Static storage objects optimization on thread context

Can a compiler optimize a static storage object in multithread program context? I'm asking it to know if a variable declared as static, for instance, generates a side effect when used in a function called in a thread.

bool flag = false; // static storage duration object

void f(){ //function called in a thread
  flag = false;
  // do some work...
  flag = true;
}

//a possible representation of the code above after optimization
void f(){
  flag = true;
  // do some work...
} // is this possible to happen?

I read some answers from here, but I didn't find anything that could help.