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

For a 4 bit group n, the value can be computed in 3 instructions using a 32bit unsigned value as a lookup table with 16 entries of two bits each:

(0xffffaa50 >> (n << 1)) & 3

This can be used to trim the end of the divide and conquer methods.

size_t blog32(uint32_t v) {    size_t r = 0, t;    t = (0 != (v >> 16)) << 4; v >>= t; r |= t;    t = (0 != (v >>  8)) << 3; v >>= t; r |= t;    t = (0 != (v >>  4)) << 2; v >>= t; r |= t;    return r + ((0xffffaa50 >> (v << 1)) & 3);}

For 64bit, add one more level:

size_t blog64(uint64_t v) {    size_t r = 0, t;    t = (0 != (v >> 32)) << 5; v >>= t; r |= t;    t = (0 != (v >> 16)) << 4; v >>= t; r |= t;    t = (0 != (v >>  8)) << 3; v >>= t; r |= t;    t = (0 != (v >>  4)) << 2; v >>= t; r |= t;    return r + ((0xffffaa50 >> (v << 1)) & 3);}