22.214.171.124. Fitter Seed
Changes in the design impact performance between compilations. This random variation is inherent in placement and routing algorithms—it is impossible to try all seeds and get the absolute best result.
If a change in optimization settings marginally affects the register-to-register timing or number of failing paths, you cannot always be certain that your change caused the improvement or degradation, or whether it is due to random effects in the Fitter. If your design is still changing, running a seed sweep (compiling your design with multiple seeds) determines whether the average result improved after an optimization change, and whether a setting that increases compilation time has benefits worth the increased time, such as with physical synthesis settings. The sweep also shows the amount of random variation to expect for your design.
If your design is finalized you can compile your design with different seeds to obtain one optimal result. However, if you subsequently make any changes to your design, you might need to perform seed sweep again.
Click Assignments > Compiler Settings to control the initial placement with the seed. You can use the DSE II to perform a seed sweep easily.
To specify a Fitter seed use the following Tcl command :
set_global_assignment -name SEED <value>