[llvm] [Linker] Do not keep a private member of a non-prevailing comdat group (PR #69143)

[Igor Kudrin via llvm-commits]

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@@ -497,6 +501,23 @@ bool ModuleLinker::run() {
   for (Function &GV : llvm::make_early_inc_range(DstM))
     dropReplacedComdat(GV, ReplacedDstComdats);
 
+  if (!NonPrevailingComdats.empty()) {
+    DenseSet<GlobalObject *> AliasedGlobals;
+    for (auto &GA : SrcM->aliases())
+      if (GlobalObject *GO = GA.getAliaseeObject(); GO && GO->getComdat())
+        AliasedGlobals.insert(GO);
+    for (const Comdat *C : NonPrevailingComdats) {
+      SmallVector<GlobalObject *> ToUpdate;
+      for (GlobalObject *GO : C->getUsers())
+        if (GO->hasPrivateLinkage() && !AliasedGlobals.contains(GO))
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igorkudrin wrote:

This change would break the `Linker/comdat-refer-to-discarded.ll` test. Originally, it was `Linker/comdat13.ll` in https://github.com/llvm/llvm-project/commit/baa3bf8f76568de6198613fb2ba7695a76c29109, which stated that the duplication of `internal` globals "matches the expectations of COFF linkers`. Since no high-level example was provided, it is now quite difficult, at least for me, to judge whether that behavior is correct or not, so I would prefer not to change it in my patch.

https://github.com/llvm/llvm-project/pull/69143