Fast way of finding most and least significant bit set in a 64-bit integer

Question

There are a lot of questions about this on StackOverflow. A lot. However I cannot find an answer that:

• Works in C#
• Works for 64-bit integers (as opposed to 32-bit)

Faster than:

private static int Obvious(ulong v)
{
    int r = 0;
    while ((v >>= 1) != 0) 
    {
        r++;
    }
    return r;
}

Or even

int r = (int)(Math.Log(v,2));

I'm assuming a 64-bit Intel CPU here.

One useful reference is the Bit Hacks page and another is fxtbook.pdf However, while these gives useful direction to approach the problem, they do not give a ready answer.

I'm after a re-usable function that can do something similar to _BitScanForward64 and _BitScanReverse64 only for C#.

Answer 1

As per my comment, this is a function in C# to count leading zero bits modified for a 64 bit integer.

public static UInt64 CountLeadingZeros(UInt64 input)
{
    if (input == 0) return 64;

    UInt64 n = 1;

    if ((input >> 32) == 0) { n = n + 32; input = input << 32; }
    if ((input >> 48) == 0) { n = n + 16; input = input << 16; }
    if ((input >> 56) == 0) { n = n + 8; input = input << 8; }
    if ((input >> 60) == 0) { n = n + 4; input = input << 4; }
    if ((input >> 62) == 0) { n = n + 2; input = input << 2; }
    n = n - (input >> 63);

    return n;
}

UPDATE:
If you're using a newer version of C#, check to see if this is built-in per the answer below. https://stackoverflow.com/a/61141435/1587755