https://github.com/libffi/libffi/issues/879
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-libs/libffi/files/libffi-3.4.7-Revert-Fix-x86-ffi64-calls-with-6-gp-and-some-sse-re.patch

Index: src/x86/ffi64.c
--- src/x86/ffi64.c.orig
+++ src/x86/ffi64.c
@@ -654,7 +654,7 @@ ffi_call_int (ffi_cif *cif, void (*fn)(void), void *rv
 		      break;
 		    default:
 		      reg_args->gpr[gprcount] = 0;
-		      memcpy (&reg_args->gpr[gprcount], a, sizeof(UINT64));
+		      memcpy (&reg_args->gpr[gprcount], a, size);
 		    }
 		  gprcount++;
 		  break;
