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Re: [PATCH] PR libstdc++/88782 avoid ODR problems in std::make_shared
- From: Jonathan Wakely <jwakely at redhat dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: libstdc++ <libstdc++ at gcc dot gnu dot org>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Mon, 21 Jan 2019 11:07:59 +0000
- Subject: Re: [PATCH] PR libstdc++/88782 avoid ODR problems in std::make_shared
- References: <20190118212843.GA7905@redhat.com> <CAFiYyc0+Acx4mO_Z0KGXPgwA5DUNjnp3khdt_JK0y=_QaAiv2g@mail.gmail.com>
On 21/01/19 09:13 +0100, Richard Biener wrote:
On Fri, Jan 18, 2019 at 10:29 PM Jonathan Wakely <firstname.lastname@example.org> wrote:
The old version of _Sp_counted_ptr_inplace::_M_get_deleter (up to GCC
8.2.0) expects to be passed a real std::typeinfo object, so mixing that
with the new definition of the __shared_ptr constructor (which always
passes the fake tag) leads to accessing the fake object as a real
std::typeinfo. Instead of trying to make it safe to mix the old and new
definitions, just stop using that function. By passing a reference to
__shared_ptr::_M_ptr to the __shared_count constructor it can be set
directly, without needing to obtain the pointer via the _M_get_deleter
back-channel. This avoids a virtual dispatch (which fixes PR 87514).
This means that code built against new libstdc++ headers doesn't use
_M_get_deleter at all, and so make_shared works the same whether RTTI is
enabled or not.
Also change _M_get_deleter so that it checks for a real type_info object
even when RTTI is disabled, by calling a library function. Unless
libstdc++ itself is built without RTTI that library function will be
able to test if it's the right type_info. This means the new definition
of _M_get_deleter can handle both the fake type_info tag and a real
type_info object, even if built without RTTI.
If linking to objects built against older versions of libstdc++ then if
all objects use -frtti or all use -fno-rtti, then the caller of
_M_get_deleter and the definition of _M_get_deleter will be consistent
and it will work. If mixing -frtti with -fno-rtti it can still fail if
the linker picks an old definition of _M_get_deleter and an old
__shared_ptr constructor that are incompatible. In that some or all
objects might need to be recompiled.
Can you try summarizing whatever caveats result from this in
the 8.3/9 changes.html parts? I have a hard time extracting that
myself from the above ;) (small example what kind of simplest code might
run into this helps)
There are *fewer* caveats compared to previous releases.
There's an example in PR 87520 showing problems mixing -frtti and
-fno-rtti code, although it's a contrived example that uses explicit
instantiation of shared_ptr internals to demonstrate the problem. The
real issue reported privately to me is harder to reproduce, and will
be even harder with this fix committed. I'll try to document this
potential problem mixing -frtti and -fno-rtti (there are still others,
e.g. PR 43105 lists one).
The problems described in PR 88782 only happen if you use 8.2.1
snapshots post-20181122 and mix code compiled by that snapshot with
code compiled by earlier releases. If you only use 8.3.0 (or 9.1.0)
and no snapshots post-20181122 then, the problem can't happen. I don't
think we need to document transient issues that only existed for two
months with snapshots.
* config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Export new symbol.
(shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...))
(allocate_shared): Change to use new tag type.
* include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_eq):
Declare new member function.
(_Sp_alloc_shared_tag): Define new type.
(_Sp_counted_ptr_inplace): Declare __shared_count<_Lp> as a friend.
(_Sp_counted_ptr_inplace::_M_get_deleter) [!__cpp_rtti]: Use
_Sp_make_shared_tag::_S_eq to check type_info.
(__shared_count(Ptr, Deleter),__shared_count(Ptr, Deleter, Alloc)):
Constrain to prevent being called with _Sp_alloc_shared_tag.
(__shared_count(_Sp_make_shared_tag, const _Alloc&, Args&&...)):
Replace constructor with ...
(__shared_count(Tp*&, _Sp_alloc_shared_tag<_Alloc>, Args&&...)): Use
reference parameter so address of the new object can be returned to
the caller. Obtain the allocator from the tag type.
(__shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...)): Replace
constructor with ...
(__shared_ptr(_Sp_alloc_shared_tag<Alloc>, Args&&...)): Pass _M_ptr
to the __shared_count constructor.
(__allocate_shared): Change to use new tag type.
* src/c++11/shared_ptr.cc (_Sp_make_shared_tag::_S_eq): Define.
Tested powerpc64le-linux, committed to trunk.
I'll backport this to gcc-8-branch without the new symbol in the library.