[Ada] Change return mechanism for variable-sized constrained array types

[Eric Botcazou <ebotcazou at adacore dot com>]

This changes the return mechanism for variable-sized constrained array types.
They used to be returned on the secondary stack, they are now returned on the
primary stack.  This is generally more efficient and, in particular, makes it
possible for the back-end to elide copies.

Tested on i586-suse-linux, applied on the mainline.


2011-03-24  Eric Botcazou  <ebotcazou@adacore.com>

	* einfo.ads (Size_Depends_On_Discriminant): Adjust description.
	* layout.adb (Compute_Size_Depends_On_Discriminant): New procedure
	to compute Set_Size_Depends_On_Discriminant.
	(Layout_Type): Call it on array types in back-end layout mode.
	* sem_util.adb (Requires_Transient_Scope): Return true for array
	types only if the size depends on the value of discriminants.
	* gcc-interface/utils2.c (build_binary_op) <MODIFY_EXPR>: Use the RHS
	type if the RHS is a call to a function that returns an unconstrained
	type with default discriminant.


2011-03-24  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/array16.ad[sb]: New test.
	* gnat.dg/array16.ads: New helper.


-- 
Eric Botcazou

Index: layout.adb
===================================================================
--- layout.adb	(revision 171345)
+++ layout.adb	(working copy)
@@ -6,7 +6,7 @@
 --                                                                          --
 --                                 B o d y                                  --
 --                                                                          --
---          Copyright (C) 2001-2010, Free Software Foundation, Inc.         --
+--          Copyright (C) 2001-2011, Free Software Foundation, Inc.         --
 --                                                                          --
 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
 -- terms of the  GNU General Public License as published  by the Free Soft- --
@@ -109,6 +109,12 @@ package body Layout is
    --  are of an enumeration type (so that the subtraction cannot be
    --  done directly) by applying the Pos operator to Hi/Lo first.
 
+   procedure Compute_Size_Depends_On_Discriminant (E : Entity_Id);
+   --  Given an array type or an array subtype E, compute whether its size
+   --  depends on the value of one or more discriminants and set the flag
+   --  Size_Depends_On_Discriminant accordingly. This need not be called
+   --  in front end layout mode since it does the computation on its own.
+
    function Expr_From_SO_Ref
      (Loc  : Source_Ptr;
       D    : SO_Ref;
@@ -1289,6 +1295,49 @@ package body Layout is
       end if;
    end Layout_Array_Type;
 
+   ------------------------------------------
+   -- Compute_Size_Depends_On_Discriminant --
+   ------------------------------------------
+
+   procedure Compute_Size_Depends_On_Discriminant (E : Entity_Id) is
+      Indx : Node_Id;
+      Ityp : Entity_Id;
+      Lo   : Node_Id;
+      Hi   : Node_Id;
+      Res  : Boolean := False;
+   begin
+      --  Loop to process array indexes
+
+      Indx := First_Index (E);
+      while Present (Indx) loop
+         Ityp := Etype (Indx);
+
+         --  If an index of the array is a generic formal type then there is
+         --  no point in determining a size for the array type.
+
+         if Is_Generic_Type (Ityp) then
+            return;
+         end if;
+
+         Lo := Type_Low_Bound (Ityp);
+         Hi := Type_High_Bound (Ityp);
+
+         if (Nkind (Lo) = N_Identifier
+               and then Ekind (Entity (Lo)) = E_Discriminant)
+           or else (Nkind (Hi) = N_Identifier
+                      and then Ekind (Entity (Hi)) = E_Discriminant)
+         then
+            Res := True;
+         end if;
+
+         Next_Index (Indx);
+      end loop;
+
+      if Res then
+         Set_Size_Depends_On_Discriminant (E);
+      end if;
+   end Compute_Size_Depends_On_Discriminant;
+
    -------------------
    -- Layout_Object --
    -------------------
@@ -2631,6 +2680,15 @@ package body Layout is
                   Set_Alignment (E, Uint_1);
                end if;
             end if;
+
+            --  We need to know whether the size depends on the value of one
+            --  or more discriminants to select the return mechanism. Skip if
+            --  errors are present, to prevent cascaded messages.
+
+            if Serious_Errors_Detected = 0 then
+               Compute_Size_Depends_On_Discriminant (E);
+            end if;
+
          end if;
       end if;
 
Index: einfo.ads
===================================================================
--- einfo.ads	(revision 171345)
+++ einfo.ads	(working copy)
@@ -6,7 +6,7 @@
 --                                                                          --
 --                                 S p e c                                  --
 --                                                                          --
---          Copyright (C) 1992-2010, Free Software Foundation, Inc.         --
+--          Copyright (C) 1992-2011, Free Software Foundation, Inc.         --
 --                                                                         --
 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
 -- terms of the  GNU General Public License as published  by the Free Soft- --
@@ -3573,8 +3573,8 @@ package Einfo is
 --    Size_Depends_On_Discriminant (Flag177)
 --       Present in all entities for types and subtypes. Indicates that the
 --       size of the type depends on the value of one or more discriminants.
---       Currently, this flag is only set in front end layout mode for arrays
---       which have one or more bounds depending on a discriminant value.
+--       Currently, this flag is only set for arrays which have one or more
+--       bounds depending on a discriminant value.
 
 --    Size_Known_At_Compile_Time (Flag92)
 --       Present in all entities for types and subtypes. Indicates that the
Index: gcc-interface/utils2.c
===================================================================
--- gcc-interface/utils2.c	(revision 171345)
+++ gcc-interface/utils2.c	(working copy)
@@ -186,7 +186,7 @@ known_alignment (tree exp)
 static tree
 find_common_type (tree t1, tree t2)
 {
-  /* ??? As of today, various constructs lead here with types of different
+  /* ??? As of today, various constructs lead to here with types of different
      sizes even when both constants (e.g. tagged types, packable vs regular
      component types, padded vs unpadded types, ...).  While some of these
      would better be handled upstream (types should be made consistent before
@@ -609,6 +609,15 @@ build_binary_op (enum tree_code op_code,
 	       && !integer_zerop (TYPE_SIZE (right_type)))
 	operation_type = left_type;
 
+      /* If we have a call to a function that returns an unconstrained type
+	 with default discriminant on the RHS, use the RHS type (which is
+	 padded) as we cannot compute the size of the actual assignment.  */
+      else if (TREE_CODE (right_operand) == CALL_EXPR
+	       && TYPE_IS_PADDING_P (right_type)
+	       && CONTAINS_PLACEHOLDER_P
+		  (TYPE_SIZE (TREE_TYPE (TYPE_FIELDS (right_type)))))
+	operation_type = right_type;
+
       /* Find the best type to use for copying between aggregate types.  */
       else if (((TREE_CODE (left_type) == ARRAY_TYPE
 		 && TREE_CODE (right_type) == ARRAY_TYPE)
Index: sem_util.adb
===================================================================
--- sem_util.adb	(revision 171345)
+++ sem_util.adb	(working copy)
@@ -6,7 +6,7 @@
 --                                                                          --
 --                                 B o d y                                  --
 --                                                                          --
---          Copyright (C) 1992-2010, Free Software Foundation, Inc.         --
+--          Copyright (C) 1992-2011, Free Software Foundation, Inc.         --
 --                                                                          --
 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
 -- terms of the  GNU General Public License as published  by the Free Soft- --
@@ -10473,11 +10473,11 @@ package body Sem_Util is
          if Requires_Transient_Scope (Component_Type (Typ)) then
             return True;
 
-         --  Otherwise, we only need a transient scope if the size is not
-         --  known at compile time.
+         --  Otherwise, we only need a transient scope if the size depends on
+         --  the value of one or more discriminants.
 
          else
-            return not Size_Known_At_Compile_Time (Typ);
+            return Size_Depends_On_Discriminant (Typ);
          end if;
 
       --  All other cases do not require a transient scope

package body Array16 is

  function F1 (A : access My_T1) return My_T1 is
  begin
    return A.all;
  end;

  function F2 (A : access My_T2) return My_T2 is
  begin
    return A.all;
  end;

  procedure Proc (A : access My_T1; B : access My_T2) is
    L1 : My_T1 := F1(A);
    L2 : My_T2 := F2(B);
  begin
    if L1.D = 0 and then L2(1) = 0 then
      raise Program_Error;
    end if;
  end;

end Array16;

package Array16_Pkg is

  function N return Integer;

end Array16_Pkg;

-- { dg-do compile }
-- { dg-options "-O -gnatn -fdump-tree-optimized" }

with Array16_Pkg;

package Array16 is

  type T1 (D : Integer) is record
    case D is
      when 1 => I : Integer;
      when others => null;
    end case;
  end record;

  type Arr is array (Integer range <>) of Integer;

  type My_T1 is new T1 (Array16_Pkg.N);
  type My_T2 is new Arr (1 .. Integer'Min (2, Array16_Pkg.N));

  function F1 (A : access My_T1) return My_T1;
  pragma Inline (F1);

  function F2 (A : access My_T2) return My_T2;
  pragma Inline (F2);

  procedure Proc (A : access My_T1; B : access My_T2);

end Array16;

-- { dg-final { scan-tree-dump-not "secondary_stack" "optimized" } }
-- { dg-final { cleanup-tree-dump "optimized" } }