When incremental mode is selected by the user, the tool might still not run the full incremental flow if it determines the design change to too much. Incremental implementation is typically run if cell reuse is above 80%.
The flow selected will be either:
- A full place and route using incremental optimized algorithms.
Placement and routing are reused as much as is possible. Target WNS is
determined by a combination of both the reference checkpoint and the directive.
Directives are selected based on the directive supplied to the
read_checkpoint -incremental -directive <directive>switch.
- A full place and route using the default algorithms. Placement
and routing are not reused. Target WNS is always 0.000. Directives are taken
from the directive switch supplied to either the
place_design -directive <directive>or
route_design -directive <directive>commands.
read_checkpoint -incrementalcommand. As this assessment is made after changes to the design are made to improve matching so it is not the same as running purely a default flow as the changes are persistent.
When incremental mode is entered the following message can be seen:
INFO: [Place 46-42] Incremental Compile tool flow is being used. Default place and route algorithms are bypassed.