Duff's device

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 realised 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."

[For maximal obscurity, the outermost pair of braces above

could be actually be removed - GLS]

[Jargon File]

(2001-06-22)