On Fri, Jan 29, 2021 at 10:41 AM Bin.Cheng <amker.ch...@gmail.com> wrote: > > On Fri, Jan 29, 2021 at 3:55 PM Richard Biener > <richard.guent...@gmail.com> wrote: > > > > 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. > Yeah, please apply the trivial patch.
OK, will do. Thanks, Richard. > Thanks, > bin > > > > > > > > > > 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
