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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/105832] [13/14 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs. 12.1.0)
Date: Wed, 12 Jul 2023 18:00:46 +0000
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X-Bugzilla-Component: tree-optimization
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X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105832

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think the simple way of fixing this is optimizing:
```
bool f(int g)
{
return (1 >> g) !=3D 0;
}
```
into
```
bool f0(int g)
{
return g =3D=3D 0;
}
```

In threadfull1 (before vrp1) we have:
```
  <bb 5> [local count: 894749065]:
  if (iftmp.0_6 <=3D 1)
    goto <bb 6>; [41.00%]
  else
    goto <bb 8>; [59.00%]

  <bb 6> [local count: 366847113]:
  _3 =3D (int) iftmp.0_6;
  _4 =3D 1 >> _3;
  if (_4 =3D=3D 0)
    goto <bb 7>; [50.00%]
  else
    goto <bb 8>; [50.00%]

  <bb 7> [local count: 183423557]:
  iftmp.1_12 =3D iftmp.0_6 << 1;

  <bb 8> [local count: 894749065]:
  # iftmp.1_7 =3D PHI <iftmp.1_12(7), iftmp.0_6(5), iftmp.0_6(6)>
```
if we optimize the bb6 to:
```
  if (iftmp.0_6 !=3D 0)
    goto <bb 7>; [50.00%]
  else
    goto <bb 8>; [50.00%]
```
We should be able to ifcombine the 2 GIMPLE to just:
iftmp.0_6 !=3D 0 && iftmp.0_6 <=3D 1

Which then will unswitch correctly ...=