[Bug tree-optimization/111694] [13/14 Regression] Wrong behavior for signbit of negative zero when optimizing

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111694

--- Comment #4 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Richard Biener from comment #3)
> Looks like some frange / relation mistake then.


l_3(D)  [frange] double [-Inf, +Inf]
Equivalence set : [l_3(D), r_4(D)]
    <bb 3> :
    _1 = __builtin_signbit (l_3(D));
    if (_1 != 0)
      goto <bb 6>; [INV]
    else
      goto <bb 4>; [INV]

3->6  (T) _1 :  [irange] int [-INF, -1][1, +INF]
3->6  (T) l_3(D) :      [frange] double [-Inf, -0.0 (-0x0.0p+0)]
3->4  (F) _1 :  [irange] int [0, 0]
3->4  (F) l_3(D) :      [frange] double [0.0 (0x0.0p+0), +Inf]

    <bb 4> :
    _2 = __builtin_signbit (r_4(D));

Yeah, we know l_3 and r_4 are equivalent, and we also know that on the edge
3->4 l_3 has the range  double [0.0 (0x0.0p+0), +Inf]

When we miss is that with an equivalence, we also have to put -0.0 back into
the range.  We currently don't so we think we can fold the second signbit call.

If I fix that, we then see 
r_4(D)  [frange] double [-0.0 (-0x0.0p+0), +Inf]
which prevents the folding.  

I need to audit to see if there are other places where we may have to "adjust"
equivalence range, or how best to deal with this in the general case.