On Thu, Jan 28, 2021 at 11:31 AM Bin.Cheng <amker.ch...@gmail.com> wrote: > > On Thu, Jan 28, 2021 at 5:08 PM Richard Biener via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: > > > > On Thu, Jan 28, 2021 at 3:49 AM bin.cheng via Gcc-patches > > <gcc-patches@gcc.gnu.org> wrote: > > > > > > Hi, > > > As described in commit message, we need to avoid computing niters info > > > for fake > > > edges. This simple patch does this by two changes. > > > > > > Bootstrap and test on X86_64, is it ok? > > > > Hmm, so I think the patch is a bit complicated and avoiding niter compute > > for fake edges would be easier when just returning false for > > fake edges in number_of_iterations_exit_assumptions? > I just grepped calls to get_loop_exit_edges, and thought there might > be cases other than niters analysis that would also like to skip fake > edges. But I didn't check the calls one by one.
My hunch is that the usual APIs always want to ignore them, but let's do a minimal fix that we can backport easily. > > > > Which pass was the problematical that had infinite loops connected to exit? > > > > I guess the cfgloop code should simply ignore fake exits - they mostly > > exist to make reverse CFG walks easy. Specifically single_exit > > and single_likely_exit but also exit edge recording should ignore them. > > > > That said, the testcase seems to be fixed with just > > > > diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c > > index 7d61ef080eb..7775bc7275c 100644 > > --- a/gcc/tree-ssa-loop-niter.c > > +++ b/gcc/tree-ssa-loop-niter.c > > @@ -2407,6 +2407,11 @@ number_of_iterations_exit_assumptions (class > > loop *loop, edge exit, > > affine_iv iv0, iv1; > > bool safe; > > > > + /* The condition at a fake exit (if it exists) does not control its > > + execution. */ > > + if (exit->flags & EDGE_FAKE) > > + return false; > > + > > /* Nothing to analyze if the loop is known to be infinite. */ > > if (loop_constraint_set_p (loop, LOOP_C_INFINITE)) > > return false; > > > > Your dfs_find_deadend change likely "breaks" post-dominance DFS order > > (this is a very fragile area). > > > > So any objection to just simplify the patch to the above hunk? > Considering we are in late stage3? No objection to this change. But I > do think dfs_find_deadend needs to be improved, if not as this patch > does. For a loop nest with the outermost loop as the infinite one, > the function adds fake (exit) edges for inner loops, which is > counter-intuitive. Sure, but then this is independent of the PR. As said, the fake exits only exist to make reverse CFG walkers easier - yes, for natural infinite loops we'd like to have "intuitive" post-dom behavior but for example for irreducible regions there's not much to do. Richard. > Thanks, > bin > > > > Thanks, > > Richard. > > > > > Thanks, > > > bin
