Estimated Wire Length
Placement algorithms are dependent on wire length estimation calculations. Constructive placement methods often use total estimated wire length for all nets as a measurement criterion. Subsequent iterative optimization algorithms may add “weighting factors” to timing-critical and high-switching-activity nets as part of the wire length minimization objective.
Describe the various net topology alternatives commonly used for wire length estimation (e.g., Steiner tree, star, bounding box). Describe the advantages and disadvantages of the different topologies in terms of computation time and accuracy trade-offs.
Constructive Placement and Physical Synthesis
The physical synthesis flow provides an initial placed netlist, and serial/parallel repowering cells are added during synthesis. Placement tools incorporate both constructive and iterative optimization steps and signal repowering features. As a result, a flow option could be provided to disregard the placement assignments from physical synthesis altogether and apply the constructive placement step on the complete block netlist.
Describe the trade-offs in exercising a constructive placement step on the full block netlist. Describe the sample experiments and quality-of-results criteria that could assist with this trade-off decision.