libffi doesn't JIT for FFI calls; and it still requires you to lay out argument values yourself, i.e. for a string argument you'd still need to write code that

libffi doesn't JIT for FFI calls; and it still requires you to lay out argument values yourself, i.e. for a string argument you'd still need to write code that converts a Ruby string object to a C string pointer. And libffi is rather slow. (the tramp.c linked in a sibling comment is for "reverse-FFI", i.e. exposing some dynamic custom operation as a function pointer; and its JITting there amounts to a total of 3 instructions to call into precompiled code)