On Sun, 29 Sep 2019, Jeff Law wrote: > On 9/26/19 12:44 AM, Richard Biener wrote: > > On Wed, 25 Sep 2019, Prathamesh Kulkarni wrote: > > > >> On Fri, 20 Sep 2019 at 15:20, Jeff Law <l...@redhat.com> wrote: > >>> > >>> On 9/19/19 10:19 AM, Prathamesh Kulkarni wrote: > >>>> Hi, > >>>> For PR91532, the dead store is trivially deleted if we place dse pass > >>>> between ifcvt and vect. Would it be OK to add another instance of dse > >>>> there ? > >>>> Or should we add an ad-hoc "basic-block dse" sub-pass to ifcvt that > >>>> will clean up the dead store ? > >>> I'd hesitate to add another DSE pass. If there's one nearby could we > >>> move the existing pass? > >> Well I think the nearest one is just after pass_warn_restrict. Not > >> sure if it's a good > >> idea to move it up from there ? > > > > You'll need it inbetween ifcvt and vect so it would be disabled > > w/o vectorization, so no, that doesn't work. > > > > ifcvt already invokes SEME region value-numbering so if we had > > MESE region DSE it could use that. Not sure if you feel like > > refactoring DSE to work on regions - it currently uses a DOM > > walk which isn't suited for that. > > > > if-conversion has a little "local" dead predicate compute removal > > thingy (not that I like that), eventually it can be enhanced to > > do the DSE you want? Eventually it should be moved after the local > > CSE invocation though. > We've speculated for a while that some of these dominator walking > optimizers could be extended to handle cases where the caller specifies > what blocks are "of interest". Bin was pretty far down this path for > DOM which is probably the most complex. Maybe that could be resurrected > in the context of DSE.
Note that DSE is way easier than DOM, it's use of a dominator walk is probably entirely historical given the fact that we only need to walk the factored virtual SSA chain. So a (SESE) region is represented by two virtual SSA names (and for a loop in the ifcvt context by its virtual PHI in the loop header). The issue with if-conversion and dominators is that if-conversion does analysis + transform loop-by-loop which means we at least wreck the fast dominator access by doing CFG transforms between analyzing separate loops in a function. The pass is in need of factoring to do analysis of all loops upfront and only then transform. The cleanup phase would then be after re-computing dominators. Richard.
