Question

I was testing the speed of an exported function in a DLL and a normal function. How is it possible that an exported function in a DLL is a lot faster?

100000000 function calls in a DLL cost: 0.572682 seconds
100000000 normal function class cost: 2.75258 seconds

This is the function in the DLL:

extern "C" __declspec (dllexport) int example()
{
    return 1;
}

This is the normal function call:

int example()
{
    return 1;
}

This is how I test it:

int main()
{
    LARGE_INTEGER frequention;
    LARGE_INTEGER dllCallStart,dllCallStop;
    LARGE_INTEGER normalStart,normalStop;
    int resultCalculation;

    //Initialize the Timer
    ::QueryPerformanceFrequency(&frequention);
    double frequency = frequention.QuadPart;
    double secondsElapsedDll = 0;
    double secondsElapsedNormal = 0;

    //Load the Dll
    HINSTANCE hDll = LoadLibraryA("example.dll");

    if(!hDll)
    {
        cout << "Dll error!" << endl;
        return 0;
    }

    dllFunction = (testFunction)GetProcAddress(hDll, "example");

    if( !dllFunction )
    {
        cout << "Dll function error!" << endl;
        return 0;
    }

    //Dll
    resultCalculation = 0;
    ::QueryPerformanceCounter(&dllCallStart);
    for(int i = 0; i < 100000000; i++)
        resultCalculation += dllFunction();
    ::QueryPerformanceCounter(&dllCallStop);

    Sleep(100);

    //Normal
    resultCalculation = 0;
    ::QueryPerformanceCounter(&normalStart);
    for(int i = 0; i < 100000000; i++)
        resultCalculation += example();
    ::QueryPerformanceCounter(&normalStop);

    //Calculate the result time
    secondsElapsedDll = ((dllCallStop.QuadPart - dllCallStart.QuadPart) / frequency);
    secondsElapsedNormal = ((normalStop.QuadPart - normalStart.QuadPart) / frequency);

    //Output
    cout << "Dll: " << secondsElapsedDll << endl; //0.572682
    cout << "Normal: " << secondsElapsedNormal << endl; //2.75258

    return 0;
}

I only test the function call speed, getting the address can be done at start-up. So the performance lost of that doesn’t mater.

Answer 1

For a very small function, the difference is in the way that a function returns/cleans up arguments.

However, that shouldn’t make that much of a difference. I think the compiler realizes that your function doesn’t do anything with resultCalcuation and optimizes it away. Try using two different variables and print their value afterwards.