Maybe the example is too simple, but it does not require `__attribute__((musttail))` for good code gen.
Also if the error handling function is unlikely, you wouldn't care too much about how fast it is to call it?
To me it seems like a structure of the form
<read data>
if (unlikely(malformed))
return error();
<prep more stuff>
switch (data_type) {
case x:
return handle_x();
case y:
return handle_y();
}