DWARF: add DW_AT_location for global decls with DECL_VALUE_EXPR

In GNAT, we materialize renamings that cannot be described in standard
DWARF as synthetic variables that describe how to fetch the renamed
object.  Look for "___XR" in gcc/ada/exp_dbug.ads for more details about
this convention.

In order to have a location for these variables in the debug info (GDB
requires it not to discard the variable) but also to avoid allocating
runtime space for them, we make these variable hold a DECL_VALUE_EXPR
tree.  However, since GCC 7, the DWARF back-end no longer generates a
DW_AT_location attribute for those.  This patch is an attempt to restore
this attribute.

gcc/

	* dwarf2out.c (dwarf2out_late_global_decl): Add locations for
	symbols that hold a DECL_VALUE_EXPR.

gcc/testsuite/

	* debug12.adb, debug12.ads: New testcase.

From-SVN: r248792

gcc/ChangeLog

+2017-06-01  Pierre-Marie de Rodat  <derodat@adacore.com>
+
+	* dwarf2out.c (dwarf2out_late_global_decl): Add locations for
+	symbols that hold a DECL_VALUE_EXPR.
+
 2017-06-01  Martin Jambor  <mjambor@suse.cz>
 
 	PR tree-optimization/80898

gcc/dwarf2out.c

@@ -25545,9 +25545,10 @@ dwarf2out_late_global_decl (tree decl)
 	{
 	  /* We get called via the symtab code invoking late_global_decl
 	     for symbols that are optimized out.  Do not add locations
-	     for those.  */
+	     for those, except if they have a DECL_VALUE_EXPR, in which case
+	     they are relevant for debuggers.  */
 	  varpool_node *node = varpool_node::get (decl);
-	  if (! node || ! node->definition)
+	  if ((! node || ! node->definition) && ! DECL_HAS_VALUE_EXPR_P (decl))
 	    tree_add_const_value_attribute_for_decl (die, decl);
 	  else
 	    add_location_or_const_value_attribute (die, decl, false);

gcc/testsuite/ChangeLog

+2017-06-01  Pierre-Marie de Rodat  <derodat@adacore.com>
+
+	* debug12.adb, debug12.ads: New testcase.
+
 2017-06-01  Martin Jambor  <mjambor@suse.cz>
 
 	PR tree-optimization/80898

gcc/testsuite/gnat.dg/debug12.adb

+--  { dg-options "-cargs -gdwarf-4 -fdebug-types-section -dA -margs" }
+--  { dg-final { scan-assembler-times "DW_AT_location" 4 } }
+
+package body Debug12 is
+   function Get_A2 return Boolean is
+   begin
+      return A2;
+   end Get_A2;
+end Debug12;

gcc/testsuite/gnat.dg/debug12.ads

+package Debug12 is
+   type Bit_Array is array (Positive range <>) of Boolean
+      with Pack;
+   A  : Bit_Array := (1 .. 10 => False);
+   A2 : Boolean renames A (2);
+
+   function Get_A2 return Boolean;
+end Debug12;