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 fanout 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


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




Place, Finalize

Device Family

  • Intel® Stratix® 10
  • Intel® Agilex™