Copy vs constref benchmark

#include <map>
#include <string>
#include <chrono>
#include <random>
#include <iostream>

__attribute__((noinline))
void test_slow(std::map<std::string, std::string> data); // copy
__attribute__((noinline))
void test_fast(const std::map<std::string, std::string>& data); // const reference

int main()
{
    // Just create a huge set of dummy data
    std::map<std::string, std::string> some_data;
    std::cout << "Initializing...\n";
    std::random_device e;
    std::mt19937_64 g(e());
    std::uniform_int_distribution d(' ','~');
    while(some_data.size() < 1000000)
    {
        std::string s1; s1.reserve(64); for(std::size_t n=0; n<64; ++n) s1 += d(g);
        std::string s2; s2.reserve(64); for(std::size_t n=0; n<64; ++n) s2 += d(g);
        some_data.emplace(std::move(s1), std::move(s2));
    }
    auto begin = std::chrono::system_clock::now();
    for(std::size_t n=0; n<32; ++n) test_slow(some_data);
    auto end   = std::chrono::system_clock::now();
    std::cout << std::fixed; std::cout.precision(12);
    std::cout << "Slow method: " << std::chrono::duration<double>(end - begin).count() << " s\n";

    auto begin2 = std::chrono::system_clock::now();

    for(std::size_t n=0; n<32; ++n) test_fast(some_data);
    auto end2   = std::chrono::system_clock::now();
    std::cout << "Fast method: " << std::chrono::duration<double>(end2 - begin2).count() << " s\n";
}

void test_slow(std::map<std::string, std::string> data) // copy
{
}
void test_fast(const std::map<std::string, std::string>& data) // const reference
{
}