What is the fastest way to calculate the number of bits needed to store a number

Question

I\'m trying to optimize some bit packing and unpacking routines. In order to do the packing I need to calculate the number of bits needed to store integer values. Here is th

Answer 1

Do a binary search instead of a linear search.

if ((n >> 16) != 0)
{
    r += 16;
    n >>= 16;
}

if ((n >> 8) != 0)
{
    r += 8;
    n >>= 8;        
}

if ((n >> 4) != 0)
{
    r += 4;
    n >>= 4;        
}

// etc.

If your hardware has bit-scan-reverse, an even faster approach would be to write your routine in assembly language. To keep your code portable, you could do

#ifdef ARCHITECTURE_WITH_BSR
   asm // ...
#else
   // Use the approach shown above
#endif