Consider the following code:
int condition = 1;
while(condition == 1)
{
std::cout<<"inside loop" <<std::endl;
}If the the optimization is enabled, the compiler will find out that the program never changes the value of variable condition, so it may change the loop into something like while(true). If the compiler doesn't optimize it, then it has to fetch the value of condition and compare it with 1, in each iteration which obviously is a little bit slow.
However, sometimes, someone else might change the value of condition from outside the program (some memcopy operations, extern variables, other thread, a hardware that directly change the memory, etc) which compiler may not be aware. In such situations the compiler's optimization is undesired.
To ensure that the compiler optimizer produce the desired result, you need to tell the compiler to avoid optimizing the while loop. That is where the volatile keyword is useful.