Loop Unrolling

Loop unrolling is the compile-time expansion of a loop into repetitions of the code, with the loop index replaced by its value in each instance.

for (i = 0; i < 3; i++)
  disp[i] = c2[i] - c1[i];

disp[0] = c2[0] - c1[0];
disp[1] = c2[1] - c1[1];
disp[2] = c2[2] - c1[2];

	Loop:	Unrolled:
Instructions:	20	12
Executed:	57	12

The second form runs faster, but it may generate more code since the code for the loop contents is multiplied by the loop count. This is a useful optimization when the size of the loop is known to be a small constant at compile time.

Modulo unrolling unrolls a loop modulo some chosen constant. This can significantly reduce the loop overhead without expanding code size too much.