Messages in this thread | | | Date | Wed, 10 Feb 2021 12:00:20 +1000 | From | Nicholas Piggin <> | Subject | Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer |
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Excerpts from Christophe Leroy's message of February 10, 2021 3:03 am: > > > 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.
It seemed to change code generation in a simple test case, oh well.
> > 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. > > > Nick, I don't understand the reason why you need that 'ti_flagsp' local var.
Just to save typing, I don't mind your patch I was just wondering if current could be improved in general.
Thanks, Nick
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