Thank you very much. I changed line 119 to an explicit cast to int and removed an unneeded cast at line 63. It now happily compiles with 64bit mode : http://dpaste.dzfl.pl/63666f07. It's kind off odd though that compiling with -inline seems to slow it a bit down.
I'm unsure if searching for the field with the least possibilities was a smart move because now I have to carry that "taken" array through all my functions and optimize has to check the whole sudoku instead of a slice. (come to think of it, taken should've been named occupied) Still, I'm really pleased with the results. I should write a prettyPrint method that prints sudoku's in a prettier format and returns the solution instead of the shorts containing the solutions hidden in bitfields :)