[PATCH PR97627]Avoid computing niters info for fake edges

[Bin.Cheng]

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.
>
> 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.

Thanks,
bin
>
> Thanks,
> Richard.
>
> > Thanks,
> > bin