[libcxx-commits] [libcxx] [libc++] Replace `__compressed_pair` with `[[no_unique_address]]` (PR #76756)

[Louis Dionne via libcxx-commits]

================
@@ -293,34 +293,19 @@ struct __shared_ptr_emplace : __shared_weak_count {
 
   // This class implements the control block for non-array shared pointers created
   // through `std::allocate_shared` and `std::make_shared`.
-  //
-  // In previous versions of the library, we used a compressed pair to store
-  // both the _Alloc and the _Tp. This implies using EBO, which is incompatible
-  // with Allocator construction for _Tp. To allow implementing P0674 in C++20,
-  // we now use a properly aligned char buffer while making sure that we maintain
-  // the same layout that we had when we used a compressed pair.
-  using _CompressedPair = __compressed_pair<_Alloc, _Tp>;
-  struct _ALIGNAS_TYPE(_CompressedPair) _Storage {
-    char __blob_[sizeof(_CompressedPair)];
+  struct _Storage {
+    union {
+      struct {
+        _LIBCPP_COMPRESSED_PAIR(_Alloc, __alloc_, _Tp, __elem_);
+      };
+    };
 
     _LIBCPP_HIDE_FROM_ABI explicit _Storage(_Alloc&& __a) { ::new ((void*)__get_alloc()) _Alloc(std::move(__a)); }
     _LIBCPP_HIDE_FROM_ABI ~_Storage() { __get_alloc()->~_Alloc(); }
-    _LIBCPP_HIDE_FROM_ABI _Alloc* __get_alloc() _NOEXCEPT {
-      _CompressedPair* __as_pair                = reinterpret_cast<_CompressedPair*>(__blob_);
-      typename _CompressedPair::_Base1* __first = _CompressedPair::__get_first_base(__as_pair);
-      _Alloc* __alloc                           = reinterpret_cast<_Alloc*>(__first);
-      return __alloc;
-    }
-    _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI _Tp* __get_elem() _NOEXCEPT {
-      _CompressedPair* __as_pair                 = reinterpret_cast<_CompressedPair*>(__blob_);
-      typename _CompressedPair::_Base2* __second = _CompressedPair::__get_second_base(__as_pair);
-      _Tp* __elem                                = reinterpret_cast<_Tp*>(__second);
-      return __elem;
-    }
+    _LIBCPP_HIDE_FROM_ABI _Alloc* __get_alloc() _NOEXCEPT { return std::addressof(__alloc_); }
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI _Tp* __get_elem() _NOEXCEPT { return std::addressof(__elem_); }
----------------
ldionne wrote:

I'm pretty sure you can (and should) drop `_LIBCPP_NO_CFI` here since this was only useful when we did type punning. It's fine to do in a separate PR but let's drop it.

https://github.com/llvm/llvm-project/pull/76756