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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/107740] [12/13 Regression] if-to-switch
 conversion happens for simple predicate function when compiled with gcc but
 not with g++
Date: Thu, 17 Nov 2022 20:59:17 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107740

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Target Milestone|---                         |12.3
   Last reconfirmed|                            |2022-11-17
             Status|UNCONFIRMED                 |NEW
            Summary|if-to-switch conversion     |[12/13 Regression]
                   |happens for simple          |if-to-switch conversion
                   |predicate function when     |happens for simple
                   |compiled with gcc but not   |predicate function when
                   |with g++                    |compiled with gcc but not
                   |                            |with g++

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The difference is PHI-OPT:
For C++ we have:

  _4 =3D c_8(D) =3D=3D 13;
  _5 =3D c_8(D) =3D=3D 9;
  _6 =3D _4 | _5;
  if (_6 !=3D 0)
    goto <bb 4>; [INV]
  else
    goto <bb 5>; [INV]

  <bb 4> :

  <bb 5> :
  # iftmp.0_7 =3D PHI <1(4), 0(3)>
Which phi-opt1 changes to:

  _4 =3D c_8(D) =3D=3D 13;
  _5 =3D c_8(D) =3D=3D 9;
  _6 =3D _4 | _5;

  <bb 4> :
  # iftmp.0_7 =3D PHI <1(2), _6(3)> (note type is bool)


But for C we have:

  if (_6 !=3D 0)
    goto <bb 4>; [INV]
  else
    goto <bb 5>; [INV]

  <bb 4> :

  <bb 5> :
  # iftmp.0_7 =3D PHI <1(4), 0(3)> (note the type is int)

phi-opt1 is the "early phi-opt" which tries not to do it here.

Also this is a regression from GCC 11.3.0.

There has been improvements to phi-opt and there was slight IR change due to
cleanup cfg after cddce1 which allowed phi-opt to do the change here. Note
PHI-OPT is not at fault really but if-to-switch not handling the slightly
different IR really.=