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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109442] Dead local copy of std::vector not
 removed from function
Date: Tue, 11 Apr 2023 13:28:53 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109442

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |jwakely.gcc at gmail dot c=
om,
                   |                            |rguenth at gcc dot gnu.org
   Last reconfirmed|                            |2023-04-11
            Version|unknown                     |13.0

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we're failing to DSE

  _13 =3D pretmp_25 - pretmp_63;
  if (_13 > 4)
    goto <bb 7>; [97.30%]
  else
    goto <bb 6>; [2.70%]

  <bb 6> [local count: 14278734]:
  if (_13 =3D=3D 4)
    goto <bb 8>; [34.00%]
  else
    goto <bb 9>; [66.00%]

  <bb 7> [local count: 913536322]:
  _23 =3D (long unsigned int) _13;
  __builtin_memmove (_37, pretmp_63, _23);
  goto <bb 9>; [100.00%]

  <bb 8> [local count: 34511373]:
  _24 =3D *pretmp_63;
  *_37 =3D _24;

  <bb 9> [local count: 542742079]:
  operator delete (_37, _49);

because I think the DTOR / overloaded global delete might inspect the vector
contents.  So I'm not sure it would be valid to elide the memmove/store.
When the stores would be elided we'd DCE the new/delete pair as well.=