turn off c++ loop optimization

Question

I am wondering how to turn off optimization for the following loop. I have searched and tried some flag but not working. Thanks.

#include <iostream>

int fun(int i) {return ++i;};

int main ()
{
    int res = 0;

    for (long i = 0; i < 10000; i++)
    {
        res += fun(i);
    }
    std::cout << res;
}

-O1 will remove the loop and directly hard code the result. I would like to use -O3 but with this optimization turned off.

Answer 1

You can use something like Google Benchmark's DoNotOptimize() function. This function uses an empty asm block to lie to the compiler that the given variable was read from. Adapted from here:

template <class Tp>
[[gnu::always_inline]] inline void DoNotOptimize(Tp const& value) {
  asm volatile("" : : "r,m"(value) : "memory");
}

template <class Tp>
[[gnu::always_inline]] inline void DoNotOptimize(Tp& value) {
#if defined(__clang__)
  asm volatile("" : "+r,m"(value) : : "memory");
#else
  asm volatile("" : "+m,r"(value) : : "memory");
#endif
}

Then, you can lie to the compiler and tell it that res is read in each iteration:

#include <iostream>

int main ()
{
    int res = 0;

    ::DoNotOptimize(res);
    for (long i = 0; i < 10; i++)
    {
        res++;
        ::DoNotOptimize(res);
    }
    std::cout << res;
}

Compiler Explorer link

In -O3, the loop is unrolled, but -O1 and -O2 keep the loop.