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

[via cfe-commits]

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@@ -911,6 +916,63 @@ static void setLiveRoot(ModuleSummaryIndex &Index, StringRef Name) {
       Summary->setLive(true);
 }
 
+// Return true if the User U is reachable from a non-vtable user
+// through the use-def chain.
+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;
+
+  return false;
+}
+
+static void analyzeRTTIVars(ModuleSummaryIndex &Index, const Module &M) {
+  Triple TT(M.getTargetTriple());
+
+  std::unique_ptr<CXXABI> ABI = CXXABI::Create(TT);
+  if (!ABI)
+    return;
+
+  for (const GlobalVariable &GV : M.globals()) {
+    if (!ABI->isTypeInfo(GV.getName()))
+      continue;
+
+    if (hasNonVTableUsers(&GV, ABI.get())) {
+      std::string TypeName =
+          ABI->getTypeNameFromTypeInfo(GV.getName());
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luxufan wrote:

Option 2 is similar to unconditionally returning the Itanium CXXABI in CXXABI::Create. For now, I proceeded with this approach and added a TODO to introduce a module flag that can distinguish between the Itanium and Microsoft ABIs. This flag will later be used to determine which ABI is currently in use. If this approach does not align with your expectations, please feel free to let me know, and I’d be happy to adjust accordingly

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