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Confusion with tree_ssa_useless_type_conversion_1


At one point, that function used to pretty much just call
lang_hooks.types_compatible_p.  Now it does more.  Specifically, it can
return TRUE even if the lang hook returns FALSE.  That's a problem for
the optimizers and causes performance regressions.

For example, I noticed that trivial trail recursion elimination isn't
being done for Ada.  I used the simple example:

function Fact (I: Integer) return Integer is
begin
   if I = 1 then
      return 1;
   else
      return I * Fact (I - 1);
   end if;
end Fact;

The problem is the operand "I - 1".  The type "Integer" is really a subtype
(of Integer'Base).  So "I" is of that type. "I - 1", being an operand, is of
type Integer'Base.  So the call above actually has a conversion from the base
type to the subtype.  However, those two types look the same to
tree_ssa_useless_type_conversion_1 even though the langhook uses the default
and says they aren't.  That means that gimplification omits the conversion
which, in turn, means that the actual and formal aren't the same type
if you just use the lang hook. Tail recursion declines to optimize
it because of that.  The following fixes it:

*** tree-tailcall.c	28 Jul 2005 16:29:56 -0000	2.47
--- tree-tailcall.c	30 Sep 2005 18:39:38 -0000
*************** find_tail_calls (basic_block bb, struct 
*** 447,452 ****
  	         we emitted a suitable type conversion statement.  */
  	      if (!is_gimple_reg_type (TREE_TYPE (param))
! 		  || !lang_hooks.types_compatible_p (TREE_TYPE (param),
! 						     TREE_TYPE (arg)))
  		break;
  
--- 447,452 ----
  	         we emitted a suitable type conversion statement.  */
  	      if (!is_gimple_reg_type (TREE_TYPE (param))
! 		  || !tree_ssa_useless_type_conversion_1 (TREE_TYPE (param),
! 							  TREE_TYPE (arg)))
  		break;
  
However, I think that *every* call to lang_hooks.types_compatible_p in the
tree-*.c files (and possibly gimplify.c) except for the one in
tree_ssa_useless_type_conversion_1 (there are 20 of them) ought to be handled
the same as above.

Comments?


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