[Bug tree-optimization/106842] [12 Regression] misleading warning : iteration X invokes undefined behavior

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106842

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
           Keywords|                            |missed-optimization,
                   |                            |wrong-code

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Huh.

static void
infer_loop_bounds_from_signedness (class loop *loop, gimple *stmt)
{
...
  low = lower_bound_in_type (type, type);
  high = upper_bound_in_type (type, type);
  value_range r;
  get_range_query (cfun)->range_of_expr (r, def);
  if (r.kind () == VR_RANGE)
    {
      low = wide_int_to_tree (type, r.lower_bound ());
      high = wide_int_to_tree (type, r.upper_bound ());
    }

  record_nonwrapping_iv (loop, base, step, stmt, low, high, false, true);

and record_nonwrapping_iv verifies that the IV stays in [low, high].  So
this essentially means we are verifying range info here.  We have

# RANGE [0, 9]
k_36 = PHI <k_20(18), 0(5)>
# RANGE [1, 9]
k_20 = k_36 + 1;

note the whole code is in a if (0) guarded block and we are executing
ranger_vrp, in the substitute and fold phase (IIRC the old [E]VRP avoided
to substitute/fold in dead code regions?).

somebody needs to double-check if the range on the stmts is wrong, but
depending on how VRP propagation treats unexecutable edges it might be
GIGO anyhow.

The "missed optimization" is that substitute-and-fold considers dead
regions.  "wrong code" is because either the range is bogus or the
niter we derive from it is.

needs a second eye