TMC-20601: Registers with High Immediate Fan-Out Tension


Violations of this rule identify registers with high immediate fan-out tension scores. This rule analyzes the final placement to identify registers with sinks that are pulling the register in various directions. The Compiler recommends these registers as candidates for duplication.

  • There are two types of sink: "immediate fan-out" and "timing path endpoint."
  • There are two types of pull: "tension" and "span."

Immediate fan-outs are the immediately connected nodes (lookup tables, other registers, RAM or DSP blocks, and others) of the register. This fan-out is equivalent to fan-outs that the Chip Planner displays, and in various high fan-out reports. Register duplication directly distributes the immediate fan-outs of a register among the duplicates.

Figure 1. Immediate Fan-Out

Tension is the sum over each sink of the distance from the sink, to the centroid of all the sinks. The value of tension is therefore dependent on the number of sinks. Register duplication can help to break up these clouds, since they may be the result of the placement solution getting "warped" by the presence of the register.

Figure 2. High Tension

Registers with high tension among their immediate fan-outs prevent the Compiler from properly localizing connections and can warp the optimization of placement and routing.


Duplicate high fanout-tension driver registers. Specify the DUPLICATE_REGISTER and DUPLICATE_HIERARCHY_DEPTH assignments for automated solutions, or edit the RTL to create duplicate copies. If you edit the RTL, apply the preserve_syn_only attribute to the duplicate registers, and assign the duplicates to individual instances in the fan-out hierarchy.




Place, Finalize

Device Family

  • Intel® Arria® 10
  • Intel® Stratix® 10
  • Intel® Agilex™