[patch] don't assume VIEW_CONVERT => !lhs in tree-nested

[Olivier Hainque <hainque at adacore dot com>]

Hello,

Ada allows view conversions to appear on the left hand side of an
assignment and this requires special care in various places in the
compiler.

In particular, VIEW_CONVERT_EXPR may appear as an assignment lhs and
this is apparently overlooked in tree-nested:

   convert_local_reference (tree *tp, int *walk_subtrees, void *data)
   { ...
     switch (TREE_CODE (t))
       {
       ...
       default:
	 if (!IS_TYPE_OR_DECL_P (t))
	   {
	     *walk_subtrees = 1;
	     wi->val_only = true;
	     wi->is_lhs = false;
	   }

... likewise in convert_nonlocal_reference.

This leads to wrong code generation for the Ada testcase below:

   procedure Lhs_View_Convert is

      type Root is tagged record
	 RV : Natural;
      end record;

      type Derived is new Root with null record;

      Root_Instance : Root := (RV => 1);

      Derived_Instance : Derived;

      procedure Process is
	 X : Natural := Derived_Instance.RV;
      begin
	 null;
      end;
   begin
      Derived_Instance.RV := 2;

      Root (Derived_Instance) := Root (Root_Instance);

      if Derived_Instance.RV /= Root_Instance.RV then
	 raise Program_Error;
      end if;
   end;


The gimple output for

  "Root (Derived_Instance) := Root (Root_Instance);"

reads

  A843b
  = VIEW_CONVERT_EXPR<struct lhs_view_convert__root>(derived_instance)._tag;

  VIEW_CONVERT_EXPR<struct lhs_view_convert__root>(derived_instance)
  = root_instance;

  VIEW_CONVERT_EXPR<struct lhs_view_convert__root>(derived_instance)._tag
  = A843b;

The reference to "Derived_Instance" in the nested "Process" subprogram
triggers various reference conversions, and the code excerpt quoted
above forces an intermediate temporary for the second gimple assignment:

  [.005t.nested]
  ...
  D.1916 = FRAME.264.derived_instance;
  VIEW_CONVERT_EXPR<struct lhs_view_convert__root>(D.1916) = root_instance;

The bulk of the assignment thus eventually doesn't affect the actual target
object and the test unexpectedly raises Program_Error.

The attached patch fixes this by handling VIEW_CONVERT_EXPR explicitly
in convert_[non]local_reference, to request walking the subtrees only and
not touch the current is_lhs/val_only indications.

Bootstrapped and regtested succesfully on i686-pc-linux-gnu.

Thanks in advance,

With Kind Regards,

Olivier


2006-12-12  Olivier Hainque  <hainque@adacore.com>

	* tree-nested.c (convert_local_reference): Handle VIEW_CONVERT_EXPR.
	Request walking the subtrees only, leaving the current is_lhs/val_only
	untouched.
	(convert_non_local_reference): Likewise.

	testsuite/
	* gnat.dg/lhs_view_convert.adb: New test.

Attachment: lhs_view_convert.dif
Description: Text document