How is RTL different from the netlist used for place and route?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
RTL expresses intended register-transfer behavior; a mapped netlist expresses an implementation using particular cells. Place and route needs concrete cell models to place instances and evaluate their timing.
Technical Explanation
An RTL statement such as adding two registers tells synthesis what operation is required. It does not uniquely determine which adder structure, drive strengths, or cell variants will implement that operation. Synthesis elaborates the design, optimizes logic under constraints, and maps it to an available technology library. The resulting netlist exposes the actual instances and connectivity presented to physical implementation. This distinction explains why a netlist alone is insufficient. A reference to NAND2_X1 has no usable size or delay until the corresponding library views are available. It also explains why simulation success at RTL is not an input-readiness test: functional intent can be correct while the mapped design has missing references, incompatible macros, or incomplete timing constraints. At handoff, compare the RTL/synthesis revision recorded by the team with the netlist revision. Identify whether scan insertion and low-power transformations have already happened. Modern integrated flows can combine stages, but in the ICC2 import flow described by these manuals the design is read as structural Verilog. Do not substitute behavioral RTL for that expected input.
Common Mistake
Claiming that physical design only moves cells and never changes the mapped circuit. Optimization can make implementation changes that require equivalence validation.
Follow-up Question & Model Response
Which input would you inspect for a wrong drive-strength cell name? Inspect the mapped netlist and the synthesis library manifest together.
Practical Example
Two libraries can map the same RTL addition into different cell networks. A netlist built against library A may contain cells absent from library B, so swapping the library directory is not a safe technology migration.
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