Using volatile to prevent compiler optimization in benchmarking code?

Question

I am creating a little program measure the performance difference between containers of types boost::shared_ptr and boost::intrusive_ptr. In order to prevent the compiler from optimizing away the copy I declare the variable as volatile. The loop looks like this:

// TestCopy measures the time required to create n copies of the given container.
// Returns time in milliseconds.
template<class Container>
time_t TestCopy(const Container & inContainer, std::size_t n) {
    Poco::Stopwatch stopwatch;
    stopwatch.start();
    for (std::size_t idx = 0; idx < n; ++idx)
    {
        volatile Container copy = inContainer; // Volatile!
    }

    // convert microseconds to milliseconds
    return static_cast<time_t>(0.5 + (double(stopwatch.elapsed()) / 1000.0));
}

The rest of the code can be found here: main.cpp.

• Will using volatile here prevent the compiler from optimizing away the copy?
• Are there any pitfalls that may invalidate the results?

Update

In response to @Neil Butterworth. Even when using the copy it still seems to me that the compiler could easily avoid the copy:

for (std::size_t idx = 0; idx < n; ++idx)
{
    // gcc won't remove this copy?
    Container copy = inContainer;
    gNumCopies += copy.size();        
}

Answer 1

The C++03 standard says that reads and writes to volatile data is observable behavior (C++ 2003, 1.9 [intro.execution] / 6). I believe this guarantees that assignment to volatile data cannot be optimized away. Another kind of observable behavior is calls to I/O functions. The C++11 standard is even more unambiguous in this regard: in 1.9/8 it explicitly says that

The least requirements on a conforming implementation are:
— Access to volatile objects are evaluated strictly according to the rules of the abstract machine.

If a compiler can prove that a code does not produce an observable behavior then it can optimize the code away. In your update (where volatile is not used), copy constructor and other function calls & overloaded operators might avoid any I/O calls and access to volatile data, and the compiler might well understand it. However if gNumCopies is a global variable that later used in an expression with observable behavior (e.g. printed), then this code will not be removed.

Answer 2

Volatile is unlikely to do what you expect for a non-POD type. I would recommend passing a char * or void * aliasing the container to an empty function in a different translation unit. Since the compiler is unable to analyze the usage of the pointer, this will act as a compiler memory barrier, forcing the object out to the processor cache at least, and preventing most dead-value-elimination optimizations.