The advent of massively integrated, multimillion-instance IC designs is driving the demand for ever-faster design convergence at a time when semiconductor companies are facing unrelenting time-to-market pressures that mandate ever-shorter tapeout schedules. Register transfer level (RTL) code for these “gigascale” systems-on-chip is often developed by geographically-dispersed teams and combined with third-party IP and blocks from previous designs that have been augmented to meet the needs of the current design. Only after the RTL and reference macros and libraries have been fully defined and are consistent with each other can designers actually begin the implementation process—comprised of RTL synthesis, design planning and place-and-route (P&R) tasks—that will ultimately determine if their design goals can indeed be met.
By this time, however, decisions about the micro-architecture have already been made and designers are essentially locked-in to their early choices. If the design does not meet its timing, area, power, test and routability requirements, substantial resources will need to be applied to achieve design closure. In extreme cases, even significant changes to the RTL are needed, though typically most of the effort is focused on the synthesis and P&R tasks. The process of convergence, characterized by time-consuming design iterations between synthesis and P&R, can consume a large fraction of total design implementation time.
Faster convergence could be achieved by assessing whether design goals can be met earlier in the design cycle, during the RTL development phase, instead of waiting until the implementation phase to discover and correct critical issues. This early assessment or “exploration” of the RTL and constraints provides designers the opportunity to determine prior to synthesis if a design will likely meet its goals and to perform what-if analyses and make changes as needed to create a better RTL heading into implementation. Fine-tuning the RTL and constraints in this manner uncovers issues earlier in the flow and reduces design iterations later, during implementation, when iterations are more resource-intensive and pose the greatest risk to tapeout schedules.
For RTL exploration to be effective when the RTL, libraries and constraints are still under development, the technology must have tolerance for incomplete and mismatching design data. When missing cells or inconsistencies are encountered in the RTL, it should not only report these discrepancies but also internally resolve them and generate a netlist to enable physical exploration of the current design. With an early netlist, designers could explore a variety of floorplan options prior to synthesis to evaluate how physical constraints impact the timing and routability of their designs. Performing physical design exploration in parallel with RTL development would not only shorten the design cycle but also create an even better starting point for synthesis, since the physical constraints would be taken into consideration in the development of the RTL.
To facilitate efficient RTL exploration, the technology must be easy to deploy and compatible with designers’ existing scripts and flows for synthesis. In addition, it must execute faster than typical synthesis runtimes while producing results similar enough to enable identification of specific timing issues in the RTL or constraints. For example, it should be able to identify critical timing paths even though the worst-case negative slack numbers may not be exactly the same as those produced by synthesis. To ensure a better starting point for synthesis, quality-of-results for RTL exploration should correlate to within about 10 percent of results produced by synthesis, while taking into account timing, area, leakage power, dynamic power and routing congestion. In the same measure, synthesis itself must correlate to within about 5 percent of P&R results. Only when there is tight correlation across the entire flow—first between RTL exploration and synthesis, then between synthesis and placement—can faster development of the RTL, constraints and floorplan lead to fewer iterations between synthesis and P&R and, in the process, faster design convergence.
In conclusion, new technology with the flexibility, ease-of-use, speed and correlation to accommodate the needs of RTL exploration would provide designers the opportunity to evaluate their RTL and to begin physical design exploration earlier, during RTL development. The added visibility into design implementation issues at this early stage would lead to development of a better RTL, constraints and floorplan which, when passed to synthesis that is tightly correlated with placement, lowers the risk of design iterations during implementation. Improving predictability of outcomes across the flow, from exploration to implementation, is essential to achieving faster design convergence in the era of gigascale systems-on-chip.
About the author:
Eyal Odiz is vice president of engineering, RTL synthesis and test automation, Synopsys, Inc. Odiz holds a bachelor of science in civil engineering and a master of science in computer science, both from Technion in Haifa, Israel.