On Fri, 5 Jul, 2019, 4:50 PM Richard Biener, <richard.guent...@gmail.com> wrote:
> 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. > Thank you, Richard and Jakub, both for giving your valuable suggestions and directions. Akshat > > > > > -Akshat >
