| Issue |
167085
|
| Summary |
`inline` variables, `static inline` member variables, and `static` member variables in a class template shouldn't be defined in the LLVM headers
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
kikairoya
|
The C++ standard guarantees `inline` variables are merged to a single instance, but on Windows, they can't be merged across DLL boundaries unless annotated with `__declspec(dllimport)` explicitly.
We do declare them without `__declspec(dllimport)`, except MSVC with `-DLLVM_BUILD_LLVM_DYLIB_VIS=ON`.
This will cause a practical problem when building a plugin or builds with `-DLLVM_LINK_LLVM_DYLIB=ON` or `-DBUILD_SHARED_LIBS=ON`.
For MSVC, the dylib builds have no problem because they require `-DLLVM_BUILD_LLVM_DYLIB_VIS=ON`.
However, plugins are supported with static builds by `-DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON`, so `inline`s will cause a symbol dup.
For MinGW and Cygwin, we currently rely on auto-export (and `-Wl,--export-all-symbols`) to export symbols, so `__declspec(dllimport)` is never attached. The `__declspec(dllexport)`/`__declspec(dllimport)` pair can be used once [the ABI annotation project](https://github.com/llvm/llvm-project/issues/109483) is completed, but not yet.
Anyway, the problem remains for builds with `-DBUILD_SHARED_LIBS` or plugins to static builds with `-DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON`.
Non-inline static member variables in a class template follow the same discussion; they can be defined in a header and will be merged by the linker, but can't cross over the DLL boundaries.
Actually, we have such a definition that cause a problem:
https://github.com/llvm/llvm-project/blob/77a194cb09e23810e449c4bc6e6be8355d2daf47/llvm/include/llvm/Support/Registry.h#L61-L62
it causes `CTTestTidyModule` test to fail due to symbol dup, though this wasn't noticed as the test is skipped in the current CI configurations (static build without plugin support).
Therefore, we should avoid defining `inline` variables, `static inline` member variables, and `static` member variables in a class template in the LLVM headers.
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