Where does voltage optimization actually sit relative to place_opt and clock_opt, and why does order matter here?

Short Answer

The voltage_opt flow runs place_opt and clock_opt first at the original voltage, then defines a scaling library group and a voltage range/target (set_vopt_range, set_vopt_target) before running voltage_opt itself -- routing and postroute optimization then happen at the newly optimized voltage. Running voltage optimization before placement/CTS are stable would mean optimizing against a design that's still going to change, wasting the analysis.

Technical Explanation

• The voltage_opt flow runs place_opt and clock_opt first, at the design's original voltage -- optimization needs a stable placed-and-clocked design to work from.
• Only after that does it define a scaling library group and a voltage range/target (set_vopt_range, set_vopt_target) before running voltage_opt itself.
• Routing and postroute optimization (route_auto, route_opt) then happen at the newly optimized voltage, not the original one.
• Running voltage optimization before placement/CTS are stable would mean optimizing against a design still about to change -- the same reasoning that keeps CTS itself from running before placement.

Follow-up Question & Model Response

"Why does routing specifically happen after voltage_opt rather than before it?"

Candidate Model Response: Because routing at the original (pre-optimization) voltage would produce routes sized and timed for a voltage the design is about to leave -- routing after voltage_opt ensures the physical implementation matches the voltage the design will actually run at.