Function popcount 

pub fn popcount(
    df: &mut DramDataflow,
    planes: &[usize],
    out_width: usize,
    zero: usize,
) -> Vec<usize>

Per-lane POPCOUNT of planes: for each lane, the number of set bits across the planes, as an out_width-plane per-lane integer (LSB first). A reduction realized ENTIRELY in the DRAM plane by folding each plane into a bit-serial counter. zero is a zero row (the initial counter value).