[IR] Allow uses of llvm.global_ctors and llvm.global_dtors

With PAuth enabled, signed function pointers in init/fini arrays are
going to be represented with `ptrauth` constants. To support address
discrimination for such signed pointers, we need to fill the storage
address with `getelementptr` referencing the array (`llvm.global_ctors`
or `llvm.global_dtors`) itself.

Such uses of these special arrays were previously disallowed since
`appendToGlobal{C|D}tors` did not update uses after construction of a new
array. This patch implements such update logic. This will be tested in a
follow-up patch introducing PAuth support for function pointers in init/fini
arrays since such signed pointers with address discrimination are
currently the only entities which have such uses needed to be updated.

Author: kovdan01

llvm/lib/IR/Verifier.cpp

@@ -850,8 +850,6 @@ void Verifier::visitGlobalVariable(const GlobalVariable &GV) {
                        GV.getName() == "llvm.global_dtors")) {
     Check(!GV.hasInitializer() || GV.hasAppendingLinkage(),
           "invalid linkage for intrinsic global variable", &GV);
-    Check(GV.materialized_use_empty(),
-          "invalid uses of intrinsic global variable", &GV);
 
     // Don't worry about emitting an error for it not being an array,
     // visitGlobalValue will complain on appending non-array.

llvm/lib/Transforms/Utils/ModuleUtils.cpp

@@ -35,15 +35,16 @@ static void appendToGlobalArray(StringRef ArrayName, Module &M, Function *F,
   // to the list.
   SmallVector<Constant *, 16> CurrentCtors;
   StructType *EltTy;
-  if (GlobalVariable *GVCtor = M.getNamedGlobal(ArrayName)) {
+  GlobalVariable *GVCtor = M.getNamedGlobal(ArrayName);
+  if (GVCtor) {
     EltTy = cast<StructType>(GVCtor->getValueType()->getArrayElementType());
     if (Constant *Init = GVCtor->getInitializer()) {
       unsigned n = Init->getNumOperands();
       CurrentCtors.reserve(n + 1);
       for (unsigned i = 0; i != n; ++i)
         CurrentCtors.push_back(cast<Constant>(Init->getOperand(i)));
     }
-    GVCtor->eraseFromParent();
+    GVCtor->removeFromParent();
   } else {
     EltTy = StructType::get(IRB.getInt32Ty(),
                             PointerType::get(FnTy, F->getAddressSpace()),
@@ -67,8 +68,13 @@ static void appendToGlobalArray(StringRef ArrayName, Module &M, Function *F,
 
   // Create the new global variable and replace all uses of
   // the old global variable with the new one.
-  (void)new GlobalVariable(M, NewInit->getType(), false,
-                           GlobalValue::AppendingLinkage, NewInit, ArrayName);
+  auto *NewGVCtor =
+      new GlobalVariable(M, NewInit->getType(), false,
+                         GlobalValue::AppendingLinkage, NewInit, ArrayName);
+  if (GVCtor)
+    for (Use &U : make_early_inc_range(GVCtor->uses()))
+      U.set(NewGVCtor);
+  delete GVCtor;
 }
 
 void llvm::appendToGlobalCtors(Module &M, Function *F, int Priority, Constant *Data) {