Find last set bit in a 64-bit word
int fls64(__u64 x);
x
the word to search
This is defined in a similar way as the libc and compiler builtin ffsll, but returns the position of the most significant set bit.
fls64(value) returns 0 if value is 0 or the position of the last set bit if value is nonzero. The last (most significant) bit is at position 64.