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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/115037] Unused std::vector is not optimized away.
Date: Fri, 10 May 2024 16:12:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115037

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at redhat dot com,
                   |                            |jwakely at redhat dot com

--- Comment #2 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
I tried to look for duplicates, but did not find one.
However I think the first problem is that we do not optimize away the store=
 of
1 to vector while clang does.  I think this is because we do not believe we=
 can
trust that delete operator is safe?

We get:
void test ()
{
  int * test$D25839$_M_impl$D25146$_M_start;
  struct vector test;
  int * _61;

  <bb 2> [local count: 1073741824]:
  _61 =3D operator new (4);

  <bb 3> [local count: 1063439392]:
  *_61 =3D 1;
  operator delete (_61, 4);
  test =3D{v} {CLOBBER};
  test =3D{v} {CLOBBER(eol)};
  return;

  <bb 4> [count: 0]:
<L1>:
  test =3D{v} {CLOBBER};
  resx 2

}
If we can not trust fact that operator delete is good, perhaps we can arran=
ge
explicit clobber before calling it? I think it is up to std::vector to deci=
de
what it will do with the stored array so in this case even instane oprator
delete has no right to expect that the data in vector will be sane :)=