Path-based analysis (PBA) for critical paths

[robtaylor]

Summary

Re-analyze critical paths with path-specific slew propagation to recover pessimism from graph-based analysis (GBA).

Current State

GEM's STA (compute_timing()) uses graph-based analysis — worst-case delay at each pin regardless of which path is being analyzed. This is pessimistic because a pin's worst-case delay may come from a different path than the one being analyzed.

Proposed Approach

• After GBA identifies top N critical paths, re-analyze each with path-specific input slew values
• Use Liberty NLDM (Non-Linear Delay Model) tables to compute delay as f(input_slew, output_load)
• Typically recovers 5-10% margin on critical paths
• Only needed for paths close to violation threshold

Impact

Medium — reduces false-positive timing violations and gives more realistic slack values.

Effort

Medium-High — requires NLDM table lookup and slew propagation along specific paths.