Find First Set instruction (ffs) for boost multiprecision uint512_t

Question

I'm developing an algorithm that uses __builtin_ffsll() with uint64_t type.

I want to switch to 512-bit field using boost multiprecision library (I'm running on a machine with avx512 support).

Is there a similar function as the mentioned builtin? Alternatively, how can I efficiently implement such functionality for 512-bit integers?

Answer 1

From the documentation:

unsigned lsb(const number-or-expression-template-type& x);

Returns the (zero-based) index of the least significant bit that is set to 1.

Throws a std::range_error if the argument is <= 0.

ffs() is one-based, so adding 1 to lsb()'s return value will make it equivalent. Edit: And as pointed out, taking the case of being passed 0 into account.

Maybe something like

unsigned ffs512(const boost::multiprecision::uint512_t &n) {
  if (n.is_zero()) {
    return 0;
  } else {
    return boost::multiprecision::lsb(n) + 1;
  }
}