Re: [RFC] LKMM: Add volatile_if()

Segher Boessenkool
2021-06-07T22:45:00Z

On Mon, Jun 07, 2021 at 01:16:33PM -0700, Paul E. McKenney wrote:
> On Mon, Jun 07, 2021 at 03:51:44PM -0400, Alan Stern wrote:
> > On Mon, Jun 07, 2021 at 01:23:35PM -0500, Segher Boessenkool wrote:
> > > On Mon, Jun 07, 2021 at 08:27:12AM -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.
> > >
> > > > > > 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? ;-)
> > >
> > > I don't know what you are referring to, sorry?
> > >
> > > I know what you asked for: literally some way to tell the compiler to
> > > emit a conditional branch. If that is what you want, the only way to
> > > make sure that is what you get is by writing exactly that in assembler.
> >
> > That's not necessarily it.
> >
> > People would be happy to have an easy way of telling the compiler that
> > all writes in the "if" branch of an if statement must be ordered after
> > any reads that the condition depends on. Or maybe all writes in either
> > the "if" branch or the "else" branch. And maybe not all reads that the
> > condition depends on, but just the reads appearing syntactically in the
> > condition. Or maybe even just the volatile reads appearing in the
> > condition. Nobody has said exactly.
> >
> > The exact method used for doing this doesn't matter. It could be
> > accomplished by treating those reads as load-acquires. Or it could be
> > done by ensuring that the object code contains a dependency (control or
> > data) from the reads to the writes. Or it could be done by treating
> > the writes as store-releases. But we do want the execution-time
> > penalty to be small.
> >
> > In short, we want to guarantee somehow that the conditional writes are
> > not re-ordered before the reads in the condition. (But note that
> > "conditional writes" includes identical writes occurring in both
> > branches.)
>
> What Alan said! ;-)

Okay, I'll think about that.

But you wrote:

> > > > > > What would you suggest as a way of instructing the compiler to emit the
> > > > > > conditional branch that we are looking for?

... and that is what I answered. I am sorry if you do not like being
taken literally, but that is how I read technical remarks: as literally
what they say. If you say you want a branch, I take it you want a
branch! :-)

Segher