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>

1 files changed, 2 insertions, 2 deletions

diff --git a/gdb/probe.h b/gdb/probe.h
index 31b0b8b..41e728a 100644
--- a/gdb/probe.h
+++ b/gdb/probe.h
@@ -143,7 +143,7 @@ public:
      corresponding to it.  The argument number is represented N.
      This function can throw an exception.  */
   virtual struct value *evaluate_argument (unsigned n,
-					   frame_info_ptr frame) = 0;
+					   const frame_info_ptr &frame) = 0;
 
   /* Compile the Nth argument of the probe to an agent expression.
      The argument number is represented by N.  */
@@ -301,7 +301,7 @@ extern struct cmd_list_element **info_probes_cmdlist_get (void);
    probe at that location, or if the probe does not have enough arguments,
    this returns NULL.  */
 
-extern struct value *probe_safe_evaluate_at_pc (frame_info_ptr frame,
+extern struct value *probe_safe_evaluate_at_pc (const frame_info_ptr &frame,
 						unsigned n);
 
 /* Return true if the PROVIDER/NAME probe from OBJFILE_NAME needs to be