Hi,

I wonder how could I find closest lower power of two to a given unsigned integer. The best way would be to get the bit position of most significant bit set. But how would I find it it with as few bitwise operations as possible? I must avoid any cycle, plus, I do not want to use the look up table as I find it uninteligent.

Have a look at this page for all sorts of bit twiddling stuff

http://graphics.stanford.edu/~seander/bithacks.html

You want to look at the section for this bit:

• Finding integer log base 2 of an integer (aka the position of the highest bit set)
  • Find the log base 2 of an integer with the MSB N set in O(N) operations (the obvious way)
  • Find the integer log base 2 of an integer with an 64-bit IEEE float
  • Find the log base 2 of an integer with a lookup table
  • Find the log base 2 of an N-bit integer in O(lg(N)) operations
  • Find the log base 2 of an N-bit integer in O(lg(N)) operations with multiply and lookup

unsigned round_down_to_power_of_2(unsigned x) {
  return 0x80000000u >> __builtin_clz(x);
}