__kernel void Sum(__global float * A, __global float *output, ulong AOffset, __local float * target ) {
    const size_t globalId = get_global_id(0);
    const size_t localId = get_local_id(0);
    target[localId] = A[globalId+AOffset];

    barrier(CLK_LOCAL_MEM_FENCE);
    size_t blockSize = get_local_size(0);
    size_t halfBlockSize = blockSize / 2;
    while (halfBlockSize>0) {
        if (localId<halfBlockSize) {
            target[localId] += target[localId + halfBlockSize];
            if ((halfBlockSize*2)<blockSize) { // uneven block division
                if (localId==0) { // when localID==0
                    target[localId] += target[localId + (blockSize-1)];
                }
            }
        }
        barrier(CLK_LOCAL_MEM_FENCE);
        blockSize = halfBlockSize;
        halfBlockSize = blockSize / 2;
    }
    if (localId==0) {
        output[get_group_id(0)] = target[0];
    }
}