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

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. This rule identifies registers with high Immediate Fan-Out Tension scores.
  • 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 is equivalent to fan-outs that the Chip Planner displays, and in various high fanout 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. 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.

Recommendation

Duplicate high fanout-tension driver registers. Refer to 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 fanout hierarchy.

Severity

Low

Stage

Place, Finalize

Device Family

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