Re: [PATCH 00/22] add support for Clang LTO

Paul E. McKenney
2020-07-06T16:26:34Z

On Fri, Jul 03, 2020 at 07:42:28AM -0700, Paul E. McKenney wrote:
> On Fri, Jul 03, 2020 at 03:13:30PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 02, 2020 at 10:59:48AM -0700, Paul E. McKenney wrote:
> > > On Thu, Jul 02, 2020 at 10:20:40AM +0200, Peter Zijlstra wrote:
> > > > On Wed, Jul 01, 2020 at 09:03:38AM -0700, Paul E. McKenney wrote:

[ . . . ]

> > Also, if C goes and specifies load dependencies, in any form, is then
> > not the corrolary that they need to specify control dependencies? How
> > else can they exclude the transformation.
>
> By requiring that any temporaries generated from variables that are
> marked _Dependent_ptr also be marked _Dependent_ptr. This is of course
> one divergence of _Dependent_ptr from the volatile keyword.
>
> > And of course, once we're there, can we get explicit support for control
> > dependencies too? :-) :-)
>
> Keep talking like this and I am going to make sure that you attend a
> standards committee meeting. If need be, by arranging for you to be
> physically dragged there. ;-)
>
> More seriously, for control dependencies, the variable that would need
> to be marked would be the program counter, which might require some
> additional syntax.

And perhaps more constructively, we do need to prioritize address and data
dependencies over control dependencies. For one thing, there are a lot
more address/data dependencies in existing code than there are control
dependencies, and (sadly, perhaps more importantly) there are a lot more
people who are convinced that address/data dependencies are important.

For another (admittedly more theoretical) thing, the OOTA scenarios
stemming from control dependencies are a lot less annoying than those
from address/data dependencies.

And address/data dependencies are as far as I know vulnerable to things
like conditional-move instructions that can cause problems for control
dependencies.

Nevertheless, yes, control dependencies also need attention.

Thanx, Paul