diff -r 0fe2b7509707 Modules/_ctypes/libffi_msvc/ffi.c
--- a/Modules/_ctypes/libffi_msvc/ffi.c	Tue Jul 19 16:41:20 2016 -0500
+++ b/Modules/_ctypes/libffi_msvc/ffi.c	Thu Jul 21 13:02:01 2016 +0100
@@ -378,7 +378,7 @@
 
   if ( cif->rtype->type == FFI_TYPE_STRUCT ) {
     *rvalue = *(void **) argp;
-    argp += 4;
+    argp += sizeof(void *);
   }
 
   p_argv = avalue;
@@ -396,6 +396,16 @@
 
       /* because we're little endian, this is what it turns into.   */
 
+#ifdef _WIN64
+      if (z > 8)
+      {
+	      /* On Win64, if a single argument takes more than 8 bytes,
+	      then it is always passed by reference. */
+	      *p_argv = *((void**)argp);
+	      z = 8;
+      }
+      else
+#endif
       *p_argv = (void*) argp;
 
       p_argv++;