I like bitfields in C structs. I think they are currently an "NYI" in LuaJIT though: that the JIT doesn't understand them and as a consequence code that uses them will be interpreted instead of JIT'd. (If that's true you would see an NYI warning in the JIT trace from -jdump.)

We should see about getting support for bitfields in LuaJIT, either by asking Mike Pall or hacking it ourselves (it will be awesome if we can develop LuaJIT-hacking expertise of our own).