The last mile timing closure stage takes the best implementation run from either of the previous two stages and attempts to close timing. Although QoR gains in this phase can be small compared to compile time, this step is still important for meeting timing. The stage only runs if the design meets the last mile timing closure entry requirements.
The last mile directive continues from an existing routed checkpoint and focuses on fixing failing paths. Timing closure is achieved in about 20 percent of designs with a WNS below –0.100 ns.
The goal of this stage is to close timing, which differs from the default tool flow that focuses on achieving the best WNS possible along with timing-closed WHS. The algorithms in this stage aim to improve timing without significantly altering the place and route results.
To achieve this, the flow applies the last mile incremental directive and QoR suggestions:
- Suggestions with the APPLIED property are reused from the reference run.
- Suggestions with the INCREMENTAL_FRIENDLY property are applied.
After routing completes, phys_opt_design can run to
further improve timing.
To enter the last mile timing closure stage from stage 1 or 2, the design must have the following:
- A fully routed run
- A WNS greater than –0.250
- An RQA score of 3 or greater