merge_64
#define merge_64(r1, r2) ((((r1) & 0xffffffffUL) << 32) + ((r2) & 0xffffffffUL))
#define merge_64(r1, r2) ((((r2) & 0xffffffffUL) << 32) + ((r1) & 0xffffffffUL))
#define merge_64(low, high) (((u64)high << 32) | low)
#define merge_64(high, low) (((u64)high << 32) | low)