sub primes {
    my $n = shift;
    return if $n < 2;

    my @primes = (2);

I:  for my $i (3 .. $n) {
        next unless 0x208a28aa & (1 << $i % 30);

        my $sqrt = int sqrt $i;
        for my $p (@primes) {
            next I unless $i % $p;
            last if $p > $sqrt;
        }
        push @primes, $i;
    }
    return @primes
}