How does Duff's device work?

Question

I\'ve read the article on Wikipedia on the Duff\'s device, and I don\'t get it. I am really interested, but I\'ve read the explanation there a couple of times and I still do

Answer 1

The point of duffs device is to reduce the number of comparisons done in a tight memcpy implementation.

Suppose you want to copy 'count' bytes from a to b, the straight forward approach is to do the following:

  do {                      
      *a = *b++;            
  } while (--count > 0);

How many times do you need to compare count to see if it's a above 0? 'count' times.

Now, the duff device uses a nasty unintentional side effect of a switch case which allows you to reduce the number of comparisons needed to count / 8.

Now suppose you want to copy 20 bytes using duffs device, how many comparisons would you need? Only 3, since you copy eight bytes at a time except the last first one where you copy just 4.

UPDATED: You don't have to do 8 comparisons/case-in-switch statements, but it's reasonable a trade-off between function size and speed.