Re: Control Dependencies vs C Compilers

Paul E. McKenney
2020-10-07T21:20:08Z

On Wed, Oct 07, 2020 at 11:07:17PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 10:11:07AM -0700, Paul E. McKenney wrote:
>
> > Challenges include:
> >
> > o Unmarked accesses. Compilers are quite aggressive about
> > moving normal code.
>
> Which is why this thread exists :-) We wants to dis-allow lifting the
> stores over our volatile-if.

Of course. But you should expect this point to be a continual source
of shock and surprise to compiler folks. ;-)

> > o Separately compiled code. For example, does the compiler have
> > unfortunatel optimization opportunities when "volatile if"
> > appears in one translation unit and the dependent stores in
> > some other translation unit?
>
> It can hardly lift anything outside a TU (barring the next point). So I
> don't see how it can go wrong here. This is in fact the case with the
> perf ringbuffer. The ctrl-dep lives in a different TU from the
> stores.

I don't see how it could either, but I have been surprised before.

> > o LTO, as has already been mentioned in this thread.
>
> So I would probably advocate the volatile-if to be a full sync point,
> and LTO would have to preserve that.

Completely agreed! And probably not the only place that LTO needs
to be reined in a bit.

Thanx, Paul