What is a 'unexpandable_clocks' in PrimeTime's check_timing report?

Hello everyone.

I have a warning in my PrimeTime check_timing report as follows.

Warning: There is 1 clock set for which periods were not expandable.

Failed clocks set Related clocks
----------------------------------------------------
1 CLK0
1 CLK1
1 CLK2
1 CLK3


Actually, I found a description about 'unexpandable_clocks' using 'man check_timing' in my pt_shell promt.
But, I can not understand exact meaning of 'unexpandable_clocks'.

Someone, could you please explain to me about this warning?

Are you already familiar with how PT will "expand" the periods of two clocks with different periods in order to find e.g. the least-common-multiple time where both clocks have a coincident edge?
(e.g. the effective period gets expended in the timing-report in order to find the tightest slack path between the two clocks)

I suspect (even though nothing shows up in SolvNet) that "unexpandable" means that PT could not eventually find an interval where "beating" of the two clocks occurs in a repeating pattern.
The assumption here is that PT gives up after iterating over larger and larger multiples of the period to find a common repeating pattern.

The bottom line is that you are either mis-defining your clock periods, or else you are overlooking e.g. false-path or set_max_delay declarations between totally unrelated clocks.

BTW, coincidentally there is an article that talks about this exact topic which you should read.
It is in the latest Xilinx Xcell Journal # 85, available on Xilinx's website, starting on page 30: "Demystifying Unexpanded Clocks".
While this is talking about Xilinx's Vivado timing analyzer, I feel the content is applicable to PT as well (noting Vivado supports the Synopsys SDC format for timing setups).