[Bug tree-optimization/126906] back threader does not thread paths into unreachable edges but DOM does

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

--- Comment #2 from aldy at quesejoda dot com ---
   The backwards threader is finding this path but silently drops it at
   BB copy time. I think it's a limitation of the generic copier:
   can_copy_bbs_p refuses to copy a region when an abnormal edge points
   back into it, and here the computed goto's abnormal edge to lab2 lands
   inside the region being duplicated. BTW, we need a way to dump these
   dropped threads, because our current SUCCESS message is deceiving.
   I can work on the dump part at least.
   On Aug 16, 2026 19:41, "pinskia at gcc dot gnu.org"
   <[email protected]> wrote:

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

     Drea Pinski <pinskia at gcc dot gnu.org> changed:

     What |Removed |Added
    
----------------------------------------------------------------------------
     Severity|normal |enhancement
     Last reconfirmed| |2026-08-16
     Status|UNCONFIRMED |NEW
     Ever confirmed|0 |1

     --- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
     Confirmed. But I am not sure it is unreachable which is causing
     the issue as
     non DOM threading does not handle this either:
     ```

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

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