c0cde79e9e
As explained in the release note, libc++ used to provide various global variables as an extension in C++03 mode. Unfortunately, that made our definition non-conforming in all standard modes. This was never a big problem until recently, since we are trying to support C++20 Modules in libc++, and that requires cleaning up the definition of these variables. This change is the first in a series of changes to achieve our end goal. This patch removes the ability for users to rely on the (incorrect) definition of those global variables inside the shared library. The plan is to then remove those definitions from the shared library (which is an ABI break but I don't think it will have impact), and finally to make our definition of those variables conforming in all standard modes. Differential Revision: https://reviews.llvm.org/D145422
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=========================================
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Libc++ 17.0.0 (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++ 17 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 17.0.0. 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++ 17.0.0?
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============================
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Implemented Papers
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------------------
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- P2520R0 - ``move_iterator<T*>`` should be a random access iterator
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- P1328R1 - ``constexpr type_info::operator==()``
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Improvements and New Features
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-----------------------------
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- ``std::equal`` and ``std::ranges::equal`` are now forwarding to ``std::memcmp`` for integral types and pointers,
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which can lead up to 40x performance improvements.
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- ``std::string_view`` now provides iterators that check for out-of-bounds accesses when the safe
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libc++ mode is enabled.
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- The performance of ``dynamic_cast`` on its hot paths is greatly improved and is as efficient as the
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``libsupc++`` implementation. Note that the performance improvements are shipped in ``libcxxabi``.
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Deprecations and Removals
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-------------------------
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- The ``<experimental/coroutine>`` header has been removed in this release. The ``<coroutine>`` header
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has been shipping since LLVM 14, so the Coroutines TS implementation is being removed per our policy
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for removing TSes.
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- Several incidental transitive includes have been removed from libc++. Those
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includes are removed based on the language version used. Incidental transitive
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inclusions of the following headers have been removed:
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- C++2b: ``atomic``, ``bit``, ``cstring``, ``type_traits``
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- The headers ``<experimental/algorithm>`` and ``<experimental/functional>`` have been removed, since all the contents
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have been implemented in namespace ``std`` for at least two releases.
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- The formatter specialization ``template<size_t N> struct formatter<const charT[N], charT>``
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has been removed. Since libc++'s format library was marked experimental there
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is no backwards compatibility option. This specialization has been removed
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from the Standard since it was never used, the proper specialization to use
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instead is ``template<size_t N> struct formatter<charT[N], charT>``.
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- Libc++ used to provide some C++11 tag type global variables in C++03 as an extension, which are removed in
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this release. Those variables were ``std::allocator_arg``, ``std::defer_lock``, ``std::try_to_lock``,
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``std::adopt_lock``, and ``std::piecewise_construct``. Note that the types associated to those variables are
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still provided in C++03 as an extension (e.g. ``std::piecewise_construct_t``). Providing those variables in
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C++03 mode made it impossible to define them properly -- C++11 mandated that they be ``constexpr`` variables,
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which is impossible in C++03 mode. Furthermore, C++17 mandated that they be ``inline constexpr`` variables,
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which led to ODR violations when mixed with the C++03 definition. Cleaning this up is required for libc++ to
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make progress on support for C++20 modules.
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Upcoming Deprecations and Removals
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----------------------------------
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- The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro will not be honored anymore in LLVM 18.
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Please see the updated documentation about the safe libc++ mode and in particular the ``_LIBCPP_VERBOSE_ABORT``
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macro for details.
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- The headers ``<experimental/deque>``, ``<experimental/forward_list>``, ``<experimental/list>``,
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``<experimental/map>``, ``<experimental/memory_resource>``, ``<experimental/regex>``, ``<experimental/set>``,
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``<experimental/string>``, ``<experimental/unordered_map>``, ``<experimental/unordered_set>``,
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and ``<experimental/vector>`` will be removed in LLVM 18, as all their contents will have been implemented in
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namespace ``std`` for at least two releases.
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API Changes
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-----------
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ABI Affecting Changes
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---------------------
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Build System Changes
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--------------------
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- Building libc++ and libc++abi for Apple platforms now requires targeting macOS 10.13 and later.
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This is relevant for vendors building the libc++ shared library and for folks statically linking
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libc++ into an application that has back-deployment requirements on Apple platforms.
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