Network flow based buffer planning

The problem of planning the locations of large number of buffers is of utmost importance in deep submicron VLSI design. Recently, [Cong, Kong, and Pan ICCAD-99, 1999, p. 358] proposed an algorithm to directly address this problem. Given a placement of circuit blocks, a key step in [1] is to use the free space between the circuit blocks for inserting as many buffers as possible. This step is very important because if all buffers can be inserted into existing spaces, no expansion of chip area would be needed. An effective greedy heuristic was used in [1] for this step. In this paper, we give a polynomial-time optimal algorithm for solving the problem of inserting maximum number of buffers into the free space between the circuit blocks. In the case where the "costs" of placing a buffer at different locations are different, we can guarantee to insert maximum number of buffers with minimum total cost. Our algorithm is based on efficient min-cost network-flow computations.