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19.9 Duff’s Device

The cases in a switch statement can be inside other control constructs. For instance, we can use a technique known as Duff’s device to optimize this simple function, 

void
copy (char *to, char *from, int count)
{
  while (count > 0)
    *to++ = *from++, count--;
}

which copies memory starting at from to memory starting at to.

Duff’s device involves unrolling the loop so that it copies several characters each time around, and using a switch statement to enter the loop body at the proper point: 

void
copy (char *to, char *from, int count)
{
  if (count <= 0)
    return;
  int n = (count + 7) / 8;
  switch (count % 8)
    {
      do {
        case 0: *to++ = *from++;
        case 7: *to++ = *from++;
        case 6: *to++ = *from++;
        case 5: *to++ = *from++;
        case 4: *to++ = *from++;
        case 3: *to++ = *from++;
        case 2: *to++ = *from++;
        case 1: *to++ = *from++;
        } while (--n > 0);
    }
}