[Bug rtl-optimization/63384] scheduler loops on endless fence list with -fselective-scheduling2 on x86

["andi-gcc at firstfloor dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63384

--- Comment #4 from Andi Kleen <andi-gcc at firstfloor dot org> ---
It loops forever in this loop in sel_sched_region_2

  while (fences)
    {
      int min_seqno, max_seqno;
      ilist_t scheduled_insns = NULL;
      ilist_t *scheduled_insns_tailp = &scheduled_insns;

      find_min_max_seqno (fences, &min_seqno, &max_seqno);
      schedule_on_fences (fences, max_seqno, &scheduled_insns_tailp);
      fences = calculate_new_fences (fences, orig_max_seqno, &max_time);
      highest_seqno_in_use = update_seqnos_and_stage (min_seqno, max_seqno,
                                                      highest_seqno_in_use,
                                                      &scheduled_insns);
    }

because calculate_new_fences always comes up with a list which is the same as
before. In move_fence_to_fences it always goes into the else

 f = flist_lookup (FLIST_TAIL_HEAD (new_fences),
                    FENCE_INSN (FLIST_FENCE (old_fences)));
  if (f)
    {
      merge_fences (f, old->insn, old->state, old->dc, old->tc,
                    old->last_scheduled_insn, old->executing_insns,
                    old->ready_ticks, old->ready_ticks_size,
                    old->sched_next, old->cycle, old->issue_more,
                    old->after_stall_p);
    }
  else
    {
      _list_add (tailp);
      FLIST_TAIL_TAILP (new_fences) = &FLIST_NEXT (*tailp);


So something is going wrong in flist_lookup.