[libc++] Replace __compressed_pair with [[no_unique_address]]

libcxx/docs/ReleaseNotes/19.rst

@@ -0,0 +1,97 @@
+===========================================
+Libc++ 19.0.0 (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++ 18.0.0 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 18.0.0. 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++ 18.0.0?
+==============================
+
+Implemented Papers
+------------------
+
+
+Improvements and New Features
+-----------------------------
+
+- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``, resulting in reduced
+  compile times and smaller debug information as well as better code gen if optimizations are disabled.
+
+Deprecations and Removals
+-------------------------
+
+
+Upcoming Deprecations and Removals
+----------------------------------
+
+LLVM 19
+~~~~~~~
+
+- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode will be deprecated and setting
+  it will trigger an error; use the ``LIBCXX_HARDENING_MODE`` variable with the value ``extensive`` instead. Similarly,
+  the ``_LIBCPP_ENABLE_ASSERTIONS`` macro will be deprecated (setting it to ``1`` still enables the extensive mode the
+  LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation
+  <using-hardening-modes>` for more details.
+
+- The base template for ``std::char_traits`` has been marked as deprecated and will be removed in LLVM 19. If you
+  are using ``std::char_traits`` with types other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``, ``char32_t``
+  or a custom character type for which you specialized ``std::char_traits``, your code will stop working when we
+  remove the base template. The Standard does not mandate that a base template is provided, and such a base template
+  is bound to be incorrect for some types, which could currently cause unexpected behavior while going undetected.
+  Note that the ``_LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION`` macro can be defined in LLVM 18 to eagerly remove
+  the specialization and prepare code bases for the unconditional removal in LLVM 19.
+
+- The ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` macro that changed the behavior for narrowing conversions
+  in ``std::variant`` will be removed in LLVM 19.
+
+- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`` macro has been deprecated in LLVM 18 and will be removed
+  entirely in LLVM 19.
+
+- The ``_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES`` and
+  ``_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES`` macros have been deprecated
+  in LLVM 18 and will be removed entirely in LLVM 19.
+
+LLVM 20
+~~~~~~~
+
+- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable
+  the safe mode will be removed.
+
+
+ABI Affecting Changes
+---------------------
+
+- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. This change results in
+  empty final types to be placed at the beginning of the object instead of where the beginning of the
+  ``__compressed_pair`` subobject was. This is only observable by checking the address of the subobject.
+
+Build System Changes
+--------------------

libcxx/include/__config

@@ -179,6 +179,8 @@
 // pointer from 16 to 8. This changes the output of std::string::max_size,
 // which makes it ABI breaking
 #    define _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
+// Don't add padding from __compressed_pair
+#    define _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
 #  elif _LIBCPP_ABI_VERSION == 1
 #    if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support

libcxx/include/__functional/function.h

@@ -138,45 +138,46 @@ class __default_alloc_func;
 
 template <class _Fp, class _Ap, class _Rp, class... _ArgTypes>
 class __alloc_func<_Fp, _Ap, _Rp(_ArgTypes...)> {
-  __compressed_pair<_Fp, _Ap> __f_;
+  _LIBCPP_COMPRESSED_PAIR(_Fp, __func_, _Ap, __alloc_);
 
 public:
   typedef _LIBCPP_NODEBUG _Fp _Target;
   typedef _LIBCPP_NODEBUG _Ap _Alloc;
 
-  _LIBCPP_HIDE_FROM_ABI const _Target& __target() const { return __f_.first(); }
+  _LIBCPP_HIDE_FROM_ABI const _Target& __target() const { return __func_; }
 
   // WIN32 APIs may define __allocator, so use __get_allocator instead.
-  _LIBCPP_HIDE_FROM_ABI const _Alloc& __get_allocator() const { return __f_.second(); }
+  _LIBCPP_HIDE_FROM_ABI const _Alloc& __get_allocator() const { return __alloc_; }
 
-  _LIBCPP_HIDE_FROM_ABI explicit __alloc_func(_Target&& __f)
-      : __f_(piecewise_construct, std::forward_as_tuple(std::move(__f)), std::forward_as_tuple()) {}
+  _LIBCPP_HIDE_FROM_ABI explicit __alloc_func(_Target&& __f) : __func_(std::move(__f)), __alloc_() {}
 
-  _LIBCPP_HIDE_FROM_ABI explicit __alloc_func(const _Target& __f, const _Alloc& __a)
-      : __f_(piecewise_construct, std::forward_as_tuple(__f), std::forward_as_tuple(__a)) {}
+  _LIBCPP_HIDE_FROM_ABI explicit __alloc_func(const _Target& __f, const _Alloc& __a) : __func_(__f), __alloc_(__a) {}
 
   _LIBCPP_HIDE_FROM_ABI explicit __alloc_func(const _Target& __f, _Alloc&& __a)
-      : __f_(piecewise_construct, std::forward_as_tuple(__f), std::forward_as_tuple(std::move(__a))) {}
+      : __func_(__f), __alloc_(std::move(__a)) {}
 
   _LIBCPP_HIDE_FROM_ABI explicit __alloc_func(_Target&& __f, _Alloc&& __a)
-      : __f_(piecewise_construct, std::forward_as_tuple(std::move(__f)), std::forward_as_tuple(std::move(__a))) {}
+      : __func_(std::move(__f)), __alloc_(std::move(__a)) {}
 
   _LIBCPP_HIDE_FROM_ABI _Rp operator()(_ArgTypes&&... __arg) {
     typedef __invoke_void_return_wrapper<_Rp> _Invoker;
-    return _Invoker::__call(__f_.first(), std::forward<_ArgTypes>(__arg)...);
+    return _Invoker::__call(__func_, std::forward<_ArgTypes>(__arg)...);
   }
 
   _LIBCPP_HIDE_FROM_ABI __alloc_func* __clone() const {
     typedef allocator_traits<_Alloc> __alloc_traits;
     typedef __rebind_alloc<__alloc_traits, __alloc_func> _AA;
-    _AA __a(__f_.second());
+    _AA __a(__alloc_);
     typedef __allocator_destructor<_AA> _Dp;
     unique_ptr<__alloc_func, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
-    ::new ((void*)__hold.get()) __alloc_func(__f_.first(), _Alloc(__a));
+    ::new ((void*)__hold.get()) __alloc_func(__func_, _Alloc(__a));
     return __hold.release();
   }
 
-  _LIBCPP_HIDE_FROM_ABI void destroy() _NOEXCEPT { __f_.~__compressed_pair<_Target, _Alloc>(); }
+  _LIBCPP_HIDE_FROM_ABI void destroy() _NOEXCEPT {
+    __func_.~_Fp();
+    __alloc_.~_Alloc();
+  }
 
   _LIBCPP_HIDE_FROM_ABI static void __destroy_and_delete(__alloc_func* __f) {
     typedef allocator_traits<_Alloc> __alloc_traits;