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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug gcov-profile/100145] missed optimization for dead code
 elimination at -O3 (vs. -O2)
Date: Thu, 20 May 2021 13:57:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100145

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note after inlining we still have a backedge into BB 3:

  <bb 2> [local count: 1073741824]:
  c =3D 0;
  b.1_7 =3D b;
  if (b.1_7 !=3D 0)
    goto <bb 3>; [34.00%]
  else
    goto <bb 7>; [66.00%]

  <bb 3> [local count: 3318838410]:
  _9 =3D 1;
  _10 =3D 0;
  _11 =3D _9 & _10;
  if (_11 !=3D 0)
    goto <bb 6>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 4> [local count: 1659419208]:
  _12 =3D 0;
  _13 =3D _10 & _12;
  if (_13 !=3D 0)
    goto <bb 6>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 5> [local count: 829709603]:
  iftmp.0_14 =3D 1;

  <bb 6> [local count: 3318838410]:
  # iftmp.0_15 =3D PHI <0(3), 0(4), iftmp.0_14(5)>
  _19 =3D iftmp.0_15;
  _8 =3D _19;
  c =3D _8;
  if (_8 !=3D 0)
    goto <bb 7>; [11.00%]
  else
    goto <bb 3>; [89.00%]

  <bb 7> [local count: 1073741824]:

but that's quickly elided by CCP after inlining producing

  <bb 2> [local count: 1073741824]:
  c =3D 0;
  b.1_7 =3D b;
  if (b.1_7 !=3D 0)
    goto <bb 3>; [34.00%]
  else
    goto <bb 7>; [66.00%]

  <bb 3> [local count: 3318838410]:
  if (0 !=3D 0)
    goto <bb 6>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 4> [local count: 1659419208]:
  if (0 !=3D 0)
    goto <bb 6>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 5> [local count: 829709603]:

  <bb 6> [local count: 3318838410]:
  c =3D 1;
  if (1 !=3D 0)
    goto <bb 7>; [11.00%]
  else
    goto <bb 3>; [89.00%]

  <bb 7> [local count: 1073741824]:

and the CFG cleanup ends up with the following:

  <bb 2> [local count: 1073741824]:
  c =3D 0;
  b.1_7 =3D b;
  if (b.1_7 !=3D 0)
    goto <bb 3>; [34.00%]
  else
    goto <bb 4>; [66.00%]

  <bb 3> [local count: 3318838410]:
  c =3D 1;

  <bb 4> [local count: 1073741824]:
  d =3D 1;
  _1 =3D c;
  if (_1 > 100)
    goto <bb 5>; [33.00%]
  else
    goto <bb 6>; [67.00%]

  <bb 5> [local count: 354334800]:
  foo ();

  <bb 6> [local count: 1073741824]:
  return 0;=