CGCXX.cpp

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//===--- CGCXX.cpp - Emit LLVM Code for declarations ----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This contains code dealing with C++ code generation.
//
//===----------------------------------------------------------------------===//
 
// We might split this into multiple files if it gets too unwieldy
 
#include "CGCXXABI.h"
#include "CodeGenFunction.h"
#include "CodeGenModule.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Attr.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/Mangle.h"
#include "clang/AST/RecordLayout.h"
#include "clang/Basic/CodeGenOptions.h"
using namespace clang;
using namespace CodeGen;
 
 
/// Try to emit a base destructor as an alias to its primary
/// base-class destructor.
bool CodeGenModule::TryEmitBaseDestructorAsAlias(const CXXDestructorDecl *D) {
  if (!getCodeGenOpts().CXXCtorDtorAliases)
    return true;
 
  // Producing an alias to a base class ctor/dtor can degrade debug quality
  // as the debugger cannot tell them apart.
  if (getCodeGenOpts().OptimizationLevel == 0)
    return true;
 
  // Disable this optimization for ARM64EC.  FIXME: This probably should work,
  // but getting the symbol table correct is complicated.
  if (getTarget().getTriple().isWindowsArm64EC())
    return true;
 
  // If sanitizing memory to check for use-after-dtor, do not emit as
  //  an alias, unless this class owns no members.
  if (getCodeGenOpts().SanitizeMemoryUseAfterDtor &&
      !D->getParent()->field_empty())
    return true;
 
  // If the destructor doesn't have a trivial body, we have to emit it
  // separately.
  if (!D->hasTrivialBody())
    return true;
 
  const CXXRecordDecl *Class = D->getParent();
 
  // We are going to instrument this destructor, so give up even if it is
  // currently empty.
  if (Class->mayInsertExtraPadding())
    return true;
 
  // If we need to manipulate a VTT parameter, give up.
  if (Class->getNumVBases()) {
    // Extra Credit:  passing extra parameters is perfectly safe
    // in many calling conventions, so only bail out if the ctor's
    // calling convention is nonstandard.
    return true;
  }
 
  // If any field has a non-trivial destructor, we have to emit the
  // destructor separately.
  for (const auto *I : Class->fields())
    if (I->getType().isDestructedType())
      return true;
 
  // Try to find a unique base class with a non-trivial destructor.
  const CXXRecordDecl *UniqueBase = nullptr;
  for (const auto &I : Class->bases()) {
 
    // We're in the base destructor, so skip virtual bases.
    if (I.isVirtual()) continue;
 
    // Skip base classes with trivial destructors.
    const auto *Base =
        cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
    if (Base->hasTrivialDestructor()) continue;
 
    // If we've already found a base class with a non-trivial
    // destructor, give up.
    if (UniqueBase) return true;
    UniqueBase = Base;
  }
 
  // If we didn't find any bases with a non-trivial destructor, then
  // the base destructor is actually effectively trivial, which can
  // happen if it was needlessly user-defined or if there are virtual
  // bases with non-trivial destructors.
  if (!UniqueBase)
    return true;
 
  // If the base is at a non-zero offset, give up.
  const ASTRecordLayout &ClassLayout = Context.getASTRecordLayout(Class);
  if (!ClassLayout.getBaseClassOffset(UniqueBase).isZero())
    return true;
 
  // Give up if the calling conventions don't match. We could update the call,
  // but it is probably not worth it.
  const CXXDestructorDecl *BaseD = UniqueBase->getDestructor();
  if (BaseD->getType()->castAs<FunctionType>()->getCallConv() !=
      D->getType()->castAs<FunctionType>()->getCallConv())
    return true;
 
  GlobalDecl AliasDecl(D, Dtor_Base);
  GlobalDecl TargetDecl(BaseD, Dtor_Base);
 
  // The alias will use the linkage of the referent.  If we can't
  // support aliases with that linkage, fail.
  llvm::GlobalValue::LinkageTypes Linkage = getFunctionLinkage(AliasDecl);
 
  // We can't use an alias if the linkage is not valid for one.
  if (!llvm::GlobalAlias::isValidLinkage(Linkage))
    return true;
 
  llvm::GlobalValue::LinkageTypes TargetLinkage =
      getFunctionLinkage(TargetDecl);
 
  // Check if we have it already.
  StringRef MangledName = getMangledName(AliasDecl);
  llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
  if (Entry && !Entry->isDeclaration())
    return false;
  if (Replacements.count(MangledName))
    return false;
 
  llvm::Type *AliasValueType = getTypes().GetFunctionType(AliasDecl);
 
  // Find the referent.
  auto *Aliasee = cast<llvm::GlobalValue>(GetAddrOfGlobal(TargetDecl));
 
  // Instead of creating as alias to a linkonce_odr, replace all of the uses
  // of the aliasee.
  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage) &&
      !(TargetLinkage == llvm::GlobalValue::AvailableExternallyLinkage &&
        TargetDecl.getDecl()->hasAttr<AlwaysInlineAttr>())) {
    // FIXME: An extern template instantiation will create functions with
    // linkage "AvailableExternally". In libc++, some classes also define
    // members with attribute "AlwaysInline" and expect no reference to
    // be generated. It is desirable to reenable this optimisation after
    // corresponding LLVM changes.
    addReplacement(MangledName, Aliasee);
    return false;
  }
 
  // If we have a weak, non-discardable alias (weak, weak_odr), like an extern
  // template instantiation or a dllexported class, avoid forming it on COFF.
  // A COFF weak external alias cannot satisfy a normal undefined symbol
  // reference from another TU. The other TU must also mark the referenced
  // symbol as weak, which we cannot rely on.
  if (llvm::GlobalValue::isWeakForLinker(Linkage) &&
      getTriple().isOSBinFormatCOFF()) {
    return true;
  }
 
  // If we don't have a definition for the destructor yet or the definition is
  // avaialable_externally, don't emit an alias.  We can't emit aliases to
  // declarations; that's just not how aliases work.
  if (Aliasee->isDeclarationForLinker())
    return true;
 
  // Don't create an alias to a linker weak symbol. This avoids producing
  // different COMDATs in different TUs. Another option would be to
  // output the alias both for weak_odr and linkonce_odr, but that
  // requires explicit comdat support in the IL.
  if (llvm::GlobalValue::isWeakForLinker(TargetLinkage))
    return true;
 
  // Create the alias with no name.
  auto *Alias = llvm::GlobalAlias::create(AliasValueType, 0, Linkage, "",
                                          Aliasee, &getModule());
 
  // Destructors are always unnamed_addr.
  Alias->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
 
  // Switch any previous uses to the alias.
  if (Entry) {
    assert(Entry->getValueType() == AliasValueType &&
           Entry->getAddressSpace() == Alias->getAddressSpace() &&
           "declaration exists with different type");
    Alias->takeName(Entry);
    Entry->replaceAllUsesWith(Alias);
    Entry->eraseFromParent();
  } else {
    Alias->setName(MangledName);
  }
 
  // Finally, set up the alias with its proper name and attributes.