How do nested or rectilinear voltage areas work, and which shape wins where they overlap?

Short Answer

Where voltage-area shapes overlap, ICC2 uses stacking order: by default the shape defined last sits on top and owns the overlap. To nest one voltage area inside another, you define the outer one first and the inner one second, so the inner shape is on top and the outer area's effective shape becomes a ring. Define them the other way round and the outer shape masks the inner one, which ends up with no placement area at all.

Technical Explanation

• Stacking order: by default the order of definition, with the last shape on top.
• Overlap between voltage areas: the top shape owns the overlap; the tool changes the interpretation but deletes no shapes.
• Effective shapes: the effective_shapes attribute read with get_attribute (ICC2) shows what each voltage area really owns after resolution.
• Merging within one voltage area: -merge_regions on create_voltage_area or create_voltage_area_shape (ICC2) merges abutting or overlapping shapes into disjoint ones.
• create_voltage_area_shape (ICC2) adds one shape per call to an existing voltage area.
• You can reorder shapes with set_voltage_area_shape -top (ICC2) and friends instead of recreating them.
• A masked inner area has no placeable region, so its cells fail placement or end up outside the intended domain.

# [ICC2]  icc2_shell
create_voltage_area -power_domains PD1 -region {{0 0} {30 30}}
create_voltage_area -power_domains PD2 -region {{10 10} {20 20}}
get_attribute -objects [get_voltage_areas PD2] -name effective_shapes
get_attribute -objects [get_voltage_areas PD1] -name effective_shapes
create_voltage_area_shape -voltage_area PD1 -region {{30 0} {40 10}} -merge_regions
report_voltage_areas -verbose

What To Check

• Every nested voltage area was created after its parent, or raised to the top afterwards.
• effective_shapes of the inner voltage area is not empty and matches its intended region.
• The outer voltage area's effective shape is a ring around the inner one, not a solid rectangle.
• After merging, each voltage area has the minimum set of disjoint shapes you expect.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): PD2 effective_shapes returns {10 10} {20 20}, and PD1 returns the ring outline around it.
• Suspicious (illustrative): Both effective_shapes are correct, but neither voltage area has a guard band, so PD1 and PD2 cells can abut at the ring edge; add one before power planning.
• Hard stop: PD2 effective_shapes is empty: the outer shape masks it, and every PD2 cell has nowhere legal to go.

What The Interviewer Is Testing

• Knows that stacking order, not size or containment, decides who owns an overlap.
• Checks effective_shapes rather than the shapes you typed.
• Can fix a wrong order in place with set_voltage_area_shape (ICC2).

Follow-up Question & Model Response

"What is the difference between merging shapes and resolving overlaps?"

Candidate Model Response: Merging works inside one voltage area: -merge_regions (ICC2) replaces abutting or overlapping shapes with a minimum set of disjoint shapes, and the merged shape takes the top shape's place. Resolving works between different voltage areas: the top shape owns the overlap, but no shape is removed; only the interpretation changes. So merging changes the shape list, while resolution changes only the effective shapes. Guard bands follow the same rules, and after merging the top shape's guard band applies.