=========================================== Libc++ 22.0.0 (In-Progress) Release Notes =========================================== .. contents:: :local: :depth: 2 Written by the `Libc++ Team `_ .. warning:: These are in-progress notes for the upcoming libc++ 22.0.0 release. Release notes for previous releases can be found on `the Download Page `_. Introduction ============ This document contains the release notes for the libc++ C++ Standard Library, part of the LLVM Compiler Infrastructure, release 22.0.0. Here we describe the status of libc++ in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see `the LLVM documentation `_. All LLVM releases may be downloaded from the `LLVM releases web site `_. For more information about libc++, please see the `Libc++ Web Site `_ or the `LLVM Web Site `_. Note that if you are reading this file from a Git checkout or the main Libc++ web page, this document applies to the *next* release, not the current one. To see the release notes for a specific release, please see the `releases page `_. What's New in Libc++ 22.0.0? ============================== Implemented Papers ------------------ - P2592R3: Hashing support for ``std::chrono`` value classes (`Github `__) - P2321R2: ``zip`` (`Github `__) - P2988R12: ``std::optional`` (`Github `__) - P3044R2: sub-``string_view`` from ``string`` (`Github `__) - P3223R2: Making ``std::istream::ignore`` less surprising (`Github `__) - P3060R3: Add ``std::views::indices(n)`` (`Github `__) - P2404R3: Move-only types for ``equality_comparable_with``, ``totally_ordered_with``, and ``three_way_comparable_with`` (`Github `__) - P2641R4: Checking if a ``union`` alternative is active (``std::is_within_lifetime``) (`Github `__) - P2835R7: Expose ``std::atomic_ref``'s object address (`Github `__) - P2944R3: Comparisons for ``reference_wrapper`` (`Github `__) - P3168R2: Give ``std::optional`` Range Support (guarded by ``-fexperimental-library``, `Github `__) - P3567R2: ``flat_meow`` Fixes (`Github `__) - P3836R2: Make ``optional`` trivially copyable (`Github `__) - P1789R3: Library Support for Expansion Statements (`Github `__) Improvements and New Features ----------------------------- - The performance of associative and unordered containers has been significantly improved. This is an overview of the different functions and by up to how much they have been improved: - ``map::map(const map&)``: 2.3x - ``map::operator=(const map&)``: 11x - ``unordered_set::unordered_set(const unordered_set&)``: 3.3x - ``unordered_set::operator=(const unordered_set&)``: 5x - ``erase`` of ``map`` and ``set``: 2x - ``find(key)`` of ``map``, ``set``, ``multimap`` and ``multiset``: 2.3x - ``(iterator, iterator)`` constructors of ``map``, ``set``, ``multimap`` and ``multiset``: 3x - ``insert(iterator, iterator)`` of ``map``, ``set``, ``multimap`` and ``multiset``: 2.5x - ``erase(iterator, iterator)`` of the unordered containers: 1.9x - ``map::insert_or_assign``: 2x - The performance of many algorithms has been improved. This is an overview of the different functions and by up to how much they have been improved: - ``std::find`` for integral types: 2x - ``std::for_each`` and ``ranges::for_each`` over the associative containers: 2x - ``std::rotate``: 3x - Some have been specifically optimized for segmented iterators: - ``std::distance`` and ``std::ranges::distance`` on non-random-access iterators: 1600x - ``{std,ranges}::{fill, fill_n}``: 10x - ``std::{generate, generate_n}`` and ``std::ranges::generate_n``: 1.8x - ``std::search_n`` for random access iterators now tries to skip elements. In contrived cases improvements of 70,000x have been observed. - There have also been performance improvements in other library facilities: - ``vector::reserve()``: 2x - ``deque::append_range``: 3.4x - ``num_get::do_get`` integral overloads: 2.8x - Some reallocations are now avoided in ``std::filesystem::path::lexically_relative``: 1.7x - ``ofstream::write`` passes large strings to system calls directly instead of copying them in chunks into a buffer - ``std::align`` is now an inline function, which allows the compiler to better optimize calls to it - ``std::atomic::wait`` has been refactored to accept more types to use platform native wait functions directly. This is guarded behind the ABI Macro ``_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE``. - Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and reduced debug information. Deprecations and Removals ------------------------- - The minimally supported deployment target on macOS has been increased to 11.0. Potentially breaking changes ---------------------------- - The algorithm for ``multi{map,set}::find`` has been modified such that it doesn't necessarily return an iterator to the first equal element in the container. This was never guaranteed by the Standard, but libc++ previously happened to always return the first equal element, like other implementations do. Starting with this release, code relying on the first element being returned from ``find`` will be broken, and ``lower_bound`` or ``equal_range`` should be used instead. - The algorithms for ``std::{map,set}`` ``lower_bound`` and ``upper_bound`` operations were modified such that their result changed for comparators that are not a strict weak order. Being a strict weak order was always a requirement of the Standard and still is, however in this release libc++ changed the behavior of ``std::{map,set}`` for such comparators. Since this may be tricky to work around in some cases, an escape hatch is provided in this release: defining ``_LIBCPP_ENABLE_LEGACY_TREE_LOWER_UPPER_BOUND`` will revert to the historical implementation of these operations. That escape hatch will be removed in LLVM 23. - The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well. Announcements About Future Releases ----------------------------------- ABI Affecting Changes --------------------- - The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well. - The internal types ``__map_value_compare``, ``__unordered_map_hasher``, ``__unordered_map_equal``, ``__hash_map_hasher`` and ``__hash_map_equal`` have been refactored to use ``_LIBCPP_COMPRESSED_ELEMENT`` instead of potentially inheriting from the types they wrap. At this point in time we are not aware of any ABI changes caused by this. - ``ranges::iota_view`` is now aware of ``__int128``. This causes ``iota_view::difference_type`` to change from ``long long`` to ``__int128`` in some cases. - ``std::allocator`` is now trivially default constructible. The behaviour can be reverted by defining ``_LIBCPP_DEPRECATED_ABI_NON_TRIVIAL_ALLOCATOR``. Please inform the libc++ team if you need this flag, since it will be removed in LLVM 24 if there is no evidence that it's required. - ``bitset::operator[]`` now returns ``bool``, making libc++ conforming. The behaviour can be reverted by defining ``_LIBCPP_DEPRECATED_ABI_BITSET_CONST_SUBSCRIPT_RETURN_REF``. Please inform the libc++ team if you need this flag, since it will be removed in LLVM 24 if there is no evidence that it's required. Build System Changes --------------------