[PATCH PR97627]Avoid computing niters info for fake edges

[Richard Biener]

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?

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?

Thanks,
Richard.

> Thanks,
> bin