[llvm-branch-commits] [libcxx] [libc++] Implement std::move_only_function (P0288R9) (PR #94670)

[via llvm-branch-commits]

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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// This header is unguarded on purpose. This header is an implementation detail of move_only_function.h
+// and generates multiple versions of std::move_only_function
+
+#include <__config>
+#include <__functional/invoke.h>
+#include <__functional/move_only_function_common.h>
+#include <__type_traits/is_trivially_destructible.h>
+#include <__utility/exchange.h>
+#include <__utility/forward.h>
+#include <__utility/in_place.h>
+#include <__utility/move.h>
+#include <__utility/pointer_int_pair.h>
+#include <__utility/small_buffer.h>
+#include <__utility/swap.h>
+#include <cstddef>
+#include <cstring>
+#include <initializer_list>
+#include <new>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#ifndef _LIBCPP_IN_MOVE_ONLY_FUNCTION_H
+#  error This header should only be included from move_only_function.h
+#endif
+
+#ifndef _LIBCPP_MOVE_ONLY_FUNCTION_CV
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_CV
+#endif
+
+#ifndef _LIBCPP_MOVE_ONLY_FUNCTION_REF
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_REF
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_INVOKE_QUALS _LIBCPP_MOVE_ONLY_FUNCTION_CV&
+#else
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_INVOKE_QUALS _LIBCPP_MOVE_ONLY_FUNCTION_CV _LIBCPP_MOVE_ONLY_FUNCTION_REF
+#endif
+
+#ifndef _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT false
+#endif
+
+#define _LIBCPP_MOVE_ONLY_FUNCTION_CVREF _LIBCPP_MOVE_ONLY_FUNCTION_CV _LIBCPP_MOVE_ONLY_FUNCTION_REF
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#ifdef _LIBCPP_ABI_MOVE_ONLY_FUNCTION_TRIVIAL_ABI
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_TRIVIAL_ABI [[_Clang::__trivial_abi__]]
+#else
+#  define _LIBCPP_MOVE_ONLY_FUNCTION_TRIVIAL_ABI
+#endif
+
+template <class...>
+class move_only_function;
+
+template <class _ReturnT, class... _ArgTypes>
+class _LIBCPP_MOVE_ONLY_FUNCTION_TRIVIAL_ABI move_only_function<_ReturnT(
+    _ArgTypes...) _LIBCPP_MOVE_ONLY_FUNCTION_CVREF noexcept(_LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT)> {
+private:
+  static constexpr size_t __buffer_size_      = 3 * sizeof(void*);
+  static constexpr size_t __buffer_alignment_ = alignof(void*);
+  using _BufferT                              = __small_buffer<__buffer_size_, __buffer_alignment_>;
+
+  using _TrivialVTable    = _MoveOnlyFunctionTrivialVTable<_BufferT, _ReturnT, _ArgTypes...>;
+  using _NonTrivialVTable = _MoveOnlyFunctionNonTrivialVTable<_BufferT, _ReturnT, _ArgTypes...>;
+
+  template <class _Functor>
+  static constexpr _TrivialVTable __trivial_vtable_ = {
+      .__call_ = [](_BufferT& __buffer, _ArgTypes... __args) noexcept(_LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT) -> _ReturnT {
+        return std::invoke_r<_ReturnT>(
+            static_cast<_Functor _LIBCPP_MOVE_ONLY_FUNCTION_INVOKE_QUALS>(*__buffer.__get<_Functor>()),
+            std::forward<_ArgTypes>(__args)...);
+      }};
+
+  template <class _Functor>
+  static constexpr _NonTrivialVTable __non_trivial_vtable_{
+      __trivial_vtable_<_Functor>,
+      [](_BufferT& __buffer) noexcept -> void {
+        std::destroy_at(__buffer.__get<_Functor>());
+        __buffer.__dealloc<_Functor>();
+      },
+  };
+
+  template <class _Functor>
+  _LIBCPP_HIDE_FROM_ABI __pointer_bool_pair<const _TrivialVTable*> __get_vptr() {
+    if constexpr (_BufferT::__fits_in_buffer<_Functor> && is_trivially_destructible_v<_Functor>) {
+      return {&__trivial_vtable_<_Functor>, false};
+    } else {
+      return {&__non_trivial_vtable_<_Functor>, true};
+    }
+  }
+
+  template <class _VT>
+  static constexpr bool __is_callable_from = [] {
+    using _DVT = decay_t<_VT>;
+    if (_LIBCPP_MOVE_ONLY_FUNCTION_NOEXCEPT) {
+      return is_nothrow_invocable_r_v<_ReturnT, _DVT _LIBCPP_MOVE_ONLY_FUNCTION_CVREF, _ArgTypes...> &&
+             is_nothrow_invocable_r_v<_ReturnT, _DVT _LIBCPP_MOVE_ONLY_FUNCTION_INVOKE_QUALS, _ArgTypes...>;
+    } else {
+      return is_invocable_r_v<_ReturnT, _DVT _LIBCPP_MOVE_ONLY_FUNCTION_CVREF, _ArgTypes...> &&
+             is_invocable_r_v<_ReturnT, _DVT _LIBCPP_MOVE_ONLY_FUNCTION_INVOKE_QUALS, _ArgTypes...>;
+    }
+  }();
+
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) {
+    static_assert(is_constructible_v<decay_t<_Func>, _Func>);
+
+    using _StoredFunc = decay_t<_Func>;
+    __vtable_         = __get_vptr<_StoredFunc>();
+    __buffer_.__construct<_StoredFunc>(std::forward<_Args>(__args)...);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void __reset() {
+    if (__vtable_.__get_value())
----------------
EricWF wrote:

There's other cost here. 

There's complexity, there's ABI risk. To speak on the ABI risk.:

 From my investigations Clang has trouble optimizing around this, where it doesn't have trouble optimizing around what @huixie90 is suggesting, which I've implemented and tested out.

 And there's two reviewers asking you to remove it.

https://github.com/llvm/llvm-project/pull/94670