Messages in this thread | | | Subject | Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer | From | Christophe Leroy <> | Date | Wed, 10 Feb 2021 09:45:56 +0100 |
| |
Le 10/02/2021 à 03:00, Nicholas Piggin a écrit : > 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,
I'll post v6 of it as a follow-up of yesterday's two remaining follow-up patches.
Christophe
| |