Original Version
Straightforward code to copy items from an array to a memory-mapped output register might look like this:
do { /* count > 0 assumed */ *to = *from++; /* Note that the 'to' pointer is NOT incremented */ } while(--count > 0);Note that this is not a memory-to-memory copy, in which you would see *to++.
While optimizing this, Duff realized that an unrolled version of his loop could be implemented by interlacing the structures of a switch and a loop.
send(to, from, count) register short *to, *from; register count; { register n = (count + 7) / 8; switch(count % 8) { case 0: do { *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); } }Notice that Duff's device can just as easily be applied with any other size for the unrolled loop, not just 8.
Read more about this topic: Duff's Device
Famous quotes containing the words original and/or version:
“He had been a lad of whom something was expected. Beyond this all had been chaos. That he would be successful in an original way, or that he would go to the dogs in an original way, seemed equally probable. The only absolute certainty about him was that he would not stand still in the circumstances amid which he was born.”
—Thomas Hardy (1840–1928)
“Exercise is the yuppie version of bulimia.”
—Barbara Ehrenreich (b. 1941)