How to check if return value optimization happened?

Question

Consider this function:

std::string 
myClass::myFunction2() {
  std::string result = myClass::myFunction1();
  return result;
}

I'm hoping that the compile performs return value optimization. How can I make sure this actually happens, and the code will not copy the result redundantly?

Answer 1

RVO is always applied, if possible. For your case, assuming myFunction1() does not return different named objects depending on the path of execution, the compiler should perform RVO. If it returns different named objects with different execution path, then the compiler is not able to perform optimization.

I recommend to do your own experiments:

To disable optimization everywhere, use pragmas:

#pragma GCC push_options
#pragma GCC optimize (ARG)

//your code

#pragma GCC pop_options

To disable optimization for a specific function, use __attribute__(()):

void __attribute__((optimize(ARG))) foo(unsigned char data) {
    // your code
}

ARG can either be numbers (i.e. an optimization level) or strings starting with 0 (i.e. an optimization option) and etc. For what you need, you can substitute ARG with "O0" Then run the both versions of your code using gcc -S to see the difference. I recommend you to read gcc 4.4 or newer doc.