Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-07T15:27:17Z

On Mon, Jun 07, 2021 at 09:12:42AM -0500, Segher Boessenkool wrote:
> 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.

Hmmm... What do you see Peter asking for that is different than what
I am asking for? ;-)

Thanx, Paul