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

[via cfe-commits]

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@@ -573,14 +574,29 @@ static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
         convertToDeclaration(GV);
       }
 
+  Triple TT(Mod.getTargetTriple());
+  std::unique_ptr<CXXABI> ABI = CXXABI::Create(TT);
+
   // Now that all dead bodies have been dropped, delete the actual objects
   // themselves when possible.
   for (GlobalValue *GV : DeadGVs) {
     GV->removeDeadConstantUsers();
-    // Might reference something defined in native object (i.e. dropped a
-    // non-prevailing IR def, but we need to keep the declaration).
-    if (GV->use_empty())
+
+    // The RTTI data consists of both type information and the type name string.
+    // Although they are considered dead, there are still users that reference them.
+    // For example, the type information might be used by a vtable, and the type name
+    // string might be used by the type info.
+    // Therefore, we need to replace these uses to null pointer before erasing them.
+    if (ABI && (ABI->isTypeInfo(GV->getName()) ||
+                            ABI->isTypeName(GV->getName()))) {
+      GV->replaceAllUsesWith(
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luxufan wrote:

Although the RTTI GlobalVariable has been inferred as dead, the vtable still references it. Without this change, the RTTI GlobalVariable cannot be eliminated.

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