When you set the enable_keep_signal variable to true, the tool preserves nets and issues a warning about the preserved nets during compilation. The tool sets an implicit size_only attribute on the logic connected to the nets to be preserved. To mark a net to be preserved, label the net with the keep_signal_name directive in the RTL and set the hdlin_keep_signal_name variable to user or user_driving. Preserving nets might cause QoR degradation.