[llvm] [llvm][test][CGPluginTest] Avoid __attribute__((constructor)) for MSVC (PR #227310)

[Fabrice de Gans via llvm-commits]

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@@ -424,12 +424,16 @@ RegisterTargetPassConfigCallback X{[](auto &TM, auto &PM, auto *TPC) {
 }};
 ```
 
-Note that passes still have to be registered:
+Note that passes still have to be registered. This can be done using a
+static object whose constructor runs when the plugin is loaded:
 
 ```cpp
-__attribute__((constructor)) static void initCodeGenPlugin() {
-  initializeExamplePass(*PassRegistry::getPassRegistry());
-}
+struct InitCodeGenPlugin {
+  InitCodeGenPlugin() {
+    initializeExamplePass(*PassRegistry::getPassRegistry());
+  }
+};
+static InitCodeGenPlugin InitCodeGenPluginInstance;
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Steelskin wrote:

It is never guaranteed, yes. I was mostly wondering if `__attribute__((constructor))` had additional guarantees. If both tests and build pass, that's good enough for me, though, thank you!

https://github.com/llvm/llvm-project/pull/227310