c++ - 为什么原生 C++ 与 C++ 互操作性表现不佳？

Question

我在下面发布了一些代码来测试从原生 c++ 和 c# 从 c++/cli 调用方法的性能(以毫秒为单位) 使用 Visual Studio 2010。我有一个单独的 native c++ 项目，它被编译成 dll。当我从 C++ 调用 C++ 时，我得到的预期结果比托管对象快得多(大约 4 倍)。但是，当我从 c++/cli 调用 c++ 时，性能降低了 10 倍。

这是从 c++/cli 调用 native c++ 时的预期行为吗？我的印象是不应该有显着差异，但这个简单的测试却表明情况并非如此。这可能是 c++ 和 c++/cli 编译器之间的优化差异吗？

更新

我对 cpp 进行了一些更新，这样我就不会在紧密循环中调用方法(正如 Reed Copsey 指出的那样)，结果表明性能差异微不足道或很小。当然，这取决于互操作的执行方式。

.h

#ifndef CPPOBJECT_H
#define CPPOBJECT_H

#ifdef CPLUSPLUSOBJECT_EXPORTING
    #define CLASS_DECLSPEC __declspec(dllexport)
#else
    #define CLASS_DECLSPEC __declspec(dllimport)
#endif

class CLASS_DECLSPEC CPlusPlusObject
{
public:
    CPlusPlusObject(){}
    ~CPlusPlusObject(){}

    void sayHello();
    double getSqrt(double n);
    // Update
    double wasteSomeTimeWithSqrt(double n);
};

#endif

.cpp

#include "CPlusPlusObject.h"
#include <iostream>

void CPlusPlusObject::sayHello(){std::cout << "Hello";}
double CPlusPlusObject::getSqrt(double n) {return std::sqrt(n);}
double CPlusPlusObject::wasteSomeTimeWithSqrt(double n)
{
    double result = 0;
    for (int x = 0; x < 10000000; x++)
    {
        result += std::sqrt(n);
    }
    return result;
}

c++/cli

const unsigned set = 100;
const unsigned repetitions = 1000000;
double cppcliTocpp()
{
    double n = 0;
    System::Diagnostics::Stopwatch^ stopWatch = gcnew System::Diagnostics::Stopwatch();

     stopWatch->Start();
     while (stopWatch->ElapsedMilliseconds < 1200){n+=0.001;}
     stopWatch->Reset();

    for (int x = 0; x < set; x++)
    {       
        stopWatch->Start();
        CPlusPlusObject cplusplusObject;
        n += cplusplusObject.wasteSomeTimeWithSqrt(123.456);
        /*for (int i = 0; i < repetitions; i++)
        {
            n += cplusplusObject.getSqrt(123.456);
        }*/
        stopWatch->Stop();
        System::Console::WriteLine("c++/cli call to native c++ took " + stopWatch->ElapsedMilliseconds + "ms.");
        stopWatch->Reset();
    }
    return n;
}

double cppcliTocSharp()
{
    double n = 0;
    System::Diagnostics::Stopwatch^ stopWatch = gcnew System::Diagnostics::Stopwatch();

    stopWatch->Start();
    while (stopWatch->ElapsedMilliseconds < 1200){n+=0.001;}
    stopWatch->Reset();

    for (int x = 0; x < set; x++)
    {       
        stopWatch->Start();
        CSharp::CSharpObject^ cSharpObject = gcnew CSharp::CSharpObject();
        for (int i = 0; i < repetitions; i++)
        {
            n += cSharpObject->GetSqrt(123.456);
        }
        stopWatch->Stop();
        System::Console::WriteLine("c++/cli call to c# took " + stopWatch->ElapsedMilliseconds + "ms.");
        stopWatch->Reset();
    }
    return n;
}

double cppcli()
{
    double n = 0;
    System::Diagnostics::Stopwatch^ stopWatch = gcnew System::Diagnostics::Stopwatch();

    stopWatch->Start();
    while (stopWatch->ElapsedMilliseconds < 1200){n+=0.001;}
    stopWatch->Reset();

    for (int x = 0; x < set; x++)
    {       
        stopWatch->Start();
        CPlusPlusCliObject cPlusPlusCliObject;
        for (int i = 0; i < repetitions; i++)
        {
            n += cPlusPlusCliObject.getSqrt(123.456);
        }
        stopWatch->Stop();
        System::Console::WriteLine("c++/cli took " + stopWatch->ElapsedMilliseconds + "ms.");
        stopWatch->Reset();
    }
    return n;
}

int main() 
{
    double n = 0;
    n += cppcliTocpp();
    n += cppcliTocSharp();
    n += cppcli();
    System::Console::WriteLine(n);
    System::Console::ReadKey();
}

Answer 1

However, when I call into c++ from c++/cli, the performance is 10x slower.

桥接 CLR 和 native 代码需要编码(marshal)处理。从 C++/CLI 转到 native 方法调用时，每个方法调用中总会有一些开销。

开销(在这种情况下)看起来如此之大的唯​​一原因是您在一个紧凑的循环中调用了一个非常快速的方法。如果您要对类进行批处理，或者调用运行时间明显更长的方法，您会发现开销非常小。