use of .fp.pg file

Thanks for this question.

I did some research on this issue and it was tricky to create a situation where a .fp.pg file is created.  But I was able to eventually force one and I think it happens when you have 

Global net connections are typically defined and stored as follows...

The first time you start a session you specify what the power and ground net names are on the Design Import form which in turn is saved in the .conf file:

set rda_Input(ui_pwrnet) {VDD1 VDD2}
set rda_Input(ui_gndnet) {VSS}

Once the design is loaded, you establish global net connections via the globalNetConnect TCL command (or the GUI) which is then saved in the .fp file:

GlobalNetConnection: VDD1 -pin VDD -inst * -type pgpin -all
GlobalNetConnection: VSS -pin VSS -inst * -type pgpin -all

Now, let's say you read in a DEF file that establishes connections that are different than what is in the .fp file.  For example one of the instances in the design has a VDD pin that's connected to VDD2 instead of VDD1:

SPECIALNETS 3 ;
- VSS  ( * VSS )
  + USE GROUND
 ;
- VDD2  ( i_c/i0 VDD )
  + USE POWER
 ;
- VDD1  ( i_b/i0 VDD )
  + USE POWER
 ;
END SPECIALNETS

The .fp file is now inconsistent with the DEF file.  When you do a "saveDesign -def" the tool saves both the .fp file and the .def file.  It uses this .fp.pg file to resolve inconsistencies between these two files.

I hope that make sense and will help you troubleshoot the situation that gave rise to this question.

Thanks,
Bob