为什么<wstring>使用自定义 wcscmp 和 wmemcmp 比较器对向量进行排序比默认快得多？

似乎使用基于自定义wcscmp和基于wmemcmp的比较器对vector<wstring>进行排序比默认行为快得多(大约 1000 毫秒(。

64 位发布版本 (VS2015(：

Default1: 3304.39 ms
wcscmp1 : 2323.26 ms
wmemcmp1: 2300.11 ms
Default2: 3239.75 ms
wcscmp2 : 2303.37 ms
wmemcmp2: 2338.55 ms
Default3: 3293.73 ms
wcscmp3 : 2303.82 ms
wmemcmp3: 2313.88 ms

可编译代码：

///////////////////////////////////////////////////////////////////////////////
//
// Compare sorting a vector<wstring> with:
//  - default comparator
//  - custom wcscmp()-based comparator
//  - custom wmemcmp()-based comparator
// 
///////////////////////////////////////////////////////////////////////////////
#include <string.h>     // wcscmp, wmemcmp
#include <algorithm>    // std::shuffle, std::sort
#include <iostream>     // std::cout
#include <random>       // std::mt19937
#include <string>       // std::wstring
#include <vector>       // std::vector
#include <Windows.h>    // Windows Platform SDK
using namespace std;

//=============================================================================
//                        Performance Counter Helpers
//=============================================================================
long long Counter()
{
    LARGE_INTEGER li;
    ::QueryPerformanceCounter(&li);
    return li.QuadPart;
}
long long Frequency()
{
    LARGE_INTEGER li;
    ::QueryPerformanceFrequency(&li);
    return li.QuadPart;
}
void PrintTime(const long long start, const long long finish, const char * const s)
{
    cout << s << ": " << (finish - start) * 1000.0 / Frequency() << " ms n";
}

//=============================================================================
//                           Performance Tests
//=============================================================================
bool CompareUsingWcscmp(const std::wstring& a, const std::wstring& b) noexcept
{
    // a < b
    return wcscmp(a.c_str(), b.c_str()) < 0;
}
bool CompareUsingWmemcmp(const std::wstring& a, const std::wstring& b) noexcept
{
    const size_t count = min(a.size(), b.size());
    return wmemcmp(a.data(), b.data(), count) < 0;
}
int main()
{
    // Build a vector of strings generated starting from "Lorem Ipsum"
    const auto shuffled = []() -> vector<wstring>
    {
        const wstring lorem[] =
        {
            L"Lorem ipsum dolor sit amet, consectetuer adipiscing elit.",
            L"Maecenas porttitor congue massa. Fusce posuere, magna sed",
            L"pulvinar ultricies, purus lectus malesuada libero,",
            L"sit amet commodo magna eros quis urna.",
            L"Nunc viverra imperdiet enim. Fusce est. Vivamus a tellus.",
            L"Pellentesque habitant morbi tristique senectus et netus et",
            L"malesuada fames ac turpis egestas. Proin pharetra nonummy pede.",
            L"Mauris et orci. [*** add more chars to prevent SSO ***]"
        };
        vector<wstring> v;
#ifdef _DEBUG
        constexpr int kTestIterationCount = 1000;
#else
        constexpr int kTestIterationCount = 200'000;
#endif
        for (int i = 0; i < kTestIterationCount; ++i)
        {
            for (const auto & s : lorem)
            {
                v.push_back(s + L" (#" + to_wstring(i) + L")");
            }
        }
        mt19937 prng(1980);
        shuffle(v.begin(), v.end(), prng);
        return v;
    }();
    long long start = 0;
    long long finish = 0;
    vector<wstring>  v1 = shuffled;
    vector<wstring>  w1 = shuffled;
    vector<wstring>  z1 = shuffled;
    vector<wstring>  v2 = shuffled;
    vector<wstring>  w2 = shuffled;
    vector<wstring>  z2 = shuffled;
    vector<wstring>  v3 = shuffled;
    vector<wstring>  w3 = shuffled;
    vector<wstring>  z3 = shuffled;
    start = Counter();
    sort(v1.begin(), v1.end());
    finish = Counter();
    PrintTime(start, finish, "Default1");
    start = Counter();
    sort(w1.begin(), w1.end(), CompareUsingWcscmp);
    finish = Counter();
    PrintTime(start, finish, "wcscmp1 ");
    start = Counter();
    sort(z1.begin(), z1.end(), CompareUsingWmemcmp);
    finish = Counter();
    PrintTime(start, finish, "wmemcmp1");
    cout << 'n';
    start = Counter();
    sort(v2.begin(), v2.end());
    finish = Counter();
    PrintTime(start, finish, "Default2");
    start = Counter();
    sort(w2.begin(), w2.end(), CompareUsingWcscmp);
    finish = Counter();
    PrintTime(start, finish, "wcscmp2 ");
    start = Counter();
    sort(z2.begin(), z2.end(), CompareUsingWmemcmp);
    finish = Counter();
    PrintTime(start, finish, "wmemcmp2");
    cout << 'n';
    start = Counter();
    sort(v3.begin(), v3.end());
    finish = Counter();
    PrintTime(start, finish, "Default3");
    start = Counter();
    sort(w3.begin(), w3.end(), CompareUsingWcscmp);
    finish = Counter();
    PrintTime(start, finish, "wcscmp3 ");
    start = Counter();
    sort(z3.begin(), z3.end(), CompareUsingWmemcmp);
    finish = Counter();
    PrintTime(start, finish, "wmemcmp3");
}
///////////////////////////////////////////////////////////////////////////////

附言我知道wcscmp不适用于包含嵌入空值的字符串。但这不是问题的重点。