This paper presents an efficient, scalable and optimal slack-driven shaping algorithm for soft blocks in non-slicing floorplan. The proposed algorithm is called <i>SDS</i>. Different from all previous approaches, <i>SDS</i> is specifically formulated for fixed-outline floorplanning. Given a fixed upper bound on the layout width, <i>SDS</i> minimizes the layout height by only shaping the soft blocks in the design. Iteratively, <i>SDS</i> shapes some soft blocks to minimize the layout height, with the guarantee that the layout width would not exceed the given upper bound. Rather than using some simple heuristic as in previous work, the amount of change on each block is determined by systematically distributing the global total amount of available slack to individual block. During the whole shaping process, the layout height is monotonically reducing, and eventually converges to an optimal solution. We also propose two optimality conditions to check the optimality of a shaping solution. To validate the efficiency and effectiveness of <i>SDS</i>, comprehensive experiments are conducted on MCNC and HB benchmarks. Compared with previous work, <i>SDS</i> is able to achieve the best experimental result with significantly faster runtime.