How does ICC2 know which cells an ECO touched?

Short Answer

Each changed cell carries the eco_change_status (ICC2) attribute, and its value says what kind of edit touched it. place_eco_cells -eco_changed_cells (ICC2) works on cells whose status is create_cell, change_link, add_buffer, size_cell or add_buffer_on_route. After legalization the status becomes eco_legalized, so the same cells are not picked up again.

Technical Explanation

• Values that mark a changed cell: create_cell, change_link, add_buffer, size_cell and add_buffer_on_route. The ICC2 guide lists the first four, and the command reference for place_eco_cells (ICC2) adds add_buffer_on_route.
• Who sets it: eco_netlist (ICC2) sets it on the cells it changes, and the command reference says Tcl editing commands set it too. If the netlist was changed some other way, set it yourself with set_attribute (ICC2).
• After placement: place_eco_cells (ICC2), with or without -legalize_only, resets the status of changed cells to eco_legalized. With -no_legalize it becomes eco_placed, and cells that were not changed keep their status.
• Other commands read it too. report_cell_feasible_space (ICC2) ignores ECO cells unless they are eco_legalized, and place_freeze_silicon -cells (ICC2) maps only cells with is_fs_eco_add true and one of the changed values.
• Why it matters: -eco_changed_cells lets you place only what the ECO touched, leaving the rest of the placement alone. If a changed cell is missing its status, it is never placed; if an old cell still shows a changed value, it is moved again.
• This page is about how ICC2 tracks the change. Comparing an ECO netlist with the golden netlist, and deciding which differences become edits, is covered on the eco-netlist page. The two meet here: every edit that eco_netlist (ICC2) writes leaves a status value behind, and that value is what later placement steps select on.

What To Check

• Count of cells with each value before place_eco_cells (ICC2).
• That the count matches the number of edits in the change file.
• All changed cells at eco_legalized after placement.
• No unplaced cells left once placement finishes.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): The change file has 57 edits and 57 cells carry changed values; after placement all 57 are eco_legalized.
• Suspicious (illustrative): More cells marked changed than the change file explains, often left over from an earlier round.
• Hard stop: Changed cells with no status, which -eco_changed_cells will skip, leaving them unplaced.

What The Interviewer Is Testing

• Knows the values that count as changed.
• Knows how legalization resets the status.
• Checks the attribute when the netlist was changed outside eco_netlist (ICC2).

Follow-up Question & Model Response

"What happens if you run place_eco_cells -no_legalize and then run it again with -legalize_only?"

Candidate Model Response: The first run places the changed cells and sets their status to eco_placed. The command reference for place_eco_cells (ICC2) says -eco_changed_cells, with or without -legalize_only, selects only cells whose status is create_cell, add_buffer, change_link, size_cell or add_buffer_on_route, so eco_placed cells are not selected. The second step must be -legalize_only -cells with an explicit collection of the cells you placed. Afterwards, check that every one of them ends at eco_legalized.