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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/103281] [10/12/13/14 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs 11.2.0)
Date: Mon, 01 May 2023 23:49:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103281

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm:
  # RANGE [irange] char [0, 2] NONZERO 0x3
  c_9 =3D (charD.7) b.4_5;
  _1 =3D c_9 <=3D 0;



Should _1 be replaced with c_9 =3D=3D 0 which then can be simplified to b.4=
_5 =3D=3D 0
That is PR 28794 I think.

And then after that we get:
b.4_5 =3D=3D (unsigned int)(b.4_5 =3D=3D 0)

Which should be optimized down to false though we don't either.
unsigned f(unsigned t)
{
        unsigned a =3D t =3D=3D 0;
        return t =3D=3D a;
}

The general CST cases (I hope I did these correctly):
a =3D=3D (a =3D=3D CST) ->
CST =3D=3D 0 -> false
CST =3D=3D 1 -> a =3D=3D 1 | a =3D=3D 0
others   -> a =3D=3D 0

a !=3D (a =3D=3D CST) ->
CST =3D=3D 0: false
CST =3D=3D 1: a !=3D 0 || a !=3D 1
others  : a !=3D 0

a !=3D (a !=3D CST) ->
CST =3D=3D 0: a =3D=3D 1 | a =3D=3D 0
CST =3D=3D 1: true
others  : a !=3D 1

a =3D=3D (a !=3D CST) ->
CST =3D=3D 0: a =3D=3D 1 | a =3D=3D 0
CST =3D=3D 1: false
others  : a =3D=3D 1=