Why do you need this branch? It case if blockSize is too small, tail should be called without loop iteration.

I would like to explain why there is a branch about j==0.

When j + 4 > blockSize, it usually means we meet a tail then we use a tail mask and let j = blockSize - 4 (compute the last 4 elements and store with mask).

Example:

• Assume blockSize=5 and output is 5*1, ignore the output channel
• The mask will be [0, 0, 0, 1]
• The first loop iteration will compute the first 4 elements in the output matrix, then the output should be [√, √, √, √, TBD]. Now, j = 4.
• The second also the last loop iteration will compute the last element with the help of the mask. Actually, we compute the last 4 elements but only store the last one because of the mask. In detail, at the beginning of this iteration, j=4 and we find that j + 4 > blockSize, so we let j = blockSize - 4, now, j=1 then we compute the last 4 elements and only store the last one.

However, there is another situation here when the blockSize is smaller than 4. In this case, we should not use mask but use vl. I will explain with an example.

Example:

• Assume blockSize=1 and output is 1*1, also ignore the output channel.
• The mask will be also [0, 0, 0, 1], which can NOT use.
• Instead, we let vl=1 and load-compute-store directly with the help of vl.