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 3EF5E3858C98; Tue,  5 Mar 2024 20:08:40 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 3EF5E3858C98
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1709669320;
	bh=PvlpQ7Oh101VvyzzOL6rbEGiMfPsynYAw64EmKcun2E=;
	h=From:To:Subject:Date:From;
	b=DThXgI/AN6NcvvE49qxuJgLF8oOkg29duhA4NYCkDbUmHl6+E5CBt339qZ3TZHTZh
	 SZpMrrhgADMJhDFCOaEOAkk44Hr4/S+WV9V2R1HqkVbusgiUJ1aESOJLx/E79PTF4N
	 iwiK+dtPei5xGvsciElL4fTuFeCInnYX1bdepboU=
From: "mwinkler at blizzard dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114245] New: Defaulted virtual destructors that do no work
 overwrite the vtable with `-O0`
Date: Tue, 05 Mar 2024 20:08:39 +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: 13.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: mwinkler at blizzard 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-114245-4@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
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114245

            Bug ID: 114245
           Summary: Defaulted virtual destructors that do no work
                    overwrite the vtable with `-O0`
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mwinkler at blizzard dot com
  Target Milestone: ---

https://godbolt.org/z/Ge135h6qh

Use the godbolt for reference against all the 3 major compilers,
GCC/Clang/MSVC.
The following bug report is only for `-O0` builds.
Optimized builds end up removing the dead vtable stores but we require our
debug builds to behave the same as release builds.

Given a virtual destructor that is defaulted and does no work GCC will
overwrite the vtable with the current objects most constructed type.

This behaviour isn't required by MSVC or Itanium CXX ABI or the C++ std but=
 it
would be nice to have all 3 major compilers agree here especially since MSVC
and Clang both have this behaviour.

```
struct Base { virtual ~Base() =3D default; };
struct Derived : Base { virtual ~Derived() =3D default; };
```

In the example above `~Derived` will overwrite the vtable with the vtable f=
or
Derived. `~Base` will overwrite the vtable with the vtable for Base.

If you have a global object of `Derived` you easily run into static de-init
fiasco issues since these objects are also registered with `__cxa_atexit`.

Clang and MSVC for defaulted virtual destructors do not overwrite the vtabl=
e in
the generated destructors even though they still register with `__cxa_atexi=
t`
so the running of the destructor ends up being a nop.
Clang also has the `[[clang::no_destroy]]` attribute.

We have a workaround on GCC by doing the following for such global objects.
```
union NoDestroy
{
    Derived v;
    constexpr NoDestroy() : v() {}
    ~NoDestroy() {}
};
```

There are two solutions that will work for our use case.

Add a similar attribute like `[[clang::no_destroy]]` to mark certain global
objects that should not be registered with `__cxa_atexit`. I couldn't find =
such
a similar attribute in the GCC docs.

Have GCC behave like MSVC and Clang here when optimizations are disabled and
have defaulted virtual destructors not overwrite the vtable.

Thanks :).=