OpenLane: Timing Signoff: SS corner seems to be the worst-case corner for hold violations

Description

We have noticed during the timing validation of the LiteX management core that min timing checks has an unexpected trend where sky130_fd_sc_hd__ss_100C_1v60.lib seems to be showing the worst case slack. Based on my checks of the openlane flow, the following libs are used for multi-corner checks:

sky130_fd_sc_hd__ss_100C_1v60.lib
sky130_fd_sc_hd__ff_n40C_1v95.lib
sky130_fd_sc_hd__tt_025C_1v80.lib

We have compiled the timing reports in the following table:

Similarly, as a sanity check, we have ran the AES design from openlane’s design suite regression tests. We noticed the same behavior:

Environment

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Git Log (Last 3 Commits)
commit 96a630e29c60f47d87c63642e2af39c19a48cdb7
Author: Vitor Bandeira <vvbandeira@users.noreply.github.com>
Date:   Tue Dec 21 21:37:30 2021 -0300
Improve Jenkins pipeline (#782
 - clean before checkout
 - fix stage names
 - add back designs
 Signed-off-by: Vitor Bandeira <vitor.vbandeira@gmail.com
 [skip ci]
 commit 1ab8d55fab363b9e10b08111a9ee14954f65e3ef
 Author: Donn <me@donn.website>
 Date:   Wed Dec 22 02:36:50 2021 +0200
 Fix or Remove Designs Failing The Extended Test Set (#777
 + Added ability to run extended test sets on a specific pull request's PR by adding `[ci ets]` anywhere in the PR body (the example there counted!)
 + Added capacity to disable certain designs in a test set by prefixing the design with a `#` 
 + Added new script that allows someone to iterate on a design's timing closure 
 ~ Decreased core utilization for some designs that just aren't routing
 ~ Fixed sizes of {BM64, blabla, y_huff} to avoid pin location issue 
 ~ Increased max buffer percentages for some designs failing to achieve timing closure 
 ~ Update all references to the design directory in config.tcl for all designs to `$::env(DESIGN_DIR)`.
 ~ core area/die area no longer have an EOL at the end of their value 
 commit 86a5e29c4f9126920174dc2f2e43a1348c743a83 
 Author: Donn <me@donn.website> 
 Date:   Tue Dec 21 20:52:09 2021 +0200
 Update Documentation for `or_issue.py` (#780
 [skip ci] 

Reproduction Material

The design is available in github here (with tar balls and all): https://github.com/efabless/caravel_mgmt_soc_litex/releases

Expected behavior

I expect the min timing checks reports to fail for the ff_n40C_1v95 corner or show worst violating slack compared to the ss_100C_1v60 corner.

@mkkassem FYI.