extern "C" {
    __global__ void twice(int* array, int* prefixPtr) {
        int i = 0;
        int prefix = *prefixPtr;
C_twice:
        if (i < prefix) {
            array[i+blockIdx.x*prefix] = array[i+blockIdx.x*prefix]*2;
            i++;
            goto C_twice;
        }
    }
}