi,
 				 int skip_unavailable);
 
 /* True if we want to allow Python-based frame filters.  */
@@ -62,7 +62,7 @@ mi_cmd_enable_frame_filters (const char *command, const char *const *argv,
 /* Like apply_ext_lang_frame_filter, but take a print_values */
 
 static enum ext_lang_bt_status
-mi_apply_ext_lang_frame_filter (frame_info_ptr frame,
+mi_apply_ext_lang_frame_filter (const frame_info_ptr &frame,
 				frame_filter_flags flags,
 				enum print_values print_values,
 				struct ui_out *out,
@@ -577,7 +577,7 @@ list_arg_or_local (const struct frame_arg *arg, enum what_to_list what,
 static void
 list_args_or_locals (const frame_print_options &fp_opts,
 		     enum what_to_list what, enum print_values values,
-		     frame_info_ptr fi, int skip_unavailable)
+		     const frame_info_ptr &fi, int skip_unavailable)
 {
   const struct block *block;
   const char *name_of_result;
diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
index db670e3..f4ab179 100644
--- a/gdb/mi/mi-main.c
+++ b/gdb/mi/mi-main.c
@@ -89,7 +89,7 @@ static void mi_execute_async_cli_command (const char *cli_command,
 					  const char *const *argv, int argc);
 static bool register_changed_p (int regnum, readonly_detached_regcache *,
 			       readonly_detached_regcache *);
-static void output_register (frame_info_ptr, int regnum, int format,
+static void output_register (const frame_info_ptr &, int regnum, int format,
 			     int skip_unavailable);
 
 /* Controls whether the frontend wants MI in async mode.  */
@@ -1097,7 +1097,7 @@ mi_cmd_data_list_register_values (const char *command, const char *const *argv,
    unavailable.  */
 
 static void
-output_register (frame_info_ptr frame, int regnum, int format,
+output_register (const frame_info_ptr &frame, int regnum, int format,
 		 int skip_unavailable)
 {
   struct ui_out *uiout = current_uiout;