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

Question

If I have some integer n, and I want to know the position of the most significant bit (that is, if the least significant bit is on the right, I want to know the position of

Answer 1

The code:

    // x>=1;
    unsigned func(unsigned x) {
    double d = x ;
    int p= (*reinterpret_cast(&d) >> 52) - 1023;
    printf( "The left-most non zero bit of %d is bit %d\n", x, p);
    }

Or get the integer part of FPU instruction FYL2X (Y*Log2 X) by setting Y=1