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

Description

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.

Parameters

Name Description Type Default Value Min Value Max Value
tension Reports a violation for drivers that have a tension score of at least the value specified in this parameter integer 100000    
span Reports a violation for drivers that have a maximum 1-dimensional span of at least the value specified in this parameter integer 250    
ignore_high_fanout_tension Filters out registers with high immediate fan-out tension bool 0    
ignore_high_fanout_span Filters out registers with high immediate fan-out span bool 0    
minimum_sinks Filters out registers with fewer than the specified number of sinks integer 2   0

Recommendation

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.

Severity

Low

Tags

Tag Description
register-duplication Design rule checks related to duplication of registers in the design, either manually or automatically.
register-spread Design rule checks related to measuring the spread of a register's sinks, as found in the "Report Register Spread" command.
place Design rule checks which pertain to the Compiler's Place stage.

Device Family

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