Date: Tue, 26 Apr 2022 18:33:15 -0700 From: Mark Millard <marklmi@yahoo.com> To: joerg@bec.de, FreeBSD Hackers <freebsd-hackers@freebsd.org> Cc: jbo@insane.engineer Subject: Re: llvm & RTTI over shared libraries Message-ID: <000ACC55-4740-4624-A0E3-24AB775929A1@yahoo.com> In-Reply-To: <A7BF3192-44EF-485C-8CB7-E7F4FFCA1E5A@yahoo.com> References: <A7BF3192-44EF-485C-8CB7-E7F4FFCA1E5A@yahoo.com>
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On 2022-Apr-26, at 17:48, Mark Millard <marklmi@yahoo.com> wrote:
>=20
> =E2=80=A2 Joerg Sonnenberger <joerg_at_bec.de> wrote on
> =E2=80=A2 Date: Tue, 26 Apr 2022 23:47:23 UTC :
>=20
>> Am Mon, Apr 25, 2022 at 03:39:48PM -0700 schrieb Mark Millard:
>>> Basically I avoid inline definitions of:
>>>=20
>>> virtual ~type_base();
>>> virtual ~type_int();
>>> virtual ~type_string();
>>=20
>> You only need to ensure that the class has one non-pure non-inline
>> function.
>=20
> I'm confused at what you are claiming that I did wrong or
> described incorrectly for the example at hand. Those are 3
> separate classes each with one virtual method that is not
> in line (and that I showed the definitions for later in
> the message). No other such functions were involved explicitly
> in those 3 classes.
>=20
> The gcc class type_info in /usr/local/lib/gcc11/include/c++/typeinfo
> describes this implementation detail for type_info itself, as its
> example, via:
>=20
> class type_info
> {
> public:
> /** Destructor first. Being the first non-inline virtual function, =
this
> * controls in which translation unit the vtable is emitted. The
> * compiler makes use of that information to know where to emit
> * the runtime-mandated type_info structures in the new-abi. */
> virtual ~type_info();
> . . .
May be the intent is tied to what happens with
a one line change, using
struct type_base
{
~type_base() =3D 0;
};
but still having the required definition:
interface::type_base::~type_base() {}
(implicitly referenced so required in order
to link). This makes type_base abstract (not
something to potentially directly create).
The original example did not have that property,
so I did not make my nearly minimal change that
way. But using the '=3D 0' as above does produce
a working program:
# ./test-core=20
t is type_int
t is type_string
done.
>> That's the key function and determines the translation unit
>> (and by extension the DSO) where the virtual table and the typeinfo =
is
>> placed.
>=20
> I'm a again confused about where the above disagrees with
> what I wrote (in text that you did not quote).
>=20
>> If there is no such function, both will be defined as weak
>> mergable symbol and that will not result in a unique address when =
using
>> RTLD_LOCAL.
>=20
> I was certainly less detailed about how multiple definitions
> are handled. Was that your point?
>=20
> Let me know if I've missed some import point --or made some
> stupid screwup in what I'd written.
>=20
=3D=3D=3D
Mark Millard
marklmi at yahoo.com
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