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From: "qinzhao at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/104550] bogus warning from -Wuninitialized +
 -ftrivial-auto-var-init=pattern
Date: Tue, 15 Feb 2022 21:33:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104550

--- Comment #3 from qinzhao at gcc dot gnu.org ---
the root cause for this bug is due to the new call to __builtin_clear_paddi=
ng=20
added by -ftrivial-auto-var-init=3Dpattern, this is treated as a use of the
uninitialized variable during early uninitialized analysis phase, in which
phase DCE hasn't been applied yet, as a result, the early uninit phase repo=
rted
this uninitialized usage.=20

the following is the details:

1. without -ftrivial-auto-var-init, the IR in gimple stage is:
void vx_set_monitor_level ()
{=20
  struct vx_audio_level info;

  try
    {

    }
  finally
    {
      info =3D {CLOBBER(eol)};
    }
}

no any usage of the auto var "info", no any warning message for
-Wuninitialized;

2. With -ftrivial-auto-var-init, the IR in gimple stage is:
void vx_set_monitor_level ()
{=20
  struct vx_audio_level info;

  try
    {
      info =3D .DEFERRED_INIT (4, 1, &"info"[0]);
      __builtin_clear_padding (&info, 1B);
    }
  finally
    {
      info =3D {CLOBBER(eol)};
    }
}

then after folding of call to "__builtin_clear_padding", the IR at early
uninitialized analysis phase is:

void vx_set_monitor_level ()
{
  struct vx_audio_level info;
  int _1;
  int _2;

  <bb 2> :
  info =3D .DEFERRED_INIT (4, 1, &"info"[0]);
  _1 =3D MEM[(struct vx_audio_level *)&info];
  _2 =3D _1 & 1;
  MEM[(struct vx_audio_level *)&info] =3D _2;
  info =3D{v} {CLOBBER(eol)};
  return;

}

The bogus warning is issued for the following fake usage:

 _1 =3D MEM[(struct vx_audio_level *)&info];

which is unavoidable during the early uninitialized stage based on the curr=
ent
IR info.=