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, 4 insertions, 4 deletions

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 9cfae10..d0256e7 100644
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -49,7 +49,7 @@ static const registry<gdbarch>::key<dwarf_gdbarch_types> dwarf_arch_cookie;
 /* Ensure that a FRAME is defined, throw an exception otherwise.  */
 
 static void
-ensure_have_frame (frame_info_ptr frame, const char *op_name)
+ensure_have_frame (const frame_info_ptr &frame, const char *op_name)
 {
   if (frame == nullptr)
     throw_error (GENERIC_ERROR,
@@ -78,7 +78,7 @@ bits_to_bytes (ULONGEST start, ULONGEST n_bits)
 /* See expr.h.  */
 
 CORE_ADDR
-read_addr_from_reg (frame_info_ptr frame, int reg)
+read_addr_from_reg (const frame_info_ptr &frame, int reg)
 {
   struct gdbarch *gdbarch = get_frame_arch (frame);
   int regnum = dwarf_reg_to_regnum_or_error (gdbarch, reg);
@@ -112,7 +112,7 @@ static piece_closure *
 allocate_piece_closure (dwarf2_per_cu_data *per_cu,
 			dwarf2_per_objfile *per_objfile,
 			std::vector<dwarf_expr_piece> &&pieces,
-			frame_info_ptr frame)
+			const frame_info_ptr &frame)
 {
   piece_closure *c = new piece_closure;
 
@@ -1076,7 +1076,7 @@ dwarf_expr_context::fetch_result (struct type *type, struct type *subobj_type,
 
 value *
 dwarf_expr_context::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,
 			      struct type *type, struct type *subobj_type,
 			      LONGEST subobj_offset)