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This ABI break only affects fancy pointer which have a different value representation when pointing to a base of T instead of T itself. This seems like a rather small set of fancy pointers, which themselves already represent a very small niche. This patch swaps a pointer to T with a pointer to base of T in a few library-internal types.
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===========================================
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Libc++ 20.0.0 (In-Progress) Release Notes
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===========================================
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.. contents::
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:local:
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:depth: 2
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Written by the `Libc++ Team <https://libcxx.llvm.org>`_
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.. warning::
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These are in-progress notes for the upcoming libc++ 20.0.0 release.
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Release notes for previous releases can be found on
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`the Download Page <https://releases.llvm.org/download.html>`_.
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Introduction
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============
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This document contains the release notes for the libc++ C++ Standard Library,
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part of the LLVM Compiler Infrastructure, release 20.0.0. Here we describe the
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status of libc++ in some detail, including major improvements from the previous
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release and new feature work. For the general LLVM release notes, see `the LLVM
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documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
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be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.
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For more information about libc++, please see the `Libc++ Web Site
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<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.
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Note that if you are reading this file from a Git checkout or the
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main Libc++ web page, this document applies to the *next* release, not
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the current one. To see the release notes for a specific release, please
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see the `releases page <https://llvm.org/releases/>`_.
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What's New in Libc++ 20.0.0?
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==============================
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Implemented Papers
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------------------
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- P2747R2: ``constexpr`` placement new (`Github <https://github.com/llvm/llvm-project/issues/105427>`__)
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- P2609R3: Relaxing Ranges Just A Smidge (`Github <https://github.com/llvm/llvm-project/issues/105253>`__)
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- P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
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- ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
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Improvements and New Features
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-----------------------------
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- The ``lexicographical_compare`` and ``ranges::lexicographical_compare`` algorithms have been optimized for trivially
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equality comparable types, resulting in a performance improvement of up to 40x.
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- The ``_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER`` macro has been added to make ``std::get_temporary_buffer`` and
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``std::return_temporary_buffer`` available.
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- The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception``
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available in C++20 and later modes.
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- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``, resulting in reduced
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compile times and smaller debug information as well as better code generation if optimizations are disabled.
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The Chromium project measured a 5% reduction in object file and debug information size.
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Deprecations and Removals
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-------------------------
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- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to
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enable the safe mode will be removed in LLVM 20.
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- Support for the C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been
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removed in language modes prior to C++20. If you are using these features prior to C++20, you will need to
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update to ``-std=c++20``.
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- TODO: The relational operators for ``std::chrono::weekday`` will be removed entirely, and the
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``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro that was used to re-enable this extension will be
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ignored in LLVM 20.
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- The ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro no longer has any effect. ``std::allocator<const T>`` is not
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supported as an extension anymore, please migrate any code that uses e.g. ``std::vector<const T>`` to be
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standards conforming.
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Upcoming Deprecations and Removals
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----------------------------------
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LLVM 20
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~~~~~~~
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- TODO
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LLVM 21
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~~~~~~~
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- The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22,
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non-critical bug fixes may not be back-ported to C++03, including LWG issues. C++03 is a legacy platform, where most
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projects are no longer actively maintained. To reduce the amount of fixes required to keep such legacy projects
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compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid
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unintended breaking changes. See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.
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If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most
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out of libc++.
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ABI Affecting Changes
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---------------------
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- The ABI breaks for removing undefined behaviour in ``std::forward_list``, ``std::list``, ``std::map``, ``std::set``,
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``std::multimap``, ``std::multiset``, ``std::unordered_map``, ``std::unordered_set``, ``std::unordered_multimap`` and
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``std::unordered_multiset`` are now applied unconditionally. This only affects fancy pointers which have a different
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value representation when pointing at the base of an internal node type instead of the type itself. A size or
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alignment difference is diagnosed, but more subtle ABI breaks may result in unexpected behaviour.
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- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. The ABI impact is:
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- When using the Itanium ABI (most non-MSVC platforms), empty types are now placed at the beginning of the enclosing
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object instead of where the beginning of the ``__compressed_pair`` subobject was. This is only observable by
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checking the address of the empty allocator, equality comparator or hasher.
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- Additionally, using an overaligned empty type as an allocator, comparator or hasher in the associative containers
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(and only those containers) may result in the container's object object size and data layout changing beyond only
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the address of the empty member.
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- When using the MSVC ABI, this change results in some classes having a completely different memory layout, so this is
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a genuine ABI break. However, the library does not currently guarantee ABI stability on MSVC platforms.
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Build System Changes
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--------------------
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- TODO
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