Copy elision on Visual C++ 2010 Beta 2

Question

I was reading Want Speed? Pass by Value on the C++ Next blog and created this program to get a feel for copy elision and move semantics in C++0x:

#include <vector>
#include <iostream>

class MoveableClass {
public:
    MoveableClass() : m_simpleData(0), instance(++Instances) {
        std::cout << "Construct instance " << instance << " (no data)" << std::endl;
    }

    MoveableClass(std::vector<double> data) : m_data(std::move(data)), m_simpleData(0), instance(++Instances) {
        std::cout << "Construct instance " << instance << " (with data)" << std::endl;
    }

    MoveableClass(int simpleData) : m_simpleData(simpleData), instance(++Instances) {
        std::cout << "Construct instance " << instance << " (with simple data)" << std::endl;
    }

    MoveableClass(const MoveableClass& other) 
        : m_data(other.m_data), m_simpleData(other.m_simpleData), instance(++Instances)
    {
        std::cout << "Construct instance " << instance << " from a copy of " << other.instance << std::endl;
        Elided = false;
    }

    MoveableClass(MoveableClass&& other) 
        : m_data(std::move(other.m_data)), m_simpleData(other.m_simpleData), instance(++Instances)
    {
        std::cout << "Construct instance " << instance << " from a move of " << other.instance << std::endl;
        Elided = false;
    }

    MoveableClass& operator=(MoveableClass other) {
        std::cout << "Assign to instance " << instance << " from " << other.instance << std::endl;
        other.Swap(*this);
        return *this;
    }

    ~MoveableClass() {
        std::cout << "Destroy instance " << instance << std::endl;
        --Instances;
    }

    void Swap(MoveableClass& other) {
        std::swap(m_data, other.m_data);
        std::swap(m_simpleData, other.m_simpleData);
    }

    static int Instances;
    static bool Elided;

private:
    int instance;
    int m_simpleData;
    std::vector<double> m_data;
};

int MoveableClass::Instances = 0;
bool MoveableClass::Elided = true;

std::vector<double> BunchOfData() {
    return std::vector<double>(9999999);
}

int SimpleData() {
    return 9999999;
}

MoveableClass CreateRVO() {
    return MoveableClass(BunchOfData());
}

MoveableClass CreateNRVO() {
    MoveableClass named(BunchOfData());
    return named;
}

MoveableClass CreateRVO_Simple() {
    return MoveableClass(SimpleData());
}

MoveableClass CreateNRVO_Simple() {
    MoveableClass named(SimpleData());
    return named;
}

int main(int argc, char* argv[]) {
    std::cout << "\nMove assign from RVO: " << '\n';
    {
        MoveableClass a;
        a = CreateRVO();
    }
    std::cout << "Move elided: " << (MoveableClass::Elided ? "Yes" : "No") << '\n';
    MoveableClass::Elided = true;   // reset for next test

    std::cout << "\nMove assign from RVO simple: " << '\n';
    {
        MoveableClass a;
        a = CreateRVO_Simple();
    }
    std::cout << "Move elided: " <<  (MoveableClass::Elided ? "Yes" : "No") << '\n';
    MoveableClass::Elided = true;   // reset for next test

    std::cout << "\nMove assign from NRVO: " << '\n';
    {
        MoveableClass a;
        a = CreateNRVO();
    }
    std::cout << "Move elided: " <<  (MoveableClass::Elided ? "Yes" : "No") << '\n';
    MoveableClass::Elided = true;   // reset for next test

    std::cout << "\nMove assign from NRVO simple: " << std::endl;
    {
        MoveableClass a;
        a = CreateNRVO_Simple();
    }
    std::cout << "Move elided: " << (MoveableClass::Elided ? "Yes" : "No") << '\n';
    MoveableClass::Elided = true;   // reset for next test
}

Here is the output I get when compiled in release mode on Visual C++ 10.0 (Beta 2):

Move assign from RVO:
Construct instance 1 (no data)
Construct instance 2 (with data)
Construct instance 3 from a move of 2
Destroy instance 2
Assign to instance 1 from 3
Destroy instance 3
Destroy instance 1
Move elided: No

Move assign from RVO simple:
Construct instance 1 (no data)
Construct instance 2 (with simple data)
Assign to instance 1 from 2
Destroy instance 2
Destroy instance 1
Move elided: Yes

Move assign from NRVO:
Construct instance 1 (no data)
Construct instance 2 (with data)
Assign to instance 1 from 2
Destroy instance 2
Destroy instance 1
Move elided: Yes

Move assign from NRVO simple:
Construct instance 1 (no data)
Construct instance 2 (with simple data)
Assign to instance 1 from 2
Destroy instance 2
Destroy instance 1
Move elided: Yes

However, I am perplexed by one thing. As you can see, all of the moves are elided except for the first one. Why can't the compiler perform RVO with a MoveableClass(std::vector) at line 86, but can with a MoveableClass(int) at line 97? Is this just a bug with MSVC or is there a good reason for this? And if there is a good reason, why can it still perform NRVO on a MoveableClass(std::vector) at line 91?

I'd like to understand it so I can go to sleep happy. :)

Damage

A very good question. For what it's worth, g++ 4.3.3 does elide all these moves, even with the -O0 flag. – Penelope

Thanks Thomas. It's interesting that it does work on GCC. Maybe that suggests is something wrong with the MSVC implementation. – Damage

I think this just underscores how vast the chasm is between "the compiler should" and "the compiler does." – Hemocyte

Answer 1

Thanks for replying Dave.

I've added my tests to that example:
pastebin.com/f7c8ca0d6

Curiously it shows that all types of elisions are not being performed except for NRVO!
Edit: Actually I suppose this is because it is the only test where the object ever has a name.

I also tried other STL types and got the same result. However when trying my own non-pod types it works as expected. I can't think what's special about the STL types that could be causing this so I don't know what else to try.

I'll submit a bug report.
Edit: Submitted here

Thanks

Damage

Answer 2

Hmm.

It seems that if you change the data constructor

MoveableClass::MoveableClass(std::vector<double> data)

to accept the vector by reference, like so,

MoveableClass::MoveableClass(const std::vector<double>& data) 

it works fine! Why does it not work if you pass the vector by value?

Also here's a version that should compile on earlier versions of MSVC, if anybody wants to run the test there. It contains no C++0x features: http://pastebin.com/f3bcb6ed1

Damage

Answer 3

Maybe it'd be a good idea to update and maintain this example from cpp-next with a version of your test that fails, so there can be one comprehensive, canonical test.

Nefarious