[cfe-dev] [RFC] Possible ABI break for libc++ built with -fno-rtti and -D_LIBCPP_BUILD_STATIC

[Eric Fiselier via cfe-dev]

On Sun, May 7, 2017 at 7:50 AM, Martin J. O'Riordan <
martin.oriordan at movidius.com> wrote:

> Hi Eric,
>
>
>
> In the email I sent on the 27th April (“[LibC++] Bug in implementation of
> 'std::shared_ptr'”), the problem I was seeing was:
>
>
>
>   o  LibC++ library built with -DLIBCXX_ENABLE_RTTI=ON and
> -D_LIBCPP_BUILD_STATIC.
>
>   o  User code compiled with ‘-fno-rtti -D_LIBCPP_BUILD_STATIC’.
>
>
>
> Having ‘_LIBCPP_BUILD_STATIC’ defined by both the library builder and the
> library user was where it broke.  I removed ‘-D_LIBCPP_BUILD_STATIC’ from
> the user and it worked.  But I wasn’t building the static library with RTTI
> disabled because it is stated as an unsupported configuration.
>
>
>
> So are you advising that with the proposed fix, that I should revert to
> defining ‘_LIBCPP_BUILD_STATIC’ as the user?
>

`_LIBCPP_BUILD_STATIC` has been removed entirely by the change. It fixes
your bug by making the vtable consistent across all configurations.

 /Eric



>
> Thanks,
>
>
>
>             MartinO
>
>
>
> *From:* Eric Fiselier [mailto:eric at efcs.ca]
> *Sent:* 04 May 2017 00:43
> *To:* clang developer list <cfe-dev at lists.llvm.org>
> *Cc:* Marshall Clow <mclow.lists at gmail.com>; Duncan P. N. Exon Smith <
> dexonsmith at apple.com>; Ed Maste <emaste at freebsd.org>; Martin J. O'Riordan
> <martin.oriordan at movidius.com>; Anton Korobeynikov <
> anton at korobeynikov.info>
> *Subject:* [RFC] Possible ABI break for libc++ built with -fno-rtti and
> -D_LIBCPP_BUILD_STATIC
>
>
>
> Hi All,
>
>
>
> Libc++ currently has an ABI incompatibility when built as a static library
> with no RTTI. Specifically some std::shared_ptr internals have different
> vtable layouts in this configuration.
>
>
>
> I would like to fix this by making the vtable definition consistent across
> all configurations, but that requires breaking the ABI of a very rare
> configuration of libc++. I suspect all users of this configuration can take
> the ABI break immediately, but I would like to confirm this before
> proceeding.
>
>
>
> The configuration in question is a static libc++ library built with
> -DLIBCXX_ENABLE_RTTI=OFF and -D_LIBCPP_BUILD_STATIC,
>
>
>
> Is anybody using this configuration and can't take an ABI break?
>
>
>
> Note that `_LIBCPP_BUILD_STATIC` must be manually defined by the user at
> both when building and using libc++, and it is not supported by the CMake
> build system. This should mean that almost nobody is defining it.
>
>
>
> The proposed changes can be found here: https://reviews.llvm.org/D32838
>
>
>
> /Eric
>
>
>
>
>
>
>
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