[libcxx-dev] should the unstable ABI support building a shared library?

[Petr Hosek via libcxx-dev]

We cross-compile libc++ for Fuchsia as a shared library with
LIBCXX_ABI_UNSTABLE in the CMake build so I'd prefer if this was
overridable/optional.

On Sat, Jun 22, 2019 at 3:26 AM Nico Weber via libcxx-dev <
libcxx-dev at lists.llvm.org> wrote:

> We depend on being able to build libc++ as a shared library
> with LIBCXX_ABI_UNSTABLE set in Chromium. We build libc++ from source as
> part of a regular chrome build, and in a component build (where each
> library of chrome becomes a .so) libc++ must be a .so as well.
>
> (We use a custom inline namespace name, and we make sure our libc++ is
> always on the search path before system libc++).
>
> We don't use libc++'s cmake files, so if this is just about libc++'s cmake
> setup, we don't have an opinion on this.
>
> On Sat, Jun 22, 2019 at 12:42 AM Richard Smith via libcxx-dev <
> libcxx-dev at lists.llvm.org> wrote:
>
>> By default, libc++ builds produce a shared library and a static library.
>> When LIBCXX_ABI_UNSTABLE is enabled, though, that seems like a mistake:
>> there is no stable interface, so dynamically linking against an
>> unstable-ABI libc++ shared library will be problematic.
>>
>> In addition, if LIBCXX_ABI_VERSION is not specified during configuration,
>> the default behavior of the build system with LIBCXX_ABI_UNSTABLE enabled
>> is to produce a libc++.so.1.0 with the unstable ABI, which causes problems
>> if it's found on the library search path for a binary built against the
>> normal-ABI libc++.so.1.0. (And in a stock monorepo build, this means that
>> enabling LIBCXX_ABI_UNSTABLE and then building, say, llvm and libc++, with
>> a host compiler that uses libc++ as the standard library, results in broken
>> LLVM binaries in the build area: they were compiled against the stable
>> libc++ ABI but will pick up an unstable-ABI version of libc++ at runtime.)
>>
>> Perhaps we should refuse to build a shared library version of libc++ with
>> the unstable ABI enabled?
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