[all-commits] [llvm/llvm-project] 505e04: [CMake] Enable CMP0179 alongside CMP0156 for dedup...

[Raul Tambre via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 505e049aa078c8961f00cacefc3983398a46fb04
      https://github.com/llvm/llvm-project/commit/505e049aa078c8961f00cacefc3983398a46fb04
  Author: Raul Tambre <raul at tambre.ee>
  Date:   2024-11-21 (Thu, 21 Nov 2024)

  Changed paths:
    M cmake/Modules/CMakePolicy.cmake

  Log Message:
  -----------
  [CMake] Enable CMP0179 alongside CMP0156 for deduplication on LLD (#116497)

LLD has a bug regarding ordering of static link libraries in the ELF backend, which has been reported as #116669.
CMake 3.31.0 started properly deduplicating static libraries for LLD causing the following linking failure for `libclang-cpp.so` with `-DLLVM_LINK_LLVM_DYLIB=ON`:
```
ld.lld: error: undefined symbol: llvm::omp::getOpenMPClauseName(llvm::omp::Clause)
>>> referenced by OpenMPKinds.cpp
>>>               tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/OpenMPKinds.cpp.o:(clang::getOpenMPSimpleClauseTypeName(llvm::omp::Clause, unsigned int))
>>> referenced by SemaOpenMP.cpp
>>>               tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaOpenMP.cpp.o:(clang::SemaOpenMP::CheckOMPRequiresDecl(clang::SourceLocation, llvm::ArrayRef<clang::OMPClause*>))
>>> referenced by SemaOpenMP.cpp
>>>               tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaOpenMP.cpp.o:(clang::SemaOpenMP::CheckOMPRequiresDecl(clang::SourceLocation, llvm::ArrayRef<clang::OMPClause*>))
>>> referenced 166 more times

[tons more]
```

CMake 3.31 also introduced CMP0179, which builds on CMP0156 and makes the deduplication consistent across platforms.
By coincidence this works around the above LLD deficiency and is the fix that CMake 3.31.1 will implement.
However, the fix is to ignore CMP0156 unless CMP0179 is also enabled, i.e. no more deduplication.
So enable CMP0179 to keep the benefits of deduplication from CMP0156 on LLD and fix the build for CMake 3.31.0.

See: #116669
See: https://gitlab.kitware.com/cmake/cmake/-/issues/26447
Fixes: cb90d5b



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