Backport from upstream, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68467

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

diff -Nur gcc-6.4.0.orig/gcc/config/m68k/m68k.c gcc-6.4.0/gcc/config/m68k/m68k.c
--- gcc-6.4.0.orig/gcc/config/m68k/m68k.c	2016-01-04 15:30:50.652828000 +0100
+++ gcc-6.4.0/gcc/config/m68k/m68k.c	2018-02-17 19:08:16.313168186 +0100
@@ -180,6 +180,8 @@
 			      const_tree, bool);
 static bool m68k_cannot_force_const_mem (machine_mode mode, rtx x);
 static bool m68k_output_addr_const_extra (FILE *, rtx);
+static machine_mode m68k_promote_function_mode (const_tree, machine_mode,
+						int *, const_tree, int);
 static void m68k_init_sync_libfuncs (void) ATTRIBUTE_UNUSED;
 
 /* Initialize the GCC target structure.  */
@@ -322,6 +324,9 @@
 #undef TARGET_ATOMIC_TEST_AND_SET_TRUEVAL
 #define TARGET_ATOMIC_TEST_AND_SET_TRUEVAL 128
 
+#undef TARGET_PROMOTE_FUNCTION_MODE
+#define TARGET_PROMOTE_FUNCTION_MODE m68k_promote_function_mode
+
 static const struct attribute_spec m68k_attribute_table[] =
 {
   /* { name, min_len, max_len, decl_req, type_req, fn_type_req, handler,
@@ -6524,4 +6529,20 @@
 	      == m68k_fk_interrupt_handler));
 }
 
+/* Implement TARGET_PROMOTE_FUNCTION_MODE.  */
+
+static machine_mode
+m68k_promote_function_mode (const_tree type, machine_mode mode,
+                            int *punsignedp ATTRIBUTE_UNUSED,
+                            const_tree fntype ATTRIBUTE_UNUSED,
+                            int for_return)
+{
+  /* Promote libcall arguments narrower than int to match the normal C
+     ABI (for which promotions are handled via
+     TARGET_PROMOTE_PROTOTYPES).  */
+  if (type == NULL_TREE && !for_return && (mode == QImode || mode == HImode))
+    return SImode;
+  return mode;
+}
+
 #include "gt-m68k.h"