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-7.2.0.orig/gcc/config/m68k/m68k.c gcc-7.2.0/gcc/config/m68k/m68k.c
--- gcc-7.2.0.orig/gcc/config/m68k/m68k.c	2017-04-03 22:30:56.274463000 +0000
+++ gcc-7.2.0/gcc/config/m68k/m68k.c	2018-01-27 02:16:53.779367849 +0000
@@ -182,6 +182,8 @@
 			      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;
 static enum flt_eval_method
 m68k_excess_precision (enum excess_precision_type);
@@ -332,6 +334,9 @@
 #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,
@@ -6571,4 +6576,20 @@
   return FLT_EVAL_METHOD_UNPREDICTABLE;
 }
 
+/* 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"
