Fix false -Wodr warnings

[Jan Hubicka <hubicka@ucw.cz>]

Hi,
this patch fixes false warning that is output when different -std
settings are used. In this case C++ FE produces same declaration in
different representations which differ by 0 sized fileds only.
The patch makes them to be ignored (and I checked we ignore them for
canonical type merging too)

Bootstrapped/regtested x86_64-linux, comitted.

Honza

	PR lto/89358
	* g++.dg/lto/pr89358_0.C: New testcase.
	* g++.dg/lto/pr89358_1.C: New testcase.
	* ipa-devirt.c (skip_in_fields_list_p): New.
	(odr_types_equivalent_p): Use it.
Index: testsuite/g++.dg/lto/pr89358_0.C
===================================================================
--- testsuite/g++.dg/lto/pr89358_0.C	(nonexistent)
+++ testsuite/g++.dg/lto/pr89358_0.C	(working copy)
@@ -0,0 +1,11 @@
+/* { dg-do link } */
+/* { dg-options "-std=c++17"  } */
+#include <map>
+
+extern void test();
+
+int main()
+{
+        std::map<int, int> m;
+        test();
+}
Index: testsuite/g++.dg/lto/pr89358_1.C
===================================================================
--- testsuite/g++.dg/lto/pr89358_1.C	(nonexistent)
+++ testsuite/g++.dg/lto/pr89358_1.C	(working copy)
@@ -0,0 +1,7 @@
+/* { dg-options "-std=c++14"  } */
+#include <map>
+
+void test()
+{
+        std::map<int, int> m;
+}
Index: ipa-devirt.c
===================================================================
--- ipa-devirt.c	(revision 270324)
+++ ipa-devirt.c	(working copy)
@@ -1282,6 +1282,24 @@ warn_types_mismatch (tree t1, tree t2, l
     inform (loc_t2, "the incompatible type is defined here");
 }
 
+/* Return true if T should be ignored in TYPE_FIELDS for ODR comparsion.  */
+
+static bool
+skip_in_fields_list_p (tree t)
+{
+  if (TREE_CODE (t) != FIELD_DECL)
+    return true;
+  /* C++ FE introduces zero sized fields depending on -std setting, see
+     PR89358.  */
+  if (DECL_SIZE (t)
+      && integer_zerop (DECL_SIZE (t))
+      && DECL_ARTIFICIAL (t)
+      && DECL_IGNORED_P (t)
+      && !DECL_NAME (t))
+    return true;
+  return false;
+}
+
 /* Compare T1 and T2, report ODR violations if WARN is true and set
    WARNED to true if anything is reported.  Return true if types match.
    If true is returned, the types are also compatible in the sense of
@@ -1548,9 +1566,9 @@ odr_types_equivalent_p (tree t1, tree t2
 		 f1 = TREE_CHAIN (f1), f2 = TREE_CHAIN (f2))
 	      {
 		/* Skip non-fields.  */
-		while (f1 && TREE_CODE (f1) != FIELD_DECL)
+		while (f1 && skip_in_fields_list_p (f1))
 		  f1 = TREE_CHAIN (f1);
-		while (f2 && TREE_CODE (f2) != FIELD_DECL)
+		while (f2 && skip_in_fields_list_p (f2))
 		  f2 = TREE_CHAIN (f2);
 		if (!f1 || !f2)
 		  break;