Duff’s device is a loop unrolling technique, specific to C and C++, to improve the performance of loops. The technique involves dividing the number of iterations in a loop by a certain factor (often 8) and using a switch statement to handle the remaining iterations. This can lead to improved performance by reducing the overhead associated with the loop control, and by allowing the compiler to better optimize the code inside the loop.

The following is an example of using Duff’s device to unroll a loop that copies a block of memory.

void duff_memcpy(char *dest, char *src, size_t count) {
    int n = (count + 7) / 8;
    switch (count % 8) {
        case 0: do {    *dest++ = *src++;
        case 7:         *dest++ = *src++;
        case 6:         *dest++ = *src++;
        case 5:         *dest++ = *src++;
        case 4:         *dest++ = *src++;
        case 3:         *dest++ = *src++;
        case 2:         *dest++ = *src++;
        case 1:         *dest++ = *src++;
        } while (--n > 0);
    }
}

In this example, the loop is unrolled by a factor of 8. The switch statement takes care of the remainder when the count of bytes to be copied is not divisible by 8. The result is that the loop runs faster, as it reduces the number of times the loop control needs to be executed. However, this technique should be used with caution as it can make the code more complex and harder to read and maintain.

A Simple Use Case

An example of how you might use the duff_memcpy() function in a program is shown below.

#include <stdio.h>
#include <string.h>

int main() {
    char src[] = "Hello, World!";
    char dest[sizeof src];
    duff_memcpy(dest, src, sizeof src);
    printf("%s\n", dest);
    return 0;
}

A “real-world” use case is when we have a large data set that needs to be copied, and performance is critical. By using Duff’s device the developer can improve the performance of the data copy operation.

Please note that in C++, the standard library already provides a highly optimized version of memcpy, which is memcpy_s in VS and memcpy in GCC/Clang. So, using duff device is not recommended in C++ if you are not doing it for educational purpose or for specific case where you have a lot of data and performance is very critical.