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

libcxx/docs/ReleaseNotes/20.rst

@@ -43,7 +43,6 @@ Implemented Papers
 - P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
 - ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
 
-
 Improvements and New Features
 -----------------------------
 
@@ -52,6 +51,9 @@ Improvements and New Features
 
 - The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception`` available in C++20 and later modes.
 
+- 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 generation if optimizations are disabled.
+  The Chromium project measured a 5% reduction in object file and debug information size.
 
 Deprecations and Removals
 -------------------------
@@ -92,8 +94,16 @@ LLVM 21
 ABI Affecting Changes
 ---------------------
 
-- TODO
+- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. The ABI impact is:
 
+  - When using the Itanium ABI (most non-MSVC platforms), empty types are now placed at the beginning of the enclosing
+    object instead of where the beginning of the ``__compressed_pair`` subobject was. This is only observable by
+    checking the address of the empty allocator, equality comparator or hasher.
+  - Additionally, using an overaligned empty type as an allocator, comparator or hasher in the associative containers
+    (and only those containers) may result in the container's object object size and data layout changing beyond only
+    the address of the empty member.
+  - When using the MSVC ABI, this change results in some classes having a completely different memory layout, so this is
+    a genuine ABI break. However, the library does not currently guarantee ABI stability on MSVC platforms.
 
 Build System Changes
 --------------------

libcxx/include/__config

@@ -31,6 +31,7 @@
 
 #  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
@@ -191,25 +192,6 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #    error "libc++ only supports C++03 with Clang-based compilers. Please enable C++11"
 #  endif
 
-// FIXME: ABI detection should be done via compiler builtin macros. This
-// is just a placeholder until Clang implements such macros. For now assume
-// that Windows compilers pretending to be MSVC++ target the Microsoft ABI,
-// and allow the user to explicitly specify the ABI to handle cases where this
-// heuristic falls short.
-#  if defined(_LIBCPP_ABI_FORCE_ITANIUM) && defined(_LIBCPP_ABI_FORCE_MICROSOFT)
-#    error "Only one of _LIBCPP_ABI_FORCE_ITANIUM and _LIBCPP_ABI_FORCE_MICROSOFT can be defined"
-#  elif defined(_LIBCPP_ABI_FORCE_ITANIUM)
-#    define _LIBCPP_ABI_ITANIUM
-#  elif defined(_LIBCPP_ABI_FORCE_MICROSOFT)
-#    define _LIBCPP_ABI_MICROSOFT
-#  else
-#    if defined(_WIN32) && defined(_MSC_VER)
-#      define _LIBCPP_ABI_MICROSOFT
-#    else
-#      define _LIBCPP_ABI_ITANIUM
-#    endif
-#  endif
-
 #  if defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_NO_VCRUNTIME)
 #    define _LIBCPP_ABI_VCRUNTIME
 #  endif

libcxx/include/__configuration/abi.h

@@ -18,6 +18,25 @@
 #  pragma GCC system_header
 #endif
 
+// FIXME: ABI detection should be done via compiler builtin macros. This
+// is just a placeholder until Clang implements such macros. For now assume
+// that Windows compilers pretending to be MSVC++ target the Microsoft ABI,
+// and allow the user to explicitly specify the ABI to handle cases where this
+// heuristic falls short.
+#if defined(_LIBCPP_ABI_FORCE_ITANIUM) && defined(_LIBCPP_ABI_FORCE_MICROSOFT)
+#  error "Only one of _LIBCPP_ABI_FORCE_ITANIUM and _LIBCPP_ABI_FORCE_MICROSOFT can be defined"
+#elif defined(_LIBCPP_ABI_FORCE_ITANIUM)
+#  define _LIBCPP_ABI_ITANIUM
+#elif defined(_LIBCPP_ABI_FORCE_MICROSOFT)
+#  define _LIBCPP_ABI_MICROSOFT
+#else
+#  if defined(_WIN32) && defined(_MSC_VER)
+#    define _LIBCPP_ABI_MICROSOFT
+#  else
+#    define _LIBCPP_ABI_ITANIUM
+#  endif
+#endif
+
 #if _LIBCPP_ABI_VERSION >= 2
 // Change short string representation so that string data starts at offset 0,
 // improving its alignment in some cases.
@@ -98,6 +117,13 @@
 // and WCHAR_MAX. This ABI setting determines whether we should instead track whether the fill
 // value has been initialized using a separate boolean, which changes the ABI.
 #  define _LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE
+// Historically, libc++ used a type called `__compressed_pair` to reduce storage needs 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
@@ -150,6 +176,11 @@
 // ABI impact: changes the iterator type of `vector` (except `vector<bool>`).
 // #define _LIBCPP_ABI_BOUNDED_ITERATORS_IN_VECTOR
 
+// [[msvc::no_unique_address]] seems to mostly affect empty classes, so the padding scheme for Itanium doesn't work.
+#if defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING)
+#  define _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
+#endif
+
 #if defined(_LIBCPP_COMPILER_CLANG_BASED)
 #  if defined(__APPLE__)
 #    if defined(__i386__) || defined(__x86_64__)

libcxx/include/__functional/function.h

@@ -143,45 +143,46 @@ 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;