[Bug tree-optimization/102879] [12 Regression] Dead Code Elimination Regression at -O3

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102879

--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Aldy Hernandez from comment #1)
>
> 
> So, presumably _62 == 0 cannot be true.  If _62 == 0, then
> 
>     0 = _30 >> _6;
> 
> But that cannot happen because _30 is 2 if you follow the series of copies
> from the "c" global.
> 
> There is no way 0 = 2 >> x can ever be true.
> 
> There's probably a couple things missing here.  Maybe
> operator_rshift::op1_range needs to be taught that ~[0,0] = x >> y implies x
> is non-zero.  But also, we can't see through the load from the c=2 global. 
> Shouldn't that c=2 have been propagated by someone at this point?  (VRP1?)


well, that depends on _6.  0 = _30 >> 32 is always true. 
now, we do happen to know that _6 is [0,1], so if we did manage to determine
that _30 is [2,2], then the we will fold [2,2] >> [0,1] into [1,2] and then
everything should fall into place as we know we can never take that breanch.

That means our core issue is
  c.2_28 = c;
  _29 = (char) c.2_28;
  _30 = (int) _29;
  if (_29 >= 0)

that we don't propagate the 2 into the c.2_28.   we are limited to a range of
[1,127] because of that.