[libcxx-commits] [libcxx] [libc++] Implement `std::function_ref` (PR #186692)

[via libcxx-commits]

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@@ -0,0 +1,190 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 function_ref.h
+// and generates multiple versions of std::function_ref
+
+#include <__assert>
+#include <__config>
+#include <__functional/function_ref_common.h>
+#include <__functional/invoke.h>
+#include <__memory/addressof.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_const.h>
+#include <__type_traits/is_convertible.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_member_pointer.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/is_pointer.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_void.h>
+#include <__type_traits/remove_cvref.h>
+#include <__type_traits/remove_pointer.h>
+#include <__type_traits/remove_reference.h>
+#include <__utility/constant_wrapper.h>
+#include <__utility/forward.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#ifndef _LIBCPP___FUNCTIONAL_FUNCTION_REF_H
+#  error This header should only be included from function_ref.h
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+template <class...>
+class function_ref;
+
+template <bool _NoExcept1, bool _NoExcept2, class _Rp, class... _ArgTypes>
+struct __is_convertible_from_specialization<
+    function_ref<_Rp(_ArgTypes...) _LIBCPP_FUNCTION_REF_CV noexcept(_NoExcept1)>,
+    function_ref<_Rp(_ArgTypes...) const noexcept(_NoExcept2)> >
+    : is_convertible<_Rp (&)(_ArgTypes...) noexcept(_NoExcept2), _Rp (&)(_ArgTypes...) noexcept(_NoExcept1)> {};
+
+template <bool _NoExcept1, bool _NoExcept2, class _Rp, class... _ArgTypes>
+struct __is_convertible_from_specialization<
+    function_ref<_Rp(_ArgTypes...) _LIBCPP_FUNCTION_REF_CV noexcept(_NoExcept1)>,
+    function_ref<_Rp(_ArgTypes...) noexcept(_NoExcept2)> >
+    : _And<is_convertible<_Rp (&)(_ArgTypes...) noexcept(_NoExcept2), _Rp (&)(_ArgTypes...) noexcept(_NoExcept1)>,
+           is_convertible<_LIBCPP_FUNCTION_REF_CV int&, int&>> {};
+
+template <class _Rp, class... _ArgTypes, bool __is_noexcept>
+class function_ref<_Rp(_ArgTypes...) _LIBCPP_FUNCTION_REF_CV noexcept(__is_noexcept)> {
+private:
+  template <class... _Tp>
+  static constexpr bool __is_invocable_using =
+      _If<__is_noexcept, is_nothrow_invocable_r<_Rp, _Tp..., _ArgTypes...>, is_invocable_r<_Rp, _Tp..., _ArgTypes...>>::
+          value;
+
+  template <class _Fn2>
+  static constexpr bool __is_convertible_from_specialization_v =
+      __is_convertible_from_specialization<function_ref, _Fn2>::value;
+
+  template <class... _Tp>
+  friend class function_ref;
+
+  template <class _Arg>
+  using __arg_t _LIBCPP_NODEBUG = typename __function_ref_arg_fwd<_Arg>::type;
+
+  using __storage_t _LIBCPP_NODEBUG = __function_ref_storage;
+
+  using __call_t _LIBCPP_NODEBUG = _Rp (*)(__storage_t, __arg_t<_ArgTypes>...) noexcept(__is_noexcept);
+
+  __storage_t __storage_;
+  __call_t __call_;
+
+public:
+  template <class _Fp>
+    requires is_function_v<_Fp> && __is_invocable_using<_Fp>
+  _LIBCPP_HIDE_FROM_ABI function_ref(_Fp* __fn_ptr) noexcept
+      : __storage_(__fn_ptr),
+        __call_([](__storage_t __storage, __arg_t<_ArgTypes>... __args) static noexcept(__is_noexcept) -> _Rp {
+          return __storage_t::template __get<_Fp>(__storage)(static_cast<__arg_t<_ArgTypes>>(__args)...);
+        }) {
+    _LIBCPP_ASSERT_NON_NULL(__fn_ptr != nullptr, "the function pointer should not be a nullptr");
+  }
+
+  template <class _Fn, class _Tp = remove_reference_t<_Fn>>
+    requires(!is_same_v<remove_cvref_t<_Fn>, function_ref> && !is_member_pointer_v<_Tp> &&
+             __is_invocable_using<_LIBCPP_FUNCTION_REF_CV _Tp&> && !__is_convertible_from_specialization_v<_Tp>)
+  _LIBCPP_HIDE_FROM_ABI constexpr function_ref(_Fn&& __obj) noexcept
+      : __storage_(std::addressof(__obj)),
----------------
Kim-J-Smith wrote:

For stateless functors, we can avoid storing their address in `function_ref`.
This optimization is expected to eliminate one `lea` instruction on x86_64.

Stateless functors fall into at least two categories:
  1. Functors with a `static operator()`.
  2. Functors that are standard operator wrappers, like std::less, std::identity, std::ranges::less, etc.

I think the added complexity is reasonable. However, the difference of a single instruction may not be reflected in the benchmark.

```cpp
template <class _Fn, class _Tp = remove_reference_t<_Fn>>
    requires(!is_same_v<remove_cvref_t<_Fn>, function_ref> && !is_member_pointer_v<_Tp> &&
             __is_invocable_using<_LIBCPP_FUNCTION_REF_CV _Tp&> && !__is_convertible_from_specialization_v<_Tp>)
  _LIBCPP_HIDE_FROM_ABI constexpr function_ref(_Fn&& __obj) noexcept {
    if constexpr (__is_static_call_v<_Tp, _ArgTypes...>) {
      __call_ = [](__storage_t, __arg_t<_ArgTypes>... __args) static noexcept(__is_noexcept) -> _Rp {
          return _Tp::operator()(static_cast<__arg_t<_ArgTypes>>(__args)...);
        };
    } else if constexpr (__is_standard_operator_wrapper_v<_Tp>) {
      __call_ = [](__storage_t, __arg_t<_ArgTypes>... __args) static noexcept(__is_noexcept) -> _Rp {
          _LIBCPP_FUNCTION_REF_CV _Tp __obj1{};
          return __obj1(static_cast<__arg_t<_ArgTypes>>(__args)...);
        };
    } else {
      __storage_ = std::addressof(__obj);
      __call_ = [](__storage_t __storage, __arg_t<_ArgTypes>... __args) static noexcept(__is_noexcept) -> _Rp {
          _LIBCPP_FUNCTION_REF_CV _Tp& __obj1 = *__storage_t::template __get<_Tp>(__storage);
          return __obj1(static_cast<__arg_t<_ArgTypes>>(__args)...);
        };
    }
  }
```

https://github.com/llvm/llvm-project/pull/186692