Improve timing and placement tuning in flow scripts (specifically in Nangate45)

[Waleed99i]

This pull request introduces improvements to the OpenROAD flow for the gcd_nangate45 design. The changes focus on enhancing timing behavior, placement flexibility, and overall flow robustness through parameter tuning and iterative refinement.

What Changes Were Made ?

The following key modifications were made:

• Improved timing optimization by enabling more accurate parasitic consideration in the flow
• Adjusted timing repair strategy to allow better buffering and utilization control
• Introduced changes in placement density to improve design space for routing and timing
• Iteratively refined the flow based on observed results to better handle timing constraints

Why These Changes Were Made?

The motivation behind these changes was to:

• Improve timing closure and reduce violations
• Provide better alignment with realistic routing and parasitic conditions
• Increase flexibility in placement to support routing and optimization
• Demonstrate practical understanding of OpenROAD flow tuning and trade-offs

These modifications reflect an exploration of how parameter tuning impacts timing, placement, and routing in a real ASIC flow.

Results & Observations

• The initial run showed timing violations and suboptimal slack values
• After applying the changes, improvements were observed in timing behavior and design feasibility
• Routing and placement quality improved due to better utilization and spacing control
• The flow demonstrates more stable and realistic optimization behavior

References / Supporting Material

• Initial Results (Terminal Output)
• Implementation Explanation Blog
• Updated Results After Improvements (Terminal Output)
• Further Improvements & Analysis Blog

Additional Notes
This work is part of my preparation for GSoC 2026 with OpenROAD. It demonstrates hands-on experience with:

• OpenROAD flow execution
• Timing-driven design optimization
• Practical flow tuning and analysis
• Understanding of real-world ASIC design trade-offs

[gemini-code-assist]

Code Review

This pull request introduces several tuning parameters to the OpenROAD flow for the gcd_nangate45 design, aiming to improve timing and placement. The changes include enabling global-routing-based parasitics for more accurate timing repair, and adjusting the repair_timing command to allow for more aggressive buffering and utilization control. While these changes are reasonable for exploring design closure, I've noted one redundancy in the configuration script that should be addressed for better maintainability.

[gemini-code-assist]

This line appears to be redundant. The variable global_place_density is already set to 0.3 in test/Nangate45/Nangate45.vars, which is sourced on line 4 of this file. To avoid redundancy and improve maintainability, you can remove this line.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"