Just some rubbish to prove the duff's loops work for n>0, when you get the do{}while condition right. I didn't in a blog post, so I investigated. And that's what I found. About to delete the file from my hard drive, but I wanted to paste the code somewhere for safe keeping.

duff-loop.cpp

#include <iostream>
#include <cstdlib>

int main(int args, const char* argv[]) {
  register size_t n,
           i=0,
           j=0;

  if (args == 2) {
    n = atoi(argv[1]);
  } else {
    n = 11;
  }

  switch(n %10) {
    case 0: do {
              ++j;
    case 9:   ++j;
    case 8:   ++j;
    case 7:   ++j;
    case 6:   ++j;
    case 5:   ++j;
    case 4:   ++j;
    case 3:   ++j;
    case 2:   ++j;
    case 1:   ++j;
            } while (i++ < n/10);
  }

  std::cout << "n: " << n << std::endl
    << "j: " << j << std::endl;

  return 0;
}