How does D deal with cases where the computation might be plaform dependent (like floating point arth. or bit kung-fu+ endianess) ? An more generally, what happens when a computation "can" be done at compile time , but "needs" (for correctness or ressource ) done at runtime ?
D doesn't run code at compile time as an optimisation. It only runs code at compile time in certain constructs like variable declarations or static if statements. If you need your code to run at runtime simply stick it in your code where it needs to run.