On July 3, 2019 5:14:58 PM GMT+02:00, "Paul E. McKenney" <paul...@linux.ibm.com> wrote: >On Wed, Jul 03, 2019 at 12:39:41AM +0530, Akshat Garg wrote: >> On Tue, Jul 2, 2019 at 8:40 PM Paul E. McKenney ><paul...@linux.ibm.com> >> wrote: >> >> > On Tue, Jul 02, 2019 at 02:15:55PM +0100, Ramana Radhakrishnan >wrote: >> > > On Tue, Jul 2, 2019 at 1:38 PM Paul E. McKenney ><paul...@linux.ibm.com> >> > wrote: >> > > >> > > > >> > > > Once a user-created non-dependent pointer is assigned to, it is >OK to >> > > > break the dependency. >> > > >> > > Ok, that's good. >> > > > >> > > > Or am I missing the point here? >> > > >> > > I was just trying to make sure we were on the same page. I wonder >if >> > > marking this volatile would be sufficient for prototyping. I >suspect >> > > we would need another flag somewhere which someone with gimple >> > > knowledge might be able to help us with. >> > >> > I expect that marking it as volatile would do the trick. ;-) >> > >> > Thanx, Paul >> > >> So, marking this pointer as volatile will not allow the compiler to >> modify/optimize the statements, the pointer is appearing in. And we >don't >> need to push any other code inside any of the passes. Due to this, we >want >> to automatically say those dependent pointers are volatile and >introduce a >> new flag for this. Am I getting you guys correctly? Kindly, let me >know? > >While I suspect that this might work, it would suppress way more >optimizations than would be good. For but one example, consider: > > _Dependent_ptr int *p; > > p = atomic_load_explicit(gp, memory_order_consume); > a = p->a; > b = p->b; > >If "p" is volatile, then the compiler will be prevented from keeping >it in a register, which would not make people coding fastpaths at >all happy. ;-) > >Still, use of volatile might be a good technique for prototyping and >analysis of _Dependent_ptr.
With this example can you quickly summarize what kind of guarantees _Dependent_ptr gives and how a compiler Could possibly break those? Richard. > > Thanx, Paul > >> Akshat >> >> > >> > > regards >> > > Ramana >> > > >> > > > >> > > > Thanx, >Paul >> > > > >> > > > > Ramana >> > > > > >> >> > > > > >> >> > > > > >> > Does this sounds like a workable plan for ? Let me know >your >> > thoughts. If this sounds good then, we can do this for all the >> > optimizations that may kill the dependencies at somepoint. >> > > > > >> > >> > > > > >> > -Akshat >> > > > > >> > > > >> > > >> > >> >
