AVX determine number of written values

Question

I have a 512 bit wide vector register (16 values) and a mask to store them to memory using _mm512_mask_i32scatter_epi32(). To determine how many values are written to memory I count the leading zeroes of the mask using __builtin_clz(). If the mask is not (!) empty, everything works fine. But when the mask is empty something strange happens:

std::cout << "mask = " << mask << " clz(mask) " << __builtin_clz(mask) << "\n";

mask = 0 clz(mask) 31

I have two questions:

1. Does anyone know, why clz is 31 and not 32?
2. Is there a better way to determine the number of written values?

Sincerely