[libcxx-commits] [libcxx] [libc++] Ensure that `std::expected` has no tail padding (PR #69673)

[Jan Kokemüller via libcxx-commits]

================
@@ -88,8 +88,263 @@ _LIBCPP_HIDE_FROM_ABI void __throw_bad_expected_access(_Arg&& __arg) {
 #  endif
 }
 
+struct __expected_invoke_tag {};
+
+template <bool _NoUnique, class _Tp>
+class __expected_conditional_no_unique_address {
+  struct __unique {
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __unique(_Args&&... __args)
+        : __v(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __unique(
+        __expected_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _Tp __v;
+  };
+
+  struct __no_unique {
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __no_unique(_Args&&... __args)
+        : __v(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __no_unique(
+        __expected_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _LIBCPP_NO_UNIQUE_ADDRESS _Tp __v;
+  };
+
+public:
+  using __type = std::conditional<_NoUnique, __no_unique, __unique>::type;
+};
+
+template <class _Union>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __expected_can_stuff_tail()
+{
+  struct __x {
+  private:
+    _LIBCPP_NO_UNIQUE_ADDRESS _Union __union_;
+    _LIBCPP_NO_UNIQUE_ADDRESS bool __has_val_;
+  };
+  return sizeof(__x) == sizeof(_Union);
+}
+
+template <class _Tp, class _Err>
+class __expected_base {
+  // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
+  // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
+  // it's not clear that it's implementable, given that the function is allowed to clobber
+  // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
+  union __union_t {
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&)
+      requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
+               is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(__union_t&&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(__union_t&&)
+      requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
+               is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(__union_t&&) = delete;
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(in_place_t, _Args&&... __args)
+        : __val_(std::forward<_Args>(__args)...) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(unexpect_t, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
+        std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __val_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
+        std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
+      requires(is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>)
+    = default;
+
+    // __repr's destructor handles this
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t() {}
+
+    _LIBCPP_NO_UNIQUE_ADDRESS _Tp __val_;
+    _LIBCPP_NO_UNIQUE_ADDRESS _Err __unex_;
+  };
+
+  static constexpr bool __can_stuff_tail = __expected_can_stuff_tail<__union_t>();
+
+  struct __repr {
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr() = delete;
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(in_place_t __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(unexpect_t __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(
+        std::__expected_construct_in_place_from_invoke_tag __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(
+        std::__expected_construct_unexpected_from_invoke_tag __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class _OtherUnion>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(bool __has_val, _OtherUnion&& __other)
----------------
jiixyj wrote:

Yeah, this is a bit unfortunate... GCC right now doesn't do guaranteed copy elision on `[[no_unique_address]]` members, so there need to be two construction strategies:

- if `repr` is `[[no_unique_address]]`, the union member inside is not. Therefore, use `__make_union` to initialize the union member using guaranteed copy elision.
- if `repr` is not `[[no_unique_address]]`, is union member _is_ (because we want to put the "has value" flag in its padding), so GCC would error out if `__make_union` were used. Therefore, use `__make_repr` to initialize the whole `repr`.

This is also the reason there has to be the `__expected_base` class, so we have a place to put the constructor in which `__make_repr` is called.

https://github.com/llvm/llvm-project/pull/69673