mirror of
https://github.com/llvm/llvm-project.git
synced 2025-04-28 07:06:05 +00:00
db381405ce
Committing on behalf of davidben. Reviewed as D146190 Patch originally by Jan Dörrie in https://reviews.llvm.org/D120064. I've just updated it to include tests, and update documentation that MSVC ABI is not stable. In the current implementation both `std::optional` and `std::variant` don't perform the EBO on MSVC's ABI. This is because both classes inherit from multiple empty base classes, which breaks the EBO for MSVC. This patch fixes this issue by applying the `empty_bases` declspec attribute, which is already used to fix a similar issue for `std::tuple`. See https://reviews.llvm.org/D120064 for discussion on MSVC ABI stability. From the discussion, libc++ doesn't have users that expect a stable ABI on MSVC. The fix is thus applied unconditionally to benefit more users. Documentation has been updated to reflect this. Fixes https://github.com/llvm/llvm-project/issues/61095.
33 lines
1.4 KiB
ReStructuredText
33 lines
1.4 KiB
ReStructuredText
|
|
====================
|
|
Libc++ ABI stability
|
|
====================
|
|
|
|
Libc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI
|
|
is linked with code built under the new ABI. At the same time, libc++ wants to make
|
|
ABI-breaking improvements and bugfixes in scenarios where the user doesn't mind ABI breaks.
|
|
|
|
To support both cases, libc++ allows specifying an ABI version at
|
|
build time. The version is defined with CMake option ``LIBCXX_ABI_VERSION``.
|
|
Currently supported values are ``1`` (the stable default)
|
|
and ``2`` (the unstable "next" version). At some point "ABI version 2" will be
|
|
frozen and new ABI-breaking changes will start being applied to version ``3``;
|
|
but this has not happened yet.
|
|
|
|
To always use the most cutting-edge, most unstable ABI (which is currently ``2``
|
|
but at some point will become ``3``), set the CMake option ``LIBCXX_ABI_UNSTABLE``.
|
|
|
|
Internally, each ABI-changing feature is placed under its own C++ macro,
|
|
``_LIBCPP_ABI_XXX``. These macros' definitions are controlled by the C++ macro
|
|
``_LIBCPP_ABI_VERSION``, which is controlled by the ``LIBCXX_ABI_VERSION`` set
|
|
at build time. Libc++ does not intend users to interact with these C++ macros
|
|
directly.
|
|
|
|
-----------------
|
|
MSVC environments
|
|
-----------------
|
|
|
|
The exception to this is MSVC environments. Libc++ does not currently have users
|
|
that require a stable ABI in MSVC environments, so MSVC-only changes may be
|
|
applied unconditionally.
|