[Ada] Spurious output on optimized default-initialized limited aggregate

[Arnaud Charlet <charlet at adacore dot com>]

When expanding a limited aggregate into individual assignments, we create a
transient scope if the type of a component requires it. This must not be done
if the context is an initialization procedure, because the target of the
assignment must be visible outside of the block, and stack cleanup will happen
on return from the initialization call. Otherwise this may result in dangling
stack references in the back-end, which produce garbled results when compiled
at higher optimization levels.

Executing the following:

   gnatmake -q -O2 cutdown
   cutdown

must yield:

   0.00000E+00

---
with Text_IO; use Text_IO;
procedure Cutdown is

   type Angle_Object_T is tagged record
      M_Value : Float := 0.0;
   end record;

   Zero : constant Angle_Object_T := (M_Value => 0.0);

   type Platform_T is record
      M_Roll : Angle_Object_T := Zero;
   end record;

   package Observable_Nongeneric is
      type Writer_T is tagged limited record
         M_Value : Platform_T;
      end record;

      function Init (Value : in Platform_T) return Writer_T;
   end Observable_Nongeneric;

   package body Observable_Nongeneric is

   --------------------------------------------------------------------------
      function Init (Value : in Platform_T) return Writer_T is
      begin
         return (M_Value => Value);
      end Init;
   --------------------------------------------------------------------------
   end Observable_Nongeneric;

   type Object_T is tagged limited record
      M_Platform : aliased Observable_Nongeneric.Writer_T :=
        Observable_Nongeneric.Init (Platform_T'(others => <>));
   end record;

   Data : Object_T;
begin
   Put_Line (Data.M_Platform.M_Value.M_Roll.M_Value'Img);

   if Data.M_Platform.M_Value.M_Roll.M_Value /= 0.0 then
      raise Program_Error;
   end if;
end Cutdown;

Tested on x86_64-pc-linux-gnu, committed on trunk

2014-10-20  Ed Schonberg  <schonberg@adacore.com>

	* exp_aggr.adb (Convert_To_Assignments): Do not create a
	transient scope for a component whose type requires it, if the
	context is an initialization procedure, because the target of
	the assignment must be visible outside of the block.

Attachment: difs
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