[libcxx-commits] [libcxx] [libc++] Implement `std::function_ref` (PR #186692)
via libcxx-commits
libcxx-commits at lists.llvm.org
Mon May 25 07:42:03 PDT 2026
================
@@ -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)),
----------------
huixie90 wrote:
I have tested the following
```cpp
// function_ref_a.h
#pragma once
#include <functional>
namespace lib {
int algo(std::function_ref<int(int, int)> f, int x);
} // namespace lib
```
```cpp
// function_ref_a.cpp
#include "func_ref_a.h"
namespace lib {
int algo(std::function_ref<int(int, int)> f, int x) {
return f(x, x);
}
} // namespace lib
```
```cpp
// function_ref_b.h
#pragma once
#include <functional>
namespace lib {
int f(int);
} // namespace lib
```
```cpp
// #include "func_ref_b.cpp"
#include "func_ref_a.h"
#include "func_ref_b.h"
namespace lib {
int f(int x) {
return algo([](int a, int b) static { return a + b; }, x);
}
} // namespace lib
```
```cpp
// function_ref.bench.pass.cpp
#include "func_ref_b.h"
#include <functional>
#include "benchmark/benchmark.h"
namespace {
void BM_func_ref_stackless(benchmark::State& state) {
std::uint64_t total_loop_test_param = state.range(0);
for (auto _ : state) {
for (std::uint64_t i = 0; i < total_loop_test_param; ++i) {
benchmark::DoNotOptimize(lib::f(42));
}
}
}
BENCHMARK(BM_func_ref_stackless)->Arg(1)->Arg(1024)->Arg(131072)->Arg(16777216);
}
BENCHMARK_MAIN();
```
Results:
```
Comparing ../../../build_bench/func_ref_non.json to ../../../build_bench/func_ref_static.json
Benchmark Time CPU Time Old Time New CPU Old CPU New
-----------------------------------------------------------------------------------------------------------------------------
BM_func_ref_stackless/1 -0.1032 -0.1623 1 1 1 1
BM_func_ref_stackless/1024 -0.1668 -0.1668 1377 1148 1377 1147
BM_func_ref_stackless/131072 -0.1597 -0.1598 175980 147871 175971 147857
BM_func_ref_stackless/16777216 -0.1590 -0.1591 22505816 18926723 22503355 18922459
OVERALL_GEOMEAN -0.1476 -0.1620 0 0 0 0
```
Roughly 15% improvement on macOS M4 Max CPU
https://github.com/llvm/llvm-project/pull/186692
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