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Duff’s Device Walkthrough

Jul 10, 2006

Duff’s device is the classic example of loop unrolling. Given a case where one might output a string bytes to a port, it would be ideal to be able to reduce the number of loop jumps by unrolling the loop to contain n number of outputs to said port where n is the length of the byte string. This of course would be impossible in a dynamic system where n is not a constant. A typical way to meet in the middle between the optimal (but impossible) and heavy (containing a loop n/8 times followed by a post-processing n%8 cleanup loop). To make this process more efficient, Tom Duff invented the following code snippet (lines numbered for easy reference)

0: int n = (len + 8 - 1) / 8;
1: switch (len % 8) {
2:      case 0: do { HAL_IO_PORT = *pSource++;
3:      case 7: HAL_IO_PORT = *pSource++;
4:      case 6: HAL_IO_PORT = *pSource++;
5:      case 5: HAL_IO_PORT = *pSource++;
6:      case 4: HAL_IO_PORT = *pSource++;
7:      case 3: HAL_IO_PORT = *pSource++;
8:      case 2: HAL_IO_PORT = *pSource++;
9:      case 1: HAL_IO_PORT = *pSource++;
A:      } while (--n > 0);
B:  }

Duff’s Device unrolls the loop 8 times (case 0-7), however it is different in that it inlines the post processing loop and makes it instead a preprocessing loop. Walking through an example string “Makes Sense To Me!” where n == 18 we start on line 0 and get an n of 3. It is easy to see that the first time through the loop the switch jumps to line 8 and preprocesses the first two bytes (’M’ and ‘a’) and then decrementing n (now 2). It then reaches the while at line A and jumps back to the to line 2 where its corresponding do statement resides. The remainder is trivial in that the do..while executes for the rest of the string “kes Sense To Me!” whose length is 16, which of course corresponds nicely with the remaining n of 2. There is a bug, in Duff’s Device however, but I will leave it to the reader to figure it out (hint: what happens to the string “”?).

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