[Bug middle-end/126906] New: back threader does not thread unreachable edges but DOM does

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126906

            Bug ID: 126906
           Summary: back threader does not thread unreachable edges but
                    DOM does
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aldyh at gcc dot gnu.org
                CC: amacleod at redhat dot com, law at gcc dot gnu.org,
                    rguenth at gcc dot gnu.org
            Blocks: 126010
  Target Milestone: ---

See gcc.dg/builtin-unreachable-6a.c with --param=dom-jump-threading=0 added:

  void
  foo (int b, int c)
  {
    void *x = &&lab;
    if (b)
      {
  lab:
        __builtin_unreachable ();
      }
  lab2:
    if (c)
      x = &&lab2;
    goto *x;
  }

With the abnormal-PHI handling now upstream, the backward threader finds and
fully resolves the path, but profitable_path_p refuses it by design:
probably_never_executed_edge_p on the taken edge:

Checking profitability of path (backwards):
    FAIL: Jump-thread path not considered: path leads to probably
    never executed edge.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126010
[Bug 126010] [meta-bug] Remove DOM