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/dwarf2/expr.h b/gdb/dwarf2/expr.h
index 6acd078..b02cc53 100644
--- a/gdb/dwarf2/expr.h
+++ b/gdb/dwarf2/expr.h
@@ -137,7 +137,7 @@ struct dwarf_expr_context
      The ADDR_INFO property can be specified to override the range of
      memory addresses with the passed in buffer.  */
   value *evaluate (const gdb_byte *addr, size_t len, bool as_lval,
-		   dwarf2_per_cu_data *per_cu, frame_info_ptr frame,
+		   dwarf2_per_cu_data *per_cu, const frame_info_ptr &frame,
 		   const struct property_addr_info *addr_info = nullptr,
 		   struct type *type = nullptr,
 		   struct type *subobj_type = nullptr,
@@ -256,7 +256,7 @@ private:
 
 /* Return the value of register number REG (a DWARF register number),
    read as an address in a given FRAME.  */
-CORE_ADDR read_addr_from_reg (frame_info_ptr frame, int reg);
+CORE_ADDR read_addr_from_reg (const frame_info_ptr &frame, int reg);
 
 void dwarf_expr_require_composition (const gdb_byte *, const gdb_byte *,
 				     const char *);