Re: Control Dependencies vs C Compilers

Willy Tarreau
2020-10-06T12:50:18Z

On Tue, Oct 06, 2020 at 12:37:06PM +0000, David Laight wrote:
> > Our Documentation/memory-barriers.txt has a Control Dependencies section
> > (which I shall not replicate here for brevity) which lists a number of
> > caveats. But in general the work-around we use is:
> >
> > x = READ_ONCE(*foo);
> > if (x > 42)
> > WRITE_ONCE(*bar, 1);
>
> An alternative is to 'persuade' the compiler that
> any 'tracked' value for a local variable is invalid.
> Rather like the way that barrier() 'invalidates' memory.
> So you generate:
>
> x = *foo
> asm ("" : "+r" (x));
> if (x > 42)
> *bar = 1;
>
> Since the "+r" constraint indicates that the value of 'x'
> might have changed it can't optimise based on any
> presumed old value.
> (Unless it looks inside the asm opcodes...)

I'm using exactly this in userland to prevent the compiler from guessing
what I'm doing with a variable, and it's also useful sometimes to shut up
certain warnings when I know a condition is satisfied but can hardly be
expressed in a way to please the compiler. Overall I find that it's no
big deal and forces the developer to think twice before doing it, which
is probably a good thing in general.

Willy