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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/102452] Structs with flexible array members are not
 optimized on stack
Date: Thu, 23 Sep 2021 06:57:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102452

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
get_three after einline w/o USE_FLEX_ARR defined (this is changing the type=
 to
int):

  <bb 2> :
  _5 =3D test_2(D)->is_a;
  if (_5 !=3D 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> :
  D.3292 =3D *test_2(D);
  goto <bb 5>; [100.00%]

  <bb 4> :
  _6 =3D test_2(D)->as.b.one;
  _7 =3D (long int) _6;
  _8 =3D test_2(D)->as.b.two;
  _9 =3D (long int) _8;
  _10 =3D test_2(D)->as.b.three;
  _11 =3D (long int) _10;
  D.3292.as.a.one =3D _7;
  D.3292.as.a.two =3D _9;
  D.3292.as.a.three =3D _11;

  <bb 5> :
  _4 =3D D.3292.as.a.three;
  return _4;

With USE_FLEX_ARR defined to 1:
  <bb 2> :
  _5 =3D test_2(D)->is_a;
  if (_5 !=3D 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> :
  D.3293 =3D *test_2(D);
  goto <bb 5>; [100.00%]

  <bb 4> :
  _6 =3D test_2(D)->as.b.one;
  _7 =3D (long int) _6;
  ret.as.a.one =3D _7;
  _8 =3D test_2(D)->as.b.two;
  _9 =3D (long int) _8;
  ret.as.a.two =3D _9;
  _10 =3D test_2(D)->as.b.three;
  _11 =3D (long int) _10;
  ret.as.a.three =3D _11;
  D.3293 =3D ret;
  ret =3D{v} {CLOBBER};

  <bb 5> :
  _4 =3D D.3293.as.a.three;
  return _4;


as far as I can tell the IR is the same before that,
even RSO:
  D.3293 =3D make_test_a (test_2(D)); [return slot optimization]


But it looks in the case of the flex array case, RSO does not actually happ=
en.=