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 D1BAD3858C60; Sun, 26 Sep 2021 11:31:11 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org D1BAD3858C60
From: "luc.briand35 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102490] New: Erroneous optimization of default constexpr
 operator== of struct with bitfields
Date: Sun, 26 Sep 2021 11:31:11 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 10.1.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: luc.briand35 at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-102490-4@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: Sun, 26 Sep 2021 11:31:11 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102490

            Bug ID: 102490
           Summary: Erroneous optimization of default constexpr operator=3D=
=3D
                    of struct with bitfields
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luc.briand35 at gmail dot com
  Target Milestone: ---

Hello,

For the following code, gcc version 10 and up wrongly optimizes the default
operator=3D=3D().
This occurs for O1 and up.
Removing the 'constexpr' qualifier fixes everything.
The size of the bitfields doesn't matter.
No warnings are appear with "-Wall -Wextra".


Godbolt link: https://gcc.godbolt.org/z/j4fG3sKze


struct A
{
    unsigned char foo : 1;
    unsigned char bar : 1;

    constexpr bool operator=3D=3D(const A&) const =3D default;
};


int main()
{
    A a{}, b{};

    a.bar =3D 0b1;

    return a =3D=3D b;
}



With the options "-std=3Dc++2a -O1", the assembly generated is simply:
main:
        mov     eax, 1
        ret



In this similar example, we can see that the generated assembly for the
equality operator ignores the 'bar' bitfield (Godbolt link:
https://gcc.godbolt.org/z/3K75xx1on) :


struct A
{
    unsigned char foo : 3;
    unsigned char bar : 1;

    constexpr bool operator=3D=3D(const A&) const =3D default;
};

void change(A& a);

int main()
{
    A a{}, b{};

    change(a);

    return a =3D=3D b;
}


The assembly for gcc version 10.X and 11.X is a bit different, but have the
same problem.=