What does clock_opt actually do, stage by stage, in ICC2?

Short Answer

clock_opt runs CTS end to end through named stages you can target with -from/-to: build_clock builds the tree structure, route_clock routes the built tree, and final_opto is a post-build optimization pass. clock_opt alone runs the full flow; clock_opt -from build_clock -to route_clock builds and routes without the final optimization pass; clock_opt -from route_clock -to route_clock only routes clocks that are already built; clock_opt -from final_opto runs only post-build optimization on an already-built, already-routed tree.

Technical Explanation

• clock_opt has named internal stages, same discipline as place_opt: build_clock (build tree structure), route_clock (route the built tree), final_opto (post-build optimization).
• clock_opt alone runs the whole flow, build_clock through final_opto.
• clock_opt -from build_clock -to route_clock builds and routes but stops before optimization -- useful to inspect the raw tree.
• clock_opt -from route_clock -to route_clock re-routes an already-built tree without rebuilding structure.
• clock_opt -from final_opto runs only post-build optimization on an already-built, already-routed tree.
• synthesize_clock_trees -propagate_only is distinct from clock_opt -- it just marks clocks as propagated for STA purposes without actually running CTS.
• Postroute clock optimization under CCD does NOT touch clock latency/skew -- it only fixes logical DRC violations at that point in the flow.

Healthy, Suspicious & Hard-stop Results

• Expected: each stage's log matches its documented job -- build_clock builds structure, route_clock routes, final_opto optimizes without rebuilding.
• Investigate: skew or latency numbers change after a postroute clock_opt -from final_opto run that you expected to only fix DRC -- confirm whether CCD was active and whether this is actually the DRC-only postroute behavior, not a latency/skew optimization you were expecting.
• Stop: a tree structure looks different after a -from route_clock -to route_clock run that should only have re-routed, not rebuilt -- that's not what that stage is documented to do.

What The Interviewer Is Testing

Whether you know clock_opt has the same kind of internal stage structure as place_opt, and specifically whether you know the postroute final_opto DRC-only limitation, which is a common source of confusion.