[Ada] Fix internal error on extension of record with representation clause

[Pierre-Marie de Rodat]

This fixes a long-standing issue present for extensions of tagged record
types with a representation clause: the clause is correctly inherited
for components inherited in the extension but the position and size are
not, which fools the logic of Is_Possibly_Unaligned_Object.

This can result in an attempt to take the address of a component not
aligned on a byte boundary, which is then flagged as an internal error.

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

2018-08-21  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * exp_util.adb (Is_Possibly_Unaligned_Object): For the case of a
        selected component inherited in a record extension and subject
        to a representation clause, retrieve the position and size from
        the original record component.

gcc/testsuite/

        * gnat.dg/rep_clause7.adb: New testcase.
--- gcc/ada/exp_util.adb
+++ gcc/ada/exp_util.adb
@@ -8402,9 +8402,26 @@ package body Exp_Util is
 
                declare
                   Align_In_Bits : constant Nat := M * System_Storage_Unit;
+                  Off : Uint;
+                  Siz : Uint;
                begin
-                  if Component_Bit_Offset (C) mod Align_In_Bits /= 0
-                    or else Esize (C) mod Align_In_Bits /= 0
+                  --  For a component inherited in a record extension, the
+                  --  clause is inherited but position and size are not set.
+
+                  if Is_Base_Type (Etype (P))
+                    and then Is_Tagged_Type (Etype (P))
+                    and then Present (Original_Record_Component (C))
+                  then
+                     Off :=
+                       Component_Bit_Offset (Original_Record_Component (C));
+                     Siz := Esize (Original_Record_Component (C));
+                  else
+                     Off := Component_Bit_Offset (C);
+                     Siz := Esize (C);
+                  end if;
+
+                  if Off mod Align_In_Bits /= 0
+                    or else Siz mod Align_In_Bits /= 0
                   then
                      return True;
                   end if;

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/rep_clause7.adb
@@ -0,0 +1,29 @@
+procedure Rep_Clause7 is
+
+   subtype Msg is String (1 .. 3);
+
+   type Root is tagged record
+     B : Boolean;
+     M : Msg;
+   end record;
+   for Root use record
+     B at 0 range 64 .. 64;
+     M at 0 range 65 .. 88;
+   end record;
+
+   type Ext is new Root with null record;
+
+   procedure Inner (T : Msg) is
+   begin
+      null;
+   end;
+
+   pragma Warnings (Off);
+   T1 : Root;
+   T2 : Ext;
+   pragma Warnings (On);
+
+begin
+   Inner (T1.M);
+   Inner (T2.M);
+end;