https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102997
--- Comment #27 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #20)
> (In reply to hubicka from comment #19)
> > The testcase would be
> >
> > void test ()
> > {
> > int i;
> > if (test())
> > i=0;
> > else
> > i=100;
> > for (;i<100 && bar ();i++)
> > do_work ();
> > }
Updated test that compiles:
int check();
int bar ();
void do_work ();
void test ()
{
int i;
if (check())
i=0;
else
i=100;
for (;i<100 && bar ();i++)
do_work ();
}
> >
> > Here I think we can thread else path to return w/o any issues with loop
> > nests even with -Os.
>
> we fail this because
>
> // If we crossed a loop into an outer loop without crossing the
> // latch, this is just an early exit from the loop.
> if (loops_crossed == 1
> && !crossed_latch
> && flow_loop_nested_p (exit->dest->loop_father,
> exit->src->loop_father))
> return false;
>
> does not trigger (loops_crossed == 2), and we then run into
>
>
> if (loops_crossed)
> {
> cancel_thread (&path, "Path crosses loops");
> return true;
> }
>
> but the loops_crossed stuff is old and not from me, not sure what that's
> useful for. We could "fix" the accounting with
>
> diff --git a/gcc/tree-ssa-threadupdate.c b/gcc/tree-ssa-threadupdate.c
> index 8aac733ac25..52ec22e85e1 100644
> --- a/gcc/tree-ssa-threadupdate.c
> +++ b/gcc/tree-ssa-threadupdate.c
> @@ -2810,7 +2810,7 @@ jt_path_registry::cancel_invalid_paths
> (vec<jump_thread_edge *> &path)
>
> // If we crossed a loop into an outer loop without crossing the
> // latch, this is just an early exit from the loop.
> - if (loops_crossed == 1
> + if (loops_crossed <= 2
> && !crossed_latch
> && flow_loop_nested_p (exit->dest->loop_father,
> exit->src->loop_father))
> return false;
The above patch doesn't cause any difference in the generated code, at least
for -O2. I also don't see any more paths registered:
$ grep Registering.jump a.c.*
a.c.126t.thread1: [1] Registering jump thread: (2, 4) incoming edge; (4, 8)
nocopy;
a.c.126t.thread1: [2] Registering jump thread: (3, 4) incoming edge; (4, 7)
nocopy;
a.c.192t.dom3: [4] Registering jump thread: (6, 5) incoming edge; (5, 4)
joiner (4, 7) normal;
>
> to also allow to thread through a loop path not crossing the latch but
> at least for the issue of "breaking loops" the loops_crossed stuff shouldn't
> be necessary. It might still prevent us from doing such transforms when
> it involves peeling.
This loops crossing stuff is indeed quite old. Are you suggesting this as
well?
- if (loops_crossed)
- {
- cancel_thread (&path, "Path crosses loops");
- return true;
- }
Which, BTW doesn't change the behavior for this test, but I'm happy to test if
you think it's not necessary.
