CTS has trouble with lack of blockage awareness

[maliberty]

CTS isn't aware of blockages when constructing the tree or adding post-opt buffers. This can lead to poor results with excessive skew. A test case is provided in https://github.com/The-OpenROAD-Project/OpenROAD/pull/2130

The Post-GRT skew is:

Clock clk
Latency      CRPR       Skew
inst_8_4/clk ^
   1.95
inst_10_4/clk ^
   1.69      0.00       0.26

Immediately after CTS this pair looks like this with the last common segment in yellow and the clkopt buffers as yellow dots. The cts buffer and the clkopt buffers are placed on the macros:

Detailed placement will push them off the macros (left & right in this example):

The global route guides have to go through the buffers leading to a circuitous route:

without instances to better see the scenic routes:

[maliberty]

I believe the same issue will show up in the resizer as well due to its reliance on Steiner trees that are blockage unaware.