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Re: [PATCH] PR libstdc++/88782 avoid ODR problems in std::make_shared


On Fri, Jan 18, 2019 at 10:29 PM Jonathan Wakely <jwakely@redhat.com> 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)

Richard.

>         PR libstdc++/87514
>         PR libstdc++/87520
>         PR libstdc++/88782
>         * config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Export new symbol.
>         * include/bits/shared_ptr.h
>         (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.
>
>


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