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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102879] [12 Regression] Dead Code Elimination
 Regression at -O3
Date: Thu, 21 Oct 2021 14:42:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102879

--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Aldy Hernandez from comment #1)
>
>=20
> So, presumably _62 =3D=3D 0 cannot be true.  If _62 =3D=3D 0, then
>=20
>     0 =3D _30 >> _6;
>=20
> But that cannot happen because _30 is 2 if you follow the series of copies
> from the "c" global.
>=20
> There is no way 0 =3D 2 >> x can ever be true.
>=20
> There's probably a couple things missing here.  Maybe
> operator_rshift::op1_range needs to be taught that ~[0,0] =3D x >> y impl=
ies x
> is non-zero.  But also, we can't see through the load from the c=3D2 glob=
al.=20
> Shouldn't that c=3D2 have been propagated by someone at this point?  (VRP=
1?)


well, that depends on _6.  0 =3D _30 >> 32 is always true.=20
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 =3D c;
  _29 =3D (char) c.2_28;
  _30 =3D (int) _29;
  if (_29 >=3D 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.=