duff's device

Slang Dictionary

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 be actually be removed -- GLS]
FOLDOC
Computing Dictionary

Duff's device definition


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)
The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
Cite This Source
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature
FAVORITES
RECENT

;