Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

[Christophe Leroy]

Le 14/04/2021 à 14:24, Segher Boessenkool a écrit :
> On Wed, Apr 14, 2021 at 12:01:21PM +1000, Nicholas Piggin wrote:
> > Would be nice if we could let the compiler deal with it all...
> >
> > static inline unsigned long lr(unsigned long *mem)
> > {
> >          unsigned long val;
> >
> >          /*
> >           * This doesn't clobber memory but want to avoid memory operations
> >           * moving ahead of it
> >           */
> >          asm volatile("ldarx     %0, %y1" : "=r"(val) : "Z"(*mem) : "memory");
> >
> >          return val;
> > }
>
> (etc.)
>
> That can not work reliably: the compiler can put random instructions
> between the larx and stcx. this way, and you then do not have guaranteed
> forward progress anymore.  It can put the two in different routines
> (after inlining and other interprocedural optimisations), duplicate
> them, make a different number of copies of them, etc.
>
> Nothing of that is okay if you want to guarantee forward progress on all
> implementations, and also not if you want to have good performance
> everywhere (or anywhere even).  Unfortunately you have to write all
> larx/stcx. loops as one block of assembler, so that you know exactly
> what instructions will end up in your binary.
>
> If you don't, it will fail mysteriously after random recompilations, or
> have performance degradations, etc.  You don't want to go there :-)

Could the kernel use GCC builtin atomic functions instead ?

https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html