unrolling a while loop

Question

original code

while(i<30){
// do something
i++;
}

unrolled while loop

while(i<15){
// do something twice
i+=2;
}

Cant we unroll it as shown above. Do we always have to do it like http://en.wikipedia.org/wiki/Loop_unrolling ?

Answer 1

In general, the answer is no. It works for 30 and 15 because 30 is even, but it would not work as easily for odd numbers. "Duff's device" was invented to deal with general case. It is quite ugly, though.