868aa5f19c
We have multiple classes with an empty base optimization that contains just a single type. This patch introduces `_LIBCPP_COMPRESSED_ELEMENT` to refactor these classes to avoid having them essentially twice, reducing the amount of code significantly.
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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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- P2321R2: ``zip`` (`Github <https://llvm.org/PR105169>`__) (The paper is partially implemented. ``zip_transform_view``
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is implemented in this release)
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- P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
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Improvements and New Features
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-----------------------------
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- The performance of ``map::map(const map&)`` has been improved up to 2.3x
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- The performance of ``map::operator=(const map&)`` has been improved by up to 11x
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- The performance of ``unordered_set::unordered_set(const unordered_set&)`` has been improved by up to 3.3x.
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- The performance of ``unordered_set::operator=(const unordered_set&)`` has been improved by up to 5x.
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- The performance of ``map::erase`` and ``set::erase`` has been improved by up to 2x
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- The performance of ``find(key)`` in ``map``, ``set``, ``multimap`` and ``multiset`` has been improved by up to 2.3x
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- Some reallocations are now avoided in `std::filesystem::path::lexically_relative`, resulting in a performance
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improvement of up to 1.7x.
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- The performance of the ``(iterator, iterator)`` constructors of ``map``, ``set``, ``multimap`` and ``multiset``
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has been improved by up to 3x
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- The performance of ``insert(iterator, iterator)`` of ``map``, ``set``, ``multimap`` and ``multiset`` has been improved
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by up to 2.5x
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- The performance of ``map::insert_or_assign`` has been improved by up to 2x
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- ``ofstream::write`` has been optimized to pass through large strings to system calls directly instead of copying them
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in chunks into a buffer.
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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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Build System Changes
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--------------------
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