Duff's device
n. The most dramatic use yet seen of {fall
through} in C, invented by Tom Duff when he was at Lucasfilm.
Trying to
bum all the instructions he could out of an inner
loop that copied data serially onto an output port, he decided to
unroll it. He then realized that the unrolled version could
be implemented by *interlacing* the structures of a switch and
a loop
register n = (count + 7) / 8; /* count > 0 assumed */
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
