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

libcxx/docs/ReleaseNotes/19.rst

@@ -143,6 +143,9 @@ ABI Affecting Changes
   ``random_device`` could throw a ``system_error`` with this value. It now
   throws ``ENOMSG``.
 
+- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. This change results in
+  empty final types being 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/docs/ReleaseNotes/20.rst

@@ -0,0 +1,77 @@
+===========================================
+Libc++ 20.0.0 (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++ 20.0.0 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 19.0.0. 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++ 20.0.0?
+==============================
+
+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
+-------------------------
+
+- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to
+  enable the safe mode will be removed in LLVM 20.
+
+Upcoming Deprecations and Removals
+----------------------------------
+
+LLVM 21
+~~~~~~~
+TODO
+
+
+ABI Affecting Changes
+---------------------
+
+- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. This change results in
+  empty final types being 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
+--------------------
+
+- The ``LIBCXX_EXECUTOR`` and ``LIBCXXABI_EXECUTOR`` CMake variables have been removed. Please
+  set ``LIBCXX_TEST_PARAMS`` to ``executor=<...>`` instead.
+
+- The Cmake variable ``LIBCXX_ENABLE_CLANG_TIDY`` has been removed. The build system has been changed
+  to automatically detect the presence of ``clang-tidy`` and the required ``Clang`` libraries.
+
+- The CMake options ``LIBCXX_INSTALL_MODULES`` now defaults to ``ON``.

libcxx/include/__config

@@ -55,6 +55,7 @@
 
 #  define _LIBCPP_CONCAT_IMPL(_X, _Y) _X##_Y
 #  define _LIBCPP_CONCAT(_X, _Y) _LIBCPP_CONCAT_IMPL(_X, _Y)
+#  define _LIBCPP_CONCAT3(X, Y, Z) _LIBCPP_CONCAT(X, _LIBCPP_CONCAT(Y, Z))
 
 #  if __STDC_HOSTED__ == 0
 #    define _LIBCPP_FREESTANDING
@@ -166,6 +167,13 @@
 // requires code not to make these assumptions.
 #    define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_ARRAY
 #    define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW
+// Historically, libc++ used a type called `__compressed_pair` to reduce storage need in cases of empty types (e.g. an
+// empty allocator in std::vector). We switched to using `[[no_unique_address]]`. However, for ABI compatibility reasons
+// we had to add artificial padding in a few places.
+//
+// This setting disables the addition of such artificial padding, leading to a more optimal
+// representation for several types.
+#    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

@@ -144,45 +144,46 @@ 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;