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: *to = *from++;
case 2: *to = *from++;
case 1: *to = *from++;
} while (--n > 0);
}
Shocking though it appears to all who encounter it for the
first time, the device is actually perfectly valid, legal C. C's
default fall through in case
statements has long been its most controversial single feature;
Duff observed that "This code forms some sort of argument in that
debate, but I'm not sure whether it's for or against." Duff has
discussed the device in detail at http://www.lysator.liu.se/c/duffs-device.html.
Note that the omission of postfix ++ from
*to was intentional (though confusing). Duff's
device can be used to implement memory copy, but the original aim
was to copy values serially into a magic IO register.
[For maximal obscurity, the outermost pair of braces above
could actually be removed -- GLS]