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

#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{}; // 사이드 이펙트를 보장함
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" : "차이점") << '\n';
    auto benchmark = [](auto fun, auto rem)
    {
        const auto start = std::chrono::high_resolution_clock::now
**변경 없음** - C++ 특정 용어와 코드 태그 내 내용은 번역하지 않음();
        for (auto size{1ULL}; size != 10'000'000ULL; ++size)
            sink = fun(gen_random());
        const std::chrono::duration
(번역 참고: HTML 태그와 속성은 번역하지 않았으며, C++ 관련 용어(std::chrono::duration)는 원문 그대로 유지했습니다)<double> diff =
            std::chrono::high_resolution_clock::now() - start;
        std::cout << "시간: " << std::fixed << std::setprecision(6) << diff.count()
                  << " 초 " << rem << std::endl; 
    };
    benchmark(with_attributes::cos, "(속성 포함)");
    benchmark(no_attributes::cos, "(속성 없음)");
    benchmark([](double t) { return std::cos(t); }, "(std::cos)");
}