# C++ attribute: likely, unlikely (since C++20)

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< cpp‎ | language‎ | attributes

Allow the compiler to optimize for the case where paths of execution including that statement are more or less likely than any alternative path of execution that does not include such a statement.

### Syntax

`[[likely]]` | (1) |
`[[unlikely]]` | (2) |

### Explanation

These attributes may be applied to labels and statements (other than declaration-statements). They may not be simultaneously applied to the same label or statement.

1) Applies to a statement to allow the compiler to optimize for the case where paths of execution including that statement are more likely than any alternative path of execution that does not include such a statement.
2) Applies to a statement to allow the compiler to optimize for the case where paths of execution including that statement are less likely than any alternative path of execution that does not include such a statement.

A path of execution is deemed to include a label if and only if it contains a jump to that label:

    int f(int i)
    {
        switch (i)
        {
            case 1: [[fallthrough]];
            [[likely]] case 2: return 1;
        }
        return 2;
    }

i == 2 is considered more likely than any other value of `i`, but the [[likely]] has no effect on the i == 1 case even though it falls through the case 2: label.

### Example

Run this code

    #include <chrono>
    #include <cmath>
    #include <iomanip>
    #include <iostream>
    #include <random>

    namespace with_attributes
    {
        constexpr double pow(double x, long long n) noexcept
        {
            if (n > 0) [[likely]]
                return x * pow(x, n - 1);
            else [[unlikely]]
                return 1;
        }
        constexpr long long fact(long long n) noexcept
        {
            if (n > 1) [[likely]]
                return n * fact(n - 1);
            else [[unlikely]]
                return 1;
        }
        constexpr double cos(double x) noexcept
        {
            constexpr long long precision{16LL};
            double y{};
            for (auto n{0LL}; n < precision; n += 2LL) [[likely]]
                y += pow(x, n) / (n & 2LL ? -fact(n) : fact(n));
            return y;
        }
    } // namespace with_attributes

    namespace no_attributes
    {
        constexpr double pow(double x, long long n) noexcept
        {
            if (n > 0)
                return x * pow(x, n - 1);
            else
                return 1;
        }
        constexpr long long fact(long long n) noexcept
        {
            if (n > 1)
                return n * fact(n - 1);
            else
                return 1;
        }
        constexpr double cos(double x) noexcept
        {
            constexpr long long precision{16LL};
            double y{};
            for (auto n{0LL}; n < precision; n += 2LL)
                y += pow(x, n) / (n & 2LL ? -fact(n) : fact(n));
            return y;
        }
    } // namespace no_attributes

    double gen_random() noexcept
    {
        static std::random_device rd;
        static std::mt19937 gen(rd());
        static std::uniform_real_distribution<double> dis(-1.0, 1.0);
        return dis(gen);
    }

    volatile double sink{}; // ensures a side effect

    int main()
    {
        for (const auto x : {0.125, 0.25, 0.5, 1. / (1 << 26)})
            std::cout
                << std::setprecision(53)
                << "x = " << x << '\n'
                << std::cos(x) << '\n'
                << with_attributes::cos(x) << '\n'
                << (std::cos(x) == with_attributes::cos(x) ? "equal" : "differ") << '\n';

        auto benchmark = [](auto fun, auto rem)
        {
            const auto start = std::chrono::high_resolution_clock::now();
            for (auto size{1ULL}; size != 10'000'000ULL; ++size)
                sink = fun(gen_random());
            const std::chrono::duration<double> diff =
                std::chrono::high_resolution_clock::now() - start;
            std::cout << "Time: " << std::fixed << std::setprecision(6) << diff.count()
                      << " sec " << rem << std::endl;
        };

        benchmark(with_attributes::cos, "(with attributes)");
        benchmark(no_attributes::cos, "(without attributes)");
        benchmark([](double t) { return std::cos(t); }, "(std::cos)");
    }

Possible output:

    x = 0.125
    0.99219766722932900560039115589461289346218109130859375
    0.99219766722932900560039115589461289346218109130859375
    equal
    x = 0.25
    0.96891242171064473343022882545483298599720001220703125
    0.96891242171064473343022882545483298599720001220703125
    equal
    x = 0.5
    0.8775825618903727587394314468838274478912353515625
    0.8775825618903727587394314468838274478912353515625
    equal
    x = 1.490116119384765625e-08
    0.99999999999999988897769753748434595763683319091796875
    0.99999999999999988897769753748434595763683319091796875
    equal
    Time: 0.579122 sec (with attributes)
    Time: 0.722553 sec (without attributes)
    Time: 0.425963 sec (std::cos)

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 9.12.7 Likelihood attributes [dcl.attr.likelihood]

- C++20 standard (ISO/IEC 14882:2020):

  - 9.12.6 Likelihood attributes [dcl.attr.likelihood]

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