https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89251
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Using a smaller than declared size for standard loads is a valid and well defined optimization (sometimes those use less bandwidth, othertimes it is better for alignment reasons). The case you are running into is that you don't want to use that kind of load. You should not using bitfields directly and really should be using volatile here. Volatile definition is exactly what is needed for hardware registers and IIRC is the real only working case for volatile these days (the other working case is a variable that changes due to an interrupt/signal).
