Reland: Dead Virtual Function Elimination

Remove dead virtual functions from vtables with
replaceNonMetadataUsesWith, so that CGProfile metadata gets cleaned up
correctly.

Original commit message:

Currently, it is hard for the compiler to remove unused C++ virtual
functions, because they are all referenced from vtables, which are referenced
by constructors. This means that if the constructor is called from any live
code, then we keep every virtual function in the final link, even if there
are no call sites which can use it.

This patch allows unused virtual functions to be removed during LTO (and
regular compilation in limited circumstances) by using type metadata to match
virtual function call sites to the vtable slots they might load from. This
information can then be used in the global dead code elimination pass instead
of the references from vtables to virtual functions, to more accurately
determine which functions are reachable.

To make this transformation safe, I have changed clang's code-generation to
always load virtual function pointers using the llvm.type.checked.load
intrinsic, instead of regular load instructions. I originally tried writing
this using clang's existing code-generation, which uses the llvm.type.test
and llvm.assume intrinsics after doing a normal load. However, it is possible
for optimisations to obscure the relationship between the GEP, load and
llvm.type.test, causing GlobalDCE to fail to find virtual function call
sites.

The existing linkage and visibility types don't accurately describe the scope
in which a virtual call could be made which uses a given vtable. This is
wider than the visibility of the type itself, because a virtual function call
could be made using a more-visible base class. I've added a new
!vcall_visibility metadata type to represent this, described in
TypeMetadata.rst. The internalization pass and libLTO have been updated to
change this metadata when linking is performed.

This doesn't currently work with ThinLTO, because it needs to see every call
to llvm.type.checked.load in the linkage unit. It might be possible to
extend this optimisation to be able to use the ThinLTO summary, as was done
for devirtualization, but until then that combination is rejected in the
clang driver.

To test this, I've written a fuzzer which generates random C++ programs with
complex class inheritance graphs, and virtual functions called through object
and function pointers of different types. The programs are spread across
multiple translation units and DSOs to test the different visibility
restrictions.

I've also tried doing bootstrap builds of LLVM to test this. This isn't
ideal, because only classes in anonymous namespaces can be optimised with
-fvisibility=default, and some parts of LLVM (plugins and bugpoint) do not
work correctly with -fvisibility=hidden. However, there are only 12 test
failures when building with -fvisibility=hidden (and an unmodified compiler),
and this change does not cause any new failures for either value of
-fvisibility.

On the 7 C++ sub-benchmarks of SPEC2006, this gives a geomean code-size
reduction of ~6%, over a baseline compiled with "-O2 -flto
-fvisibility=hidden -fwhole-program-vtables". The best cases are reductions
of ~14% in 450.soplex and 483.xalancbmk, and there are no code size
increases.

I've also run this on a set of 8 mbed-os examples compiled for Armv7M, which
show a geomean size reduction of ~3%, again with no size increases.

I had hoped that this would have no effect on performance, which would allow
it to awlays be enabled (when using -fwhole-program-vtables). However, the
changes in clang to use the llvm.type.checked.load intrinsic are causing ~1%
performance regression in the C++ parts of SPEC2006. It should be possible to
recover some of this perf loss by teaching optimisations about the
llvm.type.checked.load intrinsic, which would make it worth turning this on
by default (though it's still dependent on -fwhole-program-vtables).

Differential revision: https://reviews.llvm.org/D63932

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375094 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -6271,6 +6271,13 @@ enum is the smallest type which can represent all of its values::
     !0 = !{i32 1, !"short_wchar", i32 1}
     !1 = !{i32 1, !"short_enum", i32 0}
 
+LTO Post-Link Module Flags Metadata
+-----------------------------------
+
+Some optimisations are only when the entire LTO unit is present in the current
+module. This is represented by the ``LTOPostLink`` module flags metadata, which
+will be created with a value of ``1`` when LTO linking occurs.
+
 Automatic Linker Flags Named Metadata
 =====================================
 
@@ -16816,6 +16823,8 @@ Overview:
 The ``llvm.type.test`` intrinsic tests whether the given pointer is associated
 with the given type identifier.
 
+.. _type.checked.load:
+
 '``llvm.type.checked.load``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

docs/TypeMetadata.rst

@@ -224,3 +224,67 @@ efficiently to minimize the sizes of the underlying bitsets.
     }
 
 .. _GlobalLayoutBuilder: https://github.com/llvm/llvm-project/blob/master/llvm/include/llvm/Transforms/IPO/LowerTypeTests.h
+
+``!vcall_visibility`` Metadata
+==============================
+
+In order to allow removing unused function pointers from vtables, we need to
+know whether every virtual call which could use it is known to the compiler, or
+whether another translation unit could introduce more calls through the vtable.
+This is not the same as the linkage of the vtable, because call sites could be
+using a pointer of a more widely-visible base class. For example, consider this
+code:
+
+.. code-block:: c++
+
+  __attribute__((visibility("default")))
+  struct A {
+    virtual void f();
+  };
+
+  __attribute__((visibility("hidden")))
+  struct B : A {
+    virtual void f();
+  };
+
+With LTO, we know that all code which can see the declaration of ``B`` is
+visible to us. However, a pointer to a ``B`` could be cast to ``A*`` and passed
+to another linkage unit, which could then call ``f`` on it. This call would
+load from the vtable for ``B`` (using the object pointer), and then call
+``B::f``. This means we can't remove the function pointer from ``B``'s vtable,
+or the implementation of ``B::f``. However, if we can see all code which knows
+about any dynamic base class (which would be the case if ``B`` only inherited
+from classes with hidden visibility), then this optimisation would be valid.
+
+This concept is represented in IR by the ``!vcall_visibility`` metadata
+attached to vtable objects, with the following values:
+
+.. list-table::
+   :header-rows: 1
+   :widths: 10 90
+
+   * - Value
+     - Behavior
+
+   * - 0 (or omitted)
+     - **Public**
+           Virtual function calls using this vtable could be made from external
+           code.
+
+   * - 1
+     - **Linkage Unit**
+           All virtual function calls which might use this vtable are in the
+           current LTO unit, meaning they will be in the current module once
+           LTO linking has been performed.
+
+   * - 2
+     - **Translation Unit**
+           All virtual function calls which might use this vtable are in the
+           current module.
+
+In addition, all function pointer loads from a vtable marked with the
+``!vcall_visibility`` metadata (with a non-zero value) must be done using the
+:ref:`llvm.type.checked.load <type.checked.load>` intrinsic, so that virtual
+calls sites can be correlated with the vtables which they might load from.
+Other parts of the vtable (RTTI, offset-to-top, ...) can still be accessed with
+normal loads.

include/llvm/Analysis/TypeMetadataUtils.h

@@ -50,6 +50,8 @@ void findDevirtualizableCallsForTypeCheckedLoad(
     SmallVectorImpl<Instruction *> &LoadedPtrs,
     SmallVectorImpl<Instruction *> &Preds, bool &HasNonCallUses,
     const CallInst *CI, DominatorTree &DT);
+
+Constant *getPointerAtOffset(Constant *I, uint64_t Offset, Module &M);
 }
 
 #endif

include/llvm/IR/FixedMetadataKinds.def

@@ -40,3 +40,4 @@ LLVM_FIXED_MD_KIND(MD_access_group, "llvm.access.group", 25)
 LLVM_FIXED_MD_KIND(MD_callback, "callback", 26)
 LLVM_FIXED_MD_KIND(MD_preserve_access_index, "llvm.preserve.access.index", 27)
 LLVM_FIXED_MD_KIND(MD_misexpect, "misexpect", 28)
+LLVM_FIXED_MD_KIND(MD_vcall_visibility, "vcall_visibility", 29)

include/llvm/IR/GlobalObject.h

@@ -28,6 +28,20 @@ class MDNode;
 class Metadata;
 
 class GlobalObject : public GlobalValue {
+public:
+  // VCallVisibility - values for visibility metadata attached to vtables. This
+  // describes the scope in which a virtual call could end up being dispatched
+  // through this vtable.
+  enum VCallVisibility {
+    // Type is potentially visible to external code.
+    VCallVisibilityPublic = 0,
+    // Type is only visible to code which will be in the current Module after
+    // LTO internalization.
+    VCallVisibilityLinkageUnit = 1,
+    // Type is only visible to code in the current Module.
+    VCallVisibilityTranslationUnit = 2,
+  };
+
 protected:
   GlobalObject(Type *Ty, ValueTy VTy, Use *Ops, unsigned NumOps,
                LinkageTypes Linkage, const Twine &Name,
@@ -164,6 +178,8 @@ public:
   void copyMetadata(const GlobalObject *Src, unsigned Offset);
 
   void addTypeMetadata(unsigned Offset, Metadata *TypeID);
+  void addVCallVisibilityMetadata(VCallVisibility Visibility);
+  VCallVisibility getVCallVisibility() const;
 
 protected:
   void copyAttributesFrom(const GlobalObject *Src);

include/llvm/Transforms/IPO/GlobalDCE.h

@@ -43,11 +43,25 @@ private:
   /// Comdat -> Globals in that Comdat section.
   std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
 
+  /// !type metadata -> set of (vtable, offset) pairs
+  DenseMap<Metadata *, SmallSet<std::pair<GlobalVariable *, uint64_t>, 4>>
+      TypeIdMap;
+
+  // Global variables which are vtables, and which we have enough information
+  // about to safely do dead virtual function elimination.
+  SmallPtrSet<GlobalValue *, 32> VFESafeVTables;
+
   void UpdateGVDependencies(GlobalValue &GV);
   void MarkLive(GlobalValue &GV,
                 SmallVectorImpl<GlobalValue *> *Updates = nullptr);
   bool RemoveUnusedGlobalValue(GlobalValue &GV);
 
+  // Dead virtual function elimination.
+  void AddVirtualFunctionDependencies(Module &M);
+  void ScanVTables(Module &M);
+  void ScanTypeCheckedLoadIntrinsics(Module &M);
+  void ScanVTableLoad(Function *Caller, Metadata *TypeId, uint64_t CallOffset);
+
   void ComputeDependencies(Value *V, SmallPtrSetImpl<GlobalValue *> &U);
 };
 

lib/Analysis/TypeMetadataUtils.cpp

@@ -127,3 +127,35 @@ void llvm::findDevirtualizableCallsForTypeCheckedLoad(
     findCallsAtConstantOffset(DevirtCalls, &HasNonCallUses, LoadedPtr,
                               Offset->getZExtValue(), CI, DT);
 }
+
+Constant *llvm::getPointerAtOffset(Constant *I, uint64_t Offset, Module &M) {
+  if (I->getType()->isPointerTy()) {
+    if (Offset == 0)
+      return I;
+    return nullptr;
+  }
+
+  const DataLayout &DL = M.getDataLayout();
+
+  if (auto *C = dyn_cast<ConstantStruct>(I)) {
+    const StructLayout *SL = DL.getStructLayout(C->getType());
+    if (Offset >= SL->getSizeInBytes())
+      return nullptr;
+
+    unsigned Op = SL->getElementContainingOffset(Offset);
+    return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
+                              Offset - SL->getElementOffset(Op), M);
+  }
+  if (auto *C = dyn_cast<ConstantArray>(I)) {
+    ArrayType *VTableTy = C->getType();
+    uint64_t ElemSize = DL.getTypeAllocSize(VTableTy->getElementType());
+
+    unsigned Op = Offset / ElemSize;
+    if (Op >= C->getNumOperands())
+      return nullptr;
+
+    return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
+                              Offset % ElemSize, M);
+  }
+  return nullptr;
+}

lib/IR/Metadata.cpp

@@ -1497,6 +1497,24 @@ void GlobalObject::addTypeMetadata(unsigned Offset, Metadata *TypeID) {
                      TypeID}));
 }
 
+void GlobalObject::addVCallVisibilityMetadata(VCallVisibility Visibility) {
+  addMetadata(LLVMContext::MD_vcall_visibility,
+              *MDNode::get(getContext(),
+                           {ConstantAsMetadata::get(ConstantInt::get(
+                               Type::getInt64Ty(getContext()), Visibility))}));
+}
+
+GlobalObject::VCallVisibility GlobalObject::getVCallVisibility() const {
+  if (MDNode *MD = getMetadata(LLVMContext::MD_vcall_visibility)) {
+    uint64_t Val = cast<ConstantInt>(
+                       cast<ConstantAsMetadata>(MD->getOperand(0))->getValue())
+                       ->getZExtValue();
+    assert(Val <= 2 && "unknown vcall visibility!");
+    return (VCallVisibility)Val;
+  }
+  return VCallVisibility::VCallVisibilityPublic;
+}
+
 void Function::setSubprogram(DISubprogram *SP) {
   setMetadata(LLVMContext::MD_dbg, SP);
 }

lib/LTO/LTO.cpp

@@ -1004,6 +1004,8 @@ Error LTO::runRegularLTO(AddStreamFn AddStream) {
         GV->setLinkage(GlobalValue::InternalLinkage);
     }
 
+    RegularLTO.CombinedModule->addModuleFlag(Module::Error, "LTOPostLink", 1);
+
     if (Conf.PostInternalizeModuleHook &&
         !Conf.PostInternalizeModuleHook(0, *RegularLTO.CombinedModule))
       return Error::success();

lib/LTO/LTOCodeGenerator.cpp

@@ -463,6 +463,8 @@ void LTOCodeGenerator::applyScopeRestrictions() {
 
   internalizeModule(*MergedModule, mustPreserveGV);
 
+  MergedModule->addModuleFlag(Module::Error, "LTOPostLink", 1);
+
   ScopeRestrictionsDone = true;
 }
 

lib/Transforms/IPO/GlobalDCE.cpp

@@ -17,9 +17,11 @@
 #include "llvm/Transforms/IPO/GlobalDCE.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/TypeMetadataUtils.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
 #include "llvm/Pass.h"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/Utils/CtorUtils.h"
@@ -29,10 +31,15 @@ using namespace llvm;
 
 #define DEBUG_TYPE "globaldce"
 
+static cl::opt<bool>
+    ClEnableVFE("enable-vfe", cl::Hidden, cl::init(true), cl::ZeroOrMore,
+                cl::desc("Enable virtual function elimination"));
+
 STATISTIC(NumAliases  , "Number of global aliases removed");
 STATISTIC(NumFunctions, "Number of functions removed");
 STATISTIC(NumIFuncs,    "Number of indirect functions removed");
 STATISTIC(NumVariables, "Number of global variables removed");
+STATISTIC(NumVFuncs,    "Number of virtual functions removed");
 
 namespace {
   class GlobalDCELegacyPass : public ModulePass {
@@ -118,6 +125,15 @@ void GlobalDCEPass::UpdateGVDependencies(GlobalValue &GV) {
     ComputeDependencies(User, Deps);
   Deps.erase(&GV); // Remove self-reference.
   for (GlobalValue *GVU : Deps) {
+    // If this is a dep from a vtable to a virtual function, and we have
+    // complete information about all virtual call sites which could call
+    // though this vtable, then skip it, because the call site information will
+    // be more precise.
+    if (VFESafeVTables.count(GVU) && isa<Function>(&GV)) {
+      LLVM_DEBUG(dbgs() << "Ignoring dep " << GVU->getName() << " -> "
+                        << GV.getName() << "\n");
+      continue;
+    }
     GVDependencies[GVU].insert(&GV);
   }
 }
@@ -132,12 +148,133 @@ void GlobalDCEPass::MarkLive(GlobalValue &GV,
   if (Updates)
     Updates->push_back(&GV);
   if (Comdat *C = GV.getComdat()) {
-    for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
+    for (auto &&CM : make_range(ComdatMembers.equal_range(C))) {
       MarkLive(*CM.second, Updates); // Recursion depth is only two because only
                                      // globals in the same comdat are visited.
+    }
   }
 }
 
+void GlobalDCEPass::ScanVTables(Module &M) {
+  SmallVector<MDNode *, 2> Types;
+  LLVM_DEBUG(dbgs() << "Building type info -> vtable map\n");
+
+  auto *LTOPostLinkMD =
+      cast_or_null<ConstantAsMetadata>(M.getModuleFlag("LTOPostLink"));
+  bool LTOPostLink =
+      LTOPostLinkMD &&
+      (cast<ConstantInt>(LTOPostLinkMD->getValue())->getZExtValue() != 0);
+
+  for (GlobalVariable &GV : M.globals()) {
+    Types.clear();
+    GV.getMetadata(LLVMContext::MD_type, Types);
+    if (GV.isDeclaration() || Types.empty())
+      continue;
+
+    // Use the typeid metadata on the vtable to build a mapping from typeids to
+    // the list of (GV, offset) pairs which are the possible vtables for that
+    // typeid.
+    for (MDNode *Type : Types) {
+      Metadata *TypeID = Type->getOperand(1).get();
+
+      uint64_t Offset =
+          cast<ConstantInt>(
+              cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
+              ->getZExtValue();
+
+      TypeIdMap[TypeID].insert(std::make_pair(&GV, Offset));
+    }
+
+    // If the type corresponding to the vtable is private to this translation
+    // unit, we know that we can see all virtual functions which might use it,
+    // so VFE is safe.
+    if (auto GO = dyn_cast<GlobalObject>(&GV)) {
+      GlobalObject::VCallVisibility TypeVis = GO->getVCallVisibility();
+      if (TypeVis == GlobalObject::VCallVisibilityTranslationUnit ||
+          (LTOPostLink &&
+           TypeVis == GlobalObject::VCallVisibilityLinkageUnit)) {
+        LLVM_DEBUG(dbgs() << GV.getName() << " is safe for VFE\n");
+        VFESafeVTables.insert(&GV);
+      }
+    }
+  }
+}
+
+void GlobalDCEPass::ScanVTableLoad(Function *Caller, Metadata *TypeId,
+                                   uint64_t CallOffset) {
+  for (auto &VTableInfo : TypeIdMap[TypeId]) {
+    GlobalVariable *VTable = VTableInfo.first;
+    uint64_t VTableOffset = VTableInfo.second;
+
+    Constant *Ptr =
+        getPointerAtOffset(VTable->getInitializer(), VTableOffset + CallOffset,
+                           *Caller->getParent());
+    if (!Ptr) {
+      LLVM_DEBUG(dbgs() << "can't find pointer in vtable!\n");
+      VFESafeVTables.erase(VTable);
+      return;
+    }
+
+    auto Callee = dyn_cast<Function>(Ptr->stripPointerCasts());
+    if (!Callee) {
+      LLVM_DEBUG(dbgs() << "vtable entry is not function pointer!\n");
+      VFESafeVTables.erase(VTable);
+      return;
+    }
+
+    LLVM_DEBUG(dbgs() << "vfunc dep " << Caller->getName() << " -> "
+                      << Callee->getName() << "\n");
+    GVDependencies[Caller].insert(Callee);
+  }
+}
+
+void GlobalDCEPass::ScanTypeCheckedLoadIntrinsics(Module &M) {
+  LLVM_DEBUG(dbgs() << "Scanning type.checked.load intrinsics\n");
+  Function *TypeCheckedLoadFunc =
+      M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
+
+  if (!TypeCheckedLoadFunc)
+    return;
+
+  for (auto U : TypeCheckedLoadFunc->users()) {
+    auto CI = dyn_cast<CallInst>(U);
+    if (!CI)
+      continue;
+
+    auto *Offset = dyn_cast<ConstantInt>(CI->getArgOperand(1));
+    Value *TypeIdValue = CI->getArgOperand(2);
+    auto *TypeId = cast<MetadataAsValue>(TypeIdValue)->getMetadata();
+
+    if (Offset) {
+      ScanVTableLoad(CI->getFunction(), TypeId, Offset->getZExtValue());
+    } else {
+      // type.checked.load with a non-constant offset, so assume every entry in
+      // every matching vtable is used.
+      for (auto &VTableInfo : TypeIdMap[TypeId]) {
+        VFESafeVTables.erase(VTableInfo.first);
+      }
+    }
+  }
+}
+
+void GlobalDCEPass::AddVirtualFunctionDependencies(Module &M) {
+  if (!ClEnableVFE)
+    return;
+
+  ScanVTables(M);
+
+  if (VFESafeVTables.empty())
+    return;
+
+  ScanTypeCheckedLoadIntrinsics(M);
+
+  LLVM_DEBUG(
+    dbgs() << "VFE safe vtables:\n";
+    for (auto *VTable : VFESafeVTables)
+      dbgs() << "  " << VTable->getName() << "\n";
+  );
+}
+
 PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
   bool Changed = false;
 
@@ -163,6 +300,10 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
     if (Comdat *C = GA.getComdat())
       ComdatMembers.insert(std::make_pair(C, &GA));
 
+  // Add dependencies between virtual call sites and the virtual functions they
+  // might call, if we have that information.
+  AddVirtualFunctionDependencies(M);
+
   // Loop over the module, adding globals which are obviously necessary.
   for (GlobalObject &GO : M.global_objects()) {
     Changed |= RemoveUnusedGlobalValue(GO);
@@ -257,8 +398,17 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
   };
 
   NumFunctions += DeadFunctions.size();
-  for (Function *F : DeadFunctions)
+  for (Function *F : DeadFunctions) {
+    if (!F->use_empty()) {
+      // Virtual functions might still be referenced by one or more vtables,
+      // but if we've proven them to be unused then it's safe to replace the
+      // virtual function pointers with null, allowing us to remove the
+      // function itself.
+      ++NumVFuncs;
+      F->replaceNonMetadataUsesWith(ConstantPointerNull::get(F->getType()));
+    }
     EraseUnusedGlobalValue(F);
+  }
 
   NumVariables += DeadGlobalVars.size();
   for (GlobalVariable *GV : DeadGlobalVars)
@@ -277,6 +427,8 @@ PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
   ConstantDependenciesCache.clear();
   GVDependencies.clear();
   ComdatMembers.clear();
+  TypeIdMap.clear();
+  VFESafeVTables.clear();
 
   if (Changed)
     return PreservedAnalyses::none();

lib/Transforms/IPO/WholeProgramDevirt.cpp

@@ -492,7 +492,6 @@ struct DevirtModule {
   void buildTypeIdentifierMap(
       std::vector<VTableBits> &Bits,
       DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap);
-  Constant *getPointerAtOffset(Constant *I, uint64_t Offset);
   bool
   tryFindVirtualCallTargets(std::vector<VirtualCallTarget> &TargetsForSlot,
                             const std::set<TypeMemberInfo> &TypeMemberInfos,
@@ -810,38 +809,6 @@ void DevirtModule::buildTypeIdentifierMap(
   }
 }
 
-Constant *DevirtModule::getPointerAtOffset(Constant *I, uint64_t Offset) {
-  if (I->getType()->isPointerTy()) {
-    if (Offset == 0)
-      return I;
-    return nullptr;
-  }
-
-  const DataLayout &DL = M.getDataLayout();
-
-  if (auto *C = dyn_cast<ConstantStruct>(I)) {
-    const StructLayout *SL = DL.getStructLayout(C->getType());
-    if (Offset >= SL->getSizeInBytes())
-      return nullptr;
-
-    unsigned Op = SL->getElementContainingOffset(Offset);
-    return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
-                              Offset - SL->getElementOffset(Op));
-  }
-  if (auto *C = dyn_cast<ConstantArray>(I)) {
-    ArrayType *VTableTy = C->getType();
-    uint64_t ElemSize = DL.getTypeAllocSize(VTableTy->getElementType());
-
-    unsigned Op = Offset / ElemSize;
-    if (Op >= C->getNumOperands())
-      return nullptr;
-
-    return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
-                              Offset % ElemSize);
-  }
-  return nullptr;
-}
-
 bool DevirtModule::tryFindVirtualCallTargets(
     std::vector<VirtualCallTarget> &TargetsForSlot,
     const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset) {
@@ -850,7 +817,7 @@ bool DevirtModule::tryFindVirtualCallTargets(
       return false;
 
     Constant *Ptr = getPointerAtOffset(TM.Bits->GV->getInitializer(),
-                                       TM.Offset + ByteOffset);
+                                       TM.Offset + ByteOffset, M);
     if (!Ptr)
       return false;
 
@@ -1951,6 +1918,12 @@ bool DevirtModule::run() {
     for (VTableBits &B : Bits)
       rebuildGlobal(B);
 
+  // We have lowered or deleted the type checked load intrinsics, so we no
+  // longer have enough information to reason about the liveness of virtual
+  // function pointers in GlobalDCE.
+  for (GlobalVariable &GV : M.globals())
+    GV.eraseMetadata(LLVMContext::MD_vcall_visibility);
+
   return true;
 }
 

test/LTO/ARM/lto-linking-metadata.ll

@@ -0,0 +1,19 @@
+; RUN: opt %s -o %t1.bc
+
+; RUN: llvm-lto %t1.bc -o %t1.save.opt -save-merged-module -O1 --exported-symbol=foo
+; RUN: llvm-dis < %t1.save.opt.merged.bc | FileCheck %s
+
+; RUN: llvm-lto2 run %t1.bc -o %t.out.o -save-temps \
+; RUN:     -r=%t1.bc,foo,pxl
+; RUN: llvm-dis < %t.out.o.0.2.internalize.bc | FileCheck  %s
+
+target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7a-unknown-linux"
+
+define void @foo() {
+entry:
+  ret void
+}
+
+; CHECK: !llvm.module.flags = !{[[MD_NUM:![0-9]+]]}
+; CHECK: [[MD_NUM]] = !{i32 1, !"LTOPostLink", i32 1}

test/ThinLTO/X86/lazyload_metadata.ll

@@ -10,13 +10,13 @@
 ; RUN: llvm-lto -thinlto-action=import %t2.bc -thinlto-index=%t3.bc \
 ; RUN:          -o /dev/null -stats \
 ; RUN:  2>&1 | FileCheck %s -check-prefix=LAZY
-; LAZY: 63 bitcode-reader  - Number of Metadata records loaded
+; LAZY: 65 bitcode-reader  - Number of Metadata records loaded
 ; LAZY: 2 bitcode-reader  - Number of MDStrings loaded
 
 ; RUN: llvm-lto -thinlto-action=import %t2.bc -thinlto-index=%t3.bc \
 ; RUN:          -o /dev/null -disable-ondemand-mds-loading -stats \
 ; RUN:  2>&1 | FileCheck %s -check-prefix=NOTLAZY
-; NOTLAZY: 72 bitcode-reader  - Number of Metadata records loaded
+; NOTLAZY: 74 bitcode-reader  - Number of Metadata records loaded
 ; NOTLAZY: 7 bitcode-reader  - Number of MDStrings loaded
 
 

test/Transforms/GlobalDCE/virtual-functions-base-call.ll

@@ -0,0 +1,78 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; struct A {
+;   A();
+;   virtual int foo();
+; };
+; 
+; struct B : A {
+;   B();
+;   virtual int foo();
+; };
+; 
+; A::A() {}
+; B::B() {}
+; int A::foo() { return 42; }
+; int B::foo() { return 1337; }
+; 
+; extern "C" int test(A *p) { return p->foo(); }
+
+; The virtual call in test could be dispatched to either A::foo or B::foo, so
+; both must be retained.
+
+%struct.A = type { i32 (...)** }
+%struct.B = type { %struct.A }
+
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*)] }
+@_ZTV1A = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !2
+
+; CHECK: @_ZTV1B = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*)* @_ZN1B3fooEv to i8*)] }
+@_ZTV1B = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*)* @_ZN1B3fooEv to i8*)] }, align 8, !type !0, !type !1, !type !3, !type !4, !vcall_visibility !2
+
+; CHECK: define internal i32 @_ZN1A3fooEv(
+define internal i32 @_ZN1A3fooEv(%struct.A* nocapture readnone %this) {
+entry:
+  ret i32 42
+}
+
+; CHECK: define internal i32 @_ZN1B3fooEv(
+define internal i32 @_ZN1B3fooEv(%struct.B* nocapture readnone %this) {
+entry:
+  ret i32 1337
+}
+
+define hidden void @_ZN1AC2Ev(%struct.A* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden void @_ZN1BC2Ev(%struct.B* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.B, %struct.B* %this, i64 0, i32 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden i32 @test(%struct.A* %p) {
+entry:
+  %0 = bitcast %struct.A* %p to i8**
+  %vtable1 = load i8*, i8** %0, align 8
+  %1 = tail call { i8*, i1 } @llvm.type.checked.load(i8* %vtable1, i32 0, metadata !"_ZTS1A"), !nosanitize !10
+  %2 = extractvalue { i8*, i1 } %1, 0, !nosanitize !10
+  %3 = bitcast i8* %2 to i32 (%struct.A*)*, !nosanitize !10
+  %call = tail call i32 %3(%struct.A* %p)
+  ret i32 %call
+}
+
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata) #2
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
+!2 = !{i64 2}
+!3 = !{i64 16, !"_ZTS1B"}
+!4 = !{i64 16, !"_ZTSM1BFivE.virtual"}
+!10 = !{}

test/Transforms/GlobalDCE/virtual-functions-base-pointer-call.ll

@@ -0,0 +1,118 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; struct A {
+;   A();
+;   virtual int foo(int);
+;   virtual int bar(float);
+; };
+; 
+; struct B : A {
+;   B();
+;   virtual int foo(int);
+;   virtual int bar(float);
+; };
+; 
+; A::A() {}
+; B::B() {}
+; int A::foo(int)   { return 1; }
+; int A::bar(float) { return 2; }
+; int B::foo(int)   { return 3; }
+; int B::bar(float) { return 4; }
+; 
+; extern "C" int test(A *p, int (A::*q)(int)) { return (p->*q)(42); }
+
+; Member function pointers are tracked by the combination of their object type
+; and function type, which must both be compatible. Here, the call is through a
+; pointer of type "int (A::*q)(int)", so the call could be dispatched to A::foo
+; or B::foo. It can't be dispatched to A::bar or B::bar as the function pointer
+; does not match, so those can be removed.
+
+%struct.A = type { i32 (...)** }
+%struct.B = type { %struct.A }
+
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A3fooEi to i8*), i8* null] }
+@_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A3fooEi to i8*), i8* bitcast (i32 (%struct.A*, float)* @_ZN1A3barEf to i8*)] }, align 8, !type !0, !type !1, !type !2, !vcall_visibility !3
+; CHECK: @_ZTV1B = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*), i8* null] }
+@_ZTV1B = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*), i8* bitcast (i32 (%struct.B*, float)* @_ZN1B3barEf to i8*)] }, align 8, !type !0, !type !1, !type !2, !type !4, !type !5, !type !6, !vcall_visibility !3
+
+
+; CHECK: define internal i32 @_ZN1A3fooEi(
+define internal i32 @_ZN1A3fooEi(%struct.A* nocapture readnone %this, i32) unnamed_addr #1 align 2 {
+entry:
+  ret i32 1
+}
+
+; CHECK-NOT: define internal i32 @_ZN1A3barEf(
+define internal i32 @_ZN1A3barEf(%struct.A* nocapture readnone %this, float) unnamed_addr #1 align 2 {
+entry:
+  ret i32 2
+}
+
+; CHECK: define internal i32 @_ZN1B3fooEi(
+define internal i32 @_ZN1B3fooEi(%struct.B* nocapture readnone %this, i32) unnamed_addr #1 align 2 {
+entry:
+  ret i32 3
+}
+
+; CHECK-NOT: define internal i32 @_ZN1B3barEf(
+define internal i32 @_ZN1B3barEf(%struct.B* nocapture readnone %this, float) unnamed_addr #1 align 2 {
+entry:
+  ret i32 4
+}
+
+
+define hidden void @_ZN1AC2Ev(%struct.A* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [4 x i8*] }, { [4 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden void @_ZN1BC2Ev(%struct.B* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.B, %struct.B* %this, i64 0, i32 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [4 x i8*] }, { [4 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden i32 @test(%struct.A* %p, i64 %q.coerce0, i64 %q.coerce1) {
+entry:
+  %0 = bitcast %struct.A* %p to i8*
+  %1 = getelementptr inbounds i8, i8* %0, i64 %q.coerce1
+  %this.adjusted = bitcast i8* %1 to %struct.A*
+  %2 = and i64 %q.coerce0, 1
+  %memptr.isvirtual = icmp eq i64 %2, 0
+  br i1 %memptr.isvirtual, label %memptr.nonvirtual, label %memptr.virtual
+
+memptr.virtual:                                   ; preds = %entry
+  %3 = bitcast i8* %1 to i8**
+  %vtable = load i8*, i8** %3, align 8
+  %4 = add i64 %q.coerce0, -1
+  %5 = getelementptr i8, i8* %vtable, i64 %4, !nosanitize !12
+  %6 = tail call { i8*, i1 } @llvm.type.checked.load(i8* %5, i32 0, metadata !"_ZTSM1AFiiE.virtual"), !nosanitize !12
+  %7 = extractvalue { i8*, i1 } %6, 0, !nosanitize !12
+  %memptr.virtualfn = bitcast i8* %7 to i32 (%struct.A*, i32)*, !nosanitize !12
+  br label %memptr.end
+
+memptr.nonvirtual:                                ; preds = %entry
+  %memptr.nonvirtualfn = inttoptr i64 %q.coerce0 to i32 (%struct.A*, i32)*
+  br label %memptr.end
+
+memptr.end:                                       ; preds = %memptr.nonvirtual, %memptr.virtual
+  %8 = phi i32 (%struct.A*, i32)* [ %memptr.virtualfn, %memptr.virtual ], [ %memptr.nonvirtualfn, %memptr.nonvirtual ]
+  %call = tail call i32 %8(%struct.A* %this.adjusted, i32 42)
+  ret i32 %call
+}
+
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFiiE.virtual"}
+!2 = !{i64 24, !"_ZTSM1AFifE.virtual"}
+!3 = !{i64 2}
+!4 = !{i64 16, !"_ZTS1B"}
+!5 = !{i64 16, !"_ZTSM1BFiiE.virtual"}
+!6 = !{i64 24, !"_ZTSM1BFifE.virtual"}
+!12 = !{}

test/Transforms/GlobalDCE/virtual-functions-derived-call.ll

@@ -0,0 +1,78 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; struct A {
+;   A();
+;   virtual int foo();
+; };
+; 
+; struct B : A {
+;   B();
+;   virtual int foo();
+; };
+; 
+; A::A() {}
+; B::B() {}
+; int A::foo() { return 42; }
+; int B::foo() { return 1337; }
+; 
+; extern "C" int test(B *p) { return p->foo(); }
+
+; The virtual call in test can only be dispatched to B::foo (or a more-derived
+; class, if there was one), so A::foo can be removed.
+
+%struct.A = type { i32 (...)** }
+%struct.B = type { %struct.A }
+
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [3 x i8*] } zeroinitializer
+@_ZTV1A = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !2
+
+; CHECK: @_ZTV1B = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*)* @_ZN1B3fooEv to i8*)] }
+@_ZTV1B = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*)* @_ZN1B3fooEv to i8*)] }, align 8, !type !0, !type !1, !type !3, !type !4, !vcall_visibility !2
+
+; CHECK-NOT: define internal i32 @_ZN1A3fooEv(
+define internal i32 @_ZN1A3fooEv(%struct.A* nocapture readnone %this) {
+entry:
+  ret i32 42
+}
+
+; CHECK: define internal i32 @_ZN1B3fooEv(
+define internal i32 @_ZN1B3fooEv(%struct.B* nocapture readnone %this) {
+entry:
+  ret i32 1337
+}
+
+define hidden void @_ZN1AC2Ev(%struct.A* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden void @_ZN1BC2Ev(%struct.B* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.B, %struct.B* %this, i64 0, i32 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden i32 @test(%struct.B* %p) {
+entry:
+  %0 = bitcast %struct.B* %p to i8**
+  %vtable1 = load i8*, i8** %0, align 8
+  %1 = tail call { i8*, i1 } @llvm.type.checked.load(i8* %vtable1, i32 0, metadata !"_ZTS1B"), !nosanitize !10
+  %2 = extractvalue { i8*, i1 } %1, 0, !nosanitize !10
+  %3 = bitcast i8* %2 to i32 (%struct.B*)*, !nosanitize !10
+  %call = tail call i32 %3(%struct.B* %p)
+  ret i32 %call
+}
+
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata) #2
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
+!2 = !{i64 2}
+!3 = !{i64 16, !"_ZTS1B"}
+!4 = !{i64 16, !"_ZTSM1BFivE.virtual"}
+!10 = !{}

test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll

@@ -0,0 +1,120 @@
+
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; struct A {
+;   A();
+;   virtual int foo(int);
+;   virtual int bar(float);
+; };
+; 
+; struct B : A {
+;   B();
+;   virtual int foo(int);
+;   virtual int bar(float);
+; };
+; 
+; A::A() {}
+; B::B() {}
+; int A::foo(int)   { return 1; }
+; int A::bar(float) { return 2; }
+; int B::foo(int)   { return 3; }
+; int B::bar(float) { return 4; }
+; 
+; extern "C" int test(B *p, int (B::*q)(int)) { return (p->*q)(42); }
+
+; Member function pointers are tracked by the combination of their object type
+; and function type, which must both be compatible. Here, the call is through a
+; pointer of type "int (B::*q)(int)", so the call could only be dispatched to
+; B::foo. It can't be dispatched to A::bar or B::bar as the function pointer
+; does not match, and it can't be dispatched to A::foo as the object type
+; doesn't match, so those can be removed.
+
+%struct.A = type { i32 (...)** }
+%struct.B = type { %struct.A }
+
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } zeroinitializer
+@_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A3fooEi to i8*), i8* bitcast (i32 (%struct.A*, float)* @_ZN1A3barEf to i8*)] }, align 8, !type !0, !type !1, !type !2, !vcall_visibility !3
+; CHECK: @_ZTV1B = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*), i8* null] }
+@_ZTV1B = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*), i8* bitcast (i32 (%struct.B*, float)* @_ZN1B3barEf to i8*)] }, align 8, !type !0, !type !1, !type !2, !type !4, !type !5, !type !6, !vcall_visibility !3
+
+
+; CHECK-NOT: define internal i32 @_ZN1A3fooEi(
+define internal i32 @_ZN1A3fooEi(%struct.A* nocapture readnone %this, i32) unnamed_addr #1 align 2 {
+entry:
+  ret i32 1
+}
+
+; CHECK-NOT: define internal i32 @_ZN1A3barEf(
+define internal i32 @_ZN1A3barEf(%struct.A* nocapture readnone %this, float) unnamed_addr #1 align 2 {
+entry:
+  ret i32 2
+}
+
+; CHECK: define internal i32 @_ZN1B3fooEi(
+define internal i32 @_ZN1B3fooEi(%struct.B* nocapture readnone %this, i32) unnamed_addr #1 align 2 {
+entry:
+  ret i32 3
+}
+
+; CHECK-NOT: define internal i32 @_ZN1B3barEf(
+define internal i32 @_ZN1B3barEf(%struct.B* nocapture readnone %this, float) unnamed_addr #1 align 2 {
+entry:
+  ret i32 4
+}
+
+
+define hidden void @_ZN1AC2Ev(%struct.A* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [4 x i8*] }, { [4 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden void @_ZN1BC2Ev(%struct.B* nocapture %this) {
+entry:
+  %0 = getelementptr inbounds %struct.B, %struct.B* %this, i64 0, i32 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [4 x i8*] }, { [4 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+define hidden i32 @test(%struct.B* %p, i64 %q.coerce0, i64 %q.coerce1) {
+entry:
+  %0 = bitcast %struct.B* %p to i8*
+  %1 = getelementptr inbounds i8, i8* %0, i64 %q.coerce1
+  %this.adjusted = bitcast i8* %1 to %struct.B*
+  %2 = and i64 %q.coerce0, 1
+  %memptr.isvirtual = icmp eq i64 %2, 0
+  br i1 %memptr.isvirtual, label %memptr.nonvirtual, label %memptr.virtual
+
+memptr.virtual:                                   ; preds = %entry
+  %3 = bitcast i8* %1 to i8**
+  %vtable = load i8*, i8** %3, align 8
+  %4 = add i64 %q.coerce0, -1
+  %5 = getelementptr i8, i8* %vtable, i64 %4, !nosanitize !12
+  %6 = tail call { i8*, i1 } @llvm.type.checked.load(i8* %5, i32 0, metadata !"_ZTSM1BFiiE.virtual"), !nosanitize !12
+  %7 = extractvalue { i8*, i1 } %6, 0, !nosanitize !12
+  %memptr.virtualfn = bitcast i8* %7 to i32 (%struct.B*, i32)*, !nosanitize !12
+  br label %memptr.end
+
+memptr.nonvirtual:                                ; preds = %entry
+  %memptr.nonvirtualfn = inttoptr i64 %q.coerce0 to i32 (%struct.B*, i32)*
+  br label %memptr.end
+
+memptr.end:                                       ; preds = %memptr.nonvirtual, %memptr.virtual
+  %8 = phi i32 (%struct.B*, i32)* [ %memptr.virtualfn, %memptr.virtual ], [ %memptr.nonvirtualfn, %memptr.nonvirtual ]
+  %call = tail call i32 %8(%struct.B* %this.adjusted, i32 42)
+  ret i32 %call
+}
+
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFiiE.virtual"}
+!2 = !{i64 24, !"_ZTSM1AFifE.virtual"}
+!3 = !{i64 2}
+!4 = !{i64 16, !"_ZTS1B"}
+!5 = !{i64 16, !"_ZTSM1BFiiE.virtual"}
+!6 = !{i64 24, !"_ZTSM1BFifE.virtual"}
+!12 = !{}

test/Transforms/GlobalDCE/virtual-functions-visibility-post-lto.ll

@@ -0,0 +1,95 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+; structs A, B and C have vcall_visibility of public, linkage-unit and
+; translation-unit respectively. This test is run after LTO linking (the
+; LTOPostLink metadata is present), so B and C can be VFE'd.
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+
+%struct.A = type { i32 (...)** }
+
+@_ZTV1A = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !2
+
+define internal void @_ZN1AC2Ev(%struct.A* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK: define {{.*}} @_ZN1A3fooEv(
+define internal void @_ZN1A3fooEv(%struct.A* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Av() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.A*
+  tail call void @_ZN1AC2Ev(%struct.A* %0)
+  ret i8* %call
+}
+
+
+%struct.B = type { i32 (...)** }
+
+@_ZTV1B = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.B*)* @_ZN1B3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !3
+
+define internal void @_ZN1BC2Ev(%struct.B* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.B, %struct.B* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK-NOT: define {{.*}} @_ZN1B3fooEv(
+define internal void @_ZN1B3fooEv(%struct.B* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Bv() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.B*
+  tail call void @_ZN1BC2Ev(%struct.B* %0)
+  ret i8* %call
+}
+
+
+%struct.C = type { i32 (...)** }
+
+@_ZTV1C = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.C*)* @_ZN1C3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !4
+
+define internal void @_ZN1CC2Ev(%struct.C* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.C, %struct.C* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1C, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK-NOT: define {{.*}} @_ZN1C3fooEv(
+define internal void @_ZN1C3fooEv(%struct.C* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Cv() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.C*
+  tail call void @_ZN1CC2Ev(%struct.C* %0)
+  ret i8* %call
+}
+
+declare dso_local noalias nonnull i8* @_Znwm(i64)
+
+!llvm.module.flags = !{!5}
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
+!2 = !{i64 0} ; public vcall visibility
+!3 = !{i64 1} ; linkage-unit vcall visibility
+!4 = !{i64 2} ; translation-unit vcall visibility
+!5 = !{i32 1, !"LTOPostLink", i32 1}

test/Transforms/GlobalDCE/virtual-functions-visibility-pre-lto.ll

@@ -0,0 +1,94 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+; structs A, B and C have vcall_visibility of public, linkage-unit and
+; translation-unit respectively. This test is run before LTO linking occurs
+; (the LTOPostLink metadata is not present), so only C can be VFE'd.
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+
+%struct.A = type { i32 (...)** }
+
+@_ZTV1A = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !2
+
+define internal void @_ZN1AC2Ev(%struct.A* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK: define {{.*}} @_ZN1A3fooEv(
+define internal void @_ZN1A3fooEv(%struct.A* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Av() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.A*
+  tail call void @_ZN1AC2Ev(%struct.A* %0)
+  ret i8* %call
+}
+
+
+%struct.B = type { i32 (...)** }
+
+@_ZTV1B = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.B*)* @_ZN1B3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !3
+
+define internal void @_ZN1BC2Ev(%struct.B* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.B, %struct.B* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK: define {{.*}} @_ZN1B3fooEv(
+define internal void @_ZN1B3fooEv(%struct.B* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Bv() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.B*
+  tail call void @_ZN1BC2Ev(%struct.B* %0)
+  ret i8* %call
+}
+
+
+%struct.C = type { i32 (...)** }
+
+@_ZTV1C = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.C*)* @_ZN1C3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !4
+
+define internal void @_ZN1CC2Ev(%struct.C* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.C, %struct.C* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1C, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK-NOT: define {{.*}} @_ZN1C3fooEv(
+define internal void @_ZN1C3fooEv(%struct.C* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Cv() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.C*
+  tail call void @_ZN1CC2Ev(%struct.C* %0)
+  ret i8* %call
+}
+
+declare dso_local noalias nonnull i8* @_Znwm(i64)
+
+!llvm.module.flags = !{}
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
+!2 = !{i64 0} ; public vcall visibility
+!3 = !{i64 1} ; linkage-unit vcall visibility
+!4 = !{i64 2} ; translation-unit vcall visibility

test/Transforms/GlobalDCE/virtual-functions.ll

@@ -0,0 +1,55 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare dso_local noalias nonnull i8* @_Znwm(i64)
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+
+; %struct.A is a C++ struct with two virtual functions, A::foo and A::bar. The
+; !vcall_visibility metadata is set on the vtable, so we know that all virtual
+; calls through this vtable are visible and use the @llvm.type.checked.load
+; intrinsic. Function test_A makes a call to A::foo, but there is no call to
+; A::bar anywhere, so A::bar can be deleted, and its vtable slot replaced with
+; null.
+
+%struct.A = type { i32 (...)** }
+
+; The pointer to A::bar in the vtable can be removed, because it will never be
+; loaded. We replace it with null to keep the layout the same. Because it is at
+; the end of the vtable we could potentially shrink the vtable, but don't
+; currently do that.
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*), i8* null] }
+@_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*), i8* bitcast (i32 (%struct.A*)* @_ZN1A3barEv to i8*)] }, align 8, !type !0, !type !1, !type !2, !vcall_visibility !3
+
+; A::foo is called, so must be retained.
+; CHECK: define internal i32 @_ZN1A3fooEv(
+define internal i32 @_ZN1A3fooEv(%struct.A* nocapture readnone %this) {
+entry:
+  ret i32 42
+}
+
+; A::bar is not used, so can be deleted.
+; CHECK-NOT: define internal i32 @_ZN1A3barEv(
+define internal i32 @_ZN1A3barEv(%struct.A* nocapture readnone %this) {
+entry:
+  ret i32 1337
+}
+
+define dso_local i32 @test_A() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.A*
+  %1 = bitcast i8* %call to i32 (...)***
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [4 x i8*] }, { [4 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %1, align 8
+  %2 = tail call { i8*, i1 } @llvm.type.checked.load(i8* bitcast (i8** getelementptr inbounds ({ [4 x i8*] }, { [4 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i8*), i32 0, metadata !"_ZTS1A"), !nosanitize !9
+  %3 = extractvalue { i8*, i1 } %2, 0, !nosanitize !9
+  %4 = bitcast i8* %3 to i32 (%struct.A*)*, !nosanitize !9
+  %call1 = tail call i32 %4(%struct.A* nonnull %0)
+  ret i32 %call1
+}
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
+!2 = !{i64 24, !"_ZTSM1AFivE.virtual"}
+!3 = !{i64 2}
+!9 = !{}

test/Transforms/GlobalDCE/vtable-rtti.ll

@@ -0,0 +1,47 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+; We currently only use llvm.type.checked.load for virtual function pointers,
+; not any other part of the vtable, so we can't remove the RTTI pointer even if
+; it's never going to be loaded from.
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+
+%struct.A = type { i32 (...)** }
+
+; CHECK: @_ZTV1A = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* null] }, align 8, !type !0, !type !1, !vcall_visibility !2
+
+@_ZTV1A = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast (void (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !2
+@_ZTS1A = hidden constant [3 x i8] c"1A\00", align 1
+@_ZTI1A = hidden constant { i8*, i8* } { i8* bitcast (i8** getelementptr inbounds (i8*, i8** @_ZTVN10__cxxabiv117__class_type_infoE, i64 2) to i8*), i8* getelementptr inbounds ([3 x i8], [3 x i8]* @_ZTS1A, i32 0, i32 0) }, align 8
+
+define internal void @_ZN1AC2Ev(%struct.A* %this) {
+entry:
+  %0 = getelementptr inbounds %struct.A, %struct.A* %this, i64 0, i32 0
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV1A, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret void
+}
+
+; CHECK-NOT: define {{.*}} @_ZN1A3fooEv(
+define internal void @_ZN1A3fooEv(%struct.A* nocapture %this) {
+entry:
+  ret void
+}
+
+define dso_local i8* @_Z6make_Av() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8)
+  %0 = bitcast i8* %call to %struct.A*
+  tail call void @_ZN1AC2Ev(%struct.A* %0)
+  ret i8* %call
+}
+
+
+declare dso_local noalias nonnull i8* @_Znwm(i64)
+@_ZTVN10__cxxabiv117__class_type_infoE = external dso_local global i8*
+
+!llvm.module.flags = !{!3}
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
+!2 = !{i64 2} ; translation-unit vcall visibility
+!3 = !{i32 1, !"LTOPostLink", i32 1}

test/Transforms/Internalize/vcall-visibility.ll

@@ -0,0 +1,64 @@
+; RUN: opt < %s -internalize -S | FileCheck %s
+
+%struct.A = type { i32 (...)** }
+%struct.B = type { i32 (...)** }
+%struct.C = type { i32 (...)** }
+
+; Class A has default visibility, so has no !vcall_visibility metadata before
+; or after LTO.
+; CHECK-NOT: @_ZTV1A = {{.*}}!vcall_visibility
+@_ZTV1A = dso_local unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1
+
+; Class B has hidden visibility but public LTO visibility, so has no
+; !vcall_visibility metadata before or after LTO.
+; CHECK-NOT: @_ZTV1B = {{.*}}!vcall_visibility
+@_ZTV1B = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.B*)* @_ZN1B3fooEv to i8*)] }, align 8, !type !2, !type !3
+
+; Class C has hidden visibility, so the !vcall_visibility metadata is set to 1
+; (linkage unit) before LTO, and 2 (translation unit) after LTO.
+; CHECK: @_ZTV1C ={{.*}}!vcall_visibility [[MD_TU_VIS:![0-9]+]]
+@_ZTV1C = hidden unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.C*)* @_ZN1C3fooEv to i8*)] }, align 8, !type !4, !type !5, !vcall_visibility !6
+
+; Class D has translation unit visibility before LTO, and this is not changed
+; by LTO.
+; CHECK: @_ZTVN12_GLOBAL__N_11DE = {{.*}}!vcall_visibility [[MD_TU_VIS:![0-9]+]]
+@_ZTVN12_GLOBAL__N_11DE = internal unnamed_addr constant { [3 x i8*] } zeroinitializer, align 8, !type !7, !type !9, !vcall_visibility !11
+
+define dso_local void @_ZN1A3fooEv(%struct.A* nocapture %this) {
+entry:
+  ret void
+}
+
+define hidden void @_ZN1B3fooEv(%struct.B* nocapture %this) {
+entry:
+  ret void
+}
+
+define hidden void @_ZN1C3fooEv(%struct.C* nocapture %this) {
+entry:
+  ret void
+}
+
+define hidden noalias nonnull i8* @_Z6make_dv() {
+entry:
+  %call = tail call i8* @_Znwm(i64 8) #3
+  %0 = bitcast i8* %call to i32 (...)***
+  store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTVN12_GLOBAL__N_11DE, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8
+  ret i8* %call
+}
+
+declare dso_local noalias nonnull i8* @_Znwm(i64)
+
+; CHECK: [[MD_TU_VIS]] = !{i64 2}
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
+!2 = !{i64 16, !"_ZTS1B"}
+!3 = !{i64 16, !"_ZTSM1BFvvE.virtual"}
+!4 = !{i64 16, !"_ZTS1C"}
+!5 = !{i64 16, !"_ZTSM1CFvvE.virtual"}
+!6 = !{i64 1}
+!7 = !{i64 16, !8}
+!8 = distinct !{}
+!9 = !{i64 16, !10}
+!10 = distinct !{}
+!11 = !{i64 2}