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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103989] [12 regression] std::optional and
 bogus -Wmaybe-unitialized at -Og since r12-1992-g6feb628a706e86eb
Date: Thu, 13 Jan 2022 11:10:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103989

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The reason we warn is that at -Og we don't optimize away the dead code.  In
uninit2 we have:
  MEM[(struct _Optional_payload_base *)&D.34851]._M_engaged =3D 0;
...
  _27 =3D MEM[(const struct _Optional_payload_base &)&D.34851]._M_engaged;
  if (_27 !=3D 0)
    goto <bb 3>; [33.00%]
  else
    goto <bb 10>; [67.00%]

  <bb 10> [local count: 719407024]:
  goto <bb 9>; [100.00%]

  <bb 3> [local count: 354334800]:
  MEM[(struct __as_base  &)&b] =3D{v} {CLOBBER};
  MEM[(struct shared_ptr *)&b] =3D{v} {CLOBBER};
  MEM[(struct __shared_ptr *)&b] =3D{v} {CLOBBER};
  _30 =3D MEM[(const struct __shared_ptr &)&D.34851]._M_ptr;
  MEM[(struct __shared_ptr *)&b]._M_ptr =3D _30;
  MEM[(struct __shared_count *)&b + 8B] =3D{v} {CLOBBER};
  _31 =3D MEM[(const struct __shared_count &)&D.34851 + 8]._M_pi;
  MEM[(struct __shared_count *)&b + 8B]._M_pi =3D _31;
and we haven't figured out that bb 3 is all dead, because we store into
_M_engaged 0 and bb 3 is done only if _M_engaged is non-zero.
-Og runs fre1, but that is too early, fre1 sees
  MEM[(struct _Optional_payload_base *)&D.34851]._M_engaged =3D 0;
  D.35431 =3D{v} {CLOBBER};
  b =3D{v} {CLOBBER};
  MEM[(struct optional *)&b] =3D{v} {CLOBBER};
  std::_Optional_base<A, false, false>::_Optional_base (&MEM[(struct option=
al
*)&b].D.34640, &D.34851.D.34640);
and _Optional_base isn't inlined there.  And -Og doesn't do any FRE or PRE
after inlining, unlike e.g. -O1 which has fre3 very soon after inlining.
-Og doesn't even do forwprop after inlining.=