Question 20.35

comp.lang.c FAQ list · Question 20.35

Q: What is ``Duff's Device''?

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A: It's a devastatingly devious way of unrolling a loop, devised by Tom Duff while he was at Lucasfilm. In its ``classic'' form, it was used to copy bytes, and looked like this:

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);
 }

where count bytes are to be copied from the array pointed to by from to the memory location pointed to by to (which is a memory-mapped device output register, which is why to isn't incremented). It solves the problem of handling the leftover bytes (when count isn't a multiple of 8) by interleaving a switch statement with the loop which copies bytes 8 at a time. (Believe it or not, it is legal to have case labels buried within blocks nested in a switch statement like this. In his announcement of the technique to C's developers and the world, Duff noted that C's switch syntax, in particular its ``fall through'' behavior, had long been controversial, and that ``This code forms some sort of argument in that debate, but I'm not sure whether it's for or against.'')
Additional links: longer explanation