Primality test function doesn't work on large numbers?

[Elronnd via Digitalmars-d-learn]

Thank you!  Would you mind telling me what you changed aside from 
pow() and powm()?  diff isn't giving me readable results, since 
there was some other stuff I trimmed out of the original file.  
Also, while this is a *lot* better, I still get some lag 
generating 1024-bit primes and I can't generate larger primes in 
a reasonable amount of time.  Maybe my genbigint() function is to 
blame?  It isn't efficient:

bigint genbigint(int numbits) {
     bigint tmp;
     while (numbits --> 0) {
         tmp <<= 1;
         tmp += uniform(0, 2);
     }
     return tmp;
}