do i = 1 to 400 by 1
  a(i) = a(i) + b(i)
end

do i = 1 to 100 by 4
  a(i)   = a(i) + b(i)
  a(i+1) = a(i+1) + b(i+1)
  a(i+2) = a(i+2) + b(i+2)
  a(i+3) = a(i+3) + b(i+3)
end

Example 1: Loop unrolling.