Re: [RFC] LKMM: Add volatile_if()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 04, 2021 at 09:30:01AM -0700, Linus Torvalds wrote:
> On Fri, Jun 4, 2021 at 3:12 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > I've converted most architectures we care about, and the rest will get
> > an extra smp_mb() by means of the 'generic' fallback implementation (for
> > now).
> 
> Why is "volatile_if()" not just
> 
>        #define barier_true() ({ barrier(); 1; })
> 
>        #define volatile_if(x) if ((x) && barrier_true())
> 
> because that should essentially cause the same thing - the compiler
> should be *forced* to create one conditional branch (because "barrier"
> is an asm that can't be done on the false side, so it can't do it with
> arithmetic or other games), and after that we're done.
> 
> No need for per-architecture "asm goto" games. No new memory barriers.
> No actual new code generation (except for the empty asm volatile that
> is a barrier).

Because we weren't sure compilers weren't still allowed to optimize the
branch away. If compiler folks can guarantee us your thing (along with
maybe the BUILD_BUG_ON(__builtin_constant_p(cond)) thing) always shall
generate a conditional branch instruction, then Yay!