There are three directives that control how the incremental flow behaves. Incremental directives are set using the command:
read_checkpoint -directive <directiveName> <reference>.dcpRuntimeOptimized
The RuntimeOptimized directive tries to reuse as much placement and routing information from the reference run as possible. The timing target will be the same as the reference run. If the reference run has WNS -0.050, then the incremental run will not try to close timing on this design and instead also target -0.050. This impacts setup time only. This is the default behavior when no directive is specified.
TimingClosure
The TimingClosure directive will reuse placement and routing from the reference but it will rip up paths that do not meet timing and try to close them. Some run time intensive algorithms are run to get as much timing improvement as possible but as the placement is largely given up front gains are limited. This technique can be effective on designs with a reference WNS > -0.250 ns.
report_qor_suggestions
to generate automated design enhancements.Quick
Quick is a special mode that does not call the timer during place and route and instead uses the placement of related logic as a guide. It is the fastest mode but not applicable for most designs. Designs will need WNS > 1.000 ns to be effective. These are typically ASIC emulation or prototyping designs.