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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100171] autovectorizer
Date: Wed, 21 Apr 2021 06:58:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100171

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, the issue is that we end up with (for the simplest case):

  <bb 2> [local count: 357878152]:
  _15 =3D MEM <const double[2]> [(const value_type &)arg_3(D)][0];
  _16 =3D MEM <const double[2]> [(value_type &)out_2(D)][0];
  _17 =3D _15 + _16;
  MEM <const double[2]> [(value_type &)out_2(D)][0] =3D _17;
  _22 =3D MEM <const double[2]> [(const value_type &)arg_3(D)][1];
  _23 =3D MEM <const double[2]> [(value_type &)out_2(D)][1];
  _24 =3D _22 + _23;
  MEM <const double[2]> [(value_type &)out_2(D)][1] =3D _24;
  return;

and the first store into out[0] can end up writing to arg[1].  I don't see
what we can easily do here.  Path based disambiguation could maybe argue
that partial overlaps of value_type are not allowed.=