Figure (1) Placement flow [1]
Before the start of placement optimization all Wire Load Models (WLM) are removed. Placement uses RC values from Virtual Route (VR) to calculate timing. VR is the shortest Manhattan distance between two pins. VR RCs are more accurate than WLM RCs.
Placement is performed in four optimization phases:
1. Ire-placement optimization
2. In placement optimization
3. Post Placement Optimization (PPO) before clock tree synthesis (CTS)
4. PPO after CTS.
Pre-placement Optimization optimizes the netlist before placement, HFNs are collapsed. It can also downsize the cells.
In-placement optimization re-optimizes the logic based on VR. This can perform cell sizing, cell moving, cell bypassing, net splitting, gate duplication, buffer insertion, area recovery. Optimization performs iteration of setup fixing, incremental timing and congestion driven placement.
Post placement optimization before CTS performs netlist optimization with ideal clocks. It can fix setup, hold, max trans/cap violations. It can do placement optimization based on global routing. It re does HFN synthesis.
Post placement optimization after CTS optimizes timing with propagated clock. It tries to preserve clock skew.
Reference
[1] Astro User Guide, Version X-2005.09, September 2005
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