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 2C00E3858414; Tue, 30 Nov 2021 17:56:56 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 2C00E3858414
From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/103483] context-sensitive ranges change triggers
 stringop-overread
Date: Tue, 30 Nov 2021 17:56:56 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: msebor 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: component keywords cc
Message-ID: <bug-103483-4-KMatCcNE7u@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-103483-4@http.gcc.gnu.org/bugzilla/>
References: <bug-103483-4@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: Tue, 30 Nov 2021 17:56:56 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103483

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end
           Keywords|                            |missed-optimization
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning is only issued at -O1.  It's based on the statement in the IL a=
nd
the values or ranges of its arguments.  In this case the IL and the argument
values are below:

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 6 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Imports: _1  d_6(D)=20=20
Exports: _1  d_6(D)=20=20
_1      sizetype [0, 0][2, 15]
    <bb 6> [local count: 362271902]:
    if (_1 =3D=3D 0)
      goto <bb 8>; [100.00%]
    else
      goto <bb 7>; [0.00%]

6->8  (T) _1 :  sizetype [0, 0]
6->7  (F) _1 :  sizetype [2, 15]          >>> _1 > 1

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 7 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
    <bb 7> [local count: 346397698]:
    # _2 =3D PHI <&s.D.26133._M_local_buf(6), _19(3)>
    __builtin_memcpy (_2, &buffer, _1);   <<< -Wstringop-overread

The memcpy() call reads between 2 and 15 bytes from the one-byte buffer.  So
the warning code is working as designed.  The problem is that at -O1 the co=
de
isn't optimized sufficiently to discover that the memcpy call only reads 1 =
byte
(the function call isn't inlined and so the constant argument isn't propaga=
ted
into the call).=20

GCC 11 doesn't warn because it's unable to determine the range of the last
memcpy() argument.  Thanks to the Ranger improvements GCC 12 is able to do
better.  In some cases this should improve the S/N ratio of the middle end
diagnostics that depend on ranges.  Unfortunately, in others like this one
where other optimizations are disabled it can make things worse.=