Duff's device is a programming style that reduces the number of jumps through loop unrolling.
Suppose we have a loop of the form
do {
easy instruction; i := i - 1;
} while (i >= 0);
This generates a jump for every iteration of the loop.
If we knew that is always divisible by, say, 4, we could reduce the number of jumps by producing the following equivalent code instead, where / is integer division:
i := i / 4;
do {
easy instruction;
easy instruction;
easy instruction;
easy instruction;
} while (i >= 0);
This reduces the number of jumps by a factor of 4.
But if we don't know that divisibility, we can abuse a fall-through switch statement to cover for the remainder upon division by 4:
n := (i + 3) / 4;
switch (i mod 8) {
case 0: do { easy instruction;
case 1: easy instruction;
case 2: easy instruction;
n := n - 1;
} while (n >= 0);
This is not legal in most modern imperative programming languages, but was legal in original C.