Expose Duff's device
Duff's device Technical Terms
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); } Having verified that the device is valid portable C, Duff announced it. 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."