[Bug middle-end/126887] New: backward threader cannot peel a single-iteration loop that the DOM threader peels

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126887

            Bug ID: 126887
           Summary: backward threader cannot peel a single-iteration loop
                    that the DOM threader peels
           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, rguenth at gcc dot gnu.org
  Target Milestone: ---

I’ve been going through testcases that fail with —-param=dom-jump-threading=0
to see what fails, as a first stab at seeing what optimizations are missing or
what testcases need adjusting.

For the record, I’ve been using Claude Fable to help categorize these failures,
and I haven’t dug into them as much as I would’ve otherwise.

Here is a distilled failing testcase:

/* { dg-do compile } */
/* { dg-options "-O2 --param=dom-jump-threading=0
-fdump-tree-threadfull2-details" } */

/* When flag != 0, i enters the loop as 0 and the loop runs exactly
   once, so the entering path resolves the exit test and the first
   iteration can be peeled.  DOM's threader peels it (dom3, once
   PROP_loop_opts_done lifts the crossing-loops restriction); the
   backward threader refuses the resolvable path through the
   preheader by design (the pr14341 loop_father restriction in
   find_paths_to_names) and only ever explores the latch predecessor.
   With DOM threading disabled, no thread is registered at all.  */

volatile unsigned sink;

void
f (int flag, unsigned n)
{
  unsigned i = flag ? 0 : n;
  do
    {
      sink = i;
      i += 4;
    }
  while (i != 4);
}

/* { dg-final { scan-tree-dump-times "Registering jump thread" 1 "threadfull2"
} } */

These are the failing testcases that I believe are due to the above:

gcc.dg/vect/vect-54.c
gcc.dg/vect/vect-31-big-array.c (ppc64le only -- passes on x86_64 current
trunk)
gcc.dg/uninit-1.c (bogus warning at any -O)
gcc.dg/auto-init-uninit-1.c (same)

What do we want to do here?