Answer by rlbond for What is the fastest/most efficient way to find the highest set bit (msb) in an integer in C?

unsigned intmsb32(register unsigned int x){        x |= (x >> 1);        x |= (x >> 2);        x |= (x >> 4);        x |= (x >> 8);        x |= (x >> 16);        return(x & ~(x >> 1));}

1 register, 13 instructions. Believe it or not, this is usually faster than the BSR instruction mentioned above, which operates in linear time. This is logarithmic time.

From http://aggregate.org/MAGIC/#Most%20Significant%201%20Bit