Re: [RFC] LKMM: Add volatile_if()

[Linus Torvalds]

On Fri, Jun 4, 2021 at 9:37 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> >
> > Why is "volatile_if()" not just
> >
> >        #define barier_true() ({ barrier(); 1; })
> >
> >        #define volatile_if(x) if ((x) && barrier_true())
>
> Because we weren't sure compilers weren't still allowed to optimize the
> branch away.

This isn't about some "compiler folks think".

The above CANNOT be compiled any other way than with a branch.

A compiler that optimizes a branch away is simply broken.

Of course, the actual condition (ie "x" above) has to be something
that the compiler cannot statically determine is a constant, but since
the whole - and only - point is that there will be a READ_ONCE() or
similar there, that's not an issue.

The compiler *cannot* just say "oh, I'll do that 'volatile asm
barrier' whether the condition is true or not". That would be a
fundamental compiler bug.

It's as if we wrote

    if (x) y++;

and the compiler went "Oh, I'll just increment 'y' unconditionally by
one, I'm sure the programmer doesn't mind, the conditional on 'x' is
immaterial".

No. That's not a C compiler. That's a stinking piece of buggy shit.
The compiler has to honor the conditional.

In that "y++" case, a compiler can decide to do it without a branch,
and basically rewrite the above as

   y += !!x;

but with a "volatile asm", that would be a bug.

Of course, we might want to make sure that the compiler doesn't go
"oh, empty asm, I can ignore it", but if that's the case then it's not
about "volatile_if()" any more, at that point it's "oh, the compiler
broke our 'barrier()' implementation", and we have bigger issues.

              Linus