Why doesn't java use long[] in BigInteger's mag field?

Question

I believe in 64-bit JVM long[] is much more efficient than int[], and it can significantly speed up RSA operations.

Carry is a problem in long[], but we can use some native methods to force long as unsigned, eg long z = u64add(x, y, cr) , here cr is a boolean[] , can replace long z = x + y .

Answer 1

BigInteger supports multiply() , not just add() . Since multiply() is the more expensive operation, its speed is also what matters most for algorithms.

Multiplying two 64-bit integers gives a 128-bit result. There is no 128-bit integer type in Java, so getting the correct result isn't just "do the operation and shift some bits." You would have to split the two numbers into 4 32-bit pieces, multiply those, and then compose the result. Or add a JVM intrinsic so that a native 128-bit operation could be used. It's not that it can't be done, it's more of a cost-benefit thing.