Word:

Duff's device

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]
Browse
Duel
Dueler
Dueling
Duelist
dueller
Duelling
duellist
Duelo
Duena
Dueness
Duenna
Duet
duette
Duettino
Duetto
Duff
-- Duff's device --
Duffel
Duffel bag
duffel coat
Duffer
Duffle
duffle bag
duffle coat
Dufrenite
Dufy
Dug
Dugald Stewart
Dugong
Dugong dugon
Dugongidae
Dugout
dugout canoe
Definitions Index: # A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

About this site and copyright information - Online Dictionary Home - Privacy Policy