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 := 1 to 3 do x[i] := y[i];
is expanded into:
x[1] := y[1]; x[2] := y[2]; x[3] := y[3];
The second form may generate less code, and it runs faster. 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.