From: Christophe Leroy <christophe.le...@csgroup.eu> > Sent: 09 February 2021 17:04 > > Le 09/02/2021 à 15:31, David Laight a écrit : > > From: Segher Boessenkool > >> Sent: 09 February 2021 13:51 > >> > >> On Tue, Feb 09, 2021 at 12:36:20PM +1000, Nicholas Piggin wrote: > >>> What if you did this? > >> > >>> +static inline struct task_struct *get_current(void) > >>> +{ > >>> + register struct task_struct *task asm ("r2"); > >>> + > >>> + return task; > >>> +} > >> > >> Local register asm variables are *only* guaranteed to live in that > >> register as operands to an asm. See > >> > >> https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables > >> ("The only supported use" etc.) > >> > >> You can do something like > >> > >> static inline struct task_struct *get_current(void) > >> { > >> register struct task_struct *task asm ("r2"); > >> > >> asm("" : "+r"(task)); > >> > >> return task; > >> } > >> > >> which makes sure that "task" actually is in r2 at the point of that asm. > > > > If "r2" always contains current (and is never assigned by the compiler) > > why not use a global register variable for it? > > > > > The change proposed by Nick doesn't solve the issue. > > The problem is that at the begining of the function we have: > > unsigned long *ti_flagsp = ¤t_thread_info()->flags; > > When the function uses ti_flagsp for the first time, it does use 112(r2) > > Then the function calls some other functions. > > Most likely because the function could update 'current', GCC copies r2 into > r30, so that if r2 get > changed by the called function, ti_flagsp is still based on the previous > value of current. > > Allthough we know r2 wont change, GCC doesn't know it. And in order to save > r2 into r30, it needs to > save r30 in the stack. > > > By using ¤t_thread_info()->flags directly instead of this intermediaite > ti_flagsp pointer, GCC > uses r2 instead instead of doing a copy.
Does marking current_thread_info() 'pure' (I think that the right one) work - so that gcc knows its result doesn't depend on external data and that it doesn't change external data. Although I'm not 100% how well those attributes actually work. > Nick, I don't understand the reason why you need that 'ti_flagsp' local var. Probably to save typing. I sometimes reload locals after function calls. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)