On Thu, Jul 13, 2017 at 10:35:55PM -0600, Jeff Law wrote: > >> There's -fstack-check and -fstack-clash-protection. I think with the > >> direction we're going they are fundamentally incompatible because > >> neither the compiler nor kernel do anything to guarantee enough stack is > >> available after hitting the guard for -fstack-clash-protection. > > > > Hrm, I have to think about that. > Even if the kernel implements the reserved page stuff mentioned earlier > in the thread, I'm not sure when we'd be able to reliably depend on that > capability (or even check for it). > > ISTM that -fstack-check continues to be Ada centric and we have a new > option to deal with stack-clash protection and the two simply just > aren't allowed to be enabled together.
So this essentially means Ada programs cannot get stack clash protection at all? And if -fstack-check is really only useful for Ada, we shouldn't mix up that option with the stack clash thing. > >> And there's shrink wrapping. This was originally raised by the aarch64 > >> guys and deserves some thought. I'm particularly worried about aarch64 > >> if it was to shrink-wrap some particular register saves that we wanted > >> to use as an implicit probe. > > > > For normal shrink-wrapping, the volatile reg stores will always happen > > in the same prologue that sets up the stack frame as well, so there > > won't be any problem (except the usual stack ties you might need in the > > prologue, etc.) (*) For separate shrink-wrapping, yeah you'll need any > > bb that could potentially store to the stack require the component for > > the register you use as implicit probe. This is pretty nasty. > I'm going to try and look at this tomorrow to get a feel for the aarch64 > implementation separate shrink wrapping. > > For ppc I'm not too worried -- the implicit backchain probes are so > effective at eliminating explicit probes that I didn't bother writing > any code to track the register saves as implicit probes. I'm pretty > sure it just works on ppc with separate shrink wrapping. With what is in trunk so far, yeah ;-) In principle the "set up a stack frame" part of the prologue can be made a separate shrink-wrapping component and then we'd have to worry. But it would be complicated to do that and there is no real benefit I see so far, so it won't happen. Nothing for you to have to worry about in any case :-) Segher
