What does logic restructuring actually do during global placement, and what's cloning specifically for?

Short Answer

Physical synthesis tools combine several primary logic functions into few standard cells, decompose functional gates into equivalent primary gates, and/or duplicate combinational logic (cloning) -- all focused on reconstructing critical paths that are missing timing. Cloning specifically means duplicating a piece of combinational logic so different downstream paths can each get their own copy, rather than sharing one instance whose fanout is spreading the load (and the delay cost) across all of them.

Technical Explanation

• Logic restructuring during placement combines several primary logic functions into fewer standard cells, decomposes functional gates into equivalent primary gates, and/or duplicates combinational logic (cloning).
• All of these techniques focus specifically on reconstructing critical paths that are missing timing -- restructuring isn't a general cleanup pass, it's targeted at timing recovery.
• Cloning specifically duplicates a piece of combinational logic so different downstream paths each get their own copy, instead of sharing one instance whose fanout spreads both load and delay cost across every path that uses it.
• This is a physical-synthesis-stage technique, meaning it happens with real placement information available -- unlike a purely logical restructuring done at synthesis time with no physical context.
• set_app_options -name opt.common.advanced_logic_restructuring_mode -value timing is the real, documented option that directly implements the cloning/restructuring behavior described.

Follow-up Question & Model Response

"Why would cloning specifically help a critical path that a shared logic instance was hurting?"

Candidate Model Response: Because a shared instance's total fanout capacitance and any additional loading from other paths using it both add real delay to every path through it -- cloning gives the critical path its own dedicated copy of that logic, removing the shared loading and any interference from the other paths that used to share it.