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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104639] [12 Regression] Useless loop not
 fully optimized anymore
Date: Thu, 03 Mar 2022 11:36:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104639

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amacleod at redhat dot com

--- Comment #7 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
FWIW, thread2 does thread everything that is threadable.  That is, we thread
5->3->4 from this:

  <bb 3> [local count: 1073741824]:
  # i_1 =3D PHI <i_2(D)(2), 6(5)>
  if (i_1 =3D=3D 4)
    goto <bb 5>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 5> [local count: 955630224]:
  goto <bb 3>; [100.00%]

into this:

  <bb 2> [local count: 118111600]:
  if (i_2(D) =3D=3D 4)
    goto <bb 3>; [97.00%]
  else
    goto <bb 4>; [3.00%]

  <bb 3> [local count: 955630224]:
  # i_5 =3D PHI <6(2)>

  <bb 4> [local count: 118111600]:
  # i_6 =3D PHI <i_2(D)(2), i_5(3)>
  _3 =3D i_6 !=3D 0;
  return _3;

There are no more conditionals we can thread in the final form above.

To my untrained eye, it does seem like phiopt should be able to optimize:

  <bb 4> [local count: 118111600]:
  # i_6 =3D PHI <i_2(D)(2), 6(3)>
  _3 =3D i_6 !=3D 0;

into:

  _3 =3D i_2 !=3D 0;

Then BB2 and BB3 can just be cleaned up as dead?=