985d75a57a
P2321R2 has been implemented in various PRs. Based on the discussion in #105169, the last bit in iterator.concept.winc doesn't require any changes, so we can actually mark this as done. Fixes #105169
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===========================================
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Libc++ 22.0.0 (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++ 22.0.0 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 22.0.0. 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++ 22.0.0?
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==============================
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Implemented Papers
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------------------
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- P2592R3: Hashing support for ``std::chrono`` value classes (`Github <https://llvm.org/PR105358>`__)
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- P2321R2: ``zip`` (`Github <https://llvm.org/PR105169>`__)
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- P2988R12: ``std::optional<T&>`` (`Github <https://llvm.org/PR148131>`__)
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- P3044R2: sub-``string_view`` from ``string`` (`Github <https://llvm.org/PR148140>`__)
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- P3223R2: Making ``std::istream::ignore`` less surprising (`Github <https://llvm.org/PR148178>`__)
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- P3060R3: Add ``std::views::indices(n)`` (`Github <https://llvm.org/PR148175>`__)
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- P2404R3: Move-only types for ``equality_comparable_with``, ``totally_ordered_with``, and ``three_way_comparable_with``
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(`Github <https://llvm.org/PR105210>`__)
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- P2641R4: Checking if a ``union`` alternative is active (``std::is_within_lifetime``)
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(`Github <https://llvm.org/PR105381>`__)
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- P2835R7: Expose ``std::atomic_ref``'s object address (`Github <https://llvm.org/PR118377>`__)
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- P2944R3: Comparisons for ``reference_wrapper`` (`Github <https://llvm.org/PR105424>`__)
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- P3168R2: Give ``std::optional`` Range Support
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(guarded by ``-fexperimental-library``, `Github <https://llvm.org/PR105430>`__)
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- P3567R2: ``flat_meow`` Fixes (`Github <https://llvm.org/PR162022>`__)
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- P3836R2: Make ``optional<T&>`` trivially copyable (`Github <https://llvm.org/PR171275>`__)
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- P1789R3: Library Support for Expansion Statements (`Github <https://llvm.org/PR167184>`__)
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Improvements and New Features
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-----------------------------
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- The performance of associative and unordered containers has been significantly improved. This is an overview of the
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different functions and by up to how much they have been improved:
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- ``map::map(const map&)``: 2.3x
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- ``map::operator=(const map&)``: 11x
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- ``unordered_set::unordered_set(const unordered_set&)``: 3.3x
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- ``unordered_set::operator=(const unordered_set&)``: 5x
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- ``erase`` of ``map`` and ``set``: 2x
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- ``find(key)`` of ``map``, ``set``, ``multimap`` and ``multiset``: 2.3x
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- ``(iterator, iterator)`` constructors of ``map``, ``set``, ``multimap`` and ``multiset``: 3x
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- ``insert(iterator, iterator)`` of ``map``, ``set``, ``multimap`` and ``multiset``: 2.5x
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- ``erase(iterator, iterator)`` of the unordered containers: 1.9x
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- ``map::insert_or_assign``: 2x
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- The performance of many algorithms has been improved. This is an overview of the different functions and by up to how
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much they have been improved:
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- ``std::find`` for integral types: 2x
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- ``std::for_each`` and ``ranges::for_each`` over the associative containers: 2x
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- ``std::rotate``: 3x
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- Some have been specifically optimized for segmented iterators:
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- ``std::distance`` and ``std::ranges::distance`` on non-random-access iterators: 1600x
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- ``{std,ranges}::{fill, fill_n}``: 10x
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- ``std::{generate, generate_n}`` and ``std::ranges::generate_n``: 1.8x
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- ``std::search_n`` for random access iterators now tries to skip elements.
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In contrived cases improvements of 70,000x have been observed.
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- There have also been performance improvements in other library facilities:
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- ``vector<bool>::reserve()``: 2x
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- ``deque::append_range``: 3.4x
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- ``num_get::do_get`` integral overloads: 2.8x
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- Some reallocations are now avoided in ``std::filesystem::path::lexically_relative``: 1.7x
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- ``ofstream::write`` passes large strings to system calls directly instead of copying them in chunks into a buffer
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- ``std::align`` is now an inline function, which allows the compiler to better optimize calls to it
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- ``std::atomic::wait`` has been refactored to accept more types to use platform native wait functions directly.
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This is guarded behind the ABI Macro ``_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE``.
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- Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
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reduced debug information.
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Deprecations and Removals
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-------------------------
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Potentially breaking changes
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----------------------------
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- The algorithm for ``multi{map,set}::find`` has been modified such that it doesn't necessarily return an iterator to
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the first equal element in the container. This was never guaranteed by the Standard, but libc++ previously happened to
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always return the first equal element, like other implementations do. Starting with this release, code relying on the
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first element being returned from ``find`` will be broken, and ``lower_bound`` or ``equal_range`` should be used
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instead.
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- The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
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``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
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``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.
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Announcements About Future Releases
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-----------------------------------
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ABI Affecting Changes
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---------------------
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- The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
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``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
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``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.
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- The internal types ``__map_value_compare``, ``__unordered_map_hasher``, ``__unordered_map_equal``,
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``__hash_map_hasher`` and ``__hash_map_equal`` have been refactored to use ``_LIBCPP_COMPRESSED_ELEMENT`` instead of
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potentially inheriting from the types they wrap. At this point in time we are not aware of any ABI changes caused by
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this.
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- ``ranges::iota_view`` is now aware of ``__int128``. This causes ``iota_view::difference_type`` to change from
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``long long`` to ``__int128`` in some cases.
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- ``std::allocator`` is now trivially default constructible. The behaviour can be reverted by defining
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``_LIBCPP_DEPRECATED_ABI_NON_TRIVIAL_ALLOCATOR``. Please inform the libc++ team if you need this flag, since it will
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be removed in LLVM 24 if there is no evidence that it's required.
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- ``bitset::operator[]`` now returns ``bool``, making libc++ conforming. The behaviour can be reverted by defining
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``_LIBCPP_DEPRECATED_ABI_BITSET_CONST_SUBSCRIPT_RETURN_REF``. Please inform the libc++ team if you need this flag,
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since it will be removed in LLVM 24 if there is no evidence that it's required.
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Build System Changes
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--------------------
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