On Duff's Device and Coroutines Posted on Wednesday, January 30, 2008.
It's an 8x-unrolled while loop interlaced with a switch statement. The switch jumps into the middle of the while loop to handle the part of the count that isn't a multiple of 8. Once inside the while loop, the switch logic never executes again.
Tom Duff first described Duff's Device in a November 1983 email to Ron Gomes, Dennis Ritchie, and Rob Pike. It spread to a larger group when he revised the note and posted it to net.lang.c in May 1984. The 1984 message is the less frequently reproduced but is the one in which Duff gave it a name: “I think I'll name it after myself — ‘Duff's Device’ has a nice ring to it.” Bjarne Stroustroup used a variant as an example in The C++ Programming Language. Eventually Duff posted the 1983 message along with commentary to Usenet in 1988, in response to a heated debate about the merits of Duff's Device as an idiom.
Duff's Device is rarely worth reusing, but in the specific case that it was written for, it was an apt tool for the job. The more amazing thing about it is that it's valid C. Since a switch statement is just a computed goto and the case labels are just labels, switching into the middle of a while is just as legal as goto'ing there. In Duff's description, he wrote:
