def AllGatherv_unknown_shape(data, comm, rank, n_threads, mpi_dtype, axis=1):
    """
    Use MPI to compute shape of result array by asking every thread for their chunk's shape,
    then perform the AllGatherv operation (concatenating along the given axis) and return the result.
    """
    if axis != 1:
        data = np.swapaxes(data, axis, 1)

    # Require row-major order for concatenation along axis 0
    data = np.require(data, requirements='C')

    dim_1_len = data.shape[1]

    # Gather size of each chunk
    sizes = comm.allgather(data.size)
    offsets = np.zeros(len(sizes), dtype=np.int)
    offsets[1:] = np.cumsum(sizes)[:-1]

    comm.Barrier()

    # Create big array
    tot_size = sum(sizes)
    full_shape = tot_size/dim_1_len, dim_1_len
    all_data = np.zeros(shape=full_shape, dtype=data.dtype, order='C')

    # Perform AllGatherv
    comm.Allgatherv(data, [all_data, sizes, offsets, mpi_dtype])

    if axis != 1:
        all_data = np.swapaxes(all_data, axis, 1)

    return all_data