4.217.1 _FFS
_FFS(x)
This macro finds the first (least significant) bit set in the input value.
This macro is very similar to the function ffs() except that it evaluates its argument at compile-time, so it should only be applied to compile-time constant expressions where it will reduce to a constant itself. Application of this macro to expressions that are not constant at compile-time is not recommended, and might result in a huge amount of code generated.
Remember:
The _FFS() macro returns the position of the first (least significant) bit set in the word val, or 0 if no bits are set. The least significant bit is position 1. Only 16 bits of argument are evaluted.