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Optimize any::swap #5710

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Optimize any::swap #5710

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699d711
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achabense Sep 10, 2025
2944511
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achabense Sep 10, 2025
5567c9c
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achabense Sep 10, 2025
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1 change: 1 addition & 0 deletions benchmarks/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
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42 changes: 42 additions & 0 deletions benchmarks/src/any_swap.cpp
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Original file line number Diff line number Diff line change
@@ -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>);
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The benchmark is very simple, but I think it's enough to show the compiler cannot optimize away the copy.


BENCHMARK_MAIN();
5 changes: 4 additions & 1 deletion stl/inc/any
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Expand Up @@ -230,7 +230,10 @@ public:
}

void swap(any& _That) noexcept {
_That = _STD exchange(*this, _STD move(_That));
any _Old = _STD move(*this);
_Assign(_STD move(_That));
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@achabense achabense Sep 10, 2025 •
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We are still paying an additional copy for some insane self-swaps here...

std::any a = std::make_any<std::any>(42);
std::any& a_o = *std::any_cast<std::any>(&a);
a.swap(a_o);
a_o.swap(a);

(Also, I think _Assign(any-prvalue) is somewhat misleading (for example #5413)), and it's better to create the temporal any explicitly in each function. For example, this would be v.)

any _Old_this = _STD move(*this);
{
    any _Old_that = _STD move(_That);
    reset();
    _Move_from(_Old_that);
}
_That.reset();
_That._Move_from(_Old_this);

_That.reset();
_That._Move_from(_Old);
}

// Observers [any.observers]
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