There were many questions on why output isolation is preferred over input isolation logic, sorry could not get time to respond to all the queries related to this. Here is my view point on this

Output signal isolation method is usually a preferred choice than the input isolation method as former leads to fewer isolation cells and gives better control over how the enable signal can be  propagated to the isolation logic. 

Input isolation will be a better choice if there are very few independent domains. In such situation isolation enable signal management is very straight-forward.  Here number of power domain is not the main component, its the sequence in which they are powered down and up. In this case isolation cell is only required when the power island is active. During the power down period, the power island is no longer functional and no power is supplied to the island. Therefore, neither the floating inputs nor the input transient state matters to the power island, which is overall good from total power perspective.

For this method, probably regular standard cells (NAND and NOR) can be used as isolation cells rather than requiring a custom cell.

Sorry guys, got tied up with many projects and could not blog for almost 4 weeks.

I know we spend so much time in writing power intent of a design and validating whether its correct or not. In that process on a recent project, I did some analysis on how some of the intent generation can be pseudo automated.

Used the MV static checker, MVRC to auto generate some policies based on the xover analysis and it helped quite a bit and was amazed at how fast I was able to generate these constraints from MVRC.

For example, on one of the design, finding out what need to be isolated and excluded from isolation was not a trivial task due to multi-fanout nature of the ports/pins. Looked at the xover analysis within MVRC and used this feature to auto-generate some of the isolation policies. It was less than 30 lines of TCL code within MVRC, which made my life easier in generating some part of power intent.

just an example on what I did within MVRC

set f1 [open ${source_island}_${dest_island}.xover w]
set xs [get_crossovers -source $source_island -dest $dest_island]
foreach x $xs {
    set src_port  [get_crossover_info -object $x -boundary_source]
    set src_port [regsub -all {\{} $src_port {}]
    set src_port [regsub -all {\}} $src_port {}]
    lappend src_ports_list $src_port }}
}

if {$src_ports_list!=""} {puts $f1 "set_isolation ${source_island}_${dest_island}_ISO -domain ${source_island} -isolation_power_net $domain_vdd_net -isolation_ground_net VSS -clamp_value 0 -elements \"$src_ports_list\""}

This may not be complete, but idea is very similar to one given above.

We can debate on whether should a sign-off MV tool be used for this or not?

Happy Holidays.