What does magnet_placement actually require of the cells you name with -cells, and why do two seemingly reasonable -cells lists pull nothing at all?

Short Answer

magnet_placement declares a fixed object (a fixed macro cell, a pin of a fixed macro cell, or an I/O port) a magnet so connected standard cells get placed close to it -- best performed BEFORE standard cell placement, to improve congestion in a complex floorplan or improve timing. The key rule: cells are pulled ONLY if they form a CONTIGUOUS data path from the magnet. magnet_placement C0 -cells {C3 C4 C5} pulls NOTHING if C3/C4/C5 aren't contiguous with C0; magnet_placement C0 -cells {C3 C6 C8} also pulls nothing if that specific set isn't a contiguous data path, even though C3, C6, and C8 might each individually be reachable from C0 through some path.

Technical Explanation

• magnet_placement declares a fixed object -- a fixed macro cell, a pin of a fixed macro cell, or an I/O port -- a "magnet," pulling connected standard cells to be placed close to it.
• It is best performed BEFORE standard cell placement -- it's meant to shape where placement starts, not correct it after the fact.
• The purpose: improve congestion for a complex floorplan, or improve timing, by keeping a data path physically compact around a fixed anchor point.
• THE KEY RULE: cells named in -cells are pulled toward the magnet ONLY IF THEY FORM A CONTIGUOUS DATA PATH from the magnet -- not merely if each one is somehow reachable from the magnet through some path.
• magnet_placement C0 -cells {C3 C4 C5} pulls NOTHING -- C3/C4/C5 are not contiguous with C0 as a set.
• magnet_placement C0 -cells {C3 C6 C8} also pulls NOTHING -- despite naming real downstream cells, that specific combination is not a contiguous data path from C0.
• By contrast, magnet_placement C2 -cells {C6 C7 C8} DOES pull C6, C7, C8 toward C2 -- because that set IS a contiguous chain from C2.
• -logical_levels limits how many logic levels from the magnet get considered; -stop_by_sequential_cells/-stop_on_sequential_cells stop the tracing at sequential-cell boundaries; -only_report_magnet_cells lets you preview which cells would actually move before committing to the operation.

What To Check

• Warning sign: a magnet_placement call with -cells set to a specific list completes without error, but none of the named cells end up placed any closer to the magnet.
• Inspect: verify the named -cells set is actually a contiguous data path from the magnet, not just a set of cells that are each individually somewhere downstream of it.
• Correct: use -only_report_magnet_cells first to preview which cells the call would actually move, before committing to the real placement pull.

Healthy, Suspicious & Hard-stop Results

• Expected: a magnet_placement call's -cells list is a verified contiguous data path from the magnet, and -only_report_magnet_cells confirms the expected cells before the real run.
• Investigate: a magnet_placement call with a specific -cells list completes with no error but produces no visible placement change -- check contiguity of that exact cell set before assuming a tool bug.
• Stop: magnet_placement calls are being run repeatedly with different -cells lists by trial and error, without ever using -only_report_magnet_cells to check contiguity first.

What The Interviewer Is Testing

Whether you know the specific contiguity rule (not just that magnet_placement exists) and can predict, from a concrete -cells list, whether a call will actually move anything or silently do nothing.