Re: [RFC] LKMM: Add volatile_if()

Segher Boessenkool
2021-06-07T14:17:16Z

On Sun, Jun 06, 2021 at 04:37:29PM -0700, Paul E. McKenney wrote:
> > > The barrier() thing can work - all we need to do is to simply make it
> > > impossible for gcc to validly create anything but a conditional
> > > branch.
> >
> > And the only foolproof way of doing that is by writing a branch.

[ ... ]

> > I am saying that if you depend on that some C code you write will result
> > in some particular machine code, without actually *forcing* the compiler
> > to output that exact machine code, then you will be disappointed. Maybe
> > not today, and maybe it will take years, if you are lucky.
> >
> > (s/forcing/instructing/ of course, compilers have feelings too!)
>
> OK, I will bite...
>
> What would you suggest as a way of instructing the compiler to emit the
> conditional branch that we are looking for?

You write it in the assembler code.

Yes, it sucks. But it is the only way to get a branch if you really
want one. Now, you do not really need one here anyway, so there may be
some other way to satisfy the actual requirements.

Segher