[clang] [llvm] [ThinLTO] Support dead RTTI data elimination under -fno-split-lto-unit (PR #126336)

[Teresa Johnson via llvm-commits]

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@@ -911,6 +916,61 @@ static void setLiveRoot(ModuleSummaryIndex &Index, StringRef Name) {
       Summary->setLive(true);
 }
 
+static bool hasNonVTableUsers(const User *U, CXXABI *ABI) {
+  LLVM_DEBUG(dbgs() << "Check if " << *U << "has vtable users\n");
+  if (isa<Instruction>(U)) {
+    // If the type info is used in dynamic_cast or exception handling,
+    // its user must be the instruction.
+    return true;
+  }
+
+  // The virtual table type is either a struct of arrays. For example:
+  // @vtable = constant { [3 x ptr] } { [3 x ptr] [ ptr null, ptr @rtti, ptr @vf] }
+  //
+  // In this case, the user of @rtti is an anonymous ConstantArray.
+  // Therefore, if the user of the type information is anonymous,
+  // we need to perform a depth-first search (DFS) to locate its named users.
+  //
+  // And we also need to iterate its users if the current user is the type
+  // info global variable itself.
+  StringRef Name = U->getName();
+  if (Name.empty() || ABI->isTypeInfo(Name)) {
+    for (const User *It : U->users())
+      if (hasNonVTableUsers(It, ABI))
+        return true;
+    return false;
+  }
+
+  if (!ABI->isVTable(Name))
+    return true;
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teresajohnson wrote:

But here we are not in the thin link, so why are we not able to obtain the vtable GV?

https://github.com/llvm/llvm-project/pull/126336