CTS leaves sinks that are in no cluster degrading QoR

[arthurjolo]

Describe the bug

In designs like sky130hd/aes the CTS clustering leaves sinks alone, in no cluster. This way, these sinks that aren't in any cluster have one buffer less in their path than the sinks that are in a cluster, causing some skew between these sinks.

The image from the final clock tree viewer for sky130hd/aes, produced with OR commit 27ac731:

Expected Behavior

All sinks with the same amount of buffers in the path from the clock pin to it.

Environment

kernel: Linux 6.14.0-37-generic
os: Ubuntu 24.04.2 LTS (Noble Numbat)
cmake version 3.31.9
-- The CXX compiler identification is GNU 13.3.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- OpenROAD version: v2.0-27007-g27ac73190c
-- System name: Linux
-- Compiler: GNU 13.3.0
-- Build type: RELEASE
-- Install prefix: /usr/local
-- C++ Standard: 20
-- C++ Standard Required: ON
-- C++ Extensions: OFF
-- The C compiler identification is GNU 13.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Performing Test C_COMPILER_SUPPORTS__-Wall
-- Performing Test C_COMPILER_SUPPORTS__-Wall - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wall
-- Performing Test CXX_COMPILER_SUPPORTS__-Wall - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-array-bounds
-- Performing Test C_COMPILER_SUPPORTS__-Wno-array-bounds - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-array-bounds
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-array-bounds - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-nonnull
-- Performing Test C_COMPILER_SUPPORTS__-Wno-nonnull - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-nonnull
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-nonnull - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-maybe-uninitialized
-- Performing Test C_COMPILER_SUPPORTS__-Wno-maybe-uninitialized - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-maybe-uninitialized
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-maybe-uninitialized - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format-overflow
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format-overflow - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format-overflow
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format-overflow - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-variable
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-variable - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-variable
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-variable - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-function
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-function - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-function
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-function - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-write-strings
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-- Performing Test C_COMPILER_SUPPORTS__-Wno-sign-compare
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-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-sign-compare
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-sign-compare - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-deprecated
-- Performing Test C_COMPILER_SUPPORTS__-Wno-deprecated - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-deprecated
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-deprecated - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-c++11-narrowing
-- Performing Test C_COMPILER_SUPPORTS__-Wno-c++11-narrowing - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-c++11-narrowing
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-c++11-narrowing - Failed
-- Performing Test C_COMPILER_SUPPORTS__-Wno-register
-- Performing Test C_COMPILER_SUPPORTS__-Wno-register - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-register
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-register - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal
-- Performing Test C_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal - Failed
-- Performing Test C_COMPILER_SUPPORTS__-fpermissive
-- Performing Test C_COMPILER_SUPPORTS__-fpermissive - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-fpermissive
-- Performing Test CXX_COMPILER_SUPPORTS__-fpermissive - Success
-- Performing Test C_COMPILER_SUPPORTS__-x
-- Performing Test C_COMPILER_SUPPORTS__-x - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-x
-- Performing Test CXX_COMPILER_SUPPORTS__-x - Failed
-- Performing Test C_COMPILER_SUPPORTS__c++
-- Performing Test C_COMPILER_SUPPORTS__c++ - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__c++
-- Performing Test CXX_COMPILER_SUPPORTS__c++ - Failed
-- Performing Test C_COMPILER_SUPPORTS__-std=c++17
-- Performing Test C_COMPILER_SUPPORTS__-std=c++17 - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-std=c++17
-- Performing Test CXX_COMPILER_SUPPORTS__-std=c++17 - Success
-- Performing Test C_COMPILER_SUPPORTS__-fno-exceptions
-- Performing Test C_COMPILER_SUPPORTS__-fno-exceptions - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-fno-exceptions
-- Performing Test CXX_COMPILER_SUPPORTS__-fno-exceptions - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-but-set-variable
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-but-set-variable - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-but-set-variable
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-but-set-variable - Success
-- TCL library: /usr/lib/x86_64-linux-gnu/libtcl.so
-- TCL header: /usr/include/tcl/tcl.h
-- TCL readline library: /usr/lib/x86_64-linux-gnu/libtclreadline.so
-- TCL readline header: /usr/include/x86_64-linux-gnu
-- Found SWIG: /usr/local/bin/swig (found suitable version "4.3.0", minimum required is "4.0")
-- Using SWIG >= 4.3.0 -flatstaticmethod flag for python
-- boost: 1.80.0
-- Found GTest: /usr/local/lib/cmake/GTest/GTestConfig.cmake (found version "1.13.0")
-- GTest: 1.13.0
-- Found Python3: /usr/include/python3.12 (found version "3.12.3") found components: Development Development.Module Development.Embed
-- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.3")
-- Found Threads: TRUE
-- spdlog: 1.15.0
-- Found BISON: /usr/bin/bison (found version "3.8.2")
-- Could NOT find Doxygen (missing: DOXYGEN_EXECUTABLE) 
-- STA version: 2.7.0
-- STA git sha: 9c9b5659d6a7ecbe02ea1204aa89079a77db1d3e
-- System name: Linux
-- Compiler: GNU 13.3.0
-- Build type: RELEASE
-- Build CXX_FLAGS: -O3 -DNDEBUG
-- Install prefix: /usr/local
-- Found FLEX: /usr/bin/flex (found version "2.6.4")
-- TCL library: /usr/lib/x86_64-linux-gnu/libtcl.so
-- TCL header: /usr/include/tcl/tcl.h
-- TCL readline library: /usr/lib/x86_64-linux-gnu/libtclreadline.so
-- TCL readline header: /usr/include/x86_64-linux-gnu/tclreadline.h
-- CUDD library: /usr/local/lib/libcudd.a
-- CUDD header: /usr/local/include/cudd.h
-- SSTA: 0
-- Found SWIG: /usr/local/bin/swig (found suitable version "4.3.0", minimum required is "3.0")
-- STA library: /tmp/tmp.5y91Cd1bKv/libOpenSTA.a
-- STA executable: /tmp/tmp.5y91Cd1bKv/sta
-- Found re2: /opt/or-tools/lib/cmake/re2/re2Config.cmake (found version "11.0.0")
-- Found Clp: /opt/or-tools/lib/cmake/Clp/ClpConfig.cmake (found version "1.17.7")
-- Found Cbc: /opt/or-tools/lib/cmake/Cbc/CbcConfig.cmake (found version "2.10.7")
-- Found SCIP: /opt/or-tools/lib/cmake/scip/scip-config.cmake (found version "9.0.0")
-- Found OpenMP_CXX: -fopenmp (found version "4.5")
-- Found OpenMP: TRUE (found version "4.5")
-- Found OR-Tools: /opt/or-tools/lib/cmake/ortools (version: 9.11.4210)
-- TCL library: /usr/lib/x86_64-linux-gnu/libtcl.so
-- TCL header: /usr/include/tcl/tcl.h
-- Found OpenMP_C: -fopenmp (found version "4.5")
-- Found OpenMP: TRUE (found version "4.5")
-- Found OpenMP: TRUE (found version "4.5")
-- GUI is enabled
-- Could NOT find VTune (missing: VTune_LIBRARIES VTune_INCLUDE_DIRS) 
-- TCL library: /usr/lib/x86_64-linux-gnu/libtcl.so
-- TCL header: /usr/include/tcl/tcl.h
-- Found Eigen3: /usr/local/share/eigen3/cmake/Eigen3Config.cmake (found version "3.4.1")
-- TCL readline enabled
-- Tcl Extended disabled
-- Python3 enabled
-- Configuring done (4.2s)
-- Generating done (0.7s)
-- Build files have been written to: /tmp/tmp.5y91Cd1bKv

To Reproduce

Run with OR commit 27ac731: reproducible zip

Relevant log output

Screenshots

No response

Additional Context

No response