Optimize any::swap (#5710)

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

Author: achabense

benchmarks/CMakeLists.txt

@@ -102,6 +102,7 @@ endfunction()
 
 add_benchmark(adjacent_difference src/adjacent_difference.cpp)
 add_benchmark(adjacent_find src/adjacent_find.cpp)
+add_benchmark(any_swap src/any_swap.cpp)
 add_benchmark(bitset_from_string src/bitset_from_string.cpp)
 add_benchmark(bitset_to_string src/bitset_to_string.cpp)
 add_benchmark(efficient_nonlocking_print src/efficient_nonlocking_print.cpp)

benchmarks/src/any_swap.cpp

@@ -0,0 +1,42 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <any>
+#include <array>
+#include <benchmark/benchmark.h>
+#include <type_traits>
+
+using trivial = std::array<int, 2>;
+static_assert(std::is_trivially_copyable_v<trivial>);
+
+struct small {
+    std::array<int, 4> c{};
+    small()                            = default;
+    small(const small&)                = default;
+    small& operator=(const small&)     = default;
+    small(small&&) noexcept            = default;
+    small& operator=(small&&) noexcept = default;
+    ~small() {}
+};
+static_assert(!std::is_trivially_copyable_v<small>);
+static_assert(std::is_nothrow_move_constructible_v<small>);
+
+using large = std::array<int, 32>;
+
+template <class T>
+void bm(benchmark::State& state) {
+    std::any a = T{};
+    std::any b = T{};
+
+    for (auto _ : state) {
+        a.swap(b);
+        benchmark::DoNotOptimize(a);
+        benchmark::DoNotOptimize(b);
+    }
+}
+
+BENCHMARK(bm<trivial>);
+BENCHMARK(bm<small>);
+BENCHMARK(bm<large>);
+
+BENCHMARK_MAIN();

stl/inc/any

@@ -230,7 +230,11 @@ public:
     }
 
     void swap(any& _That) noexcept {
-        _That = _STD exchange(*this, _STD move(_That));
+        any _Old = _STD move(*this);
+        reset();
+        _Move_from(_That);
+        _That.reset();
+        _That._Move_from(_Old);
     }
 
     // Observers [any.observers]