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===========================================
Libc++ 22.0.0 (In-Progress) Release Notes
===========================================
.. contents::
:local:
:depth: 2
Written by the `Libc++ Team <https://libcxx.llvm.org>`_
.. 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 <https://releases.llvm.org/download.html>`_.
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 <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.
For more information about libc++, please see the `Libc++ Web Site
<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.
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 <https://llvm.org/releases/>`_.
What's New in Libc++ 22.0.0?
==============================
Implemented Papers
------------------
- P2321R2: ``zip`` (`Github <https://llvm.org/PR105169>`__) (The paper is partially implemented. ``zip_transform_view``
is implemented in this release)
- P3044R2: sub-``string_view`` from ``string`` (`Github <https://llvm.org/PR148140>`__)
- P3223R2: Making ``std::istream::ignore`` less surprising (`Github <https://llvm.org/PR148178>`__)
- P3060R3: Add ``std::views::indices(n)`` (`Github <https://llvm.org/PR148175>`__)
- P2641R4: Checking if a ``union`` alternative is active (``std::is_within_lifetime``)
(`Github <https://llvm.org/PR105381>`__)
- P2835R7: Expose ``std::atomic_ref``'s object address (`Github <https://llvm.org/PR118377>`__)
- P2944R3: Comparisons for ``reference_wrapper`` (`Github <https://llvm.org/PR105424>`__)
- P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
Improvements and New Features
-----------------------------
- The performance of ``map::map(const map&)`` has been improved up to 2.3x
- The performance of ``map::operator=(const map&)`` has been improved by up to 11x
- The performance of ``unordered_set::unordered_set(const unordered_set&)`` has been improved by up to 3.3x.
- The performance of ``unordered_set::operator=(const unordered_set&)`` has been improved by up to 5x.
- The performance of ``map::erase`` and ``set::erase`` has been improved by up to 2x
- The performance of ``find(key)`` in ``map``, ``set``, ``multimap`` and ``multiset`` has been improved by up to 2.3x
- Some reallocations are now avoided in `std::filesystem::path::lexically_relative`, resulting in a performance
improvement of up to 1.7x.
- The performance of the ``(iterator, iterator)`` constructors of ``map``, ``set``, ``multimap`` and ``multiset``
has been improved by up to 3x
- The performance of ``insert(iterator, iterator)`` of ``map``, ``set``, ``multimap`` and ``multiset`` has been improved
by up to 2.5x
- The performance of ``erase(iterator, iterator)`` in the unordered containers has been improved by up to 1.9x
- The performance of ``map::insert_or_assign`` has been improved by up to 2x
- ``ofstream::write`` and ``ifstream::read`` have been optimized to pass through large reads and writes to system calls
directly instead of copying them in chunks.
- Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
reduced debug information.
- The performance of ``std::find`` has been improved by up to 2x for integral types
- The ``std::distance`` and ``std::ranges::distance`` algorithms have been optimized for segmented iterators (e.g.,
``std::join_view`` iterators), reducing the complexity from ``O(n)`` to ``O(n / segment_size)``. Benchmarks show
performance improvements of over 1600x in favorable cases with large segment sizes (e.g., 1024).
- The ``std::{fill, fill_n}`` and ``std::ranges::{fill, fill_n}`` algorithms have been optimized for segmented iterators,
resulting in a performance improvement of at least 10x for ``std::deque<int>`` iterators and
``std::join_view<std::vector<std::vector<int>>>`` iterators.
- The ``std::{generate, generate_n}`` and ``std::ranges::generate_n`` algorithms have been optimized for segmented
iterators, resulting in a performance improvement for ``std::deque<short>`` and
``std::join_view<vector<vector<short>>>`` iterators.
Deprecations and Removals
-------------------------
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 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.
Build System Changes
--------------------
|
