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 i

Answer 1

The fastest way to get most significant bit at IL code should be a float conversion and accessing the exponent bits.

Save code:

int myint = 7;
int msb = (BitConverter.SingleToInt32Bits(myint) >> 23) - 0x7f;

An even faster way would be the msb and lsb cpu instructions. As mentioned by phuclv it got availibele at .Net Core 3.0 so I affffded a test that is unfortunately not much faster.

As requested here are the BenchmarkDotNet results for 10000 coverts of uint and ulong. The speedup was factor 2 so the BitScanner solution is fast, but can't beat native float conversion.

           Method |     Mean |    Error |   StdDev | Ratio
BitScannerForward | 34.37 us | 0.420 us | 0.372 us |  1.00
BitConverterULong | 18.59 us | 0.238 us | 0.223 us |  0.54
 BitConverterUInt | 18.58 us | 0.129 us | 0.121 us |  0.54
     NtdllMsbCall | 31.34 us | 0.204 us | 0.170 us |  0.91       
 LeadingZeroCount | 15.85 us | 0.169 us | 0.150 us |  0.48