How does Duff's device work?

Question

I've read the article on Wikipedia on the Duff's device, and I don't get it. I am really interested, but I've read the explanation there a couple of times and I still don't get it how the Duff's device works.

What would a more detailed explanation be?

Answer 1

There are some good explanations elsewhere, but let me give it a try. (This is a lot easier on a whiteboard!) Here's the Wikipedia example with some notations.

Let's say you're copying 20 bytes. The flow control of the program for the first pass is:

int count;                        // Set to 20
{
    int n = (count + 7) / 8;      // n is now 3.  (The "while" is going
                                  //              to be run three times.)

    switch (count % 8) {          // The remainder is 4 (20 modulo 8) so
                                  // jump to the case 4

    case 0:                       // [skipped]
             do {                 // [skipped]
                 *to = *from++;   // [skipped]
    case 7:      *to = *from++;   // [skipped]
    case 6:      *to = *from++;   // [skipped]
    case 5:      *to = *from++;   // [skipped]
    case 4:      *to = *from++;   // Start here.  Copy 1 byte  (total 1)
    case 3:      *to = *from++;   // Copy 1 byte (total 2)
    case 2:      *to = *from++;   // Copy 1 byte (total 3)
    case 1:      *to = *from++;   // Copy 1 byte (total 4)
           } while (--n > 0);     // N = 3 Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //        greater than 0 (and it is)
}

Now, start the second pass, we run just the indicated code:

int count;                        //
{
    int n = (count + 7) / 8;      //
                                  //

    switch (count % 8) {          //
                                  //

    case 0:                       //
             do {                 // The while jumps to here.
                 *to = *from++;   // Copy 1 byte (total 5)
    case 7:      *to = *from++;   // Copy 1 byte (total 6)
    case 6:      *to = *from++;   // Copy 1 byte (total 7)
    case 5:      *to = *from++;   // Copy 1 byte (total 8)
    case 4:      *to = *from++;   // Copy 1 byte (total 9)
    case 3:      *to = *from++;   // Copy 1 byte (total 10)
    case 2:      *to = *from++;   // Copy 1 byte (total 11)
    case 1:      *to = *from++;   // Copy 1 byte (total 12)
           } while (--n > 0);     // N = 2 Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //       greater than 0 (and it is)
}

Now, start the third pass:

int count;                        //
{
    int n = (count + 7) / 8;      //
                                  //

    switch (count % 8) {          //
                                  //

    case 0:                       //
             do {                 // The while jumps to here.
                 *to = *from++;   // Copy 1 byte (total 13)
    case 7:      *to = *from++;   // Copy 1 byte (total 14)
    case 6:      *to = *from++;   // Copy 1 byte (total 15)
    case 5:      *to = *from++;   // Copy 1 byte (total 16)
    case 4:      *to = *from++;   // Copy 1 byte (total 17)
    case 3:      *to = *from++;   // Copy 1 byte (total 18)
    case 2:      *to = *from++;   // Copy 1 byte (total 19)
    case 1:      *to = *from++;   // Copy 1 byte (total 20)
           } while (--n > 0);     // N = 1  Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //       greater than 0 (and it's not, so bail)
}                                 // continue here...

20 bytes are now copied.

Note: The original Duff's Device (shown above) copied to an I/O device at the to address. Thus, it wasn't necessary to increment the pointer *to. When copying between two memory buffers you'd need to use *to++.

Answer 2

The explanation in Dr. Dobb's Journal is the best that I found on the topic.

This being my AHA moment:

for (i = 0; i < len; ++i) {
    HAL_IO_PORT = *pSource++;
}

becomes:

int n = len / 8;
for (i = 0; i < n; ++i) {
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
}

n = len % 8;
for (i = 0; i < n; ++i) {
    HAL_IO_PORT = *pSource++;
}

becomes:

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

Answer 3

There are two key things to Duff's device. First, which I suspect is the easier part to understand, the loop is unrolled. This trades larger code size for more speed by avoiding some of the overhead involved in checking whether the loop is finished and jumping back to the top of the loop. The CPU can run faster when it's executing straight-line code instead of jumping.

The second aspect is the switch statement. It allows the code to jump into the middle of the loop the first time through. The surprising part to most people is that such a thing is allowed. Well, it's allowed. Execution starts at the calculated case label, and then it falls through to each successive assignment statement, just like any other switch statement. After the last case label, execution reaches the bottom of the loop, at which point it jumps back to the top. The top of the loop is inside the switch statement, so the switch is not re-evaluated anymore.

The original loop is unwound eight times, so the number of iterations is divided by eight. If the number of bytes to be copied isn't a multiple of eight, then there are some bytes left over. Most algorithms that copy blocks of bytes at a time will handle the remainder bytes at the end, but Duff's device handles them at the beginning. The function calculates count % 8 for the switch statement to figure what the remainder will be, jumps to the case label for that many bytes, and copies them. Then the loop continues to copy groups of eight bytes.

Answer 4

The point of duffs device is to reduce the number of comparisons done in a tight memcpy implementation.

Suppose you want to copy 'count' bytes from b to a, the straight forward approach is to do the following:

  do {                      
      *a = *b++;            
  } while (--count > 0);

How many times do you need to compare count to see if it's a above 0? 'count' times.

Now, the duff device uses a nasty unintentional side effect of a switch case which allows you to reduce the number of comparisons needed to count / 8.

Now suppose you want to copy 20 bytes using duffs device, how many comparisons would you need? Only 3, since you copy eight bytes at a time except the last first one where you copy just 4.

UPDATED: You don't have to do 8 comparisons/case-in-switch statements, but it's reasonable a trade-off between function size and speed.