On whitespace and stability in physical synthesis

Capo: Congestion-Driven Placement for Standard-cell and RTL Netlists with Incremental Capability

“…The existing large modules in the bin are usually kept out of this clustering. To further simplify floorplanning, soft blocks consisting of standard cells are artificially downsized, as in [4]. The clustered netlist is then passed to the fixed-outline floorplanner Parquet [2], which sizes soft blocks and optimizes block orientations.…”

“…Instead, a simple analytical placement technique, such as Successive Over-Relaxation (SOR), is used to determine reasonable locations for the large macros. 4 It has been shown that analytical techniques are good at finding general areas where objects should be placed [6], so this is a reasonable and efficient solution for placing a large macro or macros in this situation. As such, this technique may also be useful in regions with large amounts of whitespace as block-packing often overlooks good solutions in such situations.…”

“…In these situations, we allow ECO-system to shift the cut-line to legalize the derived partition with respect to area. Cut-line shifting is a technique commonly used in the top-down min-cut placement for allocation of whitespace [4,27,28,35,38]. The cut-line is shifted as little as possible to make the derived partitioning legal with respect to area.…”

“…However more recent work [4] introduces tunable whitespace distribution for improved wirelength, while preserving a minimum amount of local whitespace in most regions to ensure routability. Whitespace allocation and D R A F T Fig.…”

“…This variety of algorithms facilitates partitioning with small tolerance, allowing Capo to distribute the available whitespace uniformly [15] so as to facilitate routing. This provides a convenient baseline for further wirelength improvement [4] by non-uniform distribution (this configuration is now used by default).…”