[clang][RelativeVTablesABI] Use dso_local_equivalent rather than emitting stubs

Thanks to D77248, we can bypass the use of stubs altogether and use PLT
relocations if they are available for the target. LLVM and LLD support the
R_AARCH64_PLT32 relocation, so we can also guarantee a static PLT relocation on AArch64.
Not emitting these stubs saves a lot of extra binary size.

Differential Revision: https://reviews.llvm.org/D83812

1 files changed, 1 insertions, 69 deletions

diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 75afc860..bef9a29 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -641,7 +641,7 @@ void CodeGenVTables::addRelativeComponent(ConstantArrayBuilder &builder,
 
   llvm::Constant *target;
   if (auto *func = dyn_cast<llvm::Function>(globalVal)) {
-    target = getOrCreateRelativeStub(func, stubLinkage, isCompleteDtor);
+    target = llvm::DSOLocalEquivalent::get(func);
   } else {
     llvm::SmallString<16> rttiProxyName(globalVal->getName());
     rttiProxyName.append(".rtti_proxy");
@@ -669,74 +669,6 @@ void CodeGenVTables::addRelativeComponent(ConstantArrayBuilder &builder,
                                       /*position=*/vtableAddressPoint);
 }
 
-llvm::Function *CodeGenVTables::getOrCreateRelativeStub(
-    llvm::Function *func, llvm::GlobalValue::LinkageTypes stubLinkage,
-    bool isCompleteDtor) const {
-  // A complete object destructor can later be substituted in the vtable for an
-  // appropriate base object destructor when optimizations are enabled. This can
-  // happen for child classes that don't have their own destructor. In the case
-  // where a parent virtual destructor is not guaranteed to be in the same
-  // linkage unit as the child vtable, it's possible for an external reference
-  // for this destructor to be substituted into the child vtable, preventing it
-  // from being in rodata. If this function is a complete virtual destructor, we
-  // can just force a stub to be emitted for it.
-  if (func->isDSOLocal() && !isCompleteDtor)
-    return func;
-
-  llvm::SmallString<16> stubName(func->getName());
-  stubName.append(".stub");
-
-  // Instead of taking the offset between the vtable and virtual function
-  // directly, we emit a dso_local stub that just contains a tail call to the
-  // original virtual function and take the offset between that and the
-  // vtable. We do this because there are some cases where the original
-  // function that would've been inserted into the vtable is not dso_local
-  // which may require some kind of dynamic relocation which prevents the
-  // vtable from being readonly. On x86_64, taking the offset between the
-  // function and the vtable gets lowered to the offset between the PLT entry
-  // for the function and the vtable which gives us a PLT32 reloc. On AArch64,
-  // right now only CALL26 and JUMP26 instructions generate PLT relocations,
-  // so we manifest them with stubs that are just jumps to the original
-  // function.
-  auto &module = CGM.getModule();
-  llvm::Function *stub = module.getFunction(stubName);
-  if (stub) {
-    assert(stub->isDSOLocal() &&
-           "The previous definition of this stub should've been dso_local.");
-    return stub;
-  }
-
-  stub = llvm::Function::Create(func->getFunctionType(), stubLinkage, stubName,
-                                module);
-
-  // Propogate function attributes.
-  stub->setAttributes(func->getAttributes());
-
-  stub->setDSOLocal(true);
-  stub->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
-  if (!stub->hasLocalLinkage()) {
-    stub->setVisibility(llvm::GlobalValue::HiddenVisibility);
-    stub->setComdat(module.getOrInsertComdat(stubName));
-  }
-
-  // Fill the stub with a tail call that will be optimized.
-  llvm::BasicBlock *block =
-      llvm::BasicBlock::Create(module.getContext(), "entry", stub);
-  llvm::IRBuilder<> block_builder(block);
-  llvm::SmallVector<llvm::Value *, 8> args;
-  for (auto &arg : stub->args())
-    args.push_back(&arg);
-  llvm::CallInst *call = block_builder.CreateCall(func, args);
-  call->setAttributes(func->getAttributes());
-  call->setTailCall();
-  if (call->getType()->isVoidTy())
-    block_builder.CreateRetVoid();
-  else
-    block_builder.CreateRet(call);
-
-  return stub;
-}
-
 bool CodeGenVTables::useRelativeLayout() const {
   return CGM.getTarget().getCXXABI().isItaniumFamily() &&
          CGM.getItaniumVTableContext().isRelativeLayout();