[PATCH] prevent dwarf2 collapsing of nested arrays for Ada

[Olivier Hainque <hainque at adacore dot com>]

Hello,

add_subscript_info and gen_array_type_die currently work together to
collapse nested array types into a single array type DIE with multiple
subscripts in most circumstances (except for MIPS_DEBUGGING_INFO).

The bulk of this is:

<< add_subscript_info (dw_die_ref type_die, tree type)
  { ...
  /* The GNU compilers represent multidimensional array types as sequences of
     one dimensional array types whose element types are themselves array
     types.  Here we squish that down, so that each multidimensional array
     type gets only one array_type DIE in the Dwarf debugging info. The draft
     Dwarf specification say that we are allowed to do this kind of
     compression in C (because there is no difference between an array or
     arrays and a multidimensional array in C) but for other source languages
     (e.g. Ada) we probably shouldn't do this.  */
  [...]
>>

It turns out that the comment is right in saying "we probably
shouldn't do this for Ada". Ada is very flexible in allowing
compositions of arrays and records of variable size, and this
transformation is indeed problematic in this respect.

The visible effect is variations of debugger misbehavior trying to
print "My_Game" from the testcase below:

   with Games; use Games;
   procedure Foo is
      My_Game : Game;
   begin
      null;
   end;

   package Games is
      Name_Size : Natural := 4;
      Pragma Volatile (Name_Size);

      type Name_Table is array (1 .. 10) of String (1 .. Name_Size);

      type Game is record
	 Players : Name_Table := (others => (others => ' '));
      end record;
   end;

With GCC mainline and GDB 6.8 on x86_64-suse-linux, for instance, we
get

   << $1 = (players => ()) >>

instead of the expected

   << $1 = (players => ("    ", "    ", "    ", "    ", "    ", "    ",
           "    ", "    ", "    ", "    ")) >>

The attached patch is a suggestion to address this by disabling the
transformation for Ada, reorganizing the add_subscript_info/
gen_array_type_die synchronization along the way.

Bootstrapped and regression tested on x86_64-suse-linux plus
mips-sgi-irix6.5 as the MIPS_DEBUGGING_INFO control is performed in a
slightly different manner.

I also checked that the GDB testsuite yields identical results before
and after the patch on both targets.

Thanks in advance,

Olivier

2008-05-22  Olivier Hainque  <hainque@adacore.com>

	* dwarf2out.c (add_subscript_info): New explicit COLLAPSE_P
	argument, saying whether nested array are to be collapsed
	into a single array type DIE with multiple subscripts.
	(gen_array_type_die): Factorize comments about the MIPS_DEBUG_INFO
	issues, centralize the nested array types collapsing control and
	disable the transformation for Ada.

Attachment: dw2-arrays.dif
Description: Text document