From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 5DE183858010; Thu,  2 Dec 2021 21:53:01 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5DE183858010
From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/103534] [12 regression] Spurious -Wstringop-overflow
 warning with std::string concatencation
Date: Thu, 02 Dec 2021 21:53:01 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: alias, diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: pinskia at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: keywords
Message-ID: <bug-103534-4-O4desJJiQn@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-103534-4@http.gcc.gnu.org/bugzilla/>
References: <bug-103534-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Thu, 02 Dec 2021 21:53:01 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103534

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |alias, missed-optimization

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note there is one missing optimization:
  D.42086._M_string_length =3D 0;
  MEM[(char_type &)&D.42086 + 16] =3D 0;
  MEM[(struct basic_string *)&D.41925] =3D{v} {CLOBBER};
  MEM[(struct _Alloc_hider *)&D.41925] =3D{v} {CLOBBER};
  MEM[(struct _Alloc_hider *)&D.41925]._M_p =3D &D.41925.D.33308._M_local_b=
uf;
  _69 =3D operator new (17);

  <bb 3> [local count: 1003797485]:
  D.41925._M_dataplus._M_p =3D _69;
  D.41925.D.33308._M_allocated_capacity =3D 16;
  __builtin_memcpy (_69, "1234567890123456", 16);
  D.41925._M_string_length =3D 16;
  MEM[(char_type &)_69 + 16] =3D 0;
  _75 =3D D.42086._M_string_length;
  __size_76 =3D _75 + 16;
  pretmp_56 =3D D.42086._M_dataplus._M_p;
  if (__size_76 > 16)
    goto <bb 4>; [67.00%]
  else
    goto <bb 12>; [33.00%]


This might be related to that GCC does not handle the operator new better.
Obvious if that missed optimization is fixed, then the warning will be gone.

I also noticed that with -std=3Dc++17 (or before), the constructor for
std::string is not inlined fully for both std::string objects which avoids =
the
false positive.

This is another case where it is a "maybe" exceeds maximum object size and =
we
need to come up with a solution for it.=