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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/102402] Seemingly suboptimal optimization of
 jmp/cmovcc for conditionally loading constants
Date: Sat, 18 Sep 2021 18:35:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102402

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
          Component|target                      |tree-optimization
   Last reconfirmed|                            |2021-09-18
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, note the best code is generated by:
void ClearModM1(struct MusicPlayerTrack *track, uint8_t modT)
{
    int t =3D0;
    if (track->modT =3D=3D 0)
        t =3D 3;
    else
        t =3D 12;
    track->flags |=3D t;
}

This is then a gimple level issue where:
  if (_1 =3D=3D 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _3 =3D pretmp_6 | 3;
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 536870913]:
  _5 =3D pretmp_6 | 12;

  <bb 5> [local count: 1073741824]:
  # cstore_12 =3D PHI <_3(3), _5(4)>
is not transformed into:
    if (track->modT =3D=3D 0)
        t =3D 3;
    else
        t =3D 12;
cstore_12 =3D pretmp_6  | t;

There is also a sinking issue though I don't know if has a wider effect (and
does not matter in this case really).
Take:
void ClearModM1(struct MusicPlayerTrack *track, uint8_t modT)
{
    int t =3D0;
    int t2 =3D track->modT;
    int t1 =3D track->flags;
    if (t2 =3D=3D 0)
        t =3D 3;
    else
        t =3D 12;
    track->flags =3D t1|t;
}
The load for track->flags is not sunk after the if.=