<functional>: make move_only_function do less work (#5328)

Co-authored-by: Stephan T. Lavavej <[email protected]>

Author: AlexGuteniev

benchmarks/CMakeLists.txt

@@ -117,6 +117,7 @@ add_benchmark(iota src/iota.cpp)
 add_benchmark(locale_classic src/locale_classic.cpp)
 add_benchmark(minmax_element src/minmax_element.cpp)
 add_benchmark(mismatch src/mismatch.cpp)
+add_benchmark(move_only_function src/move_only_function.cpp)
 add_benchmark(path_lexically_normal src/path_lexically_normal.cpp)
 add_benchmark(priority_queue_push_range src/priority_queue_push_range.cpp)
 add_benchmark(random_integer_generation src/random_integer_generation.cpp)

benchmarks/src/move_only_function.cpp

@@ -0,0 +1,47 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <benchmark/benchmark.h>
+#include <functional>
+#include <utility>
+
+using namespace std;
+
+void mof_none(benchmark::State& state) {
+    move_only_function<void()> mof;
+    for (auto _ : state) {
+        benchmark::DoNotOptimize(mof);
+    }
+}
+
+void mof_construct(benchmark::State& state) {
+    for (auto _ : state) {
+        move_only_function<void()> mof;
+        benchmark::DoNotOptimize(mof);
+    }
+}
+
+void mof_move(benchmark::State& state) {
+    move_only_function<void()> mof;
+    for (auto _ : state) {
+        benchmark::DoNotOptimize(mof);
+        auto moved_mof = move(mof);
+        benchmark::DoNotOptimize(moved_mof);
+    }
+}
+
+void mof_construct_and_move(benchmark::State& state) {
+    for (auto _ : state) {
+        move_only_function<void()> mof;
+        benchmark::DoNotOptimize(mof);
+        auto moved_mof = move(mof);
+        benchmark::DoNotOptimize(moved_mof);
+    }
+}
+
+BENCHMARK(mof_none);
+BENCHMARK(mof_construct);
+BENCHMARK(mof_move);
+BENCHMARK(mof_construct_and_move);
+
+BENCHMARK_MAIN();

stl/inc/functional

@@ -1506,11 +1506,6 @@ public:
         _Other._Reset_to_null();
     }
 
-    void _Construct_with_null() noexcept {
-        _Data._Impl = nullptr;
-        _Data._Set_large_fn_ptr(nullptr); // initialize, since we'll be copying it
-    }
-
     void _Reset_to_null() noexcept {
         _Data._Impl = nullptr;
     }
@@ -1888,11 +1883,11 @@ public:
     using typename _Call::result_type;
 
     move_only_function() noexcept {
-        this->_Construct_with_null();
+        this->_Reset_to_null();
     }
 
     move_only_function(nullptr_t) noexcept {
-        this->_Construct_with_null();
+        this->_Reset_to_null();
     }
 
     move_only_function(move_only_function&&) noexcept = default;
@@ -1906,7 +1901,7 @@ public:
 
         if constexpr (is_member_pointer_v<_Vt> || is_pointer_v<_Vt> || _Is_specialization_v<_Vt, move_only_function>) {
             if (_Callable == nullptr) {
-                this->_Construct_with_null();
+                this->_Reset_to_null();
                 return;
             }
         }