On Wed, May 20, 2015 at 03:15:48PM +0100, Ramana Radhakrishnan wrote: > > > On 20/05/15 15:03, Paul E. McKenney wrote: > >On Wed, May 20, 2015 at 02:44:30PM +0100, Ramana Radhakrishnan wrote: > >> > >> > >>On 20/05/15 14:37, David Howells wrote: > >>>Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > >>> > >>>>I was thinking of "y" as a simple variable, but if it is something more > >>>>complex, then the compiler could do this, right? > >>>> > >>>> char *x; > >>>> > >>>> y; > >>>> x = z; > >>> > >>>Yeah. I presume it has to maintain the ordering, though. > >> > >>The scheduler for e.g. is free to reorder if it can prove there is > >>no dependence (or indeed side-effects for y) between insns produced > >>for y and `x = z'. > > > >So for example, if y is independent of z, the compiler can do the > >following: > > > > char *x; > > > > x = z; > > y; > > > >But the dependency ordering is still maintained from z to x, so this > >is not a problem. > > > Well, reads if any of x (assuming x was initialized elsewhere) would > need to happen before x got assigned to z. Agreed, there needs to be a memory_order_consume load up there somewhere. (AKA rcu_dereference().) > I understood the original "maintain the ordering" as between the > statements `x = z' and `y'. Ah, I was assuming between x and z. David, what was your intent? ;-) > >Or am I missing something subtle here? > > No, it sounds like we are on the same page here. Whew! ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
