Bitboard 64-bit machine: should I use int16 or int64 for a 4x4 board?

I'm programming a 4x4 board game in C++ using bitboards in a 64-bit machine. I only need 16 bits to implement the board. Should I use:

• uint16_t - to reduce used space?
• uint64_t - if operations are(?) faster with 64-bit integers, should I use them and mask the value with 0xFFFF (bitwise AND) when necessary?
• uint_fast16_t - I just discovered this integer type, but I'm not sure how it works and if I would need a mask too?

I don't know if it helps, but my processor is: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz 1.99 GHz

• If you are looking to save space, use uint16_t.
• If you are looking to save time, use uint_fast16_t.
• uint64_t may be useful, too, for making an array of values aligned at 8-byte boundaries. This is unlikely to give you much benefits, though, because it comes at the price of wasting 75% of memory allocated for the array, along with the associated loss in cache performance.

Note: you may end up using the same type as uint64_t if your library maps uint_fast16_t to uint64_t.