Number of 1 Bits - Brian Kernighan's Trick

The expression n & (n - 1) clears the lowest set bit.

Example: $n = 12$ (binary 1100). $n - 1 = 11$ (binary 1011). $n \& (n-1) = 1000$ (binary 1000).

Count how many times you can apply this operation until $n$ becomes $0$. Each operation removes exactly one set bit.