On Thu, Apr 09, 2020 at 09:51:28AM -0700, Linus Torvalds wrote: > On Wed, Apr 8, 2020 at 11:23 PM Luc Van Oostenryck > <luc.vanoostenryck@xxxxxxxxx> wrote: > > > > Yes, the problem is caused at expand_conditional() where one of > > the sides is throwed away if the condition is known. So the label > > doesn't exist anymore and at linearization Sparse ends with a > > jump to an unexisting BB. > > Yes. > > And I don't think that's really a problem per se. I think the gcc > model of saying "you jumped to an invalid place, go away" is fine - > particularly since this can only happen if you use a gcc extension to > begin with. > > So I don't think sparse is wrong, except for the total lack of any > error messages. > > > I tried to simply discard the early optimization in expand but > > then when testing the kernel I got a whole bunch of warnings > > I don't think we want to get rid of the early tree-level > simplifications. They are sensible and help avoid unnecessary work > later. Yes, sure. I had hopped to be able to keep the advantages of the value-expansion while keeping the original information but it would need quite a bit changes and is certainly not worth this "jump inside a (discarded) expression statement". It's interesting, though, that a simple if (0) ...stuff... can't be discarded at expand time because of the gotos/labels. > So I'd much rather just figure out some way to say "hmm, this goto is > to something that was removed earlier, let's just say so". > > > I tried also to warn on gotos jumping into an expression statement. > > The idea was to give a new 'label_scope' for each such statement. > > Things are a bit complicated because the labels are implicitly > > declared by the gotos. > > Yes. I think the gcc warning is nice, but I also think that it would > be entirely sufficient to not notice at an early stage, but only when > linearizing and hitting the "I'm branching to something that I can't > generate code for", and report it at that point, instead of being > clever and analyzing scopes up front. Yes, that's certainly much easier and avoids the current garbage with the IR and the diagnostic will, I think, still be informative enough. -- Luc
