this_cannot_happen
this_cannot_happen(5, inst.generic_format.opcode);
this_cannot_happen(3, inst.bits);
this_cannot_happen(4, inst.bits);
this_cannot_happen(2, inst.bits);
this_cannot_happen(3, inst.bits);
this_cannot_happen(1, inst.bits);
this_cannot_happen(6, -1);