The Last Mile Timing Closure stage takes the best implementation run result from either of the previous two stages and attempts to close timing on it. The QoR gain in this phase might be small compared to compile time. The design must have met the Last Mile Timing Closure requirements for the stage to be run.
The Last Mile directive continues on from an existed routed checkpoint and attempts to work on paths that are failing. Timing closure is achieved in approximately 20% of designs with a WNS < -0.100 ns.
The goal of the Last Mile Timing Closure stage is to close timing on the design.
This is slightly different when compared to the default tool flow, which aims to achieve
the best WNS possible and timing closed WHS. Algorithms must strike a balance between
trying to improve timing but not altering the place and route results significantly. To
achieve this, the Last Mile incremental directive and QoR suggestions are used to close
timing. Suggestions with the APPLIED property are reused from the reference run and
suggestions that have the INCREMENTAL_FRIENDLY property set are applied. After routing
is complete, phys_opt_design can be run to further
attempt to close 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 > -0.250
- An RQA score of 3 or greater