[clang] [llvm] [Windows] Fix plugin registry symbols not exported/linked with CLANG_LINK_CLANG_DYLIB (PR #163391)

[Hans Wennborg via cfe-commits]

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@@ -105,6 +105,11 @@ def should_keep_microsoft_symbol(symbol, calling_convention_decoration):
     # Skip X86GenMnemonicTables functions, they are not exposed from llvm/include/.
     elif re.match(r"\?is[A-Z0-9]*@X86 at llvm", symbol):
         return None
+    # Keep Registry<T>::Head and Registry<T>::Tail static members for plugin support.
+    # Pattern matches: ?Head@?$Registry@<template_args>@llvm@@ or ?Tail@?$Registry at ...
+    elif ("?$Registry@" in symbol and "@llvm@@" in symbol and
+          (symbol.startswith("?Head@") or symbol.startswith("?Tail@"))):
+        return symbol
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zmodem wrote:

Just targeting these two feels a bit hacky, but I suppose the alternative of exporting all global variables might be too much.

Ideally we'd rely on the dllimport/export annotations (the plugin registry is marked imported/exported here:) https://github.com/llvm/llvm-project/blob/7bbb4a51035aa491313b1242a779d52fd12904c6/clang/include/clang/Frontend/FrontendPluginRegistry.h#L28

Can you check if your build might work without `-DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON`?

https://github.com/llvm/llvm-project/pull/163391