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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109443] missed optimization of std::vector access (Related
 to issue 35269)
Date: Tue, 11 Apr 2023 10:16:13 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: alias, missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109443

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-04-11
             Status|UNCONFIRMED                 |NEW
            Version|unknown                     |13.0
          Component|tree-optimization           |c++
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=3D35269

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, but v.size () is not loop invariant.  As Andrew says 'v' is passed by
(const?) reference but the PTA solution doesn't exactly reflect this
so we do think the call clobbers it:

  # USE =3D nonlocal escaped
  # CLB =3D nonlocal escaped
  f (_1);
...
  # PT =3D nonlocal escaped null
  _10 =3D MEM[(const struct vector *)v_5(D)].D.35644._M_impl.D.34955._M_fin=
ish;
  # PT =3D nonlocal escaped null
  _11 =3D MEM[(const struct vector *)v_5(D)].D.35644._M_impl.D.34955._M_sta=
rt;
  _12 =3D _10 - _11;
  _13 =3D _12 /[ex] 4;
  _14 =3D (long unsigned int) _13;
  if (i_8 < _14)
    goto <bb 3>; [89.00%]

so somehow the C++ handling of value-as-reference passing doesn't work (it
should set TYPE_RESTRICT on the argument type used.=