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
