calls: Introduce cxx17_empty_base_field_p [PR94383]

As multiple targets are affected apparently, I believe at least
aarch64, arm, powerpc64le, s390{,x} and ia64,
I think we should have a middle-end predicate for this, so that if we need
to tweak it, we can do it in one spot.

2020-04-22  Jakub Jelinek  <jakub@redhat.com>

	PR target/94383
	* calls.h (cxx17_empty_base_field_p): Declare.
	* calls.c (cxx17_empty_base_field_p): Define.

gcc/ChangeLog

+2020-04-22  Jakub Jelinek  <jakub@redhat.com>
+
+	PR target/94383
+	* calls.h (cxx17_empty_base_field_p): Declare.
+	* calls.c (cxx17_empty_base_field_p): Define.
+
 2020-04-22  Christophe Lyon  <christophe.lyon@linaro.org>
 
 	* doc/sourcebuild.texi (arm_softfp_ok, arm_hard_ok): Document.

gcc/calls.c

@@ -6261,5 +6261,23 @@ must_pass_va_arg_in_stack (tree type)
   return targetm.calls.must_pass_in_stack (arg);
 }
 
+/* Return true if FIELD is the C++17 empty base field that should
+   be ignored for ABI calling convention decisions in order to
+   maintain ABI compatibility between C++14 and earlier, which doesn't
+   add this FIELD to classes with empty bases, and C++17 and later
+   which does.  */
+
+bool
+cxx17_empty_base_field_p (const_tree field)
+{
+  return (TREE_CODE (field) == FIELD_DECL
+	  && DECL_ARTIFICIAL (field)
+	  && RECORD_OR_UNION_TYPE_P (TREE_TYPE (field))
+	  && DECL_SIZE (field)
+	  && integer_zerop (DECL_SIZE (field))
+	  && TYPE_SIZE (TREE_TYPE (field))
+	  && !integer_zerop (TYPE_SIZE (TREE_TYPE (field))));
+}
+
 /* Tell the garbage collector about GTY markers in this source file.  */
 #include "gt-calls.h"

gcc/calls.h

@@ -135,5 +135,6 @@ extern tree get_attr_nonstring_decl (tree, tree * = NULL);
 extern void maybe_warn_nonstring_arg (tree, tree);
 extern bool get_size_range (tree, tree[2], bool = false);
 extern rtx rtx_for_static_chain (const_tree, bool);
+extern bool cxx17_empty_base_field_p (const_tree);
 
 #endif // GCC_CALLS_H