[clang] Frontend components for the relative vtables ABI

This patch contains all of the clang changes from D72959.

- Generalize the relative vtables ABI such that it can be used by other targets.
- Add an enum VTableComponentLayout which controls whether components in the
  vtable should be pointers to other structs or relative offsets to those structs.
  Other ABIs can change this enum to restructure how components in the vtable
  are laid out/accessed.
- Add methods to ConstantInitBuilder for inserting relative offsets to a
  specified position in the aggregate being constructed.

See D72959 for background info.

Differential Revision: https://reviews.llvm.org/D77592

1 files changed, 275 insertions, 39 deletions

diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 6a0a848..73a74ca3 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -618,29 +618,178 @@ void CodeGenVTables::EmitThunks(GlobalDecl GD) {
     maybeEmitThunk(GD, Thunk, /*ForVTable=*/false);
 }
 
-void CodeGenVTables::addVTableComponent(
-    ConstantArrayBuilder &builder, const VTableLayout &layout,
-    unsigned idx, llvm::Constant *rtti, unsigned &nextVTableThunkIndex) {
-  auto &component = layout.vtable_components()[idx];
+void CodeGenVTables::addRelativeComponent(ConstantArrayBuilder &builder,
+                                          llvm::Constant *component,
+                                          unsigned vtableAddressPoint,
+                                          bool vtableHasLocalLinkage,
+                                          bool isCompleteDtor) const {
+  // No need to get the offset of a nullptr.
+  if (component->isNullValue())
+    return builder.add(llvm::ConstantInt::get(CGM.Int32Ty, 0));
+
+  auto *globalVal =
+      cast<llvm::GlobalValue>(component->stripPointerCastsAndAliases());
+  llvm::Module &module = CGM.getModule();
+
+  // We don't want to copy the linkage of the vtable exactly because we still
+  // want the stub/proxy to be emitted for properly calculating the offset.
+  // Examples where there would be no symbol emitted are available_externally
+  // and private linkages.
+  auto stubLinkage = vtableHasLocalLinkage ? llvm::GlobalValue::InternalLinkage
+                                           : llvm::GlobalValue::ExternalLinkage;
+
+  llvm::Constant *target;
+  if (auto *func = dyn_cast<llvm::Function>(globalVal)) {
+    target = getOrCreateRelativeStub(func, stubLinkage, isCompleteDtor);
+  } else {
+    llvm::SmallString<16> rttiProxyName(globalVal->getName());
+    rttiProxyName.append(".rtti_proxy");
+
+    // The RTTI component may not always be emitted in the same linkage unit as
+    // the vtable. As a general case, we can make a dso_local proxy to the RTTI
+    // that points to the actual RTTI struct somewhere. This will result in a
+    // GOTPCREL relocation when taking the relative offset to the proxy.
+    llvm::GlobalVariable *proxy = module.getNamedGlobal(rttiProxyName);
+    if (!proxy) {
+      proxy = new llvm::GlobalVariable(module, globalVal->getType(),
+                                       /*isConstant=*/true, stubLinkage,
+                                       globalVal, rttiProxyName);
+      proxy->setDSOLocal(true);
+      proxy->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
+      if (!proxy->hasLocalLinkage()) {
+        proxy->setVisibility(llvm::GlobalValue::HiddenVisibility);
+        proxy->setComdat(module.getOrInsertComdat(rttiProxyName));
+      }
+    }
+    target = proxy;
+  }
+
+  builder.addRelativeOffsetToPosition(CGM.Int32Ty, target,
+                                      /*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();
+}
+
+llvm::Type *CodeGenVTables::getVTableComponentType() const {
+  if (useRelativeLayout())
+    return CGM.Int32Ty;
+  return CGM.Int8PtrTy;
+}
+
+static void AddPointerLayoutOffset(const CodeGenModule &CGM,
+                                   ConstantArrayBuilder &builder,
+                                   CharUnits offset) {
+  builder.add(llvm::ConstantExpr::getIntToPtr(
+      llvm::ConstantInt::get(CGM.PtrDiffTy, offset.getQuantity()),
+      CGM.Int8PtrTy));
+}
 
-  auto addOffsetConstant = [&](CharUnits offset) {
-    builder.add(llvm::ConstantExpr::getIntToPtr(
-        llvm::ConstantInt::get(CGM.PtrDiffTy, offset.getQuantity()),
-        CGM.Int8PtrTy));
-  };
+static void AddRelativeLayoutOffset(const CodeGenModule &CGM,
+                                    ConstantArrayBuilder &builder,
+                                    CharUnits offset) {
+  builder.add(llvm::ConstantInt::get(CGM.Int32Ty, offset.getQuantity()));
+}
+
+void CodeGenVTables::addVTableComponent(ConstantArrayBuilder &builder,
+                                        const VTableLayout &layout,
+                                        unsigned componentIndex,
+                                        llvm::Constant *rtti,
+                                        unsigned &nextVTableThunkIndex,
+                                        unsigned vtableAddressPoint,
+                                        bool vtableHasLocalLinkage) {
+  auto &component = layout.vtable_components()[componentIndex];
+
+  auto addOffsetConstant =
+      useRelativeLayout() ? AddRelativeLayoutOffset : AddPointerLayoutOffset;
 
   switch (component.getKind()) {
   case VTableComponent::CK_VCallOffset:
-    return addOffsetConstant(component.getVCallOffset());
+    return addOffsetConstant(CGM, builder, component.getVCallOffset());
 
   case VTableComponent::CK_VBaseOffset:
-    return addOffsetConstant(component.getVBaseOffset());
+    return addOffsetConstant(CGM, builder, component.getVBaseOffset());
 
   case VTableComponent::CK_OffsetToTop:
-    return addOffsetConstant(component.getOffsetToTop());
+    return addOffsetConstant(CGM, builder, component.getOffsetToTop());
 
   case VTableComponent::CK_RTTI:
-    return builder.add(llvm::ConstantExpr::getBitCast(rtti, CGM.Int8PtrTy));
+    if (useRelativeLayout())
+      return addRelativeComponent(builder, rtti, vtableAddressPoint,
+                                  vtableHasLocalLinkage,
+                                  /*isCompleteDtor=*/false);
+    else
+      return builder.add(llvm::ConstantExpr::getBitCast(rtti, CGM.Int8PtrTy));
 
   case VTableComponent::CK_FunctionPointer:
   case VTableComponent::CK_CompleteDtorPointer:
@@ -674,11 +823,21 @@ void CodeGenVTables::addVTableComponent(
               ? MD->hasAttr<CUDADeviceAttr>()
               : (MD->hasAttr<CUDAHostAttr>() || !MD->hasAttr<CUDADeviceAttr>());
       if (!CanEmitMethod)
-        return builder.addNullPointer(CGM.Int8PtrTy);
+        return builder.add(llvm::ConstantExpr::getNullValue(CGM.Int8PtrTy));
       // Method is acceptable, continue processing as usual.
     }
 
     auto getSpecialVirtualFn = [&](StringRef name) -> llvm::Constant * {
+      // FIXME(PR43094): When merging comdat groups, lld can select a local
+      // symbol as the signature symbol even though it cannot be accessed
+      // outside that symbol's TU. The relative vtables ABI would make
+      // __cxa_pure_virtual and __cxa_deleted_virtual local symbols, and
+      // depending on link order, the comdat groups could resolve to the one
+      // with the local symbol. As a temporary solution, fill these components
+      // with zero. We shouldn't be calling these in the first place anyway.
+      if (useRelativeLayout())
+        return llvm::ConstantPointerNull::get(CGM.Int8PtrTy);
+
       // For NVPTX devices in OpenMP emit special functon as null pointers,
       // otherwise linking ends up with unresolved references.
       if (CGM.getLangOpts().OpenMP && CGM.getLangOpts().OpenMPIsDevice &&
@@ -699,19 +858,20 @@ void CodeGenVTables::addVTableComponent(
     if (cast<CXXMethodDecl>(GD.getDecl())->isPure()) {
       if (!PureVirtualFn)
         PureVirtualFn =
-          getSpecialVirtualFn(CGM.getCXXABI().GetPureVirtualCallName());
+            getSpecialVirtualFn(CGM.getCXXABI().GetPureVirtualCallName());
       fnPtr = PureVirtualFn;
 
     // Deleted virtual member functions.
     } else if (cast<CXXMethodDecl>(GD.getDecl())->isDeleted()) {
       if (!DeletedVirtualFn)
         DeletedVirtualFn =
-          getSpecialVirtualFn(CGM.getCXXABI().GetDeletedVirtualCallName());
+            getSpecialVirtualFn(CGM.getCXXABI().GetDeletedVirtualCallName());
       fnPtr = DeletedVirtualFn;
 
     // Thunks.
     } else if (nextVTableThunkIndex < layout.vtable_thunks().size() &&
-               layout.vtable_thunks()[nextVTableThunkIndex].first == idx) {
+               layout.vtable_thunks()[nextVTableThunkIndex].first ==
+                   componentIndex) {
       auto &thunkInfo = layout.vtable_thunks()[nextVTableThunkIndex].second;
 
       nextVTableThunkIndex++;
@@ -723,13 +883,19 @@ void CodeGenVTables::addVTableComponent(
       fnPtr = CGM.GetAddrOfFunction(GD, fnTy, /*ForVTable=*/true);
     }
 
-    fnPtr = llvm::ConstantExpr::getBitCast(fnPtr, CGM.Int8PtrTy);
-    builder.add(fnPtr);
-    return;
+    if (useRelativeLayout()) {
+      return addRelativeComponent(
+          builder, fnPtr, vtableAddressPoint, vtableHasLocalLinkage,
+          component.getKind() == VTableComponent::CK_CompleteDtorPointer);
+    } else
+      return builder.add(llvm::ConstantExpr::getBitCast(fnPtr, CGM.Int8PtrTy));
   }
 
   case VTableComponent::CK_UnusedFunctionPointer:
-    return builder.addNullPointer(CGM.Int8PtrTy);
+    if (useRelativeLayout())
+      return builder.add(llvm::ConstantExpr::getNullValue(CGM.Int32Ty));
+    else
+      return builder.addNullPointer(CGM.Int8PtrTy);
   }
 
   llvm_unreachable("Unexpected vtable component kind");
@@ -737,34 +903,41 @@ void CodeGenVTables::addVTableComponent(
 
 llvm::Type *CodeGenVTables::getVTableType(const VTableLayout &layout) {
   SmallVector<llvm::Type *, 4> tys;
-  for (unsigned i = 0, e = layout.getNumVTables(); i != e; ++i) {
-    tys.push_back(llvm::ArrayType::get(CGM.Int8PtrTy, layout.getVTableSize(i)));
-  }
+  llvm::Type *componentType = getVTableComponentType();
+  for (unsigned i = 0, e = layout.getNumVTables(); i != e; ++i)
+    tys.push_back(llvm::ArrayType::get(componentType, layout.getVTableSize(i)));
 
   return llvm::StructType::get(CGM.getLLVMContext(), tys);
 }
 
 void CodeGenVTables::createVTableInitializer(ConstantStructBuilder &builder,
                                              const VTableLayout &layout,
-                                             llvm::Constant *rtti) {
+                                             llvm::Constant *rtti,
+                                             bool vtableHasLocalLinkage) {
+  llvm::Type *componentType = getVTableComponentType();
+
+  const auto &addressPoints = layout.getAddressPointIndices();
   unsigned nextVTableThunkIndex = 0;
-  for (unsigned i = 0, e = layout.getNumVTables(); i != e; ++i) {
-    auto vtableElem = builder.beginArray(CGM.Int8PtrTy);
-    size_t thisIndex = layout.getVTableOffset(i);
-    size_t nextIndex = thisIndex + layout.getVTableSize(i);
-    for (unsigned i = thisIndex; i != nextIndex; ++i) {
-      addVTableComponent(vtableElem, layout, i, rtti, nextVTableThunkIndex);
+  for (unsigned vtableIndex = 0, endIndex = layout.getNumVTables();
+       vtableIndex != endIndex; ++vtableIndex) {
+    auto vtableElem = builder.beginArray(componentType);
+
+    size_t vtableStart = layout.getVTableOffset(vtableIndex);
+    size_t vtableEnd = vtableStart + layout.getVTableSize(vtableIndex);
+    for (size_t componentIndex = vtableStart; componentIndex < vtableEnd;
+         ++componentIndex) {
+      addVTableComponent(vtableElem, layout, componentIndex, rtti,
+                         nextVTableThunkIndex, addressPoints[vtableIndex],
+                         vtableHasLocalLinkage);
     }
     vtableElem.finishAndAddTo(builder);
   }
 }
 
-llvm::GlobalVariable *
-CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
-                                      const BaseSubobject &Base,
-                                      bool BaseIsVirtual,
-                                   llvm::GlobalVariable::LinkageTypes Linkage,
-                                      VTableAddressPointsMapTy& AddressPoints) {
+llvm::GlobalVariable *CodeGenVTables::GenerateConstructionVTable(
+    const CXXRecordDecl *RD, const BaseSubobject &Base, bool BaseIsVirtual,
+    llvm::GlobalVariable::LinkageTypes Linkage,
+    VTableAddressPointsMapTy &AddressPoints) {
   if (CGDebugInfo *DI = CGM.getModuleDebugInfo())
     DI->completeClassData(Base.getBase());
 
@@ -781,7 +954,14 @@ CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
   cast<ItaniumMangleContext>(CGM.getCXXABI().getMangleContext())
       .mangleCXXCtorVTable(RD, Base.getBaseOffset().getQuantity(),
                            Base.getBase(), Out);
-  StringRef Name = OutName.str();
+  SmallString<256> Name(OutName);
+
+  bool UsingRelativeLayout = getItaniumVTableContext().isRelativeLayout();
+  bool VTableAliasExists = UsingRelativeLayout && CGM.getModule().getNamedAlias(Name);
+  if (VTableAliasExists) {
+    // We previously made the vtable hidden and changed its name.
+    Name.append(".local");
+  }
 
   llvm::Type *VTType = getVTableType(*VTLayout);
 
@@ -808,7 +988,8 @@ CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
   // Create and set the initializer.
   ConstantInitBuilder builder(CGM);
   auto components = builder.beginStruct();
-  createVTableInitializer(components, *VTLayout, RTTI);
+  createVTableInitializer(components, *VTLayout, RTTI,
+                          VTable->hasLocalLinkage());
   components.finishAndSetAsInitializer(VTable);
 
   // Set properties only after the initializer has been set to ensure that the
@@ -818,9 +999,64 @@ CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
 
   CGM.EmitVTableTypeMetadata(RD, VTable, *VTLayout.get());
 
+  if (UsingRelativeLayout && !VTable->isDSOLocal())
+    GenerateRelativeVTableAlias(VTable, OutName);
+
   return VTable;
 }
 
+// If the VTable is not dso_local, then we will not be able to indicate that
+// the VTable does not need a relocation and move into rodata. An frequent
+// time this can occur is for classes that should be made public from a DSO
+// (like in libc++). For cases like these, we can make the vtable hidden or
+// private and create a public alias with the same visibility and linkage as
+// the original vtable type.
+void CodeGenVTables::GenerateRelativeVTableAlias(llvm::GlobalVariable *VTable,
+                                                 llvm::StringRef AliasName) {
+  assert(getItaniumVTableContext().isRelativeLayout() &&
+         "Can only use this if the relative vtable ABI is used");
+  assert(!VTable->isDSOLocal() && "This should be called only if the vtable is "
+                                  "not guaranteed to be dso_local");
+
+  // If the vtable is available_externally, we shouldn't (or need to) generate
+  // an alias for it in the first place since the vtable won't actually by
+  // emitted in this compilation unit.
+  if (VTable->hasAvailableExternallyLinkage())
+    return;
+
+  VTable->setName(AliasName + ".local");
+
+  auto Linkage = VTable->getLinkage();
+  assert(llvm::GlobalAlias::isValidLinkage(Linkage) &&
+         "Invalid vtable alias linkage");
+
+  llvm::GlobalAlias *VTableAlias = CGM.getModule().getNamedAlias(AliasName);
+  if (!VTableAlias) {
+    VTableAlias = llvm::GlobalAlias::create(
+        VTable->getValueType(), VTable->getAddressSpace(), Linkage, AliasName,
+        &CGM.getModule());
+  } else {
+    assert(VTableAlias->getValueType() == VTable->getValueType());
+    assert(VTableAlias->getLinkage() == Linkage);
+  }
+  VTableAlias->setVisibility(VTable->getVisibility());
+  VTableAlias->setUnnamedAddr(VTable->getUnnamedAddr());
+
+  // Both of these imply dso_local for the vtable.
+  if (!VTable->hasComdat()) {
+    // If this is in a comdat, then we shouldn't make the linkage private due to
+    // an issue in lld where private symbols can be used as the key symbol when
+    // choosing the prevelant group. This leads to "relocation refers to a
+    // symbol in a discarded section".
+    VTable->setLinkage(llvm::GlobalValue::PrivateLinkage);
+  } else {
+    // We should at least make this hidden since we don't want to expose it.
+    VTable->setVisibility(llvm::GlobalValue::HiddenVisibility);
+  }
+
+  VTableAlias->setAliasee(VTable);
+}
+
 static bool shouldEmitAvailableExternallyVTable(const CodeGenModule &CGM,
                                                 const CXXRecordDecl *RD) {
   return CGM.getCodeGenOpts().OptimizationLevel > 0 &&