[MS] Emit vftable thunks for functions with incomplete prototypes

[rnk]

Summary

The following class hierarchy requires that we be able to emit a
this-adjusting thunk for B::foo in C's vftable:

struct Incomplete;
struct A {
  virtual A* foo(Incomplete p) = 0;
};
struct B : virtual A {
  void foo(Incomplete p) override;
};
struct C : B { int c; };

This TU is valid, but lacks a definition of 'Incomplete', which makes it
hard to build a thunk for the final overrider, B::foo.

Before this change, Clang gives up attempting to emit the thunk, because
it assumes that if the parameter types are incomplete, it must be
emitting the thunk for optimization purposes. This is untrue for the MS
ABI, where the implementation of B::foo has no idea what thunks C's
vftable may require. Clang needs to emit the thunk without necessarily
having access to the complete prototype of foo.

This change makes Clang emit a musttail variadic call when it needs such
a thunk. I call these "unprototyped" thunks, because they only prototype
the "this" parameter, which must always come first in the MS C++ ABI.

These thunks work, but they create ugly LLVM IR. If the call to the
thunk is devirtualized, it will be a call to a bitcast of a function
pointer. Today, LLVM cannot inline through such a call, but I want to
address that soon, because we also use this pattern for virtual member
pointer thunks.

This change also implements an old FIXME in the code about reusing the
thunk's computed CGFunctionInfo as much as possible. Now we don't end up
computing the thunk's mangled name and arranging it's prototype up to
around three times.

Fixes PR36952

[majnemer]

Does this help PR25641?

[rjmccall]

LGTM.

[rnk]

In D45112#1053539, @majnemer wrote:

Does this help PR25641?

I think so, these are probably duplicate PRs. I think your analysis on that bug is slightly off, because the TU that provides ImplCanvas::createColor *can't* provide the thunks that ImplBitmapCanvas needs. It has no way to predict how the offset from the virtual base to the final overrider will change in derived classes.

[sberg]

see https://bugs.llvm.org/show_bug.cgi?id=37161 "clang-cl triggers ASTContext::getASTRecordLayout Assertion `D && 'Cannot get layout of forward declarations!''" for what appears to be fallout from this change

clang/lib/CodeGen/CGCall.cpp

 }
 
 /// Arrange a thunk that takes 'this' as the first parameter followed by
 /// varargs.  Return a void pointer, regardless of the actual return type.
 /// The body of the thunk will end in a musttail call to a function of the
 /// correct type, and the caller will bitcast the function to the correct
 /// prototype.
 const CGFunctionInfo &
-CodeGenTypes::arrangeMSMemberPointerThunk(const CXXMethodDecl *MD) {
-  assert(MD->isVirtual() && "only virtual memptrs have thunks");
+CodeGenTypes::arrangeUnprototypedMustTailThunk(const CXXMethodDecl *MD) {
+  assert(MD->isVirtual() && "only methods have thunks");
   CanQual<FunctionProtoType> FTP = GetFormalType(MD);
   CanQualType ArgTys[] = { GetThisType(Context, MD->getParent()) };
   return arrangeLLVMFunctionInfo(Context.VoidTy, /*instanceMethod=*/false,
                                  /*chainCall=*/false, ArgTys,
                                  FTP->getExtInfo(), {}, RequiredArgs(1));
 }
 
 const CGFunctionInfo &

clang/lib/CodeGen/CGVTables.h

class CodeGenVTables {
   SecondaryVirtualPointerIndicesMapTy SecondaryVirtualPointerIndices;
 
   /// Cache for the pure virtual member call function.
   llvm::Constant *PureVirtualFn = nullptr;
 
   /// Cache for the deleted virtual member call function.
   llvm::Constant *DeletedVirtualFn = nullptr;
 
-  /// emitThunk - Emit a single thunk.
-  void emitThunk(GlobalDecl GD, const ThunkInfo &Thunk, bool ForVTable);
-
-  /// maybeEmitThunkForVTable - Emit the given thunk for the vtable if needed by
-  /// the ABI.
-  void maybeEmitThunkForVTable(GlobalDecl GD, const ThunkInfo &Thunk);
+  /// Get the address of a thunk and emit it if necessary.
+  llvm::Constant *maybeEmitThunk(GlobalDecl GD,
+                                 const ThunkInfo &ThunkAdjustments,
+                                 bool ForVTable);
 
   void addVTableComponent(ConstantArrayBuilder &builder,
                           const VTableLayout &layout, unsigned idx,
                           llvm::Constant *rtti,
                           unsigned &nextVTableThunkIndex);
 
 public:
   /// Add vtable components for the given vtable layout to the given

clang/lib/CodeGen/CGVTables.cpp

 #include <cstdio>
 
 using namespace clang;
 using namespace CodeGen;
 
 CodeGenVTables::CodeGenVTables(CodeGenModule &CGM)
     : CGM(CGM), VTContext(CGM.getContext().getVTableContext()) {}
 
-llvm::Constant *CodeGenModule::GetAddrOfThunk(GlobalDecl GD,
-                                              const ThunkInfo &Thunk) {
-  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
-
-  // Compute the mangled name.
-  SmallString<256> Name;
-  llvm::raw_svector_ostream Out(Name);
-  if (const CXXDestructorDecl* DD = dyn_cast<CXXDestructorDecl>(MD))
-    getCXXABI().getMangleContext().mangleCXXDtorThunk(DD, GD.getDtorType(),
-                                                      Thunk.This, Out);
-  else
-    getCXXABI().getMangleContext().mangleThunk(MD, Thunk, Out);
-
-  llvm::Type *Ty = getTypes().GetFunctionTypeForVTable(GD);
-  return GetOrCreateLLVMFunction(Name, Ty, GD, /*ForVTable=*/true,
+llvm::Constant *CodeGenModule::GetAddrOfThunk(StringRef Name, llvm::Type *FnTy,
+                                              GlobalDecl GD) {
+  return GetOrCreateLLVMFunction(Name, FnTy, GD, /*ForVTable=*/true,
                                  /*DontDefer=*/true, /*IsThunk=*/true);
 }
 
 static void setThunkProperties(CodeGenModule &CGM, const ThunkInfo &Thunk,
                                llvm::Function *ThunkFn, bool ForVTable,
                                GlobalDecl GD) {
   CGM.setFunctionLinkage(GD, ThunkFn);
   CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
for (llvm::BasicBlock &BB : *Fn) {
       }
     }
   }
 
   return Fn;
 }
 
 void CodeGenFunction::StartThunk(llvm::Function *Fn, GlobalDecl GD,
-                                 const CGFunctionInfo &FnInfo) {
+                                 const CGFunctionInfo &FnInfo,
+                                 bool IsUnprototyped) {
   assert(!CurGD.getDecl() && "CurGD was already set!");
   CurGD = GD;
   CurFuncIsThunk = true;
 
   // Build FunctionArgs.
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
   QualType ThisType = MD->getThisType(getContext());
   const FunctionProtoType *FPT = MD->getType()->getAs<FunctionProtoType>();
   QualType ResultType = CGM.getCXXABI().HasThisReturn(GD)
                             ? ThisType
                             : CGM.getCXXABI().hasMostDerivedReturn(GD)
                                   ? CGM.getContext().VoidPtrTy
                                   : FPT->getReturnType();
   FunctionArgList FunctionArgs;
 
   // Create the implicit 'this' parameter declaration.
   CGM.getCXXABI().buildThisParam(*this, FunctionArgs);
 
-  // Add the rest of the parameters.
+  // Add the rest of the parameters, if we have a prototype to work with.
+  if (!IsUnprototyped) {
     FunctionArgs.append(MD->param_begin(), MD->param_end());
 
     if (isa<CXXDestructorDecl>(MD))
-    CGM.getCXXABI().addImplicitStructorParams(*this, ResultType, FunctionArgs);
+      CGM.getCXXABI().addImplicitStructorParams(*this, ResultType,
+                                                FunctionArgs);
+  }
 
   // Start defining the function.
   auto NL = ApplyDebugLocation::CreateEmpty(*this);
   StartFunction(GlobalDecl(), ResultType, Fn, FnInfo, FunctionArgs,
                 MD->getLocation());
   // Create a scope with an artificial location for the body of this function.
   auto AL = ApplyDebugLocation::CreateArtificial(*this);
 
void CodeGenFunction::FinishThunk() {
   // StartFunction/FinishFunction.
   CurCodeDecl = nullptr;
   CurFuncDecl = nullptr;
 
   FinishFunction();
 }
 
 void CodeGenFunction::EmitCallAndReturnForThunk(llvm::Constant *CalleePtr,
-                                                const ThunkInfo *Thunk) {
+                                                const ThunkInfo *Thunk,
+                                                bool IsUnprototyped) {
   assert(isa<CXXMethodDecl>(CurGD.getDecl()) &&
          "Please use a new CGF for this thunk");
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(CurGD.getDecl());
 
   // Adjust the 'this' pointer if necessary
   llvm::Value *AdjustedThisPtr =
     Thunk ? CGM.getCXXABI().performThisAdjustment(
                           *this, LoadCXXThisAddress(), Thunk->This)
           : LoadCXXThis();
 
-  if (CurFnInfo->usesInAlloca()) {
+  if (CurFnInfo->usesInAlloca() || IsUnprototyped) {
     // We don't handle return adjusting thunks, because they require us to call
     // the copy constructor.  For now, fall through and pretend the return
     // adjustment was empty so we don't crash.
     if (Thunk && !Thunk->Return.isEmpty()) {
+      if (IsUnprototyped)
+        CGM.ErrorUnsupported(
+            MD, "return-adjusting thunk with incomplete parameter type");
+      else
         CGM.ErrorUnsupported(
             MD, "non-trivial argument copy for return-adjusting thunk");
     }
     EmitMustTailThunk(MD, AdjustedThisPtr, CalleePtr);
     return;
   }
 
   // Start building CallArgs.
   CallArgList CallArgs;
   QualType ThisType = MD->getThisType(getContext());
void CodeGenFunction::EmitMustTailThunk(const CXXMethodDecl *MD,
 
   // Finish the function to maintain CodeGenFunction invariants.
   // FIXME: Don't emit unreachable code.
   EmitBlock(createBasicBlock());
   FinishFunction();
 }
 
 void CodeGenFunction::generateThunk(llvm::Function *Fn,
-                                    const CGFunctionInfo &FnInfo,
-                                    GlobalDecl GD, const ThunkInfo &Thunk) {
-  StartThunk(Fn, GD, FnInfo);
+                                    const CGFunctionInfo &FnInfo, GlobalDecl GD,
+                                    const ThunkInfo &Thunk,
+                                    bool IsUnprototyped) {
+  StartThunk(Fn, GD, FnInfo, IsUnprototyped);
   // Create a scope with an artificial location for the body of this function.
   auto AL = ApplyDebugLocation::CreateArtificial(*this);
 
-  // Get our callee.
-  llvm::Type *Ty =
-    CGM.getTypes().GetFunctionType(CGM.getTypes().arrangeGlobalDeclaration(GD));
+  // Get our callee. Use a placeholder type if this method is unprototyped so
+  // that CodeGenModule doesn't try to set attributes.
+  llvm::Type *Ty;
+  if (IsUnprototyped)
+    Ty = llvm::StructType::get(getLLVMContext());
+  else
+    Ty = CGM.getTypes().GetFunctionType(FnInfo);
+
   llvm::Constant *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
 
+  // Fix up the function type for an unprototyped musttail call.
+  if (IsUnprototyped)
+    Callee = llvm::ConstantExpr::getBitCast(Callee, Fn->getType());
+
   // Make the call and return the result.
-  EmitCallAndReturnForThunk(Callee, &Thunk);
+  EmitCallAndReturnForThunk(Callee, &Thunk, IsUnprototyped);
 }
 
-void CodeGenVTables::emitThunk(GlobalDecl GD, const ThunkInfo &Thunk,
-                               bool ForVTable) {
-  const CGFunctionInfo &FnInfo = CGM.getTypes().arrangeGlobalDeclaration(GD);
+static bool shouldEmitVTableThunk(CodeGenModule &CGM, const CXXMethodDecl *MD,
+                                  bool IsUnprototyped, bool ForVTable) {
+  // Always emit thunks in the MS C++ ABI. We cannot rely on other TUs to
+  // provide thunks for us.
+  if (CGM.getTarget().getCXXABI().isMicrosoft())
+    return true;
 
-  // FIXME: re-use FnInfo in this computation.
-  llvm::Constant *C = CGM.GetAddrOfThunk(GD, Thunk);
-  llvm::GlobalValue *Entry;
-
-  // Strip off a bitcast if we got one back.
-  if (llvm::ConstantExpr *CE = dyn_cast<llvm::ConstantExpr>(C)) {
-    assert(CE->getOpcode() == llvm::Instruction::BitCast);
-    Entry = cast<llvm::GlobalValue>(CE->getOperand(0));
-  } else {
-    Entry = cast<llvm::GlobalValue>(C);
-  }
+  // In the Itanium C++ ABI, vtable thunks are provided by TUs that provide
+  // definitions of the main method. Therefore, emitting thunks with the vtable
+  // is purely an optimization. Emit the thunk if optimizations are enabled and
+  // all of the parameter types are complete.
+  if (ForVTable)
+    return CGM.getCodeGenOpts().OptimizationLevel && !IsUnprototyped;
+
+  // Always emit thunks along with the method definition.
+  return true;
 
-  // There's already a declaration with the same name, check if it has the same
-  // type or if we need to replace it.
-  if (Entry->getType()->getElementType() !=
-      CGM.getTypes().GetFunctionTypeForVTable(GD)) {
-    llvm::GlobalValue *OldThunkFn = Entry;
-
-    // If the types mismatch then we have to rewrite the definition.
-    assert(OldThunkFn->isDeclaration() &&
-           "Shouldn't replace non-declaration");
+llvm::Constant *CodeGenVTables::maybeEmitThunk(GlobalDecl GD,
+                                               const ThunkInfo &TI,
+                                               bool ForVTable) {
+  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
+
+  // First, get a declaration. Compute the mangled name. Don't worry about
+  // getting the function prototype right, since we may only need this
+  // declaration to fill in a vtable slot.
+  SmallString<256> Name;
+  MangleContext &MCtx = CGM.getCXXABI().getMangleContext();
+  llvm::raw_svector_ostream Out(Name);
+  if (const CXXDestructorDecl *DD = dyn_cast<CXXDestructorDecl>(MD))
+    MCtx.mangleCXXDtorThunk(DD, GD.getDtorType(), TI.This, Out);
+  else
+    MCtx.mangleThunk(MD, TI, Out);
+  llvm::Type *ThunkVTableTy = CGM.getTypes().GetFunctionTypeForVTable(GD);
+  llvm::Constant *Thunk = CGM.GetAddrOfThunk(Name, ThunkVTableTy, GD);
+
+  // If we don't need to emit a definition, return this declaration as is.
+  bool IsUnprototyped = !CGM.getTypes().isFuncTypeConvertible(
+      MD->getType()->castAs<FunctionType>());
+  if (!shouldEmitVTableThunk(CGM, MD, IsUnprototyped, ForVTable))
+    return Thunk;
+
+  // Arrange a function prototype appropriate for a function definition. In some
+  // cases in the MS ABI, we may need to build an unprototyped musttail thunk.
+  const CGFunctionInfo &FnInfo =
+      IsUnprototyped ? CGM.getTypes().arrangeUnprototypedMustTailThunk(MD)
+                     : CGM.getTypes().arrangeGlobalDeclaration(GD);
+  llvm::FunctionType *ThunkFnTy = CGM.getTypes().GetFunctionType(FnInfo);
+
+  // If the type of the underlying GlobalValue is wrong, we'll have to replace
+  // it. It should be a declaration.
+  llvm::Function *ThunkFn = cast<llvm::Function>(Thunk->stripPointerCasts());
+  if (ThunkFn->getFunctionType() != ThunkFnTy) {
+    llvm::GlobalValue *OldThunkFn = ThunkFn;
+
+    assert(OldThunkFn->isDeclaration() && "Shouldn't replace non-declaration");
 
     // Remove the name from the old thunk function and get a new thunk.
     OldThunkFn->setName(StringRef());
-    Entry = cast<llvm::GlobalValue>(CGM.GetAddrOfThunk(GD, Thunk));
+    ThunkFn = llvm::Function::Create(ThunkFnTy, llvm::Function::ExternalLinkage,
+                                     Name.str(), &CGM.getModule());
+    CGM.SetLLVMFunctionAttributes(MD, FnInfo, ThunkFn);
 
     // If needed, replace the old thunk with a bitcast.
     if (!OldThunkFn->use_empty()) {
       llvm::Constant *NewPtrForOldDecl =
-        llvm::ConstantExpr::getBitCast(Entry, OldThunkFn->getType());
+          llvm::ConstantExpr::getBitCast(ThunkFn, OldThunkFn->getType());
       OldThunkFn->replaceAllUsesWith(NewPtrForOldDecl);
     }
 
     // Remove the old thunk.
     OldThunkFn->eraseFromParent();
   }
 
-  llvm::Function *ThunkFn = cast<llvm::Function>(Entry);
   bool ABIHasKeyFunctions = CGM.getTarget().getCXXABI().hasKeyFunctions();
   bool UseAvailableExternallyLinkage = ForVTable && ABIHasKeyFunctions;
 
   if (!ThunkFn->isDeclaration()) {
     if (!ABIHasKeyFunctions || UseAvailableExternallyLinkage) {
       // There is already a thunk emitted for this function, do nothing.
-      return;
+      return ThunkFn;
     }
 
-    setThunkProperties(CGM, Thunk, ThunkFn, ForVTable, GD);
-    return;
+    setThunkProperties(CGM, TI, ThunkFn, ForVTable, GD);
+    return ThunkFn;
   }
 
+  // If this will be unprototyped, add the "thunk" attribute so that LLVM knows
+  // that the return type is meaningless. These thunks can be used to call
+  // functions with differing return types, and the caller is required to cast
+  // the prototype appropriately to extract the correct value.
+  if (IsUnprototyped)
+    ThunkFn->addFnAttr("thunk");
+
   CGM.SetLLVMFunctionAttributesForDefinition(GD.getDecl(), ThunkFn);
 
-  if (ThunkFn->isVarArg()) {
+  if (!IsUnprototyped && ThunkFn->isVarArg()) {
     // Varargs thunks are special; we can't just generate a call because
     // we can't copy the varargs.  Our implementation is rather
     // expensive/sucky at the moment, so don't generate the thunk unless
     // we have to.
     // FIXME: Do something better here; GenerateVarArgsThunk is extremely ugly.
     if (UseAvailableExternallyLinkage)
-      return;
-    ThunkFn =
-        CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, Thunk);
+      return ThunkFn;
+    ThunkFn = CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD,
+                                                        TI);
   } else {
     // Normal thunk body generation.
-    CodeGenFunction(CGM).generateThunk(ThunkFn, FnInfo, GD, Thunk);
+    CodeGenFunction(CGM).generateThunk(ThunkFn, FnInfo, GD, TI, IsUnprototyped);
   }
 
-  setThunkProperties(CGM, Thunk, ThunkFn, ForVTable, GD);
-}
-
-void CodeGenVTables::maybeEmitThunkForVTable(GlobalDecl GD,
-                                             const ThunkInfo &Thunk) {
-  // If the ABI has key functions, only the TU with the key function should emit
-  // the thunk. However, we can allow inlining of thunks if we emit them with
-  // available_externally linkage together with vtables when optimizations are
-  // enabled.
-  if (CGM.getTarget().getCXXABI().hasKeyFunctions() &&
-      !CGM.getCodeGenOpts().OptimizationLevel)
-    return;
-
-  // We can't emit thunks for member functions with incomplete types.
-  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
-  if (!CGM.getTypes().isFuncTypeConvertible(
-           MD->getType()->castAs<FunctionType>()))
-    return;
-
-  emitThunk(GD, Thunk, /*ForVTable=*/true);
+  setThunkProperties(CGM, TI, ThunkFn, ForVTable, GD);
+  return ThunkFn;
 }
 
-void CodeGenVTables::EmitThunks(GlobalDecl GD)
-{
+void CodeGenVTables::EmitThunks(GlobalDecl GD) {
   const CXXMethodDecl *MD =
     cast<CXXMethodDecl>(GD.getDecl())->getCanonicalDecl();
 
   // We don't need to generate thunks for the base destructor.
   if (isa<CXXDestructorDecl>(MD) && GD.getDtorType() == Dtor_Base)
     return;
 
   const VTableContextBase::ThunkInfoVectorTy *ThunkInfoVector =
       VTContext->getThunkInfo(GD);
 
   if (!ThunkInfoVector)
     return;
 
   for (const ThunkInfo& Thunk : *ThunkInfoVector)
-    emitThunk(GD, Thunk, /*ForVTable=*/false);
+    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];
 
   auto addOffsetConstant = [&](CharUnits offset) {
// Deleted virtual member functions.
           getSpecialVirtualFn(CGM.getCXXABI().GetDeletedVirtualCallName());
       fnPtr = DeletedVirtualFn;
 
     // Thunks.
     } else if (nextVTableThunkIndex < layout.vtable_thunks().size() &&
                layout.vtable_thunks()[nextVTableThunkIndex].first == idx) {
       auto &thunkInfo = layout.vtable_thunks()[nextVTableThunkIndex].second;
 
-      maybeEmitThunkForVTable(GD, thunkInfo);
       nextVTableThunkIndex++;
-      fnPtr = CGM.GetAddrOfThunk(GD, thunkInfo);
+      fnPtr = maybeEmitThunk(GD, thunkInfo, /*ForVTable=*/true);
 
     // Otherwise we can use the method definition directly.
     } else {
       llvm::Type *fnTy = CGM.getTypes().GetFunctionTypeForVTable(GD);
       fnPtr = CGM.GetAddrOfFunction(GD, fnTy, /*ForVTable=*/true);
     }
 
     fnPtr = llvm::ConstantExpr::getBitCast(fnPtr, CGM.Int8PtrTy);

clang/lib/CodeGen/CodeGenFunction.h

public:
   /// return block is is avoided.
   llvm::DebugLoc EmitReturnBlock();
 
   /// FinishFunction - Complete IR generation of the current function. It is
   /// legal to call this function even if there is no current insertion point.
   void FinishFunction(SourceLocation EndLoc=SourceLocation());
 
   void StartThunk(llvm::Function *Fn, GlobalDecl GD,
-                  const CGFunctionInfo &FnInfo);
+                  const CGFunctionInfo &FnInfo, bool IsUnprototyped);
 
-  void EmitCallAndReturnForThunk(llvm::Constant *Callee,
-                                 const ThunkInfo *Thunk);
+  void EmitCallAndReturnForThunk(llvm::Constant *Callee, const ThunkInfo *Thunk,
+                                 bool IsUnprototyped);
 
   void FinishThunk();
 
   /// Emit a musttail call for a thunk with a potentially adjusted this pointer.
   void EmitMustTailThunk(const CXXMethodDecl *MD, llvm::Value *AdjustedThisPtr,
                          llvm::Value *Callee);
 
   /// Generate a thunk for the given method.
   void generateThunk(llvm::Function *Fn, const CGFunctionInfo &FnInfo,
-                     GlobalDecl GD, const ThunkInfo &Thunk);
+                     GlobalDecl GD, const ThunkInfo &Thunk,
+                     bool IsUnprototyped);
 
   llvm::Function *GenerateVarArgsThunk(llvm::Function *Fn,
                                        const CGFunctionInfo &FnInfo,
                                        GlobalDecl GD, const ThunkInfo &Thunk);
 
   void EmitCtorPrologue(const CXXConstructorDecl *CD, CXXCtorType Type,
                         FunctionArgList &Args);
 

clang/lib/CodeGen/CodeGenModule.h

public:
 
   /// Get the address of the RTTI descriptor for the given type.
   llvm::Constant *GetAddrOfRTTIDescriptor(QualType Ty, bool ForEH = false);
 
   /// Get the address of a uuid descriptor .
   ConstantAddress GetAddrOfUuidDescriptor(const CXXUuidofExpr* E);
 
   /// Get the address of the thunk for the given global decl.
-  llvm::Constant *GetAddrOfThunk(GlobalDecl GD, const ThunkInfo &Thunk);
+  llvm::Constant *GetAddrOfThunk(StringRef Name, llvm::Type *FnTy,
+                                 GlobalDecl GD);
 
   /// Get a reference to the target of VD.
   ConstantAddress GetWeakRefReference(const ValueDecl *VD);
 
   /// Returns the assumed alignment of an opaque pointer to the given class.
   CharUnits getClassPointerAlignment(const CXXRecordDecl *CD);
 
   /// Returns the assumed alignment of a virtual base of a class.

clang/lib/CodeGen/CodeGenTypes.h

const CGFunctionInfo &arrangeCXXConstructorCall(const CallArgList &Args,
                                                   unsigned ExtraPrefixArgs,
                                                   unsigned ExtraSuffixArgs,
                                                   bool PassProtoArgs = true);
 
   const CGFunctionInfo &arrangeCXXMethodCall(const CallArgList &args,
                                              const FunctionProtoType *type,
                                              RequiredArgs required,
                                              unsigned numPrefixArgs);
-  const CGFunctionInfo &arrangeMSMemberPointerThunk(const CXXMethodDecl *MD);
+  const CGFunctionInfo &
+  arrangeUnprototypedMustTailThunk(const CXXMethodDecl *MD);
   const CGFunctionInfo &arrangeMSCtorClosure(const CXXConstructorDecl *CD,
                                                  CXXCtorType CT);
   const CGFunctionInfo &arrangeCXXMethodType(const CXXRecordDecl *RD,
                                              const FunctionProtoType *FTP,
                                              const CXXMethodDecl *MD);
 
   /// "Arrange" the LLVM information for a call or type with the given
   /// signature.  This is largely an internal method; other clients

clang/lib/CodeGen/MicrosoftCXXABI.cpp

MicrosoftCXXABI::EmitVirtualMemPtrThunk(const CXXMethodDecl *MD,
   llvm::raw_svector_ostream Out(ThunkName);
   getMangleContext().mangleVirtualMemPtrThunk(MD, ML, Out);
 
   // If the thunk has been generated previously, just return it.
   if (llvm::GlobalValue *GV = CGM.getModule().getNamedValue(ThunkName))
     return cast<llvm::Function>(GV);
 
   // Create the llvm::Function.
-  const CGFunctionInfo &FnInfo = CGM.getTypes().arrangeMSMemberPointerThunk(MD);
+  const CGFunctionInfo &FnInfo =
+      CGM.getTypes().arrangeUnprototypedMustTailThunk(MD);
   llvm::FunctionType *ThunkTy = CGM.getTypes().GetFunctionType(FnInfo);
   llvm::Function *ThunkFn =
       llvm::Function::Create(ThunkTy, llvm::Function::ExternalLinkage,
                              ThunkName.str(), &CGM.getModule());
   assert(ThunkFn->getName() == ThunkName && "name was uniqued!");
 
   ThunkFn->setLinkage(MD->isExternallyVisible()
                           ? llvm::GlobalValue::LinkOnceODRLinkage

clang/test/CodeGenCXX/ms-thunks-unprototyped-return.cpp

+// RUN: %clang_cc1 -fno-rtti-data -triple x86_64-windows-msvc -emit-llvm-only %s -verify
+
+// Verify that we error out on this return adjusting thunk that we can't emit.
+
+struct Incomplete;
+
+struct A {
+  virtual A * clone(Incomplete p) = 0;
+};
+struct B : virtual A {
+  // expected-error@+1 2 {{cannot compile this return-adjusting thunk with incomplete parameter type yet}}
+  B * clone(Incomplete p) override;
+};
+struct C : B { int c; };
+C c;

clang/test/CodeGenCXX/ms-thunks-unprototyped.cpp

+// RUN: %clang_cc1 -fno-rtti-data -triple x86_64-windows-msvc -emit-llvm %s -o - | FileCheck %s
+
+// In this example, C does not override B::foo, but it needs to emit a thunk to
+// adjust for the relative difference of position between A-in-B and A-in-C.
+
+struct Incomplete;
+template <typename T>
+struct DoNotInstantiate {
+  typename T::does_not_exist field;
+};
+template <typename T>
+struct InstantiateLater;
+
+struct A {
+  virtual void foo(Incomplete p) = 0;
+  virtual void bar(DoNotInstantiate<int> p) = 0;
+  virtual int baz(InstantiateLater<int> p) = 0;
+};
+struct B : virtual A {
+  void foo(Incomplete p) override;
+  void bar(DoNotInstantiate<int> p) override;
+  inline int baz(InstantiateLater<int> p) override;
+};
+struct C : B { int c; };
+C c;
+
+// CHECK: @"??_7C@@6B@" = linkonce_odr unnamed_addr constant
+// CHECK-SAME: void (%struct.B*, ...)* @"?foo@B@@W7EAAXUIncomplete@@@Z"
+// CHECK-SAME: void (%struct.B*, ...)* @"?bar@B@@W7EAAXU?$DoNotInstantiate@H@@@Z"
+// CHECK-SAME: i32 (i8*, i32)* @"?baz@B@@W7EAAHU?$InstantiateLater@H@@@Z"
+
+// The thunks should have a -8 adjustment.
+
+// CHECK-LABEL: define linkonce_odr dso_local void @"?foo@B@@W7EAAXUIncomplete@@@Z"(%struct.B* %this, ...)
+// CHECK: %[[THIS_ADJ_i8:[^ ]*]] = getelementptr i8, i8* {{.*}}, i32 -8
+// CHECK: %[[THIS_ADJ:[^ ]*]] = bitcast i8* %[[THIS_ADJ_i8]] to %struct.B*
+// CHECK: musttail call void (%struct.B*, ...) {{.*}}@"?foo@B@@UEAAXUIncomplete@@@Z"
+// CHECK-SAME: (%struct.B* %[[THIS_ADJ]], ...)
+// CHECK-NEXT: ret void
+
+// CHECK-LABEL: define linkonce_odr dso_local void @"?bar@B@@W7EAAXU?$DoNotInstantiate@H@@@Z"(%struct.B* %this, ...)
+// CHECK: %[[THIS_ADJ_i8:[^ ]*]] = getelementptr i8, i8* {{.*}}, i32 -8
+// CHECK: %[[THIS_ADJ:[^ ]*]] = bitcast i8* %[[THIS_ADJ_i8]] to %struct.B*
+// CHECK: musttail call void (%struct.B*, ...) {{.*}}@"?bar@B@@UEAAXU?$DoNotInstantiate@H@@@Z"
+// CHECK-SAME: (%struct.B* %[[THIS_ADJ]], ...)
+// CHECK-NEXT: ret void
+
+// If we complete the definition later, things work out.
+template <typename T> struct InstantiateLater { T x; };
+inline int B::baz(InstantiateLater<int> p) { return p.x; }
+
+// CHECK-LABEL: define linkonce_odr dso_local i32 @"?baz@B@@W7EAAHU?$InstantiateLater@H@@@Z"(i8* %this.coerce, i32 %p.coerce)
+// CHECK: = getelementptr i8, i8* {{.*}}, i32 -8
+// CHECK: tail call i32 @"?baz@B@@UEAAHU?$InstantiateLater@H@@@Z"(i8* {{[^,]*}}, i32 {{.*}})
+
+// CHECK-LABEL: define linkonce_odr dso_local i32 @"?baz@B@@UEAAHU?$InstantiateLater@H@@@Z"(i8* %this.coerce, i32 %p.coerce)