手工编码的快速排序在较小的整数上比较慢

当比较我的快速排序实现与std::sort在我的编译器和我的归并排序实现时，我注意到一个奇怪的模式在大型数据集上:当操作64位整数时，快速排序始终比归并排序快;然而，对于较小的int大小，快速排序变慢，归并排序变快。

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <utility>
#include <random>
#include <chrono>
#include <limits>
#include <functional>
#include <cstdint>

template <typename Iterator>
void insertion_sort(Iterator first, Iterator last)
{
    using namespace std;
    Iterator head = first;
    Iterator new_position;
    while(head != last)
    {
        new_position = head;
        while(new_position != first && *new_position < *prev(new_position))
        {
            swap(*new_position, *prev(new_position));
            --new_position;
        }
        ++head;
    }
}
template <typename Iterator>
void recursive_mergesort_impl(Iterator first, Iterator last, std::vector<typename Iterator::value_type>& temp)
{
    if(last - first > 32)
    {
        auto middle = first + (last-first)/2;
        recursive_mergesort_impl(first, middle, temp);
        recursive_mergesort_impl(middle, last, temp);
        auto last_merged = merge_move(first, middle, middle, last, temp.begin());
        std::move(temp.begin(), last_merged, first);
    }
    else
    {
        insertion_sort(first, last);
    }
}
template <typename Iterator>
void recursive_mergesort(Iterator first, Iterator last)
{
    std::vector<typename Iterator::value_type> temp(last-first);
    recursive_mergesort_impl(first, last, temp);
}
// Pick a pivot and move it to front of range
template <typename Iterator>
template <typename Iterator>
void quicksort_pivot_back(Iterator first, Iterator last)
{
    using namespace std;
    auto middle = first + (last-first)/2;
    auto last_elem = prev(last);
    Iterator pivot;
    if(*first < *middle)
    {
        if(*middle < *last_elem)
            pivot = middle;
        else if(*first < *last_elem)
            pivot = last_elem;
        else
            pivot = first;
    }
    else if(*first < *last_elem)
        pivot = first;
    else if(*middle < *last_elem)
        pivot = last_elem;
    else
        pivot = middle;
    swap(*last_elem, *pivot);
}
template <typename Iterator, typename Function>
std::pair<Iterator, Iterator> quicksort_partition(Iterator first, Iterator last, Function pivot_select)
{
    using namespace std;
    pivot_select(first, last);
    auto pivot = prev(last);
    auto bottom = first;
    auto top = pivot;
    while(bottom != top)
    {
        if(*bottom < *pivot) ++bottom;
        else swap(*bottom, *--top);
    }
    swap(*pivot, *top++);
    return make_pair(bottom, top);
}
template <typename Iterator>
void quicksort_loop(Iterator first, Iterator last)
{
    using namespace std;
    while(last - first > 32)
    {
        auto bounds = quicksort_partition(first, last, quicksort_pivot_back<Iterator>);
        quicksort_loop(bounds.second, last);
        last = bounds.first;
    }
}

template <typename Iterator>
void quicksort(Iterator first, Iterator last)
{
    quicksort_loop(first, last);
    insertion_sort(first, last);
}
template <typename IntType = uint64_t, typename Duration = std::chrono::microseconds, typename Timer = std::chrono::high_resolution_clock, typename Function, typename Generator>
void run_trial(Function sort_func, Generator gen, std::string name, std::size_t trial_size, std::size_t trial_count)
{
    using namespace std;
    using namespace chrono;
    vector<IntType> data(trial_size);
    Duration elapsed(0);
    cout << "Sorting with " << name << endl;
    for(unsigned int i = 0; i < trial_count; ++i)
    {
        generate(data.begin(), data.end(), gen);
        auto start = Timer::now();
        sort_func(data.begin(), data.end());
        auto finish = Timer::now();
        elapsed += duration_cast<Duration>(finish-start);
    }
    cout << "Done. Average elapsed time: " << elapsed.count() / trial_count << endl;
    cout << "Is correct: " << is_sorted(data.begin(), data.end()) << endl << endl;
}
int main()
{
    using namespace std;
    using namespace chrono;
    using int_type = uint64_t;
    const size_t trial_size = 12800000;
    const int trial_count = 15;
    vector<int_type> data(trial_size);
    uniform_int_distribution<int_type> distr;
    mt19937_64 rnd;
    run_trial<int_type>(recursive_mergesort<vector<int_type>::iterator>, bind(distr, rnd), "recursive mergesort", trial_size, trial_count);
    run_trial<int_type>(quicksort<vector<int_type>::iterator>, bind(distr, rnd), "quicksort", trial_size, trial_count);
    run_trial<int_type>(sort<vector<int_type>::iterator>, bind(distr, rnd), "std::sort", trial_size, trial_count);
}

下面是对12800000个元素进行15次试验的次数:

uint64_t:

Sorting with recursive mergesort
Done. Average elapsed time: 1725431
Is correct: 1
Sorting with quicksort
Done. Average elapsed time: 1238070
Is correct: 1
Sorting with std::sort
Done. Average elapsed time: 1131464
Is correct: 1

uint16_t:

Sorting with recursive mergesort
Done. Average elapsed time: 1186467
Is correct: 1
Sorting with quicksort
Done. Average elapsed time: 2368535
Is correct: 1
Sorting with std::sort
Done. Average elapsed time: 888517
Is correct: 1

我有一种感觉，这个问题与未对齐的内存访问有关，然而，这仍然让我想知道为什么其他算法得到加速，而快速排序变慢。