[Bug middle-end/126896] New: backward threader cannot duplicate a PHI-of-compares join block

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126896

            Bug ID: 126896
           Summary: backward threader cannot duplicate a PHI-of-compares
                    join block
           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
  Target Milestone: ---

Another missing optimization.  I don’t know who’s responsibility this is, cause
I don’t think this maps to ranger.

Fixing this category fixes the following regressions from disabling DOM jump
threading:

    gcc.dg/tree-ssa/phi_on_compare-1.c
    gcc.dg/tree-ssa/phi_on_compare-2.c
    gcc.dg/tree-ssa/phi_on_compare-3.c

/* { dg-do compile } */
/* { dg-options "-O2 --param=dom-jump-threading=0 -fdump-tree-optimized" } */

/* Each arm computes a boolean relational; out-of-ssa leaves
   t = PHI <t_9, t_6>; if (t).  The exit conditional is [0,1] varying
   on every path, so the backward threader rejects every candidate
   path: it only registers a path with a resolved single out-edge.
   DOM's forward threader duplicates the join block into each
   predecessor as a "joiner" and its simplifier replaces the copied
   if (t) with the arm's own comparison, so the bool PHI and the join
   block die.  With DOM threading disabled they survive.  */

void g (void);

void
f (int x, int a, int b, int c, int d)
{
  _Bool t;
  if (x)
    t = a < b;
  else
    t = c < d;
  if (t)
    g ();
}

/* { dg-final { scan-tree-dump-not "PHI" "optimized" } } */