<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/163367>163367</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Windows] Plugin registry symbols not exported/linked with CLANG_LINK_CLANG_DYLIB, breaking external plugins
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          zond
      </td>
    </tr>
</table>

<pre>
    **Summary**

On Windows, when building LLVM with `CLANG_LINK_CLANG_DYLIB=ON`, external plugins cannot register themselves through `llvm::Registry<clang::PluginASTAction>` due to two issues:
1. Static data members (`Head`, `Tail`) are filtered out during symbol export
2. Static methods (`add_node()`, `begin()`) are not linked into the executable

This breaks the plugin registration mechanism that relies on static initialization.

**Affected Versions**

- LLVM 19, 20, 21, and main branch (verified 2025-01-14)
- Specifically affects Windows builds with `CLANG_LINK_CLANG_DYLIB=ON`

**Problem Description**

**Issue 1: Registry symbols filtered from DLL exports**

The script `llvm/utils/extract_symbols.py` generates `.def` files for Windows DLL symbol exports. At line 126, it filters symbols with this regex:

```python
elif re.search(r"(llvm|clang)@@[A-Z][A-Z0-9_]*[A-JQ].+(X|.+@|.*Z)$", symbol):
```

This pattern matches MSVC-mangled function symbols but excludes static data members, which have `@@0` or `@@3` suffixes. The `Registry<T>` static members `Head` and `Tail` are therefore filtered out:

```
?Head@?$Registry@VPluginASTAction@clang@@@llvm@@0PEAVnode@12@EA
?Tail@?$Registry@VPluginASTAction@clang@@@llvm@@0PEAPEAVnode@12@EA
```

Without these symbols exported, plugins cannot access the registry's linked list.

**Issue 2: Registry methods not linked into executable**

Even if the symbols are exported from the DLL, the `Registry<T>` methods `add_node()` and `begin()` exist in libraries but aren't linked into `clang.exe` by default because nothing in the main executable references them. When plugins try to call these functions, they fail because the symbols aren't present in the executable's import address table.

**Reproduction**

1. Build LLVM with `-DCLANG_LINK_CLANG_DYLIB=ON -DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON` on Windows
2. Create an external plugin that registers via `Registry<PluginASTAction>`
3. Try to load the plugin with `clang.exe -Xclang -load -Xclang plugin.dll`

Result: Plugin fails to register, either due to missing symbols or unresolved externals.

**Expected Behavior**

Plugins should be able to register themselves using the `Registry<T>` pattern, as they can on Linux/macOS and as documented in the LLVM plugin documentation.

**Context**

This issue was discovered while enabling Clang 20 support for Firefox's clang-plugin on Windows. Firefox has been working around similar issues with local patches, but this affects any downstream project trying to build Windows plugins with dynamic linking.

The `Registry<T>` pattern is fundamental to LLVM's plugin infrastructure and is used throughout the codebase for various plugin types (`PluginASTAction`, `PassBuilderOptPluginPass`, etc.).

**Related**

- `llvm/utils/extract_symbols.py` - Symbol export script
- `clang/tools/driver/driver.cpp` - Main executable
- `llvm/include/llvm/Support/Registry.h` - Registry template definition

</pre>
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