diff -r e3a773fefddf Modules/_ctypes/libffi_msvc/ffi.c
--- a/Modules/_ctypes/libffi_msvc/ffi.c	Sat Jul 23 15:17:19 2011 +0200
+++ b/Modules/_ctypes/libffi_msvc/ffi.c	Sun Aug 14 13:45:21 2011 -0700
@@ -65,6 +65,15 @@
 	argp = (char *) ALIGN(argp, sizeof(void *));
 
       z = (*p_arg)->size;
+
+#ifdef _WIN64
+	// MSVC on x64 will always pass arguments larger than 8 bytes by reference
+    if (z > sizeof(void *))
+    {
+        z = sizeof(void *);
+    }
+#endif
+
       if (z < sizeof(int))
 	{
 	  z = sizeof(int);