为什么在我的测试程序中`std :: copy` 5x(!)比`memcpy`慢？

Question

这是我发布此程序的这个问题的后续行动:

#include <algorithm>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <iomanip>
#include <iostream>
#include <vector>
#include <chrono>

class Stopwatch
{
public:
    typedef std::chrono::high_resolution_clock Clock;

    //! Constructor starts the stopwatch
    Stopwatch() : mStart(Clock::now())
    {
    }

    //! Returns elapsed number of seconds in decimal form.
    double elapsed()
    {
        return 1.0 * (Clock::now() - mStart).count() / Clock::period::den;
    }

    Clock::time_point mStart;
};

struct test_cast
{
    int operator()(const char * data) const
    {
        return *((int*)data);
    }
};

struct test_memcpy
{
    int operator()(const char * data) const
    {
        int result;
        memcpy(&result, data, sizeof(result));
        return result;
    }
};

struct test_memmove
{
    int operator()(const char * data) const
    {
        int result;
        memmove(&result, data, sizeof(result));
        return result;
    }
};

struct test_std_copy
{
    int operator()(const char * data) const
    {
        int result;
        std::copy(data, data + sizeof(int), reinterpret_cast<char *>(&result));
        return result;
    }
};

enum
{
    iterations = 2000,
    container_size = 2000
};

//! Returns a list of integers in binary form.
std::vector<char> get_binary_data()
{
    std::vector<char> bytes(sizeof(int) * container_size);
    for (std::vector<int>::size_type i = 0; i != bytes.size(); i += sizeof(int))
    {
        memcpy(&bytes[i], &i, sizeof(i));
    }
    return bytes;
}

template<typename Function>
unsigned benchmark(const Function & function, unsigned & counter)
{
    std::vector<char> binary_data = get_binary_data();
    Stopwatch sw;
    for (unsigned iter = 0; iter != iterations; ++iter)
    {
        for (unsigned i = 0; i != binary_data.size(); i += 4)
        {
            const char * c = reinterpret_cast<const char*>(&binary_data[i]);
            counter += function(c);
        }
    }
    return unsigned(0.5 + 1000.0 * sw.elapsed());
}

int main()
{
    srand(time(0));
    unsigned counter = 0;

    std::cout << "cast:      " << benchmark(test_cast(),     counter) << " ms" << std::endl;
    std::cout << "memcpy:    " << benchmark(test_memcpy(),   counter) << " ms" << std::endl;
    std::cout << "memmove:   " << benchmark(test_memmove(),  counter) << " ms" << std::endl;
    std::cout << "std::copy: " << benchmark(test_std_copy(), counter) << " ms" << std::endl;
    std::cout << "(counter:  " << counter << ")" << std::endl << std::endl;

}

我注意到由于某些原因,std::copy它比memcpy更糟糕.使用gcc 4.7,我的Mac上的输出看起来像这样.

g++ -o test -std=c++0x -O0 -Wall -Werror -Wextra -pedantic-errors main.cpp
cast:      41 ms
memcpy:    46 ms
memmove:   53 ms
std::copy: 211 ms
(counter:  3838457856)

g++ -o test -std=c++0x -O1 -Wall -Werror -Wextra -pedantic-errors main.cpp
cast:      8 ms
memcpy:    7 ms
memmove:   8 ms
std::copy: 19 ms
(counter:  3838457856)

g++ -o test -std=c++0x -O2 -Wall -Werror -Wextra -pedantic-errors main.cpp
cast:      3 ms
memcpy:    2 ms
memmove:   3 ms
std::copy: 27 ms
(counter:  3838457856)

g++ -o test -std=c++0x -O3 -Wall -Werror -Wextra -pedantic-errors main.cpp
cast:      2 ms
memcpy:    2 ms
memmove:   3 ms
std::copy: 16 ms
(counter:  3838457856)

正如您所看到的,即使使用-O3它也比memcpy慢5倍(!).

Linux上的结果相似.

有谁知道为什么？

Answer 1

我同意@ RICI的评论关于制定一个更有意义的基准,所以我用你的改写测试两个向量的基准拷贝memcpy(),memmove(),std::copy()和std::vector赋值运算符:

#include <algorithm>
#include <iostream>
#include <vector>
#include <chrono>
#include <random>
#include <cstring>
#include <cassert>

typedef std::vector<int> vector_type;

void test_memcpy(vector_type & destv, vector_type const & srcv)
{
    vector_type::pointer       const dest = destv.data();
    vector_type::const_pointer const src  = srcv.data();

    std::memcpy(dest, src, srcv.size() * sizeof(vector_type::value_type));
}

void test_memmove(vector_type & destv, vector_type const & srcv)
{
    vector_type::pointer       const dest = destv.data();
    vector_type::const_pointer const src  = srcv.data();

    std::memmove(dest, src, srcv.size() * sizeof(vector_type::value_type));
}

void test_std_copy(vector_type & dest, vector_type const & src)
{
    std::copy(src.begin(), src.end(), dest.begin());
}

void test_assignment(vector_type & dest, vector_type const & src)
{
    dest = src;
}

auto
benchmark(std::function<void(vector_type &, vector_type const &)> copy_func)
    ->decltype(std::chrono::milliseconds().count())
{
    std::random_device rd;
    std::mt19937 generator(rd());
    std::uniform_int_distribution<vector_type::value_type> distribution;

    static vector_type::size_type const num_elems = 2000;

    vector_type dest(num_elems);
    vector_type src(num_elems);

    // Fill the source and destination vectors with random data.
    for (vector_type::size_type i = 0; i < num_elems; ++i) {
        src.push_back(distribution(generator));
        dest.push_back(distribution(generator));
    }

    static int const iterations = 50000;

    std::chrono::time_point<std::chrono::system_clock> start, end;

    start = std::chrono::system_clock::now();

    for (int i = 0; i != iterations; ++i)
        copy_func(dest, src);

    end = std::chrono::system_clock::now();

    assert(src == dest);

    return
        std::chrono::duration_cast<std::chrono::milliseconds>(
            end - start).count();
}

int main()
{
    std::cout
        << "memcpy:     " << benchmark(test_memcpy)     << " ms" << std::endl
        << "memmove:    " << benchmark(test_memmove)    << " ms" << std::endl
        << "std::copy:  " << benchmark(test_std_copy)   << " ms" << std::endl
        << "assignment: " << benchmark(test_assignment) << " ms" << std::endl
        << std::endl;
}

为了好玩,我对C++ 11有点过分了.

以下是我使用g ++ 4.6.3在我的64位Ubuntu盒子上得到的结果:

$ g++ -O3 -std=c++0x foo.cpp ; ./a.out 
memcpy:     33 ms
memmove:    33 ms
std::copy:  33 ms
assignment: 34 ms

结果都很可比!我得到可比的时间在所有试验情况下,当我改变整数类型,例如到long long,在向量为好.

除非我的基准重写被破坏,否则您自己的基准测试似乎没有执行有效的比较.HTH!

Answer 2

在我看来,答案是gcc可以优化对memmove和memcpy的这些特定调用,而不是std :: copy.gcc知道memmove和memcpy的语义,在这种情况下可以利用已知大小(sizeof(int))将调用转换为单个mov指令的事实.

std :: copy是用memcpy实现的,但显然gcc优化器并没有设法弄清楚data + sizeof(int) - 数据正好是sizeof(int).所以基准调用memcpy.

通过调用gcc -S并快速翻转输出,我得到了所有这些; 我很容易弄错了,但我所看到的似乎与你的测量结果一致.

顺便说一句,我认为测试或多或少没有意义.一个更合理的现实世界的测试可能会创建一个实际的vector<int> src和int[N] dst,然后比较memcpy(dst, src.data(), sizeof(int)*src.size())有std::copy(src.begin(), src.end(), &dst).

Answer 3

这不是我得到的结果：

> g++ -O3 XX.cpp 
> ./a.out
cast:      5 ms
memcpy:    4 ms
std::copy: 3 ms
(counter:  1264720400)

Hardware: 2GHz Intel Core i7
Memory:   8G 1333 MHz DDR3
OS:       Max OS X 10.7.5
Compiler: i686-apple-darwin11-llvm-g++-4.2 (GCC) 4.2.1

在 Linux 机器上我得到不同的结果：

> g++ -std=c++0x -O3 XX.cpp 
> ./a.out 
cast:      3 ms
memcpy:    4 ms
std::copy: 21 ms
(counter:  731359744)


Hardware:  Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz
Memory:    61363780 kB
OS:        Linux ip-10-58-154-83 3.2.0-29-virtual #46-Ubuntu SMP
Compiler:  g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3