[1] Duff implemented this technique in C by using C's case label fall-through feature to jump into the unrolled body.
[2] Duff's problem was to copy 16-bit unsigned integers ("shorts" in most C implementations) from an array into a memory-mapped output register, denoted in C by a pointer.
Based on an algorithm used widely by programmers coding in assembly for minimizing the number of tests and branches during a copy, Duff's device appears out of place when implemented in C. The device is valid C by virtue of two attributes in C: This leads to what the Jargon File calls "the most dramatic use yet seen of fall through in C".
[5] C's default fall-through in case statements has long been one of its most controversial features; Duff himself said that "This code forms some sort of argument in that debate, but I'm not sure whether it's for or against.
The problem appears to come down to the ability of the compiler to correctly optimize the device; it may also interfere with pipelining and branch prediction on some architectures.
[6] When numerous instances of Duff's device were removed from the XFree86 Server in version 4.0, there was an improvement in performance and a noticeable reduction in size of the executable.