how much data past the end of the array we could write? say, data_size == 1, we have 15 bytes of padding to the right (because elements have size of 1-byte) and we write 64 bytes at a time. looks like potential segfault on these other 48 bytes, am I missing smth?

Quick answer: data access will not across the end of array.

For Container & data, we need to load it into register. If the data_size == 1, we use mask load (L425-427), to make sure we will not read across boundary, only 1 bytes loaded:

        /// one single mask load for table size <= 64
        __mmask64 last_mask = MASK64 >> (64 - data_size);
        __m512i table1 = _mm512_maskz_loadu_epi8(last_mask, data_pos);

For Container & res_data, we need to store into memory. Let's say if limit == 1, we use mask store (L461-467) to make sure not to write across boundary, here only 1 bytes stored to res_data:

        /// tail handling
        if (limit > limit64)
        {
            __mmask64 tail_mask = MASK64 >> (limit64 + 64 - limit);
            __m512i vidx = _mm512_maskz_loadu_epi8(tail_mask, indexes_pos + pos);
            __m512i out = _mm512_permutexvar_epi8(vidx, table1);
            _mm512_mask_storeu_epi8(res_pos + pos, tail_mask, out);
        }