DWARF: no location for non-definition DECLs with non-trivial DECL_VALUE_EXPR

This patch restricts the set of cases in which we allow the generation of
location attributes for variables that are not defined in the current unit.
For such variables with complex DECL_VALUE_EXPR trees, generating a location
attribute can end up creating relocations to text symbols in the debug section
of LTO object files, which is not valid.

gcc/
	PR lto/84213
	* dwarf2out.c (is_trivial_indirect_ref): New function.
	(dwarf2out_late_global_decl): Do not generate a location
	attribute for variables that have a non-trivial DECL_VALUE_EXPR
	and that are not defined in the current unit.

From-SVN: r257526

gcc/ChangeLog

+2018-02-09  Pierre-Marie de Rodat  <derodat@adacore.com>
+
+	PR lto/84213
+	* dwarf2out.c (is_trivial_indirect_ref): New function.
+	(dwarf2out_late_global_decl): Do not generate a location
+	attribute for variables that have a non-trivial DECL_VALUE_EXPR
+	and that are not defined in the current unit.
+
 2018-02-09  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* optabs.c (prepare_cmp_insn): Try harder to emit a direct comparison

gcc/dwarf2out.c

@@ -26235,6 +26235,23 @@ dwarf2out_early_global_decl (tree decl)
   symtab->global_info_ready = save;
 }
 
+/* Return whether EXPR is an expression with the following pattern:
+   INDIRECT_REF (NOP_EXPR (INTEGER_CST)).  */
+
+static bool
+is_trivial_indirect_ref (tree expr)
+{
+  if (expr == NULL_TREE || TREE_CODE (expr) != INDIRECT_REF)
+    return false;
+
+  tree nop = TREE_OPERAND (expr, 0);
+  if (nop == NULL_TREE || TREE_CODE (nop) != NOP_EXPR)
+    return false;
+
+  tree int_cst = TREE_OPERAND (nop, 0);
+  return int_cst != NULL_TREE && TREE_CODE (int_cst) == INTEGER_CST;
+}
+
 /* Output debug information for global decl DECL.  Called from
    toplev.c after compilation proper has finished.  */
 
@@ -26259,11 +26276,17 @@ dwarf2out_late_global_decl (tree decl)
       if (die)
 	{
 	  /* We get called via the symtab code invoking late_global_decl
-	     for symbols that are optimized out.  Do not add locations
-	     for those, except if they have a DECL_VALUE_EXPR, in which case
-	     they are relevant for debuggers.  */
+	     for symbols that are optimized out.
+
+	     Do not add locations for those, except if they have a
+	     DECL_VALUE_EXPR, in which case they are relevant for debuggers.
+	     Still don't add a location if the DECL_VALUE_EXPR is not a trivial
+	     INDIRECT_REF expression, as this could generate relocations to
+	     text symbols in LTO object files, which is invalid.  */
 	  varpool_node *node = varpool_node::get (decl);
-	  if ((! node || ! node->definition) && ! DECL_HAS_VALUE_EXPR_P (decl))
+	  if ((! node || ! node->definition)
+	      && ! (DECL_HAS_VALUE_EXPR_P (decl)
+		    && is_trivial_indirect_ref (DECL_VALUE_EXPR (decl))))
 	    tree_add_const_value_attribute_for_decl (die, decl);
 	  else
 	    add_location_or_const_value_attribute (die, decl, false);