clang/lib/CodeGen/CGVTables.cpp

   1 //===--- CGVTables.cpp - Emit LLVM Code for C++ vtables -------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This contains code dealing with C++ code generation of virtual tables.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "CGCXXABI.h"
  14 #include "CodeGenFunction.h"
  15 #include "CodeGenModule.h"
  16 #include "clang/AST/Attr.h"
  17 #include "clang/AST/CXXInheritance.h"
  18 #include "clang/AST/RecordLayout.h"
  19 #include "clang/Basic/CodeGenOptions.h"
  20 #include "clang/CodeGen/CGFunctionInfo.h"
  21 #include "clang/CodeGen/ConstantInitBuilder.h"
  22 #include "llvm/IR/IntrinsicInst.h"
  23 #include "llvm/Support/Format.h"
  24 #include "llvm/Transforms/Utils/Cloning.h"
  25 #include <algorithm>
  26 #include <cstdio>
  27
  28 using namespace clang;
  29 using namespace CodeGen;
  30
  31 CodeGenVTables::CodeGenVTables(CodeGenModule &CGM)
  32     : CGM(CGM), VTContext(CGM.getContext().getVTableContext()) {}
  33
  34 llvm::Constant *CodeGenModule::GetAddrOfThunk(StringRef Name, llvm::Type *FnTy,
  35                                               GlobalDecl GD) {
  36   return GetOrCreateLLVMFunction(Name, FnTy, GD, /*ForVTable=*/true,
  37                                  /*DontDefer=*/true, /*IsThunk=*/true);
  38 }
  39
  40 static void setThunkProperties(CodeGenModule &CGM, const ThunkInfo &Thunk,
  41                                llvm::Function *ThunkFn, bool ForVTable,
  42                                GlobalDecl GD) {
  43   CGM.setFunctionLinkage(GD, ThunkFn);
  44   CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
  45                                   !Thunk.Return.isEmpty());
  46
  47   // Set the right visibility.
  48   CGM.setGVProperties(ThunkFn, GD);
  49
  50   if (!CGM.getCXXABI().exportThunk()) {
  51     ThunkFn->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
  52     ThunkFn->setDSOLocal(true);
  53   }
  54
  55   if (CGM.supportsCOMDAT() && ThunkFn->isWeakForLinker())
  56     ThunkFn->setComdat(CGM.getModule().getOrInsertComdat(ThunkFn->getName()));
  57 }
  58
  59 #ifndef NDEBUG
  60 static bool similar(const ABIArgInfo &infoL, CanQualType typeL,
  61                     const ABIArgInfo &infoR, CanQualType typeR) {
  62   return (infoL.getKind() == infoR.getKind() &&
  63           (typeL == typeR ||
  64            (isa<PointerType>(typeL) && isa<PointerType>(typeR)) ||
  65            (isa<ReferenceType>(typeL) && isa<ReferenceType>(typeR))));
  66 }
  67 #endif
  68
  69 static RValue PerformReturnAdjustment(CodeGenFunction &CGF,
  70                                       QualType ResultType, RValue RV,
  71                                       const ThunkInfo &Thunk) {
  72   // Emit the return adjustment.
  73   bool NullCheckValue = !ResultType->isReferenceType();
  74
  75   llvm::BasicBlock *AdjustNull = nullptr;
  76   llvm::BasicBlock *AdjustNotNull = nullptr;
  77   llvm::BasicBlock *AdjustEnd = nullptr;
  78
  79   llvm::Value *ReturnValue = RV.getScalarVal();
  80
  81   if (NullCheckValue) {
  82     AdjustNull = CGF.createBasicBlock("adjust.null");
  83     AdjustNotNull = CGF.createBasicBlock("adjust.notnull");
  84     AdjustEnd = CGF.createBasicBlock("adjust.end");
  85
  86     llvm::Value *IsNull = CGF.Builder.CreateIsNull(ReturnValue);
  87     CGF.Builder.CreateCondBr(IsNull, AdjustNull, AdjustNotNull);
  88     CGF.EmitBlock(AdjustNotNull);
  89   }
  90
  91   auto ClassDecl = ResultType->getPointeeType()->getAsCXXRecordDecl();
  92   auto ClassAlign = CGF.CGM.getClassPointerAlignment(ClassDecl);
  93   ReturnValue = CGF.CGM.getCXXABI().performReturnAdjustment(
  94       CGF,
  95       Address(ReturnValue, CGF.ConvertTypeForMem(ResultType->getPointeeType()),
  96               ClassAlign),
  97       Thunk.Return);
  98
  99   if (NullCheckValue) {
 100     CGF.Builder.CreateBr(AdjustEnd);
 101     CGF.EmitBlock(AdjustNull);
 102     CGF.Builder.CreateBr(AdjustEnd);
 103     CGF.EmitBlock(AdjustEnd);
 104
 105     llvm::PHINode *PHI = CGF.Builder.CreatePHI(ReturnValue->getType(), 2);
 106     PHI->addIncoming(ReturnValue, AdjustNotNull);
 107     PHI->addIncoming(llvm::Constant::getNullValue(ReturnValue->getType()),
 108                      AdjustNull);
 109     ReturnValue = PHI;
 110   }
 111
 112   return RValue::get(ReturnValue);
 113 }
 114
 115 /// This function clones a function's DISubprogram node and enters it into
 116 /// a value map with the intent that the map can be utilized by the cloner
 117 /// to short-circuit Metadata node mapping.
 118 /// Furthermore, the function resolves any DILocalVariable nodes referenced
 119 /// by dbg.value intrinsics so they can be properly mapped during cloning.
 120 static void resolveTopLevelMetadata(llvm::Function *Fn,
 121                                     llvm::ValueToValueMapTy &VMap) {
 122   // Clone the DISubprogram node and put it into the Value map.
 123   auto *DIS = Fn->getSubprogram();
 124   if (!DIS)
 125     return;
 126   auto *NewDIS = DIS->replaceWithDistinct(DIS->clone());
 127   VMap.MD()[DIS].reset(NewDIS);
 128
 129   // Find all llvm.dbg.declare intrinsics and resolve the DILocalVariable nodes
 130   // they are referencing.
 131   for (auto &BB : *Fn) {
 132     for (auto &I : BB) {
 133       if (auto *DII = dyn_cast<llvm::DbgVariableIntrinsic>(&I)) {
 134         auto *DILocal = DII->getVariable();
 135         if (!DILocal->isResolved())
 136           DILocal->resolve();
 137       }
 138     }
 139   }
 140 }
 141
 142 // This function does roughly the same thing as GenerateThunk, but in a
 143 // very different way, so that va_start and va_end work correctly.
 144 // FIXME: This function assumes "this" is the first non-sret LLVM argument of
 145 //        a function, and that there is an alloca built in the entry block
 146 //        for all accesses to "this".
 147 // FIXME: This function assumes there is only one "ret" statement per function.
 148 // FIXME: Cloning isn't correct in the presence of indirect goto!
 149 // FIXME: This implementation of thunks bloats codesize by duplicating the
 150 //        function definition.  There are alternatives:
 151 //        1. Add some sort of stub support to LLVM for cases where we can
 152 //           do a this adjustment, then a sibcall.
 153 //        2. We could transform the definition to take a va_list instead of an
 154 //           actual variable argument list, then have the thunks (including a
 155 //           no-op thunk for the regular definition) call va_start/va_end.
 156 //           There's a bit of per-call overhead for this solution, but it's
 157 //           better for codesize if the definition is long.
 158 llvm::Function *
 159 CodeGenFunction::GenerateVarArgsThunk(llvm::Function *Fn,
 160                                       const CGFunctionInfo &FnInfo,
 161                                       GlobalDecl GD, const ThunkInfo &Thunk) {
 162   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
 163   const FunctionProtoType *FPT = MD->getType()->castAs<FunctionProtoType>();
 164   QualType ResultType = FPT->getReturnType();
 165
 166   // Get the original function
 167   assert(FnInfo.isVariadic());
 168   llvm::Type *Ty = CGM.getTypes().GetFunctionType(FnInfo);
 169   llvm::Value *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
 170   llvm::Function *BaseFn = cast<llvm::Function>(Callee);
 171
 172   // Cloning can't work if we don't have a definition. The Microsoft ABI may
 173   // require thunks when a definition is not available. Emit an error in these
 174   // cases.
 175   if (!MD->isDefined()) {
 176     CGM.ErrorUnsupported(MD, "return-adjusting thunk with variadic arguments");
 177     return Fn;
 178   }
 179   assert(!BaseFn->isDeclaration() && "cannot clone undefined variadic method");
 180
 181   // Clone to thunk.
 182   llvm::ValueToValueMapTy VMap;
 183
 184   // We are cloning a function while some Metadata nodes are still unresolved.
 185   // Ensure that the value mapper does not encounter any of them.
 186   resolveTopLevelMetadata(BaseFn, VMap);
 187   llvm::Function *NewFn = llvm::CloneFunction(BaseFn, VMap);
 188   Fn->replaceAllUsesWith(NewFn);
 189   NewFn->takeName(Fn);
 190   Fn->eraseFromParent();
 191   Fn = NewFn;
 192
 193   // "Initialize" CGF (minimally).
 194   CurFn = Fn;
 195
 196   // Get the "this" value
 197   llvm::Function::arg_iterator AI = Fn->arg_begin();
 198   if (CGM.ReturnTypeUsesSRet(FnInfo))
 199     ++AI;
 200
 201   // Find the first store of "this", which will be to the alloca associated
 202   // with "this".
 203   Address ThisPtr =
 204       Address(&*AI, ConvertTypeForMem(MD->getThisType()->getPointeeType()),
 205               CGM.getClassPointerAlignment(MD->getParent()));
 206   llvm::BasicBlock *EntryBB = &Fn->front();
 207   llvm::BasicBlock::iterator ThisStore =
 208       llvm::find_if(*EntryBB, [&](llvm::Instruction &I) {
 209         return isa<llvm::StoreInst>(I) &&
 210                I.getOperand(0) == ThisPtr.getPointer();
 211       });
 212   assert(ThisStore != EntryBB->end() &&
 213          "Store of this should be in entry block?");
 214   // Adjust "this", if necessary.
 215   Builder.SetInsertPoint(&*ThisStore);
 216   llvm::Value *AdjustedThisPtr =
 217       CGM.getCXXABI().performThisAdjustment(*this, ThisPtr, Thunk.This);
 218   AdjustedThisPtr = Builder.CreateBitCast(AdjustedThisPtr,
 219                                           ThisStore->getOperand(0)->getType());
 220   ThisStore->setOperand(0, AdjustedThisPtr);
 221
 222   if (!Thunk.Return.isEmpty()) {
 223     // Fix up the returned value, if necessary.
 224     for (llvm::BasicBlock &BB : *Fn) {
 225       llvm::Instruction *T = BB.getTerminator();
 226       if (isa<llvm::ReturnInst>(T)) {
 227         RValue RV = RValue::get(T->getOperand(0));
 228         T->eraseFromParent();
 229         Builder.SetInsertPoint(&BB);
 230         RV = PerformReturnAdjustment(*this, ResultType, RV, Thunk);
 231         Builder.CreateRet(RV.getScalarVal());
 232         break;
 233       }
 234     }
 235   }
 236
 237   return Fn;
 238 }
 239
 240 void CodeGenFunction::StartThunk(llvm::Function *Fn, GlobalDecl GD,
 241                                  const CGFunctionInfo &FnInfo,
 242                                  bool IsUnprototyped) {
 243   assert(!CurGD.getDecl() && "CurGD was already set!");
 244   CurGD = GD;
 245   CurFuncIsThunk = true;
 246
 247   // Build FunctionArgs.
 248   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
 249   QualType ThisType = MD->getThisType();
 250   QualType ResultType;
 251   if (IsUnprototyped)
 252     ResultType = CGM.getContext().VoidTy;
 253   else if (CGM.getCXXABI().HasThisReturn(GD))
 254     ResultType = ThisType;
 255   else if (CGM.getCXXABI().hasMostDerivedReturn(GD))
 256     ResultType = CGM.getContext().VoidPtrTy;
 257   else
 258     ResultType = MD->getType()->castAs<FunctionProtoType>()->getReturnType();
 259   FunctionArgList FunctionArgs;
 260
 261   // Create the implicit 'this' parameter declaration.
 262   CGM.getCXXABI().buildThisParam(*this, FunctionArgs);
 263
 264   // Add the rest of the parameters, if we have a prototype to work with.
 265   if (!IsUnprototyped) {
 266     FunctionArgs.append(MD->param_begin(), MD->param_end());
 267
 268     if (isa<CXXDestructorDecl>(MD))
 269       CGM.getCXXABI().addImplicitStructorParams(*this, ResultType,
 270                                                 FunctionArgs);
 271   }
 272
 273   // Start defining the function.
 274   auto NL = ApplyDebugLocation::CreateEmpty(*this);
 275   StartFunction(GlobalDecl(), ResultType, Fn, FnInfo, FunctionArgs,
 276                 MD->getLocation());
 277   // Create a scope with an artificial location for the body of this function.
 278   auto AL = ApplyDebugLocation::CreateArtificial(*this);
 279
 280   // Since we didn't pass a GlobalDecl to StartFunction, do this ourselves.
 281   CGM.getCXXABI().EmitInstanceFunctionProlog(*this);
 282   CXXThisValue = CXXABIThisValue;
 283   CurCodeDecl = MD;
 284   CurFuncDecl = MD;
 285 }
 286
 287 void CodeGenFunction::FinishThunk() {
 288   // Clear these to restore the invariants expected by
 289   // StartFunction/FinishFunction.
 290   CurCodeDecl = nullptr;
 291   CurFuncDecl = nullptr;
 292
 293   FinishFunction();
 294 }
 295
 296 void CodeGenFunction::EmitCallAndReturnForThunk(llvm::FunctionCallee Callee,
 297                                                 const ThunkInfo *Thunk,
 298                                                 bool IsUnprototyped) {
 299   assert(isa<CXXMethodDecl>(CurGD.getDecl()) &&
 300          "Please use a new CGF for this thunk");
 301   const CXXMethodDecl *MD = cast<CXXMethodDecl>(CurGD.getDecl());
 302
 303   // Adjust the 'this' pointer if necessary
 304   llvm::Value *AdjustedThisPtr =
 305     Thunk ? CGM.getCXXABI().performThisAdjustment(
 306                           *this, LoadCXXThisAddress(), Thunk->This)
 307           : LoadCXXThis();
 308
 309   // If perfect forwarding is required a variadic method, a method using
 310   // inalloca, or an unprototyped thunk, use musttail. Emit an error if this
 311   // thunk requires a return adjustment, since that is impossible with musttail.
 312   if (CurFnInfo->usesInAlloca() || CurFnInfo->isVariadic() || IsUnprototyped) {
 313     if (Thunk && !Thunk->Return.isEmpty()) {
 314       if (IsUnprototyped)
 315         CGM.ErrorUnsupported(
 316             MD, "return-adjusting thunk with incomplete parameter type");
 317       else if (CurFnInfo->isVariadic())
 318         llvm_unreachable("shouldn't try to emit musttail return-adjusting "
 319                          "thunks for variadic functions");
 320       else
 321         CGM.ErrorUnsupported(
 322             MD, "non-trivial argument copy for return-adjusting thunk");
 323     }
 324     EmitMustTailThunk(CurGD, AdjustedThisPtr, Callee);
 325     return;
 326   }
 327
 328   // Start building CallArgs.
 329   CallArgList CallArgs;
 330   QualType ThisType = MD->getThisType();
 331   CallArgs.add(RValue::get(AdjustedThisPtr), ThisType);
 332
 333   if (isa<CXXDestructorDecl>(MD))
 334     CGM.getCXXABI().adjustCallArgsForDestructorThunk(*this, CurGD, CallArgs);
 335
 336 #ifndef NDEBUG
 337   unsigned PrefixArgs = CallArgs.size() - 1;
 338 #endif
 339   // Add the rest of the arguments.
 340   for (const ParmVarDecl *PD : MD->parameters())
 341     EmitDelegateCallArg(CallArgs, PD, SourceLocation());
 342
 343   const FunctionProtoType *FPT = MD->getType()->castAs<FunctionProtoType>();
 344
 345 #ifndef NDEBUG
 346   const CGFunctionInfo &CallFnInfo = CGM.getTypes().arrangeCXXMethodCall(
 347       CallArgs, FPT, RequiredArgs::forPrototypePlus(FPT, 1), PrefixArgs);
 348   assert(CallFnInfo.getRegParm() == CurFnInfo->getRegParm() &&
 349          CallFnInfo.isNoReturn() == CurFnInfo->isNoReturn() &&
 350          CallFnInfo.getCallingConvention() == CurFnInfo->getCallingConvention());
 351   assert(isa<CXXDestructorDecl>(MD) || // ignore dtor return types
 352          similar(CallFnInfo.getReturnInfo(), CallFnInfo.getReturnType(),
 353                  CurFnInfo->getReturnInfo(), CurFnInfo->getReturnType()));
 354   assert(CallFnInfo.arg_size() == CurFnInfo->arg_size());
 355   for (unsigned i = 0, e = CurFnInfo->arg_size(); i != e; ++i)
 356     assert(similar(CallFnInfo.arg_begin()[i].info,
 357                    CallFnInfo.arg_begin()[i].type,
 358                    CurFnInfo->arg_begin()[i].info,
 359                    CurFnInfo->arg_begin()[i].type));
 360 #endif
 361
 362   // Determine whether we have a return value slot to use.
 363   QualType ResultType = CGM.getCXXABI().HasThisReturn(CurGD)
 364                             ? ThisType
 365                             : CGM.getCXXABI().hasMostDerivedReturn(CurGD)
 366                                   ? CGM.getContext().VoidPtrTy
 367                                   : FPT->getReturnType();
 368   ReturnValueSlot Slot;
 369   if (!ResultType->isVoidType() &&
 370       (CurFnInfo->getReturnInfo().getKind() == ABIArgInfo::Indirect ||
 371        hasAggregateEvaluationKind(ResultType)))
 372     Slot = ReturnValueSlot(ReturnValue, ResultType.isVolatileQualified(),
 373                            /*IsUnused=*/false, /*IsExternallyDestructed=*/true);
 374
 375   // Now emit our call.
 376   llvm::CallBase *CallOrInvoke;
 377   RValue RV = EmitCall(*CurFnInfo, CGCallee::forDirect(Callee, CurGD), Slot,
 378                        CallArgs, &CallOrInvoke);
 379
 380   // Consider return adjustment if we have ThunkInfo.
 381   if (Thunk && !Thunk->Return.isEmpty())
 382     RV = PerformReturnAdjustment(*this, ResultType, RV, *Thunk);
 383   else if (llvm::CallInst* Call = dyn_cast<llvm::CallInst>(CallOrInvoke))
 384     Call->setTailCallKind(llvm::CallInst::TCK_Tail);
 385
 386   // Emit return.
 387   if (!ResultType->isVoidType() && Slot.isNull())
 388     CGM.getCXXABI().EmitReturnFromThunk(*this, RV, ResultType);
 389
 390   // Disable the final ARC autorelease.
 391   AutoreleaseResult = false;
 392
 393   FinishThunk();
 394 }
 395
 396 void CodeGenFunction::EmitMustTailThunk(GlobalDecl GD,
 397                                         llvm::Value *AdjustedThisPtr,
 398                                         llvm::FunctionCallee Callee) {
 399   // Emitting a musttail call thunk doesn't use any of the CGCall.cpp machinery
 400   // to translate AST arguments into LLVM IR arguments.  For thunks, we know
 401   // that the caller prototype more or less matches the callee prototype with
 402   // the exception of 'this'.
 403   SmallVector<llvm::Value *, 8> Args(llvm::make_pointer_range(CurFn->args()));
 404
 405   // Set the adjusted 'this' pointer.
 406   const ABIArgInfo &ThisAI = CurFnInfo->arg_begin()->info;
 407   if (ThisAI.isDirect()) {
 408     const ABIArgInfo &RetAI = CurFnInfo->getReturnInfo();
 409     int ThisArgNo = RetAI.isIndirect() && !RetAI.isSRetAfterThis() ? 1 : 0;
 410     llvm::Type *ThisType = Args[ThisArgNo]->getType();
 411     if (ThisType != AdjustedThisPtr->getType())
 412       AdjustedThisPtr = Builder.CreateBitCast(AdjustedThisPtr, ThisType);
 413     Args[ThisArgNo] = AdjustedThisPtr;
 414   } else {
 415     assert(ThisAI.isInAlloca() && "this is passed directly or inalloca");
 416     Address ThisAddr = GetAddrOfLocalVar(CXXABIThisDecl);
 417     llvm::Type *ThisType = ThisAddr.getElementType();
 418     if (ThisType != AdjustedThisPtr->getType())
 419       AdjustedThisPtr = Builder.CreateBitCast(AdjustedThisPtr, ThisType);
 420     Builder.CreateStore(AdjustedThisPtr, ThisAddr);
 421   }
 422
 423   // Emit the musttail call manually.  Even if the prologue pushed cleanups, we
 424   // don't actually want to run them.
 425   llvm::CallInst *Call = Builder.CreateCall(Callee, Args);
 426   Call->setTailCallKind(llvm::CallInst::TCK_MustTail);
 427
 428   // Apply the standard set of call attributes.
 429   unsigned CallingConv;
 430   llvm::AttributeList Attrs;
 431   CGM.ConstructAttributeList(Callee.getCallee()->getName(), *CurFnInfo, GD,
 432                              Attrs, CallingConv, /*AttrOnCallSite=*/true,
 433                              /*IsThunk=*/false);
 434   Call->setAttributes(Attrs);
 435   Call->setCallingConv(static_cast<llvm::CallingConv::ID>(CallingConv));
 436
 437   if (Call->getType()->isVoidTy())
 438     Builder.CreateRetVoid();
 439   else
 440     Builder.CreateRet(Call);
 441
 442   // Finish the function to maintain CodeGenFunction invariants.
 443   // FIXME: Don't emit unreachable code.
 444   EmitBlock(createBasicBlock());
 445
 446   FinishThunk();
 447 }
 448
 449 void CodeGenFunction::generateThunk(llvm::Function *Fn,
 450                                     const CGFunctionInfo &FnInfo, GlobalDecl GD,
 451                                     const ThunkInfo &Thunk,
 452                                     bool IsUnprototyped) {
 453   StartThunk(Fn, GD, FnInfo, IsUnprototyped);
 454   // Create a scope with an artificial location for the body of this function.
 455   auto AL = ApplyDebugLocation::CreateArtificial(*this);
 456
 457   // Get our callee. Use a placeholder type if this method is unprototyped so
 458   // that CodeGenModule doesn't try to set attributes.
 459   llvm::Type *Ty;
 460   if (IsUnprototyped)
 461     Ty = llvm::StructType::get(getLLVMContext());
 462   else
 463     Ty = CGM.getTypes().GetFunctionType(FnInfo);
 464
 465   llvm::Constant *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
 466
 467   // Fix up the function type for an unprototyped musttail call.
 468   if (IsUnprototyped)
 469     Callee = llvm::ConstantExpr::getBitCast(Callee, Fn->getType());
 470
 471   // Make the call and return the result.
 472   EmitCallAndReturnForThunk(llvm::FunctionCallee(Fn->getFunctionType(), Callee),
 473                             &Thunk, IsUnprototyped);
 474 }
 475
 476 static bool shouldEmitVTableThunk(CodeGenModule &CGM, const CXXMethodDecl *MD,
 477                                   bool IsUnprototyped, bool ForVTable) {
 478   // Always emit thunks in the MS C++ ABI. We cannot rely on other TUs to
 479   // provide thunks for us.
 480   if (CGM.getTarget().getCXXABI().isMicrosoft())
 481     return true;
 482
 483   // In the Itanium C++ ABI, vtable thunks are provided by TUs that provide
 484   // definitions of the main method. Therefore, emitting thunks with the vtable
 485   // is purely an optimization. Emit the thunk if optimizations are enabled and
 486   // all of the parameter types are complete.
 487   if (ForVTable)
 488     return CGM.getCodeGenOpts().OptimizationLevel && !IsUnprototyped;
 489
 490   // Always emit thunks along with the method definition.
 491   return true;
 492 }
 493
 494 llvm::Constant *CodeGenVTables::maybeEmitThunk(GlobalDecl GD,
 495                                                const ThunkInfo &TI,
 496                                                bool ForVTable) {
 497   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
 498
 499   // First, get a declaration. Compute the mangled name. Don't worry about
 500   // getting the function prototype right, since we may only need this
 501   // declaration to fill in a vtable slot.
 502   SmallString<256> Name;
 503   MangleContext &MCtx = CGM.getCXXABI().getMangleContext();
 504   llvm::raw_svector_ostream Out(Name);
 505   if (const CXXDestructorDecl *DD = dyn_cast<CXXDestructorDecl>(MD))
 506     MCtx.mangleCXXDtorThunk(DD, GD.getDtorType(), TI.This, Out);
 507   else
 508     MCtx.mangleThunk(MD, TI, Out);
 509   llvm::Type *ThunkVTableTy = CGM.getTypes().GetFunctionTypeForVTable(GD);
 510   llvm::Constant *Thunk = CGM.GetAddrOfThunk(Name, ThunkVTableTy, GD);
 511
 512   // If we don't need to emit a definition, return this declaration as is.
 513   bool IsUnprototyped = !CGM.getTypes().isFuncTypeConvertible(
 514       MD->getType()->castAs<FunctionType>());
 515   if (!shouldEmitVTableThunk(CGM, MD, IsUnprototyped, ForVTable))
 516     return Thunk;
 517
 518   // Arrange a function prototype appropriate for a function definition. In some
 519   // cases in the MS ABI, we may need to build an unprototyped musttail thunk.
 520   const CGFunctionInfo &FnInfo =
 521       IsUnprototyped ? CGM.getTypes().arrangeUnprototypedMustTailThunk(MD)
 522                      : CGM.getTypes().arrangeGlobalDeclaration(GD);
 523   llvm::FunctionType *ThunkFnTy = CGM.getTypes().GetFunctionType(FnInfo);
 524
 525   // If the type of the underlying GlobalValue is wrong, we'll have to replace
 526   // it. It should be a declaration.
 527   llvm::Function *ThunkFn = cast<llvm::Function>(Thunk->stripPointerCasts());
 528   if (ThunkFn->getFunctionType() != ThunkFnTy) {
 529     llvm::GlobalValue *OldThunkFn = ThunkFn;
 530
 531     assert(OldThunkFn->isDeclaration() && "Shouldn't replace non-declaration");
 532
 533     // Remove the name from the old thunk function and get a new thunk.
 534     OldThunkFn->setName(StringRef());
 535     ThunkFn = llvm::Function::Create(ThunkFnTy, llvm::Function::ExternalLinkage,
 536                                      Name.str(), &CGM.getModule());
 537     CGM.SetLLVMFunctionAttributes(MD, FnInfo, ThunkFn, /*IsThunk=*/false);
 538
 539     // If needed, replace the old thunk with a bitcast.
 540     if (!OldThunkFn->use_empty()) {
 541       llvm::Constant *NewPtrForOldDecl =
 542           llvm::ConstantExpr::getBitCast(ThunkFn, OldThunkFn->getType());
 543       OldThunkFn->replaceAllUsesWith(NewPtrForOldDecl);
 544     }
 545
 546     // Remove the old thunk.
 547     OldThunkFn->eraseFromParent();
 548   }
 549
 550   bool ABIHasKeyFunctions = CGM.getTarget().getCXXABI().hasKeyFunctions();
 551   bool UseAvailableExternallyLinkage = ForVTable && ABIHasKeyFunctions;
 552
 553   if (!ThunkFn->isDeclaration()) {
 554     if (!ABIHasKeyFunctions || UseAvailableExternallyLinkage) {
 555       // There is already a thunk emitted for this function, do nothing.
 556       return ThunkFn;
 557     }
 558
 559     setThunkProperties(CGM, TI, ThunkFn, ForVTable, GD);
 560     return ThunkFn;
 561   }
 562
 563   // If this will be unprototyped, add the "thunk" attribute so that LLVM knows
 564   // that the return type is meaningless. These thunks can be used to call
 565   // functions with differing return types, and the caller is required to cast
 566   // the prototype appropriately to extract the correct value.
 567   if (IsUnprototyped)
 568     ThunkFn->addFnAttr("thunk");
 569
 570   CGM.SetLLVMFunctionAttributesForDefinition(GD.getDecl(), ThunkFn);
 571
 572   // Thunks for variadic methods are special because in general variadic
 573   // arguments cannot be perfectly forwarded. In the general case, clang
 574   // implements such thunks by cloning the original function body. However, for
 575   // thunks with no return adjustment on targets that support musttail, we can
 576   // use musttail to perfectly forward the variadic arguments.
 577   bool ShouldCloneVarArgs = false;
 578   if (!IsUnprototyped && ThunkFn->isVarArg()) {
 579     ShouldCloneVarArgs = true;
 580     if (TI.Return.isEmpty()) {
 581       switch (CGM.getTriple().getArch()) {
 582       case llvm::Triple::x86_64:
 583       case llvm::Triple::x86:
 584       case llvm::Triple::aarch64:
 585         ShouldCloneVarArgs = false;
 586         break;
 587       default:
 588         break;
 589       }
 590     }
 591   }
 592
 593   if (ShouldCloneVarArgs) {
 594     if (UseAvailableExternallyLinkage)
 595       return ThunkFn;
 596     ThunkFn =
 597         CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, TI);
 598   } else {
 599     // Normal thunk body generation.
 600     CodeGenFunction(CGM).generateThunk(ThunkFn, FnInfo, GD, TI, IsUnprototyped);
 601   }
 602
 603   setThunkProperties(CGM, TI, ThunkFn, ForVTable, GD);
 604   return ThunkFn;
 605 }
 606
 607 void CodeGenVTables::EmitThunks(GlobalDecl GD) {
 608   const CXXMethodDecl *MD =
 609     cast<CXXMethodDecl>(GD.getDecl())->getCanonicalDecl();
 610
 611   // We don't need to generate thunks for the base destructor.
 612   if (isa<CXXDestructorDecl>(MD) && GD.getDtorType() == Dtor_Base)
 613     return;
 614
 615   const VTableContextBase::ThunkInfoVectorTy *ThunkInfoVector =
 616       VTContext->getThunkInfo(GD);
 617
 618   if (!ThunkInfoVector)
 619     return;
 620
 621   for (const ThunkInfo& Thunk : *ThunkInfoVector)
 622     maybeEmitThunk(GD, Thunk, /*ForVTable=*/false);
 623 }
 624
 625 void CodeGenVTables::addRelativeComponent(ConstantArrayBuilder &builder,
 626                                           llvm::Constant *component,
 627                                           unsigned vtableAddressPoint,
 628                                           bool vtableHasLocalLinkage,
 629                                           bool isCompleteDtor) const {
 630   // No need to get the offset of a nullptr.
 631   if (component->isNullValue())
 632     return builder.add(llvm::ConstantInt::get(CGM.Int32Ty, 0));
 633
 634   auto *globalVal =
 635       cast<llvm::GlobalValue>(component->stripPointerCastsAndAliases());
 636   llvm::Module &module = CGM.getModule();
 637
 638   // We don't want to copy the linkage of the vtable exactly because we still
 639   // want the stub/proxy to be emitted for properly calculating the offset.
 640   // Examples where there would be no symbol emitted are available_externally
 641   // and private linkages.
 642   auto stubLinkage = vtableHasLocalLinkage ? llvm::GlobalValue::InternalLinkage
 643                                            : llvm::GlobalValue::ExternalLinkage;
 644
 645   llvm::Constant *target;
 646   if (auto *func = dyn_cast<llvm::Function>(globalVal)) {
 647     target = llvm::DSOLocalEquivalent::get(func);
 648   } else {
 649     llvm::SmallString<16> rttiProxyName(globalVal->getName());
 650     rttiProxyName.append(".rtti_proxy");
 651
 652     // The RTTI component may not always be emitted in the same linkage unit as
 653     // the vtable. As a general case, we can make a dso_local proxy to the RTTI
 654     // that points to the actual RTTI struct somewhere. This will result in a
 655     // GOTPCREL relocation when taking the relative offset to the proxy.
 656     llvm::GlobalVariable *proxy = module.getNamedGlobal(rttiProxyName);
 657     if (!proxy) {
 658       proxy = new llvm::GlobalVariable(module, globalVal->getType(),
 659                                        /*isConstant=*/true, stubLinkage,
 660                                        globalVal, rttiProxyName);
 661       proxy->setDSOLocal(true);
 662       proxy->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
 663       if (!proxy->hasLocalLinkage()) {
 664         proxy->setVisibility(llvm::GlobalValue::HiddenVisibility);
 665         proxy->setComdat(module.getOrInsertComdat(rttiProxyName));
 666       }
 667       // Do not instrument the rtti proxies with hwasan to avoid a duplicate
 668       // symbol error. Aliases generated by hwasan will retain the same namebut
 669       // the addresses they are set to may have different tags from different
 670       // compilation units. We don't run into this without hwasan because the
 671       // proxies are in comdat groups, but those aren't propagated to the alias.
 672       RemoveHwasanMetadata(proxy);
 673     }
 674     target = proxy;
 675   }
 676
 677   builder.addRelativeOffsetToPosition(CGM.Int32Ty, target,
 678                                       /*position=*/vtableAddressPoint);
 679 }
 680
 681 static bool UseRelativeLayout(const CodeGenModule &CGM) {
 682   return CGM.getTarget().getCXXABI().isItaniumFamily() &&
 683          CGM.getItaniumVTableContext().isRelativeLayout();
 684 }
 685
 686 bool CodeGenVTables::useRelativeLayout() const {
 687   return UseRelativeLayout(CGM);
 688 }
 689
 690 llvm::Type *CodeGenModule::getVTableComponentType() const {
 691   if (UseRelativeLayout(*this))
 692     return Int32Ty;
 693   return Int8PtrTy;
 694 }
 695
 696 llvm::Type *CodeGenVTables::getVTableComponentType() const {
 697   return CGM.getVTableComponentType();
 698 }
 699
 700 static void AddPointerLayoutOffset(const CodeGenModule &CGM,
 701                                    ConstantArrayBuilder &builder,
 702                                    CharUnits offset) {
 703   builder.add(llvm::ConstantExpr::getIntToPtr(
 704       llvm::ConstantInt::get(CGM.PtrDiffTy, offset.getQuantity()),
 705       CGM.Int8PtrTy));
 706 }
 707
 708 static void AddRelativeLayoutOffset(const CodeGenModule &CGM,
 709                                     ConstantArrayBuilder &builder,
 710                                     CharUnits offset) {
 711   builder.add(llvm::ConstantInt::get(CGM.Int32Ty, offset.getQuantity()));
 712 }
 713
 714 void CodeGenVTables::addVTableComponent(ConstantArrayBuilder &builder,
 715                                         const VTableLayout &layout,
 716                                         unsigned componentIndex,
 717                                         llvm::Constant *rtti,
 718                                         unsigned &nextVTableThunkIndex,
 719                                         unsigned vtableAddressPoint,
 720                                         bool vtableHasLocalLinkage) {
 721   auto &component = layout.vtable_components()[componentIndex];
 722
 723   auto addOffsetConstant =
 724       useRelativeLayout() ? AddRelativeLayoutOffset : AddPointerLayoutOffset;
 725
 726   switch (component.getKind()) {
 727   case VTableComponent::CK_VCallOffset:
 728     return addOffsetConstant(CGM, builder, component.getVCallOffset());
 729
 730   case VTableComponent::CK_VBaseOffset:
 731     return addOffsetConstant(CGM, builder, component.getVBaseOffset());
 732
 733   case VTableComponent::CK_OffsetToTop:
 734     return addOffsetConstant(CGM, builder, component.getOffsetToTop());
 735
 736   case VTableComponent::CK_RTTI:
 737     if (useRelativeLayout())
 738       return addRelativeComponent(builder, rtti, vtableAddressPoint,
 739                                   vtableHasLocalLinkage,
 740                                   /*isCompleteDtor=*/false);
 741     else
 742       return builder.add(llvm::ConstantExpr::getBitCast(rtti, CGM.Int8PtrTy));
 743
 744   case VTableComponent::CK_FunctionPointer:
 745   case VTableComponent::CK_CompleteDtorPointer:
 746   case VTableComponent::CK_DeletingDtorPointer: {
 747     GlobalDecl GD = component.getGlobalDecl();
 748
 749     if (CGM.getLangOpts().CUDA) {
 750       // Emit NULL for methods we can't codegen on this
 751       // side. Otherwise we'd end up with vtable with unresolved
 752       // references.
 753       const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
 754       // OK on device side: functions w/ __device__ attribute
 755       // OK on host side: anything except __device__-only functions.
 756       bool CanEmitMethod =
 757           CGM.getLangOpts().CUDAIsDevice
 758               ? MD->hasAttr<CUDADeviceAttr>()
 759               : (MD->hasAttr<CUDAHostAttr>() || !MD->hasAttr<CUDADeviceAttr>());
 760       if (!CanEmitMethod)
 761         return builder.add(llvm::ConstantExpr::getNullValue(CGM.Int8PtrTy));
 762       // Method is acceptable, continue processing as usual.
 763     }
 764
 765     auto getSpecialVirtualFn = [&](StringRef name) -> llvm::Constant * {
 766       // FIXME(PR43094): When merging comdat groups, lld can select a local
 767       // symbol as the signature symbol even though it cannot be accessed
 768       // outside that symbol's TU. The relative vtables ABI would make
 769       // __cxa_pure_virtual and __cxa_deleted_virtual local symbols, and
 770       // depending on link order, the comdat groups could resolve to the one
 771       // with the local symbol. As a temporary solution, fill these components
 772       // with zero. We shouldn't be calling these in the first place anyway.
 773       if (useRelativeLayout())
 774         return llvm::ConstantPointerNull::get(CGM.Int8PtrTy);
 775
 776       // For NVPTX devices in OpenMP emit special functon as null pointers,
 777       // otherwise linking ends up with unresolved references.
 778       if (CGM.getLangOpts().OpenMP && CGM.getLangOpts().OpenMPIsDevice &&
 779           CGM.getTriple().isNVPTX())
 780         return llvm::ConstantPointerNull::get(CGM.Int8PtrTy);
 781       llvm::FunctionType *fnTy =
 782           llvm::FunctionType::get(CGM.VoidTy, /*isVarArg=*/false);
 783       llvm::Constant *fn = cast<llvm::Constant>(
 784           CGM.CreateRuntimeFunction(fnTy, name).getCallee());
 785       if (auto f = dyn_cast<llvm::Function>(fn))
 786         f->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
 787       return llvm::ConstantExpr::getBitCast(fn, CGM.Int8PtrTy);
 788     };
 789
 790     llvm::Constant *fnPtr;
 791
 792     // Pure virtual member functions.
 793     if (cast<CXXMethodDecl>(GD.getDecl())->isPure()) {
 794       if (!PureVirtualFn)
 795         PureVirtualFn =
 796             getSpecialVirtualFn(CGM.getCXXABI().GetPureVirtualCallName());
 797       fnPtr = PureVirtualFn;
 798
 799     // Deleted virtual member functions.
 800     } else if (cast<CXXMethodDecl>(GD.getDecl())->isDeleted()) {
 801       if (!DeletedVirtualFn)
 802         DeletedVirtualFn =
 803             getSpecialVirtualFn(CGM.getCXXABI().GetDeletedVirtualCallName());
 804       fnPtr = DeletedVirtualFn;
 805
 806     // Thunks.
 807     } else if (nextVTableThunkIndex < layout.vtable_thunks().size() &&
 808                layout.vtable_thunks()[nextVTableThunkIndex].first ==
 809                    componentIndex) {
 810       auto &thunkInfo = layout.vtable_thunks()[nextVTableThunkIndex].second;
 811
 812       nextVTableThunkIndex++;
 813       fnPtr = maybeEmitThunk(GD, thunkInfo, /*ForVTable=*/true);
 814
 815     // Otherwise we can use the method definition directly.
 816     } else {
 817       llvm::Type *fnTy = CGM.getTypes().GetFunctionTypeForVTable(GD);
 818       fnPtr = CGM.GetAddrOfFunction(GD, fnTy, /*ForVTable=*/true);
 819     }
 820
 821     if (useRelativeLayout()) {
 822       return addRelativeComponent(
 823           builder, fnPtr, vtableAddressPoint, vtableHasLocalLinkage,
 824           component.getKind() == VTableComponent::CK_CompleteDtorPointer);
 825     } else
 826       return builder.add(llvm::ConstantExpr::getBitCast(fnPtr, CGM.Int8PtrTy));
 827   }
 828
 829   case VTableComponent::CK_UnusedFunctionPointer:
 830     if (useRelativeLayout())
 831       return builder.add(llvm::ConstantExpr::getNullValue(CGM.Int32Ty));
 832     else
 833       return builder.addNullPointer(CGM.Int8PtrTy);
 834   }
 835
 836   llvm_unreachable("Unexpected vtable component kind");
 837 }
 838
 839 llvm::Type *CodeGenVTables::getVTableType(const VTableLayout &layout) {
 840   SmallVector<llvm::Type *, 4> tys;
 841   llvm::Type *componentType = getVTableComponentType();
 842   for (unsigned i = 0, e = layout.getNumVTables(); i != e; ++i)
 843     tys.push_back(llvm::ArrayType::get(componentType, layout.getVTableSize(i)));
 844
 845   return llvm::StructType::get(CGM.getLLVMContext(), tys);
 846 }
 847
 848 void CodeGenVTables::createVTableInitializer(ConstantStructBuilder &builder,
 849                                              const VTableLayout &layout,
 850                                              llvm::Constant *rtti,
 851                                              bool vtableHasLocalLinkage) {
 852   llvm::Type *componentType = getVTableComponentType();
 853
 854   const auto &addressPoints = layout.getAddressPointIndices();
 855   unsigned nextVTableThunkIndex = 0;
 856   for (unsigned vtableIndex = 0, endIndex = layout.getNumVTables();
 857        vtableIndex != endIndex; ++vtableIndex) {
 858     auto vtableElem = builder.beginArray(componentType);
 859
 860     size_t vtableStart = layout.getVTableOffset(vtableIndex);
 861     size_t vtableEnd = vtableStart + layout.getVTableSize(vtableIndex);
 862     for (size_t componentIndex = vtableStart; componentIndex < vtableEnd;
 863          ++componentIndex) {
 864       addVTableComponent(vtableElem, layout, componentIndex, rtti,
 865                          nextVTableThunkIndex, addressPoints[vtableIndex],
 866                          vtableHasLocalLinkage);
 867     }
 868     vtableElem.finishAndAddTo(builder);
 869   }
 870 }
 871
 872 llvm::GlobalVariable *CodeGenVTables::GenerateConstructionVTable(
 873     const CXXRecordDecl *RD, const BaseSubobject &Base, bool BaseIsVirtual,
 874     llvm::GlobalVariable::LinkageTypes Linkage,
 875     VTableAddressPointsMapTy &AddressPoints) {
 876   if (CGDebugInfo *DI = CGM.getModuleDebugInfo())
 877     DI->completeClassData(Base.getBase());
 878
 879   std::unique_ptr<VTableLayout> VTLayout(
 880       getItaniumVTableContext().createConstructionVTableLayout(
 881           Base.getBase(), Base.getBaseOffset(), BaseIsVirtual, RD));
 882
 883   // Add the address points.
 884   AddressPoints = VTLayout->getAddressPoints();
 885
 886   // Get the mangled construction vtable name.
 887   SmallString<256> OutName;
 888   llvm::raw_svector_ostream Out(OutName);
 889   cast<ItaniumMangleContext>(CGM.getCXXABI().getMangleContext())
 890       .mangleCXXCtorVTable(RD, Base.getBaseOffset().getQuantity(),
 891                            Base.getBase(), Out);
 892   SmallString<256> Name(OutName);
 893
 894   bool UsingRelativeLayout = getItaniumVTableContext().isRelativeLayout();
 895   bool VTableAliasExists =
 896       UsingRelativeLayout && CGM.getModule().getNamedAlias(Name);
 897   if (VTableAliasExists) {
 898     // We previously made the vtable hidden and changed its name.
 899     Name.append(".local");
 900   }
 901
 902   llvm::Type *VTType = getVTableType(*VTLayout);
 903
 904   // Construction vtable symbols are not part of the Itanium ABI, so we cannot
 905   // guarantee that they actually will be available externally. Instead, when
 906   // emitting an available_externally VTT, we provide references to an internal
 907   // linkage construction vtable. The ABI only requires complete-object vtables
 908   // to be the same for all instances of a type, not construction vtables.
 909   if (Linkage == llvm::GlobalVariable::AvailableExternallyLinkage)
 910     Linkage = llvm::GlobalVariable::InternalLinkage;
 911
 912   llvm::Align Align = CGM.getDataLayout().getABITypeAlign(VTType);
 913
 914   // Create the variable that will hold the construction vtable.
 915   llvm::GlobalVariable *VTable =
 916       CGM.CreateOrReplaceCXXRuntimeVariable(Name, VTType, Linkage, Align);
 917
 918   // V-tables are always unnamed_addr.
 919   VTable->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
 920
 921   llvm::Constant *RTTI = CGM.GetAddrOfRTTIDescriptor(
 922       CGM.getContext().getTagDeclType(Base.getBase()));
 923
 924   // Create and set the initializer.
 925   ConstantInitBuilder builder(CGM);
 926   auto components = builder.beginStruct();
 927   createVTableInitializer(components, *VTLayout, RTTI,
 928                           VTable->hasLocalLinkage());
 929   components.finishAndSetAsInitializer(VTable);
 930
 931   // Set properties only after the initializer has been set to ensure that the
 932   // GV is treated as definition and not declaration.
 933   assert(!VTable->isDeclaration() && "Shouldn't set properties on declaration");
 934   CGM.setGVProperties(VTable, RD);
 935
 936   CGM.EmitVTableTypeMetadata(RD, VTable, *VTLayout.get());
 937
 938   if (UsingRelativeLayout) {
 939     RemoveHwasanMetadata(VTable);
 940     if (!VTable->isDSOLocal())
 941       GenerateRelativeVTableAlias(VTable, OutName);
 942   }
 943
 944   return VTable;
 945 }
 946
 947 // Ensure this vtable is not instrumented by hwasan. That is, a global alias is
 948 // not generated for it. This is mainly used by the relative-vtables ABI where
 949 // vtables instead contain 32-bit offsets between the vtable and function
 950 // pointers. Hwasan is disabled for these vtables for now because the tag in a
 951 // vtable pointer may fail the overflow check when resolving 32-bit PLT
 952 // relocations. A future alternative for this would be finding which usages of
 953 // the vtable can continue to use the untagged hwasan value without any loss of
 954 // value in hwasan.
 955 void CodeGenVTables::RemoveHwasanMetadata(llvm::GlobalValue *GV) const {
 956   if (CGM.getLangOpts().Sanitize.has(SanitizerKind::HWAddress)) {
 957     llvm::GlobalValue::SanitizerMetadata Meta;
 958     if (GV->hasSanitizerMetadata())
 959       Meta = GV->getSanitizerMetadata();
 960     Meta.NoHWAddress = true;
 961     GV->setSanitizerMetadata(Meta);
 962   }
 963 }
 964
 965 // If the VTable is not dso_local, then we will not be able to indicate that
 966 // the VTable does not need a relocation and move into rodata. A frequent
 967 // time this can occur is for classes that should be made public from a DSO
 968 // (like in libc++). For cases like these, we can make the vtable hidden or
 969 // private and create a public alias with the same visibility and linkage as
 970 // the original vtable type.
 971 void CodeGenVTables::GenerateRelativeVTableAlias(llvm::GlobalVariable *VTable,
 972                                                  llvm::StringRef AliasNameRef) {
 973   assert(getItaniumVTableContext().isRelativeLayout() &&
 974          "Can only use this if the relative vtable ABI is used");
 975   assert(!VTable->isDSOLocal() && "This should be called only if the vtable is "
 976                                   "not guaranteed to be dso_local");
 977
 978   // If the vtable is available_externally, we shouldn't (or need to) generate
 979   // an alias for it in the first place since the vtable won't actually by
 980   // emitted in this compilation unit.
 981   if (VTable->hasAvailableExternallyLinkage())
 982     return;
 983
 984   // Create a new string in the event the alias is already the name of the
 985   // vtable. Using the reference directly could lead to use of an inititialized
 986   // value in the module's StringMap.
 987   llvm::SmallString<256> AliasName(AliasNameRef);
 988   VTable->setName(AliasName + ".local");
 989
 990   auto Linkage = VTable->getLinkage();
 991   assert(llvm::GlobalAlias::isValidLinkage(Linkage) &&
 992          "Invalid vtable alias linkage");
 993
 994   llvm::GlobalAlias *VTableAlias = CGM.getModule().getNamedAlias(AliasName);
 995   if (!VTableAlias) {
 996     VTableAlias = llvm::GlobalAlias::create(VTable->getValueType(),
 997                                             VTable->getAddressSpace(), Linkage,
 998                                             AliasName, &CGM.getModule());
 999   } else {
1000     assert(VTableAlias->getValueType() == VTable->getValueType());
1001     assert(VTableAlias->getLinkage() == Linkage);
1002   }
1003   VTableAlias->setVisibility(VTable->getVisibility());
1004   VTableAlias->setUnnamedAddr(VTable->getUnnamedAddr());
1005
1006   // Both of these imply dso_local for the vtable.
1007   if (!VTable->hasComdat()) {
1008     // If this is in a comdat, then we shouldn't make the linkage private due to
1009     // an issue in lld where private symbols can be used as the key symbol when
1010     // choosing the prevelant group. This leads to "relocation refers to a
1011     // symbol in a discarded section".
1012     VTable->setLinkage(llvm::GlobalValue::PrivateLinkage);
1013   } else {
1014     // We should at least make this hidden since we don't want to expose it.
1015     VTable->setVisibility(llvm::GlobalValue::HiddenVisibility);
1016   }
1017
1018   VTableAlias->setAliasee(VTable);
1019 }
1020
1021 static bool shouldEmitAvailableExternallyVTable(const CodeGenModule &CGM,
1022                                                 const CXXRecordDecl *RD) {
1023   return CGM.getCodeGenOpts().OptimizationLevel > 0 &&
1024          CGM.getCXXABI().canSpeculativelyEmitVTable(RD);
1025 }
1026
1027 /// Compute the required linkage of the vtable for the given class.
1028 ///
1029 /// Note that we only call this at the end of the translation unit.
1030 llvm::GlobalVariable::LinkageTypes
1031 CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
1032   if (!RD->isExternallyVisible())
1033     return llvm::GlobalVariable::InternalLinkage;
1034
1035   // We're at the end of the translation unit, so the current key
1036   // function is fully correct.
1037   const CXXMethodDecl *keyFunction = Context.getCurrentKeyFunction(RD);
1038   if (keyFunction && !RD->hasAttr<DLLImportAttr>()) {
1039     // If this class has a key function, use that to determine the
1040     // linkage of the vtable.
1041     const FunctionDecl *def = nullptr;
1042     if (keyFunction->hasBody(def))
1043       keyFunction = cast<CXXMethodDecl>(def);
1044
1045     switch (keyFunction->getTemplateSpecializationKind()) {
1046       case TSK_Undeclared:
1047       case TSK_ExplicitSpecialization:
1048       assert(
1049           (def || CodeGenOpts.OptimizationLevel > 0 ||
1050            CodeGenOpts.getDebugInfo() != llvm::codegenoptions::NoDebugInfo) &&
1051           "Shouldn't query vtable linkage without key function, "
1052           "optimizations, or debug info");
1053       if (!def && CodeGenOpts.OptimizationLevel > 0)
1054         return llvm::GlobalVariable::AvailableExternallyLinkage;
1055
1056       if (keyFunction->isInlined())
1057         return !Context.getLangOpts().AppleKext
1058                    ? llvm::GlobalVariable::LinkOnceODRLinkage
1059                    : llvm::Function::InternalLinkage;
1060
1061       return llvm::GlobalVariable::ExternalLinkage;
1062
1063       case TSK_ImplicitInstantiation:
1064         return !Context.getLangOpts().AppleKext ?
1065                  llvm::GlobalVariable::LinkOnceODRLinkage :
1066                  llvm::Function::InternalLinkage;
1067
1068       case TSK_ExplicitInstantiationDefinition:
1069         return !Context.getLangOpts().AppleKext ?
1070                  llvm::GlobalVariable::WeakODRLinkage :
1071                  llvm::Function::InternalLinkage;
1072
1073       case TSK_ExplicitInstantiationDeclaration:
1074         llvm_unreachable("Should not have been asked to emit this");
1075     }
1076   }
1077
1078   // -fapple-kext mode does not support weak linkage, so we must use
1079   // internal linkage.
1080   if (Context.getLangOpts().AppleKext)
1081     return llvm::Function::InternalLinkage;
1082
1083   llvm::GlobalVariable::LinkageTypes DiscardableODRLinkage =
1084       llvm::GlobalValue::LinkOnceODRLinkage;
1085   llvm::GlobalVariable::LinkageTypes NonDiscardableODRLinkage =
1086       llvm::GlobalValue::WeakODRLinkage;
1087   if (RD->hasAttr<DLLExportAttr>()) {
1088     // Cannot discard exported vtables.
1089     DiscardableODRLinkage = NonDiscardableODRLinkage;
1090   } else if (RD->hasAttr<DLLImportAttr>()) {
1091     // Imported vtables are available externally.
1092     DiscardableODRLinkage = llvm::GlobalVariable::AvailableExternallyLinkage;
1093     NonDiscardableODRLinkage = llvm::GlobalVariable::AvailableExternallyLinkage;
1094   }
1095
1096   switch (RD->getTemplateSpecializationKind()) {
1097     case TSK_Undeclared:
1098     case TSK_ExplicitSpecialization:
1099     case TSK_ImplicitInstantiation:
1100       return DiscardableODRLinkage;
1101
1102     case TSK_ExplicitInstantiationDeclaration:
1103       // Explicit instantiations in MSVC do not provide vtables, so we must emit
1104       // our own.
1105       if (getTarget().getCXXABI().isMicrosoft())
1106         return DiscardableODRLinkage;
1107       return shouldEmitAvailableExternallyVTable(*this, RD)
1108                  ? llvm::GlobalVariable::AvailableExternallyLinkage
1109                  : llvm::GlobalVariable::ExternalLinkage;
1110
1111     case TSK_ExplicitInstantiationDefinition:
1112       return NonDiscardableODRLinkage;
1113   }
1114
1115   llvm_unreachable("Invalid TemplateSpecializationKind!");
1116 }
1117
1118 /// This is a callback from Sema to tell us that a particular vtable is
1119 /// required to be emitted in this translation unit.
1120 ///
1121 /// This is only called for vtables that _must_ be emitted (mainly due to key
1122 /// functions).  For weak vtables, CodeGen tracks when they are needed and
1123 /// emits them as-needed.
1124 void CodeGenModule::EmitVTable(CXXRecordDecl *theClass) {
1125   VTables.GenerateClassData(theClass);
1126 }
1127
1128 void
1129 CodeGenVTables::GenerateClassData(const CXXRecordDecl *RD) {
1130   if (CGDebugInfo *DI = CGM.getModuleDebugInfo())
1131     DI->completeClassData(RD);
1132
1133   if (RD->getNumVBases())
1134     CGM.getCXXABI().emitVirtualInheritanceTables(RD);
1135
1136   CGM.getCXXABI().emitVTableDefinitions(*this, RD);
1137 }
1138
1139 /// At this point in the translation unit, does it appear that can we
1140 /// rely on the vtable being defined elsewhere in the program?
1141 ///
1142 /// The response is really only definitive when called at the end of
1143 /// the translation unit.
1144 ///
1145 /// The only semantic restriction here is that the object file should
1146 /// not contain a vtable definition when that vtable is defined
1147 /// strongly elsewhere.  Otherwise, we'd just like to avoid emitting
1148 /// vtables when unnecessary.
1149 bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
1150   assert(RD->isDynamicClass() && "Non-dynamic classes have no VTable.");
1151
1152   // We always synthesize vtables if they are needed in the MS ABI. MSVC doesn't
1153   // emit them even if there is an explicit template instantiation.
1154   if (CGM.getTarget().getCXXABI().isMicrosoft())
1155     return false;
1156
1157   // If we have an explicit instantiation declaration (and not a
1158   // definition), the vtable is defined elsewhere.
1159   TemplateSpecializationKind TSK = RD->getTemplateSpecializationKind();
1160   if (TSK == TSK_ExplicitInstantiationDeclaration)
1161     return true;
1162
1163   // Otherwise, if the class is an instantiated template, the
1164   // vtable must be defined here.
1165   if (TSK == TSK_ImplicitInstantiation ||
1166       TSK == TSK_ExplicitInstantiationDefinition)
1167     return false;
1168
1169   // Otherwise, if the class doesn't have a key function (possibly
1170   // anymore), the vtable must be defined here.
1171   const CXXMethodDecl *keyFunction = CGM.getContext().getCurrentKeyFunction(RD);
1172   if (!keyFunction)
1173     return false;
1174
1175   const FunctionDecl *Def;
1176   // Otherwise, if we don't have a definition of the key function, the
1177   // vtable must be defined somewhere else.
1178   if (!keyFunction->hasBody(Def))
1179     return true;
1180
1181   assert(Def && "The body of the key function is not assigned to Def?");
1182   // If the non-inline key function comes from another module unit, the vtable
1183   // must be defined there.
1184   return Def->isInAnotherModuleUnit() && !Def->isInlineSpecified();
1185 }
1186
1187 /// Given that we're currently at the end of the translation unit, and
1188 /// we've emitted a reference to the vtable for this class, should
1189 /// we define that vtable?
1190 static bool shouldEmitVTableAtEndOfTranslationUnit(CodeGenModule &CGM,
1191                                                    const CXXRecordDecl *RD) {
1192   // If vtable is internal then it has to be done.
1193   if (!CGM.getVTables().isVTableExternal(RD))
1194     return true;
1195
1196   // If it's external then maybe we will need it as available_externally.
1197   return shouldEmitAvailableExternallyVTable(CGM, RD);
1198 }
1199
1200 /// Given that at some point we emitted a reference to one or more
1201 /// vtables, and that we are now at the end of the translation unit,
1202 /// decide whether we should emit them.
1203 void CodeGenModule::EmitDeferredVTables() {
1204 #ifndef NDEBUG
1205   // Remember the size of DeferredVTables, because we're going to assume
1206   // that this entire operation doesn't modify it.
1207   size_t savedSize = DeferredVTables.size();
1208 #endif
1209
1210   for (const CXXRecordDecl *RD : DeferredVTables)
1211     if (shouldEmitVTableAtEndOfTranslationUnit(*this, RD))
1212       VTables.GenerateClassData(RD);
1213     else if (shouldOpportunisticallyEmitVTables())
1214       OpportunisticVTables.push_back(RD);
1215
1216   assert(savedSize == DeferredVTables.size() &&
1217          "deferred extra vtables during vtable emission?");
1218   DeferredVTables.clear();
1219 }
1220
1221 bool CodeGenModule::AlwaysHasLTOVisibilityPublic(const CXXRecordDecl *RD) {
1222   if (RD->hasAttr<LTOVisibilityPublicAttr>() || RD->hasAttr<UuidAttr>() ||
1223       RD->hasAttr<DLLExportAttr>() || RD->hasAttr<DLLImportAttr>())
1224     return true;
1225
1226   if (!getCodeGenOpts().LTOVisibilityPublicStd)
1227     return false;
1228
1229   const DeclContext *DC = RD;
1230   while (true) {
1231     auto *D = cast<Decl>(DC);
1232     DC = DC->getParent();
1233     if (isa<TranslationUnitDecl>(DC->getRedeclContext())) {
1234       if (auto *ND = dyn_cast<NamespaceDecl>(D))
1235         if (const IdentifierInfo *II = ND->getIdentifier())
1236           if (II->isStr("std") || II->isStr("stdext"))
1237             return true;
1238       break;
1239     }
1240   }
1241
1242   return false;
1243 }
1244
1245 bool CodeGenModule::HasHiddenLTOVisibility(const CXXRecordDecl *RD) {
1246   LinkageInfo LV = RD->getLinkageAndVisibility();
1247   if (!isExternallyVisible(LV.getLinkage()))
1248     return true;
1249
1250   if (!getTriple().isOSBinFormatCOFF() &&
1251       LV.getVisibility() != HiddenVisibility)
1252     return false;
1253
1254   return !AlwaysHasLTOVisibilityPublic(RD);
1255 }
1256
1257 llvm::GlobalObject::VCallVisibility CodeGenModule::GetVCallVisibilityLevel(
1258     const CXXRecordDecl *RD, llvm::DenseSet<const CXXRecordDecl *> &Visited) {
1259   // If we have already visited this RD (which means this is a recursive call
1260   // since the initial call should have an empty Visited set), return the max
1261   // visibility. The recursive calls below compute the min between the result
1262   // of the recursive call and the current TypeVis, so returning the max here
1263   // ensures that it will have no effect on the current TypeVis.
1264   if (!Visited.insert(RD).second)
1265     return llvm::GlobalObject::VCallVisibilityTranslationUnit;
1266
1267   LinkageInfo LV = RD->getLinkageAndVisibility();
1268   llvm::GlobalObject::VCallVisibility TypeVis;
1269   if (!isExternallyVisible(LV.getLinkage()))
1270     TypeVis = llvm::GlobalObject::VCallVisibilityTranslationUnit;
1271   else if (HasHiddenLTOVisibility(RD))
1272     TypeVis = llvm::GlobalObject::VCallVisibilityLinkageUnit;
1273   else
1274     TypeVis = llvm::GlobalObject::VCallVisibilityPublic;
1275
1276   for (const auto &B : RD->bases())
1277     if (B.getType()->getAsCXXRecordDecl()->isDynamicClass())
1278       TypeVis = std::min(
1279           TypeVis,
1280           GetVCallVisibilityLevel(B.getType()->getAsCXXRecordDecl(), Visited));
1281
1282   for (const auto &B : RD->vbases())
1283     if (B.getType()->getAsCXXRecordDecl()->isDynamicClass())
1284       TypeVis = std::min(
1285           TypeVis,
1286           GetVCallVisibilityLevel(B.getType()->getAsCXXRecordDecl(), Visited));
1287
1288   return TypeVis;
1289 }
1290
1291 void CodeGenModule::EmitVTableTypeMetadata(const CXXRecordDecl *RD,
1292                                            llvm::GlobalVariable *VTable,
1293                                            const VTableLayout &VTLayout) {
1294   if (!getCodeGenOpts().LTOUnit)
1295     return;
1296
1297   CharUnits ComponentWidth = GetTargetTypeStoreSize(getVTableComponentType());
1298
1299   typedef std::pair<const CXXRecordDecl *, unsigned> AddressPoint;
1300   std::vector<AddressPoint> AddressPoints;
1301   for (auto &&AP : VTLayout.getAddressPoints())
1302     AddressPoints.push_back(std::make_pair(
1303         AP.first.getBase(), VTLayout.getVTableOffset(AP.second.VTableIndex) +
1304                                 AP.second.AddressPointIndex));
1305
1306   // Sort the address points for determinism.
1307   llvm::sort(AddressPoints, [this](const AddressPoint &AP1,
1308                                    const AddressPoint &AP2) {
1309     if (&AP1 == &AP2)
1310       return false;
1311
1312     std::string S1;
1313     llvm::raw_string_ostream O1(S1);
1314     getCXXABI().getMangleContext().mangleTypeName(
1315         QualType(AP1.first->getTypeForDecl(), 0), O1);
1316     O1.flush();
1317
1318     std::string S2;
1319     llvm::raw_string_ostream O2(S2);
1320     getCXXABI().getMangleContext().mangleTypeName(
1321         QualType(AP2.first->getTypeForDecl(), 0), O2);
1322     O2.flush();
1323
1324     if (S1 < S2)
1325       return true;
1326     if (S1 != S2)
1327       return false;
1328
1329     return AP1.second < AP2.second;
1330   });
1331
1332   ArrayRef<VTableComponent> Comps = VTLayout.vtable_components();
1333   for (auto AP : AddressPoints) {
1334     // Create type metadata for the address point.
1335     AddVTableTypeMetadata(VTable, ComponentWidth * AP.second, AP.first);
1336
1337     // The class associated with each address point could also potentially be
1338     // used for indirect calls via a member function pointer, so we need to
1339     // annotate the address of each function pointer with the appropriate member
1340     // function pointer type.
1341     for (unsigned I = 0; I != Comps.size(); ++I) {
1342       if (Comps[I].getKind() != VTableComponent::CK_FunctionPointer)
1343         continue;
1344       llvm::Metadata *MD = CreateMetadataIdentifierForVirtualMemPtrType(
1345           Context.getMemberPointerType(
1346               Comps[I].getFunctionDecl()->getType(),
1347               Context.getRecordType(AP.first).getTypePtr()));
1348       VTable->addTypeMetadata((ComponentWidth * I).getQuantity(), MD);
1349     }
1350   }
1351
1352   if (getCodeGenOpts().VirtualFunctionElimination ||
1353       getCodeGenOpts().WholeProgramVTables) {
1354     llvm::DenseSet<const CXXRecordDecl *> Visited;
1355     llvm::GlobalObject::VCallVisibility TypeVis =
1356         GetVCallVisibilityLevel(RD, Visited);
1357     if (TypeVis != llvm::GlobalObject::VCallVisibilityPublic)
1358       VTable->setVCallVisibilityMetadata(TypeVis);
1359   }
1360 }