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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/110903] [12/13/14 Regression] Dead Code
 Elimination Regression
Date: Fri, 04 Aug 2023 18:37:42 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: missed-optimization, needs-bisection
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X-Bugzilla-Who: pinskia at gcc dot gnu.org
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X-Bugzilla-Target-Milestone: 12.4
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110903

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-08-04
             Status|UNCONFIRMED                 |NEW
   Target Milestone|---                         |12.4
     Ever confirmed|0                           |1
           Keywords|                            |needs-bisection
            Summary|[14 Regression] Dead Code   |[12/13/14 Regression] Dead
                   |Elimination Regression      |Code Elimination Regression
                   |since r14-1597-g64d90d06d2d |

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. before r14-1597, there was a jump threading happening with respe=
ct
to:
  if (j_32 <=3D 0)
    goto <bb 9>; [50.00%]
  else
    goto <bb 8>; [50.00%]

  <bb 8> [local count: 238907556]:

  <bb 9> [local count: 477815112]:
  # iftmp.10_37 =3D PHI <_11(7), 0(8)>

But after, we change that into
iftmp.10_37 =3D _11 & (j_32 <=3D 0);

It just happens we depend on that due to:
  _43 =3D l_22 | _25;
  _39 =3D j_32 <=3D 0;
  _12 =3D ~_43;
  _44 =3D _12 & _39;


If we change the code to be:
```
void foo(void);
static signed char b, c;
static short e, f;
static int g =3D 41317;
static int(a)(int h, int i) { return h + i; }
static int(d)(int h, int i) { return i & h;}//i ? h : 0; }
short t =3D 10;
int main() {
    {
        signed char j;
        short k =3D t;
        for (; g >=3D 10; g =3D (short)g) {
            _Bool l =3D 1;
            int m =3D 0;
            j =3D 8 * k;
            k =3D j <=3D 0;
            f =3D c + 3;
            for (; c < 2; c =3D f) {
                signed char n =3D 4073709551615;
                if (!(((m) >=3D 0) && ((m) <=3D 0))) {
                    __builtin_unreachable();
                }
                if (g)
                    ;
                else {
                    if ((m =3D k, (b =3D a(d(l, k), e) && n) || l) < k) foo=
();
                    e =3D l =3D 0;
                }
            }
        }
    }
}
```

GCC 11 is able to remove the call to foo but GCC 12 cannot.
the IR for the part where the phiopt2 changes on the trunk is similar enoug=
h.

So this is instead a regression from GCC 11.=