Function _mm256_castsi128_si256

Casts vector of type __m128i to type __m256i; the upper 128 bits of the result are indeterminate.

In the Intel documentation, the upper bits are declared to be "undefined". This is not equivalent to mem::MaybeUninit; instead, these bits are non-deterministically set to some valid value. In practice, this is typically equivalent to mem::zeroed.

Intel's documentation