On Fri, Jul 5, 2019 at 1:08 AM Akshat Garg <xks...@gmail.com> wrote: > > On Thu, Jul 4, 2019 at 11:39 PM Jakub Jelinek <ja...@redhat.com> wrote: >> >> On Thu, Jul 04, 2019 at 10:40:15AM -0700, Paul E. McKenney wrote: >> > > I think fully guaranteeing this is hard (besides when you use >> > > volatile), we have the very same issue when dealing with >> > > pointer provenance rules, known for years and not fixed >> > > (and I don't see a good way to fix these issues without >> > > sacrifying performance everywhere). >> > > >> > > Good luck ;) >> > >> > Thank you, we will need it! ;-) >> >> Well, luck probably isn't all you need, you'll need some way to represent >> those dependencies in the IL (both GIMPLE and RTL) that would ensure that >> they aren't optimized away, because just hoping that optimization don't mess >> that up or just patching a few optimizations you discover first isn't going >> to be very reliable. Say don't rewrite those into SSA form and represent >> them close to how volatile ptrs are handled at least for the beginning, and >> if there are safe optimizations special case those, rather than allowing all >> optimizations on those and hope it will work out. >> >> Jakub > > I don't understand this statement completely "Say don't rewrite those into > SSA form and represent them close to how volatile ptrs are handled at least > for the beginning, and if there are safe optimizations special case those, > rather than allowing all optimizations on those and hope it will work out.". > Are you saying that we should try to represent dependent ptrs as volatile > ptrs for some places? We should figure out all the places where a check for > volatile ptrs happens and we check for dependent ptrs there also and see if > we can allow the optimization or not. > Am I getting you correctly, please tell?
I think Jakub means not to use volatile as is but mimic some of its properties (like we do not rewrite volatile vars into SSA form which fends off most optimizations turing data dependences into crontrol dependences, not all speculation though). I think "fixing" the GIMPLE optimizers to not mess up is possible even with SSA (just a bit tedious and fragile), but real issues will start at the RTL level where you completely lose properties like "this is a [dependent] pointer". Richard. > > -Akshat
