The Most Significant Bit (LSB) is the bit position of the last bit set in a value.

There are many ways to determine the MSB, but the goal is to do that as quickly as possible.

• Due to the number of Chess positions possible, many millions, even a small saving for the lookup of the MSB can provide much further lookups in the same time.

int msbPerformanceJunkie32(u32 x)
{
  static const unsigned int bval[] =
    { 0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4 };
  unsigned int r = 0;
  if (x & 0xFFFF0000) 
  {
    r += 16 / 1;
    x >>= 16 / 1;
  }
  if (x & 0x0000FF00) 
  {
    r += 16 / 2;
    x >>= 16 / 2;
  }
  if (x & 0x000000F0) 
  {
    r += 16 / 4;
    x >>= 16 / 4;
  }
  return r + bval[x];
}

u32 msbDeBruijn32(u32 v) {

static const int MultiplyDeBruijnBitPosition[32] =
{
  0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30,
  8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31
};

v |= v >> 1; // first round down to one less than a power of 2
v |= v >> 2;
v |= v >> 4;
v |= v >> 8;
v |= v >> 16;

return MultiplyDeBruijnBitPosition[( u32 )( v * 0x07C4ACDDU ) >> 27];

} </code>

#include <intrin.h>
 
unsigned char _BitScanForward(
   unsigned long * Index,
   unsigned long Mask
);
unsigned char _BitScanForward64(
   unsigned long * Index,
   unsigned __int64 Mask
);

https://www.chessprogramming.org/De_Bruijn_Sequence_Generator.

https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64?redirectedfrom=MSDN&view=msvc-160