Physical Lint: Physical Quality Metrics For Your RTL

Why Analyze Physical Metrics at RTL?

The quality of the logic structures generated from RTL has a direct impact on the number of design iterations required to close a design. Additionally, the quality of logic structures generated from RTL has a direct impact on design utilization. These trends are illustrated in Figure 1.

Essentially, improving the quality of the logic structures in a design will result in fewer design iterations and increased utilization.

Figure 1

Typically, the analysis of design physical feasibility starts during latter stages of physical synthesis and often carries through several cycles of physical prototyping. Facing reality, until the design is closed through physical design (Place & Route and Static Timing Analysis) the potential for hitting new issues will exist.

Physical design tools optimize according to a set of fixed constraints. When the constraints for area, timing and routability cannot be met by the physical design tools, the design team will face iterations to close the design. Figure 2 illustrates the high-level process for analyzing and resolving design closure issues. The investigation to narrow down and resolve the design issue often remains with the Physical Design team. Design teams will generally acknowledge that these iterations can be reduced if logic changes are made along critical timing paths, but backend engineers generally lack insight into the design functionality, and therefore rely on floorplan changes, physical constraints, or high effort design optimization techniques to resolve the issue. It’s not until all these methods and techniques are exhausted that the attention turns back to the frontend design or RTL. Once back in the hands of the RTL designer, the relevant logic changes are made and the cycle starts again.

Figure 2

Imagine if the frontend designer was given insight into the quality of the RTL, or more explicitly, provided metrics that assesses the physical complexity of the logic structures in the design. The frontend designer, by reviewing the complexity metrics gains insight into the RTL code that will negatively impact downstream design convergence. While most frontend designers have general awareness of these logic structures, they rarely have visibility into their impact on physical design. Armed with this awareness, frontend designers can make changes to the RTL and rerun physical lint checks to get immediate feedback on new complexity metrics and the resulting downstream impact on physical design.

Improving the quality of the RTL, by addressing complex logic structures with Physical Lint, leads to more predictable design convergence and better overall design metrics, namely higher design utilization.