TMC-20603: Registers with High Immediate Fan-Out Span

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 Span 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 fan-out reports. Register Duplication directly distributes the immediate fan-outs of a register among the duplicates.

Figure 1. Immediate Fan-Out

Span is the maximum one dimensional delta between the left-bottom-most sink, and the right-top-most sink. The Span value is therefore independent of the number of sinks, and it is good at detecting registers that drive a long distance in opposite directions. Register duplication can allow duplicates to travel in each direction to more efficiently disperse the signal.

Figure 2. Span

Recommendation

Duplicate high fanout-span 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® Agilex™
  • Intel® Stratix® 10
  • Intel® Arria® 10