Function _mm256_mask_lzcnt_epi64 

pub fn _mm256_mask_lzcnt_epi64(src: __m256i, k: __mmask8, a: __m256i) -> __m256i

Available on x86-64 and target feature avx512cd and target feature avx512vl only.

Counts the number of leading zero bits in each packed 64-bit integer in a, and store the results in dst using writemask k (elements are copied from src when the corresponding mask bit is not set).

Intel’s documentation