clang/lib/CodeGen/CGExprAgg.cpp

//===--- CGExprAgg.cpp - Emit LLVM Code from Aggregate Expressions --------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This contains code to emit Aggregate Expr nodes as LLVM code.
//
//===----------------------------------------------------------------------===//

#include "CodeGenFunction.h"
#include "CGObjCRuntime.h"
#include "CodeGenModule.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclTemplate.h"
#include "clang/AST/StmtVisitor.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Intrinsics.h"
using namespace clang;
using namespace CodeGen;

//===----------------------------------------------------------------------===//
//                        Aggregate Expression Emitter
//===----------------------------------------------------------------------===//

namespace  {
class AggExprEmitter : public StmtVisitor<AggExprEmitter> {
  CodeGenFunction &CGF;
  CGBuilderTy &Builder;
  AggValueSlot Dest;
  bool IsResultUnused;

  /// We want to use 'dest' as the return slot except under two
  /// conditions:
  ///   - The destination slot requires garbage collection, so we
  ///     need to use the GC API.
  ///   - The destination slot is potentially aliased.
  bool shouldUseDestForReturnSlot() const {
    return !(Dest.requiresGCollection() || Dest.isPotentiallyAliased());
  }

  ReturnValueSlot getReturnValueSlot() const {
    if (!shouldUseDestForReturnSlot())
      return ReturnValueSlot();

    return ReturnValueSlot(Dest.getAddr(), Dest.isVolatile(), IsResultUnused);
  }

  AggValueSlot EnsureSlot(QualType T) {
    if (!Dest.isIgnored()) return Dest;
    return CGF.CreateAggTemp(T, "agg.tmp.ensured");
  }
  void EnsureDest(QualType T) {
    if (!Dest.isIgnored()) return;
    Dest = CGF.CreateAggTemp(T, "agg.tmp.ensured");
  }

public:
  AggExprEmitter(CodeGenFunction &cgf, AggValueSlot Dest, bool IsResultUnused)
    : CGF(cgf), Builder(CGF.Builder), Dest(Dest),
    IsResultUnused(IsResultUnused) { }

  //===--------------------------------------------------------------------===//
  //                               Utilities
  //===--------------------------------------------------------------------===//

  /// EmitAggLoadOfLValue - Given an expression with aggregate type that
  /// represents a value lvalue, this method emits the address of the lvalue,
  /// then loads the result into DestPtr.
  void EmitAggLoadOfLValue(const Expr *E);

  /// EmitFinalDestCopy - Perform the final copy to DestPtr, if desired.
  void EmitFinalDestCopy(QualType type, const LValue &src);
  void EmitFinalDestCopy(QualType type, RValue src,
                         CharUnits srcAlignment = CharUnits::Zero());
  void EmitCopy(QualType type, const AggValueSlot &dest,
                const AggValueSlot &src);

  void EmitMoveFromReturnSlot(const Expr *E, RValue Src);

  void EmitArrayInit(llvm::Value *DestPtr, llvm::ArrayType *AType,
                     QualType elementType, InitListExpr *E);

  AggValueSlot::NeedsGCBarriers_t needsGC(QualType T) {
    if (CGF.getLangOpts().getGC() && TypeRequiresGCollection(T))
      return AggValueSlot::NeedsGCBarriers;
    return AggValueSlot::DoesNotNeedGCBarriers;
  }

  bool TypeRequiresGCollection(QualType T);

  //===--------------------------------------------------------------------===//
  //                            Visitor Methods
  //===--------------------------------------------------------------------===//

  void Visit(Expr *E) {
    ApplyDebugLocation DL(CGF, E);
    StmtVisitor<AggExprEmitter>::Visit(E);
  }

  void VisitStmt(Stmt *S) {
    CGF.ErrorUnsupported(S, "aggregate expression");
  }
  void VisitParenExpr(ParenExpr *PE) { Visit(PE->getSubExpr()); }
  void VisitGenericSelectionExpr(GenericSelectionExpr *GE) {
    Visit(GE->getResultExpr());
  }
  void VisitUnaryExtension(UnaryOperator *E) { Visit(E->getSubExpr()); }
  void VisitSubstNonTypeTemplateParmExpr(SubstNonTypeTemplateParmExpr *E) {
    return Visit(E->getReplacement());
  }

  // l-values.
  void VisitDeclRefExpr(DeclRefExpr *E) {
    // For aggregates, we should always be able to emit the variable
    // as an l-value unless it's a reference.  This is due to the fact
    // that we can't actually ever see a normal l2r conversion on an
    // aggregate in C++, and in C there's no language standard
    // actively preventing us from listing variables in the captures
    // list of a block.
    if (E->getDecl()->getType()->isReferenceType()) {
      if (CodeGenFunction::ConstantEmission result
            = CGF.tryEmitAsConstant(E)) {
        EmitFinalDestCopy(E->getType(), result.getReferenceLValue(CGF, E));
        return;
      }
    }

    EmitAggLoadOfLValue(E);
  }

  void VisitMemberExpr(MemberExpr *ME) { EmitAggLoadOfLValue(ME); }
  void VisitUnaryDeref(UnaryOperator *E) { EmitAggLoadOfLValue(E); }
  void VisitStringLiteral(StringLiteral *E) { EmitAggLoadOfLValue(E); }
  void VisitCompoundLiteralExpr(CompoundLiteralExpr *E);
  void VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
    EmitAggLoadOfLValue(E);
  }
  void VisitPredefinedExpr(const PredefinedExpr *E) {
    EmitAggLoadOfLValue(E);
  }

  // Operators.
  void VisitCastExpr(CastExpr *E);
  void VisitCallExpr(const CallExpr *E);
  void VisitStmtExpr(const StmtExpr *E);
  void VisitBinaryOperator(const BinaryOperator *BO);
  void VisitPointerToDataMemberBinaryOperator(const BinaryOperator *BO);
  void VisitBinAssign(const BinaryOperator *E);
  void VisitBinComma(const BinaryOperator *E);

  void VisitObjCMessageExpr(ObjCMessageExpr *E);
  void VisitObjCIvarRefExpr(ObjCIvarRefExpr *E) {
    EmitAggLoadOfLValue(E);
  }

  void VisitAbstractConditionalOperator(const AbstractConditionalOperator *CO);
  void VisitChooseExpr(const ChooseExpr *CE);
  void VisitInitListExpr(InitListExpr *E);
  void VisitImplicitValueInitExpr(ImplicitValueInitExpr *E);
  void VisitCXXDefaultArgExpr(CXXDefaultArgExpr *DAE) {
    Visit(DAE->getExpr());
  }
  void VisitCXXDefaultInitExpr(CXXDefaultInitExpr *DIE) {
    CodeGenFunction::CXXDefaultInitExprScope Scope(CGF);
    Visit(DIE->getExpr());
  }
  void VisitCXXBindTemporaryExpr(CXXBindTemporaryExpr *E);
  void VisitCXXConstructExpr(const CXXConstructExpr *E);
  void VisitLambdaExpr(LambdaExpr *E);
  void VisitCXXStdInitializerListExpr(CXXStdInitializerListExpr *E);
  void VisitExprWithCleanups(ExprWithCleanups *E);
  void VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *E);
  void VisitCXXTypeidExpr(CXXTypeidExpr *E) { EmitAggLoadOfLValue(E); }
  void VisitMaterializeTemporaryExpr(MaterializeTemporaryExpr *E);
  void VisitOpaqueValueExpr(OpaqueValueExpr *E);

  void VisitPseudoObjectExpr(PseudoObjectExpr *E) {
    if (E->isGLValue()) {
      LValue LV = CGF.EmitPseudoObjectLValue(E);
      return EmitFinalDestCopy(E->getType(), LV);
    }

    CGF.EmitPseudoObjectRValue(E, EnsureSlot(E->getType()));
  }

  void VisitVAArgExpr(VAArgExpr *E);

  void EmitInitializationToLValue(Expr *E, LValue Address);
  void EmitNullInitializationToLValue(LValue Address);
  //  case Expr::ChooseExprClass:
  void VisitCXXThrowExpr(const CXXThrowExpr *E) { CGF.EmitCXXThrowExpr(E); }
  void VisitAtomicExpr(AtomicExpr *E) {
    CGF.EmitAtomicExpr(E, EnsureSlot(E->getType()).getAddr());
  }
};
}  // end anonymous namespace.

//===----------------------------------------------------------------------===//
//                                Utilities
//===----------------------------------------------------------------------===//

/// EmitAggLoadOfLValue - Given an expression with aggregate type that
/// represents a value lvalue, this method emits the address of the lvalue,
/// then loads the result into DestPtr.
void AggExprEmitter::EmitAggLoadOfLValue(const Expr *E) {
  LValue LV = CGF.EmitLValue(E);

  // If the type of the l-value is atomic, then do an atomic load.
  if (LV.getType()->isAtomicType() || CGF.LValueIsSuitableForInlineAtomic(LV)) {
    CGF.EmitAtomicLoad(LV, E->getExprLoc(), Dest);
    return;
  }

  EmitFinalDestCopy(E->getType(), LV);
}

/// \brief True if the given aggregate type requires special GC API calls.
bool AggExprEmitter::TypeRequiresGCollection(QualType T) {
  // Only record types have members that might require garbage collection.
  const RecordType *RecordTy = T->getAs<RecordType>();
  if (!RecordTy) return false;

  // Don't mess with non-trivial C++ types.
  RecordDecl *Record = RecordTy->getDecl();
  if (isa<CXXRecordDecl>(Record) &&
      (cast<CXXRecordDecl>(Record)->hasNonTrivialCopyConstructor() ||
       !cast<CXXRecordDecl>(Record)->hasTrivialDestructor()))
    return false;

  // Check whether the type has an object member.
  return Record->hasObjectMember();
}

/// \brief Perform the final move to DestPtr if for some reason
/// getReturnValueSlot() didn't use it directly.
///
/// The idea is that you do something like this:
///   RValue Result = EmitSomething(..., getReturnValueSlot());
///   EmitMoveFromReturnSlot(E, Result);
///
/// If nothing interferes, this will cause the result to be emitted
/// directly into the return value slot.  Otherwise, a final move
/// will be performed.
void AggExprEmitter::EmitMoveFromReturnSlot(const Expr *E, RValue src) {
  if (shouldUseDestForReturnSlot()) {
    // Logically, Dest.getAddr() should equal Src.getAggregateAddr().
    // The possibility of undef rvalues complicates that a lot,
    // though, so we can't really assert.
    return;
  }

  // Otherwise, copy from there to the destination.
  assert(Dest.getAddr() != src.getAggregateAddr());
  std::pair<CharUnits, CharUnits> typeInfo = 
    CGF.getContext().getTypeInfoInChars(E->getType());
  EmitFinalDestCopy(E->getType(), src, typeInfo.second);
}

/// EmitFinalDestCopy - Perform the final copy to DestPtr, if desired.
void AggExprEmitter::EmitFinalDestCopy(QualType type, RValue src,
                                       CharUnits srcAlign) {
  assert(src.isAggregate() && "value must be aggregate value!");
  LValue srcLV = CGF.MakeAddrLValue(src.getAggregateAddr(), type, srcAlign);
  EmitFinalDestCopy(type, srcLV);
}

/// EmitFinalDestCopy - Perform the final copy to DestPtr, if desired.
void AggExprEmitter::EmitFinalDestCopy(QualType type, const LValue &src) {
  // If Dest is ignored, then we're evaluating an aggregate expression
  // in a context that doesn't care about the result.  Note that loads
  // from volatile l-values force the existence of a non-ignored
  // destination.
  if (Dest.isIgnored())
    return;

  AggValueSlot srcAgg =
    AggValueSlot::forLValue(src, AggValueSlot::IsDestructed,
                            needsGC(type), AggValueSlot::IsAliased);
  EmitCopy(type, Dest, srcAgg);
}

/// Perform a copy from the source into the destination.
///
/// \param type - the type of the aggregate being copied; qualifiers are
///   ignored
void AggExprEmitter::EmitCopy(QualType type, const AggValueSlot &dest,
                              const AggValueSlot &src) {
  if (dest.requiresGCollection()) {
    CharUnits sz = CGF.getContext().getTypeSizeInChars(type);
    llvm::Value *size = llvm::ConstantInt::get(CGF.SizeTy, sz.getQuantity());
    CGF.CGM.getObjCRuntime().EmitGCMemmoveCollectable(CGF,
                                                      dest.getAddr(),
                                                      src.getAddr(),
                                                      size);
    return;
  }

  // If the result of the assignment is used, copy the LHS there also.
  // It's volatile if either side is.  Use the minimum alignment of
  // the two sides.
  CGF.EmitAggregateCopy(dest.getAddr(), src.getAddr(), type,
                        dest.isVolatile() || src.isVolatile(),
                        std::min(dest.getAlignment(), src.getAlignment()));
}

/// \brief Emit the initializer for a std::initializer_list initialized with a
/// real initializer list.
void
AggExprEmitter::VisitCXXStdInitializerListExpr(CXXStdInitializerListExpr *E) {
  // Emit an array containing the elements.  The array is externally destructed
  // if the std::initializer_list object is.
  ASTContext &Ctx = CGF.getContext();
  LValue Array = CGF.EmitLValue(E->getSubExpr());
  assert(Array.isSimple() && "initializer_list array not a simple lvalue");
  llvm::Value *ArrayPtr = Array.getAddress();

  const ConstantArrayType *ArrayType =
      Ctx.getAsConstantArrayType(E->getSubExpr()->getType());
  assert(ArrayType && "std::initializer_list constructed from non-array");

  // FIXME: Perform the checks on the field types in SemaInit.
  RecordDecl *Record = E->getType()->castAs<RecordType>()->getDecl();
  RecordDecl::field_iterator Field = Record->field_begin();
  if (Field == Record->field_end()) {
    CGF.ErrorUnsupported(E, "weird std::initializer_list");
    return;
  }

  // Start pointer.
  if (!Field->getType()->isPointerType() ||
      !Ctx.hasSameType(Field->getType()->getPointeeType(),
                       ArrayType->getElementType())) {
    CGF.ErrorUnsupported(E, "weird std::initializer_list");
    return;
  }

  AggValueSlot Dest = EnsureSlot(E->getType());
  LValue DestLV = CGF.MakeAddrLValue(Dest.getAddr(), E->getType(),
                                     Dest.getAlignment());
  LValue Start = CGF.EmitLValueForFieldInitialization(DestLV, *Field);
  llvm::Value *Zero = llvm::ConstantInt::get(CGF.PtrDiffTy, 0);
  llvm::Value *IdxStart[] = { Zero, Zero };
  llvm::Value *ArrayStart =
      Builder.CreateInBoundsGEP(ArrayPtr, IdxStart, "arraystart");
  CGF.EmitStoreThroughLValue(RValue::get(ArrayStart), Start);
  ++Field;

  if (Field == Record->field_end()) {
    CGF.ErrorUnsupported(E, "weird std::initializer_list");
    return;
  }

  llvm::Value *Size = Builder.getInt(ArrayType->getSize());
  LValue EndOrLength = CGF.EmitLValueForFieldInitialization(DestLV, *Field);
  if (Field->getType()->isPointerType() &&
      Ctx.hasSameType(Field->getType()->getPointeeType(),
                      ArrayType->getElementType())) {
    // End pointer.
    llvm::Value *IdxEnd[] = { Zero, Size };
    llvm::Value *ArrayEnd =
        Builder.CreateInBoundsGEP(ArrayPtr, IdxEnd, "arrayend");
    CGF.EmitStoreThroughLValue(RValue::get(ArrayEnd), EndOrLength);
  } else if (Ctx.hasSameType(Field->getType(), Ctx.getSizeType())) {
    // Length.
    CGF.EmitStoreThroughLValue(RValue::get(Size), EndOrLength);
  } else {
    CGF.ErrorUnsupported(E, "weird std::initializer_list");
    return;
  }
}

/// \brief Determine if E is a trivial array filler, that is, one that is
/// equivalent to zero-initialization.
static bool isTrivialFiller(Expr *E) {
  if (!E)
    return true;

  if (isa<ImplicitValueInitExpr>(E))
    return true;

  if (auto *ILE = dyn_cast<InitListExpr>(E)) {
    if (ILE->getNumInits())
      return false;
    return isTrivialFiller(ILE->getArrayFiller());
  }

  if (auto *Cons = dyn_cast_or_null<CXXConstructExpr>(E))
    return Cons->getConstructor()->isDefaultConstructor() &&
           Cons->getConstructor()->isTrivial();

  // FIXME: Are there other cases where we can avoid emitting an initializer?
  return false;
}

/// \brief Emit initialization of an array from an initializer list.
void AggExprEmitter::EmitArrayInit(llvm::Value *DestPtr, llvm::ArrayType *AType,
                                   QualType elementType, InitListExpr *E) {
  uint64_t NumInitElements = E->getNumInits();

  uint64_t NumArrayElements = AType->getNumElements();
  assert(NumInitElements <= NumArrayElements);

  // DestPtr is an array*.  Construct an elementType* by drilling
  // down a level.
  llvm::Value *zero = llvm::ConstantInt::get(CGF.SizeTy, 0);
  llvm::Value *indices[] = { zero, zero };
  llvm::Value *begin =
    Builder.CreateInBoundsGEP(DestPtr, indices, "arrayinit.begin");

  // Exception safety requires us to destroy all the
  // already-constructed members if an initializer throws.
  // For that, we'll need an EH cleanup.
  QualType::DestructionKind dtorKind = elementType.isDestructedType();
  llvm::AllocaInst *endOfInit = nullptr;
  EHScopeStack::stable_iterator cleanup;
  llvm::Instruction *cleanupDominator = nullptr;
  if (CGF.needsEHCleanup(dtorKind)) {
    // In principle we could tell the cleanup where we are more
    // directly, but the control flow can get so varied here that it
    // would actually be quite complex.  Therefore we go through an
    // alloca.
    endOfInit = CGF.CreateTempAlloca(begin->getType(),
                                     "arrayinit.endOfInit");
    cleanupDominator = Builder.CreateStore(begin, endOfInit);
    CGF.pushIrregularPartialArrayCleanup(begin, endOfInit, elementType,
                                         CGF.getDestroyer(dtorKind));
    cleanup = CGF.EHStack.stable_begin();

  // Otherwise, remember that we didn't need a cleanup.
  } else {
    dtorKind = QualType::DK_none;
  }

  llvm::Value *one = llvm::ConstantInt::get(CGF.SizeTy, 1);

  // The 'current element to initialize'.  The invariants on this
  // variable are complicated.  Essentially, after each iteration of
  // the loop, it points to the last initialized element, except
  // that it points to the beginning of the array before any
  // elements have been initialized.
  llvm::Value *element = begin;

  // Emit the explicit initializers.
  for (uint64_t i = 0; i != NumInitElements; ++i) {
    // Advance to the next element.
    if (i > 0) {
      element = Builder.CreateInBoundsGEP(element, one, "arrayinit.element");

      // Tell the cleanup that it needs to destroy up to this
      // element.  TODO: some of these stores can be trivially
      // observed to be unnecessary.
      if (endOfInit) Builder.CreateStore(element, endOfInit);
    }

    LValue elementLV = CGF.MakeAddrLValue(element, elementType);
    EmitInitializationToLValue(E->getInit(i), elementLV);
  }

  // Check whether there's a non-trivial array-fill expression.
  Expr *filler = E->getArrayFiller();
  bool hasTrivialFiller = isTrivialFiller(filler);

  // Any remaining elements need to be zero-initialized, possibly
  // using the filler expression.  We can skip this if the we're
  // emitting to zeroed memory.
  if (NumInitElements != NumArrayElements &&
      !(Dest.isZeroed() && hasTrivialFiller &&
        CGF.getTypes().isZeroInitializable(elementType))) {

    // Use an actual loop.  This is basically
    //   do { *array++ = filler; } while (array != end);

    // Advance to the start of the rest of the array.
    if (NumInitElements) {
      element = Builder.CreateInBoundsGEP(element, one, "arrayinit.start");
      if (endOfInit) Builder.CreateStore(element, endOfInit);
    }

    // Compute the end of the array.
    llvm::Value *end = Builder.CreateInBoundsGEP(begin,
                      llvm::ConstantInt::get(CGF.SizeTy, NumArrayElements),
                                                 "arrayinit.end");

    llvm::BasicBlock *entryBB = Builder.GetInsertBlock();
    llvm::BasicBlock *bodyBB = CGF.createBasicBlock("arrayinit.body");

    // Jump into the body.
    CGF.EmitBlock(bodyBB);
    llvm::PHINode *currentElement =
      Builder.CreatePHI(element->getType(), 2, "arrayinit.cur");
    currentElement->addIncoming(element, entryBB);

    // Emit the actual filler expression.
    LValue elementLV = CGF.MakeAddrLValue(currentElement, elementType);
    if (filler)
      EmitInitializationToLValue(filler, elementLV);
    else
      EmitNullInitializationToLValue(elementLV);

    // Move on to the next element.
    llvm::Value *nextElement =
      Builder.CreateInBoundsGEP(currentElement, one, "arrayinit.next");

    // Tell the EH cleanup that we finished with the last element.
    if (endOfInit) Builder.CreateStore(nextElement, endOfInit);

    // Leave the loop if we're done.
    llvm::Value *done = Builder.CreateICmpEQ(nextElement, end,
                                             "arrayinit.done");
    llvm::BasicBlock *endBB = CGF.createBasicBlock("arrayinit.end");
    Builder.CreateCondBr(done, endBB, bodyBB);
    currentElement->addIncoming(nextElement, Builder.GetInsertBlock());

    CGF.EmitBlock(endBB);
  }

  // Leave the partial-array cleanup if we entered one.
  if (dtorKind) CGF.DeactivateCleanupBlock(cleanup, cleanupDominator);
}

//===----------------------------------------------------------------------===//
//                            Visitor Methods
//===----------------------------------------------------------------------===//

void AggExprEmitter::VisitMaterializeTemporaryExpr(MaterializeTemporaryExpr *E){
  Visit(E->GetTemporaryExpr());
}

void AggExprEmitter::VisitOpaqueValueExpr(OpaqueValueExpr *e) {
  EmitFinalDestCopy(e->getType(), CGF.getOpaqueLValueMapping(e));
}

void
AggExprEmitter::VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
  if (Dest.isPotentiallyAliased() &&
      E->getType().isPODType(CGF.getContext())) {
    // For a POD type, just emit a load of the lvalue + a copy, because our
    // compound literal might alias the destination.
    EmitAggLoadOfLValue(E);
    return;
  }
  
  AggValueSlot Slot = EnsureSlot(E->getType());
  CGF.EmitAggExpr(E->getInitializer(), Slot);
}

/// Attempt to look through various unimportant expressions to find a
/// cast of the given kind.
static Expr *findPeephole(Expr *op, CastKind kind) {
  while (true) {
    op = op->IgnoreParens();
    if (CastExpr *castE = dyn_cast<CastExpr>(op)) {
      if (castE->getCastKind() == kind)
        return castE->getSubExpr();
      if (castE->getCastKind() == CK_NoOp)
        continue;
    }
    return nullptr;
  }
}

void AggExprEmitter::VisitCastExpr(CastExpr *E) {
  switch (E->getCastKind()) {
  case CK_Dynamic: {
    // FIXME: Can this actually happen? We have no test coverage for it.
    assert(isa<CXXDynamicCastExpr>(E) && "CK_Dynamic without a dynamic_cast?");
    LValue LV = CGF.EmitCheckedLValue(E->getSubExpr(),
                                      CodeGenFunction::TCK_Load);
    // FIXME: Do we also need to handle property references here?
    if (LV.isSimple())
      CGF.EmitDynamicCast(LV.getAddress(), cast<CXXDynamicCastExpr>(E));
    else
      CGF.CGM.ErrorUnsupported(E, "non-simple lvalue dynamic_cast");
    
    if (!Dest.isIgnored())
      CGF.CGM.ErrorUnsupported(E, "lvalue dynamic_cast with a destination");
    break;
  }
      
  case CK_ToUnion: {
    // Evaluate even if the destination is ignored.
    if (Dest.isIgnored()) {
      CGF.EmitAnyExpr(E->getSubExpr(), AggValueSlot::ignored(),
                      /*ignoreResult=*/true);
      break;
    }

    // GCC union extension
    QualType Ty = E->getSubExpr()->getType();
    QualType PtrTy = CGF.getContext().getPointerType(Ty);
    llvm::Value *CastPtr = Builder.CreateBitCast(Dest.getAddr(),
                                                 CGF.ConvertType(PtrTy));
    EmitInitializationToLValue(E->getSubExpr(),
                               CGF.MakeAddrLValue(CastPtr, Ty));
    break;
  }

  case CK_DerivedToBase:
  case CK_BaseToDerived:
  case CK_UncheckedDerivedToBase: {
    llvm_unreachable("cannot perform hierarchy conversion in EmitAggExpr: "
                "should have been unpacked before we got here");
  }

  case CK_NonAtomicToAtomic:
  case CK_AtomicToNonAtomic: {
    bool isToAtomic = (E->getCastKind() == CK_NonAtomicToAtomic);

    // Determine the atomic and value types.
    QualType atomicType = E->getSubExpr()->getType();
    QualType valueType = E->getType();
    if (isToAtomic) std::swap(atomicType, valueType);

    assert(atomicType->isAtomicType());
    assert(CGF.getContext().hasSameUnqualifiedType(valueType,
                          atomicType->castAs<AtomicType>()->getValueType()));

    // Just recurse normally if we're ignoring the result or the
    // atomic type doesn't change representation.
    if (Dest.isIgnored() || !CGF.CGM.isPaddedAtomicType(atomicType)) {
      return Visit(E->getSubExpr());
    }

    CastKind peepholeTarget =
      (isToAtomic ? CK_AtomicToNonAtomic : CK_NonAtomicToAtomic);

    // These two cases are reverses of each other; try to peephole them.
    if (Expr *op = findPeephole(E->getSubExpr(), peepholeTarget)) {
      assert(CGF.getContext().hasSameUnqualifiedType(op->getType(),
                                                     E->getType()) &&
           "peephole significantly changed types?");
      return Visit(op);
    }

    // If we're converting an r-value of non-atomic type to an r-value
    // of atomic type, just emit directly into the relevant sub-object.
    if (isToAtomic) {
      AggValueSlot valueDest = Dest;
      if (!valueDest.isIgnored() && CGF.CGM.isPaddedAtomicType(atomicType)) {
        // Zero-initialize.  (Strictly speaking, we only need to intialize
        // the padding at the end, but this is simpler.)
        if (!Dest.isZeroed())
          CGF.EmitNullInitialization(Dest.getAddr(), atomicType);

        // Build a GEP to refer to the subobject.
        llvm::Value *valueAddr =
            CGF.Builder.CreateStructGEP(nullptr, valueDest.getAddr(), 0);
        valueDest = AggValueSlot::forAddr(valueAddr,
                                          valueDest.getAlignment(),
                                          valueDest.getQualifiers(),
                                          valueDest.isExternallyDestructed(),
                                          valueDest.requiresGCollection(),
                                          valueDest.isPotentiallyAliased(),
                                          AggValueSlot::IsZeroed);
      }
      
      CGF.EmitAggExpr(E->getSubExpr(), valueDest);
      return;
    }

    // Otherwise, we're converting an atomic type to a non-atomic type.
    // Make an atomic temporary, emit into that, and then copy the value out.
    AggValueSlot atomicSlot =
      CGF.CreateAggTemp(atomicType, "atomic-to-nonatomic.temp");
    CGF.EmitAggExpr(E->getSubExpr(), atomicSlot);

    llvm::Value *valueAddr =
        Builder.CreateStructGEP(nullptr, atomicSlot.getAddr(), 0);
    RValue rvalue = RValue::getAggregate(valueAddr, atomicSlot.isVolatile());
    return EmitFinalDestCopy(valueType, rvalue);
  }

  case CK_LValueToRValue:
    // If we're loading from a volatile type, force the destination
    // into existence.
    if (E->getSubExpr()->getType().isVolatileQualified()) {
      EnsureDest(E->getType());
      return Visit(E->getSubExpr());
    }

    // fallthrough

  case CK_NoOp:
  case CK_UserDefinedConversion:
  case CK_ConstructorConversion:
    assert(CGF.getContext().hasSameUnqualifiedType(E->getSubExpr()->getType(),
                                                   E->getType()) &&
           "Implicit cast types must be compatible");
    Visit(E->getSubExpr());
    break;
      
  case CK_LValueBitCast:
    llvm_unreachable("should not be emitting lvalue bitcast as rvalue");

  case CK_Dependent:
  case CK_BitCast:
  case CK_ArrayToPointerDecay:
  case CK_FunctionToPointerDecay:
  case CK_NullToPointer:
  case CK_NullToMemberPointer:
  case CK_BaseToDerivedMemberPointer:
  case CK_DerivedToBaseMemberPointer:
  case CK_MemberPointerToBoolean:
  case CK_ReinterpretMemberPointer:
  case CK_IntegralToPointer:
  case CK_PointerToIntegral:
  case CK_PointerToBoolean:
  case CK_ToVoid:
  case CK_VectorSplat:
  case CK_IntegralCast:
  case CK_IntegralToBoolean:
  case CK_IntegralToFloating:
  case CK_FloatingToIntegral:
  case CK_FloatingToBoolean:
  case CK_FloatingCast:
  case CK_CPointerToObjCPointerCast:
  case CK_BlockPointerToObjCPointerCast:
  case CK_AnyPointerToBlockPointerCast:
  case CK_ObjCObjectLValueCast:
  case CK_FloatingRealToComplex:
  case CK_FloatingComplexToReal:
  case CK_FloatingComplexToBoolean:
  case CK_FloatingComplexCast:
  case CK_FloatingComplexToIntegralComplex:
  case CK_IntegralRealToComplex:
  case CK_IntegralComplexToReal:
  case CK_IntegralComplexToBoolean:
  case CK_IntegralComplexCast:
  case CK_IntegralComplexToFloatingComplex:
  case CK_ARCProduceObject:
  case CK_ARCConsumeObject:
  case CK_ARCReclaimReturnedObject:
  case CK_ARCExtendBlockObject:
  case CK_CopyAndAutoreleaseBlockObject:
  case CK_BuiltinFnToFnPtr:
  case CK_ZeroToOCLEvent:
  case CK_AddressSpaceConversion:
    llvm_unreachable("cast kind invalid for aggregate types");
  }
}

void AggExprEmitter::VisitCallExpr(const CallExpr *E) {
  if (E->getCallReturnType(CGF.getContext())->isReferenceType()) {
    EmitAggLoadOfLValue(E);
    return;
  }

  RValue RV = CGF.EmitCallExpr(E, getReturnValueSlot());
  EmitMoveFromReturnSlot(E, RV);
}

void AggExprEmitter::VisitObjCMessageExpr(ObjCMessageExpr *E) {
  RValue RV = CGF.EmitObjCMessageExpr(E, getReturnValueSlot());
  EmitMoveFromReturnSlot(E, RV);
}

void AggExprEmitter::VisitBinComma(const BinaryOperator *E) {
  CGF.EmitIgnoredExpr(E->getLHS());
  Visit(E->getRHS());
}

void AggExprEmitter::VisitStmtExpr(const StmtExpr *E) {
  CodeGenFunction::StmtExprEvaluation eval(CGF);
  CGF.EmitCompoundStmt(*E->getSubStmt(), true, Dest);
}

void AggExprEmitter::VisitBinaryOperator(const BinaryOperator *E) {
  if (E->getOpcode() == BO_PtrMemD || E->getOpcode() == BO_PtrMemI)
    VisitPointerToDataMemberBinaryOperator(E);
  else
    CGF.ErrorUnsupported(E, "aggregate binary expression");
}

void AggExprEmitter::VisitPointerToDataMemberBinaryOperator(
                                                    const BinaryOperator *E) {
  LValue LV = CGF.EmitPointerToDataMemberBinaryExpr(E);
  EmitFinalDestCopy(E->getType(), LV);
}

/// Is the value of the given expression possibly a reference to or
/// into a __block variable?
static bool isBlockVarRef(const Expr *E) {
  // Make sure we look through parens.
  E = E->IgnoreParens();

  // Check for a direct reference to a __block variable.
  if (const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E)) {
    const VarDecl *var = dyn_cast<VarDecl>(DRE->getDecl());
    return (var && var->hasAttr<BlocksAttr>());
  }

  // More complicated stuff.

  // Binary operators.
  if (const BinaryOperator *op = dyn_cast<BinaryOperator>(E)) {
    // For an assignment or pointer-to-member operation, just care
    // about the LHS.
    if (op->isAssignmentOp() || op->isPtrMemOp())
      return isBlockVarRef(op->getLHS());

    // For a comma, just care about the RHS.
    if (op->getOpcode() == BO_Comma)
      return isBlockVarRef(op->getRHS());

    // FIXME: pointer arithmetic?
    return false;

  // Check both sides of a conditional operator.
  } else if (const AbstractConditionalOperator *op
               = dyn_cast<AbstractConditionalOperator>(E)) {
    return isBlockVarRef(op->getTrueExpr())
        || isBlockVarRef(op->getFalseExpr());

  // OVEs are required to support BinaryConditionalOperators.
  } else if (const OpaqueValueExpr *op
               = dyn_cast<OpaqueValueExpr>(E)) {
    if (const Expr *src = op->getSourceExpr())
      return isBlockVarRef(src);

  // Casts are necessary to get things like (*(int*)&var) = foo().
  // We don't really care about the kind of cast here, except
  // we don't want to look through l2r casts, because it's okay
  // to get the *value* in a __block variable.
  } else if (const CastExpr *cast = dyn_cast<CastExpr>(E)) {
    if (cast->getCastKind() == CK_LValueToRValue)
      return false;
    return isBlockVarRef(cast->getSubExpr());

  // Handle unary operators.  Again, just aggressively look through
  // it, ignoring the operation.
  } else if (const UnaryOperator *uop = dyn_cast<UnaryOperator>(E)) {
    return isBlockVarRef(uop->getSubExpr());

  // Look into the base of a field access.
  } else if (const MemberExpr *mem = dyn_cast<MemberExpr>(E)) {
    return isBlockVarRef(mem->getBase());

  // Look into the base of a subscript.
  } else if (const ArraySubscriptExpr *sub = dyn_cast<ArraySubscriptExpr>(E)) {
    return isBlockVarRef(sub->getBase());
  }

  return false;
}

void AggExprEmitter::VisitBinAssign(const BinaryOperator *E) {
  // For an assignment to work, the value on the right has
  // to be compatible with the value on the left.
  assert(CGF.getContext().hasSameUnqualifiedType(E->getLHS()->getType(),
                                                 E->getRHS()->getType())
         && "Invalid assignment");

  // If the LHS might be a __block variable, and the RHS can
  // potentially cause a block copy, we need to evaluate the RHS first
  // so that the assignment goes the right place.
  // This is pretty semantically fragile.
  if (isBlockVarRef(E->getLHS()) &&
      E->getRHS()->HasSideEffects(CGF.getContext())) {
    // Ensure that we have a destination, and evaluate the RHS into that.
    EnsureDest(E->getRHS()->getType());
    Visit(E->getRHS());

    // Now emit the LHS and copy into it.
    LValue LHS = CGF.EmitCheckedLValue(E->getLHS(), CodeGenFunction::TCK_Store);

    // That copy is an atomic copy if the LHS is atomic.
    if (LHS.getType()->isAtomicType() ||
        CGF.LValueIsSuitableForInlineAtomic(LHS)) {
      CGF.EmitAtomicStore(Dest.asRValue(), LHS, /*isInit*/ false);
      return;
    }

    EmitCopy(E->getLHS()->getType(),
             AggValueSlot::forLValue(LHS, AggValueSlot::IsDestructed,
                                     needsGC(E->getLHS()->getType()),
                                     AggValueSlot::IsAliased),
             Dest);
    return;
  }
  
  LValue LHS = CGF.EmitLValue(E->getLHS());

  // If we have an atomic type, evaluate into the destination and then
  // do an atomic copy.
  if (LHS.getType()->isAtomicType() ||
      CGF.LValueIsSuitableForInlineAtomic(LHS)) {
    EnsureDest(E->getRHS()->getType());
    Visit(E->getRHS());
    CGF.EmitAtomicStore(Dest.asRValue(), LHS, /*isInit*/ false);
    return;
  }

  // Codegen the RHS so that it stores directly into the LHS.
  AggValueSlot LHSSlot =
    AggValueSlot::forLValue(LHS, AggValueSlot::IsDestructed, 
                            needsGC(E->getLHS()->getType()),
                            AggValueSlot::IsAliased);
  // A non-volatile aggregate destination might have volatile member.
  if (!LHSSlot.isVolatile() &&
      CGF.hasVolatileMember(E->getLHS()->getType()))
    LHSSlot.setVolatile(true);
      
  CGF.EmitAggExpr(E->getRHS(), LHSSlot);

  // Copy into the destination if the assignment isn't ignored.
  EmitFinalDestCopy(E->getType(), LHS);
}

void AggExprEmitter::
VisitAbstractConditionalOperator(const AbstractConditionalOperator *E) {
  llvm::BasicBlock *LHSBlock = CGF.createBasicBlock("cond.true");
  llvm::BasicBlock *RHSBlock = CGF.createBasicBlock("cond.false");
  llvm::BasicBlock *ContBlock = CGF.createBasicBlock("cond.end");

  // Bind the common expression if necessary.
  CodeGenFunction::OpaqueValueMapping binding(CGF, E);

  CodeGenFunction::ConditionalEvaluation eval(CGF);
  CGF.EmitBranchOnBoolExpr(E->getCond(), LHSBlock, RHSBlock,
                           CGF.getProfileCount(E));

  // Save whether the destination's lifetime is externally managed.
  bool isExternallyDestructed = Dest.isExternallyDestructed();

  eval.begin(CGF);
  CGF.EmitBlock(LHSBlock);
  CGF.incrementProfileCounter(E);
  Visit(E->getTrueExpr());
  eval.end(CGF);

  assert(CGF.HaveInsertPoint() && "expression evaluation ended with no IP!");
  CGF.Builder.CreateBr(ContBlock);

  // If the result of an agg expression is unused, then the emission
  // of the LHS might need to create a destination slot.  That's fine
  // with us, and we can safely emit the RHS into the same slot, but
  // we shouldn't claim that it's already being destructed.
  Dest.setExternallyDestructed(isExternallyDestructed);

  eval.begin(CGF);
  CGF.EmitBlock(RHSBlock);
  Visit(E->getFalseExpr());
  eval.end(CGF);

  CGF.EmitBlock(ContBlock);
}

void AggExprEmitter::VisitChooseExpr(const ChooseExpr *CE) {
  Visit(CE->getChosenSubExpr());
}

void AggExprEmitter::VisitVAArgExpr(VAArgExpr *VE) {
  llvm::Value *ArgValue = CGF.EmitVAListRef(VE->getSubExpr());
  llvm::Value *ArgPtr = CGF.EmitVAArg(ArgValue, VE->getType());

  if (!ArgPtr) {
    // If EmitVAArg fails, we fall back to the LLVM instruction.
    llvm::Value *Val =
        Builder.CreateVAArg(ArgValue, CGF.ConvertType(VE->getType()));
    if (!Dest.isIgnored())
      Builder.CreateStore(Val, Dest.getAddr());
    return;
  }

  EmitFinalDestCopy(VE->getType(), CGF.MakeAddrLValue(ArgPtr, VE->getType()));
}

void AggExprEmitter::VisitCXXBindTemporaryExpr(CXXBindTemporaryExpr *E) {
  // Ensure that we have a slot, but if we already do, remember
  // whether it was externally destructed.
  bool wasExternallyDestructed = Dest.isExternallyDestructed();
  EnsureDest(E->getType());

  // We're going to push a destructor if there isn't already one.
  Dest.setExternallyDestructed();

  Visit(E->getSubExpr());

  // Push that destructor we promised.
  if (!wasExternallyDestructed)
    CGF.EmitCXXTemporary(E->getTemporary(), E->getType(), Dest.getAddr());
}

void
AggExprEmitter::VisitCXXConstructExpr(const CXXConstructExpr *E) {
  AggValueSlot Slot = EnsureSlot(E->getType());
  CGF.EmitCXXConstructExpr(E, Slot);
}

void
AggExprEmitter::VisitLambdaExpr(LambdaExpr *E) {
  AggValueSlot Slot = EnsureSlot(E->getType());
  CGF.EmitLambdaExpr(E, Slot);
}

void AggExprEmitter::VisitExprWithCleanups(ExprWithCleanups *E) {
  CGF.enterFullExpression(E);
  CodeGenFunction::RunCleanupsScope cleanups(CGF);
  Visit(E->getSubExpr());
}

void AggExprEmitter::VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *E) {
  QualType T = E->getType();
  AggValueSlot Slot = EnsureSlot(T);
  EmitNullInitializationToLValue(CGF.MakeAddrLValue(Slot.getAddr(), T));
}

void AggExprEmitter::VisitImplicitValueInitExpr(ImplicitValueInitExpr *E) {
  QualType T = E->getType();
  AggValueSlot Slot = EnsureSlot(T);
  EmitNullInitializationToLValue(CGF.MakeAddrLValue(Slot.getAddr(), T));
}

/// isSimpleZero - If emitting this value will obviously just cause a store of
/// zero to memory, return true.  This can return false if uncertain, so it just
/// handles simple cases.
static bool isSimpleZero(const Expr *E, CodeGenFunction &CGF) {
  E = E->IgnoreParens();

  // 0
  if (const IntegerLiteral *IL = dyn_cast<IntegerLiteral>(E))
    return IL->getValue() == 0;
  // +0.0
  if (const FloatingLiteral *FL = dyn_cast<FloatingLiteral>(E))
    return FL->getValue().isPosZero();
  // int()
  if ((isa<ImplicitValueInitExpr>(E) || isa<CXXScalarValueInitExpr>(E)) &&
      CGF.getTypes().isZeroInitializable(E->getType()))
    return true;
  // (int*)0 - Null pointer expressions.
  if (const CastExpr *ICE = dyn_cast<CastExpr>(E))
    return ICE->getCastKind() == CK_NullToPointer;
  // '\0'
  if (const CharacterLiteral *CL = dyn_cast<CharacterLiteral>(E))
    return CL->getValue() == 0;
  
  // Otherwise, hard case: conservatively return false.
  return false;
}


void 
AggExprEmitter::EmitInitializationToLValue(Expr *E, LValue LV) {
  QualType type = LV.getType();
  // FIXME: Ignore result?
  // FIXME: Are initializers affected by volatile?
  if (Dest.isZeroed() && isSimpleZero(E, CGF)) {
    // Storing "i32 0" to a zero'd memory location is a noop.
    return;
  } else if (isa<ImplicitValueInitExpr>(E) || isa<CXXScalarValueInitExpr>(E)) {
    return EmitNullInitializationToLValue(LV);
  } else if (type->isReferenceType()) {
    RValue RV = CGF.EmitReferenceBindingToExpr(E);
    return CGF.EmitStoreThroughLValue(RV, LV);
  }
  
  switch (CGF.getEvaluationKind(type)) {
  case TEK_Complex:
    CGF.EmitComplexExprIntoLValue(E, LV, /*isInit*/ true);
    return;
  case TEK_Aggregate:
    CGF.EmitAggExpr(E, AggValueSlot::forLValue(LV,
                                               AggValueSlot::IsDestructed,
                                      AggValueSlot::DoesNotNeedGCBarriers,
                                               AggValueSlot::IsNotAliased,
                                               Dest.isZeroed()));
    return;
  case TEK_Scalar:
    if (LV.isSimple()) {
      CGF.EmitScalarInit(E, /*D=*/nullptr, LV, /*Captured=*/false);
    } else {
      CGF.EmitStoreThroughLValue(RValue::get(CGF.EmitScalarExpr(E)), LV);
    }
    return;
  }
  llvm_unreachable("bad evaluation kind");
}

void AggExprEmitter::EmitNullInitializationToLValue(LValue lv) {
  QualType type = lv.getType();

  // If the destination slot is already zeroed out before the aggregate is
  // copied into it, we don't have to emit any zeros here.
  if (Dest.isZeroed() && CGF.getTypes().isZeroInitializable(type))
    return;
  
  if (CGF.hasScalarEvaluationKind(type)) {
    // For non-aggregates, we can store the appropriate null constant.
    llvm::Value *null = CGF.CGM.EmitNullConstant(type);
    // Note that the following is not equivalent to
    // EmitStoreThroughBitfieldLValue for ARC types.
    if (lv.isBitField()) {
      CGF.EmitStoreThroughBitfieldLValue(RValue::get(null), lv);
    } else {
      assert(lv.isSimple());
      CGF.EmitStoreOfScalar(null, lv, /* isInitialization */ true);
    }
  } else {
    // There's a potential optimization opportunity in combining
    // memsets; that would be easy for arrays, but relatively
    // difficult for structures with the current code.
    CGF.EmitNullInitialization(lv.getAddress(), lv.getType());
  }
}

void AggExprEmitter::VisitInitListExpr(InitListExpr *E) {
#if 0
  // FIXME: Assess perf here?  Figure out what cases are worth optimizing here
  // (Length of globals? Chunks of zeroed-out space?).
  //
  // If we can, prefer a copy from a global; this is a lot less code for long
  // globals, and it's easier for the current optimizers to analyze.
  if (llvm::Constant* C = CGF.CGM.EmitConstantExpr(E, E->getType(), &CGF)) {
    llvm::GlobalVariable* GV =
    new llvm::GlobalVariable(CGF.CGM.getModule(), C->getType(), true,
                             llvm::GlobalValue::InternalLinkage, C, "");
    EmitFinalDestCopy(E->getType(), CGF.MakeAddrLValue(GV, E->getType()));
    return;
  }
#endif
  if (E->hadArrayRangeDesignator())
    CGF.ErrorUnsupported(E, "GNU array range designator extension");

  AggValueSlot Dest = EnsureSlot(E->getType());

  LValue DestLV = CGF.MakeAddrLValue(Dest.getAddr(), E->getType(),
                                     Dest.getAlignment());

  // Handle initialization of an array.
  if (E->getType()->isArrayType()) {
    if (E->isStringLiteralInit())
      return Visit(E->getInit(0));

    QualType elementType =
        CGF.getContext().getAsArrayType(E->getType())->getElementType();

    llvm::PointerType *APType =
      cast<llvm::PointerType>(Dest.getAddr()->getType());
    llvm::ArrayType *AType =
      cast<llvm::ArrayType>(APType->getElementType());

    EmitArrayInit(Dest.getAddr(), AType, elementType, E);
    return;
  }

  if (E->getType()->isAtomicType()) {
    // An _Atomic(T) object can be list-initialized from an expression
    // of the same type.
    assert(E->getNumInits() == 1 &&
           CGF.getContext().hasSameUnqualifiedType(E->getInit(0)->getType(),
                                                   E->getType()) &&
           "unexpected list initialization for atomic object");
    return Visit(E->getInit(0));
  }

  assert(E->getType()->isRecordType() && "Only support structs/unions here!");

  // Do struct initialization; this code just sets each individual member
  // to the approprate value.  This makes bitfield support automatic;
  // the disadvantage is that the generated code is more difficult for
  // the optimizer, especially with bitfields.
  unsigned NumInitElements = E->getNumInits();
  RecordDecl *record = E->getType()->castAs<RecordType>()->getDecl();

  // Prepare a 'this' for CXXDefaultInitExprs.
  CodeGenFunction::FieldConstructionScope FCS(CGF, Dest.getAddr());

  if (record->isUnion()) {
    // Only initialize one field of a union. The field itself is
    // specified by the initializer list.
    if (!E->getInitializedFieldInUnion()) {
      // Empty union; we have nothing to do.

#ifndef NDEBUG
      // Make sure that it's really an empty and not a failure of
      // semantic analysis.
      for (const auto *Field : record->fields())
        assert(Field->isUnnamedBitfield() && "Only unnamed bitfields allowed");
#endif
      return;
    }

    // FIXME: volatility
    FieldDecl *Field = E->getInitializedFieldInUnion();

    LValue FieldLoc = CGF.EmitLValueForFieldInitialization(DestLV, Field);
    if (NumInitElements) {
      // Store the initializer into the field
      EmitInitializationToLValue(E->getInit(0), FieldLoc);
    } else {
      // Default-initialize to null.
      EmitNullInitializationToLValue(FieldLoc);
    }

    return;
  }

  // We'll need to enter cleanup scopes in case any of the member
  // initializers throw an exception.
  SmallVector<EHScopeStack::stable_iterator, 16> cleanups;
  llvm::Instruction *cleanupDominator = nullptr;

  // Here we iterate over the fields; this makes it simpler to both
  // default-initialize fields and skip over unnamed fields.
  unsigned curInitIndex = 0;
  for (const auto *field : record->fields()) {
    // We're done once we hit the flexible array member.
    if (field->getType()->isIncompleteArrayType())
      break;

    // Always skip anonymous bitfields.
    if (field->isUnnamedBitfield())
      continue;

    // We're done if we reach the end of the explicit initializers, we
    // have a zeroed object, and the rest of the fields are
    // zero-initializable.
    if (curInitIndex == NumInitElements && Dest.isZeroed() &&
        CGF.getTypes().isZeroInitializable(E->getType()))
      break;
    

    LValue LV = CGF.EmitLValueForFieldInitialization(DestLV, field);
    // We never generate write-barries for initialized fields.
    LV.setNonGC(true);
    
    if (curInitIndex < NumInitElements) {
      // Store the initializer into the field.
      EmitInitializationToLValue(E->getInit(curInitIndex++), LV);
    } else {
      // We're out of initalizers; default-initialize to null
      EmitNullInitializationToLValue(LV);
    }

    // Push a destructor if necessary.
    // FIXME: if we have an array of structures, all explicitly
    // initialized, we can end up pushing a linear number of cleanups.
    bool pushedCleanup = false;
    if (QualType::DestructionKind dtorKind
          = field->getType().isDestructedType()) {
      assert(LV.isSimple());
      if (CGF.needsEHCleanup(dtorKind)) {
        if (!cleanupDominator)
          cleanupDominator = CGF.Builder.CreateUnreachable(); // placeholder

        CGF.pushDestroy(EHCleanup, LV.getAddress(), field->getType(),
                        CGF.getDestroyer(dtorKind), false);
        cleanups.push_back(CGF.EHStack.stable_begin());
        pushedCleanup = true;
      }
    }
    
    // If the GEP didn't get used because of a dead zero init or something
    // else, clean it up for -O0 builds and general tidiness.
    if (!pushedCleanup && LV.isSimple()) 
      if (llvm::GetElementPtrInst *GEP =
            dyn_cast<llvm::GetElementPtrInst>(LV.getAddress()))
        if (GEP->use_empty())
          GEP->eraseFromParent();
  }

  // Deactivate all the partial cleanups in reverse order, which
  // generally means popping them.
  for (unsigned i = cleanups.size(); i != 0; --i)
    CGF.DeactivateCleanupBlock(cleanups[i-1], cleanupDominator);

  // Destroy the placeholder if we made one.
  if (cleanupDominator)
    cleanupDominator->eraseFromParent();
}

//===----------------------------------------------------------------------===//
//                        Entry Points into this File
//===----------------------------------------------------------------------===//

/// GetNumNonZeroBytesInInit - Get an approximate count of the number of
/// non-zero bytes that will be stored when outputting the initializer for the
/// specified initializer expression.
static CharUnits GetNumNonZeroBytesInInit(const Expr *E, CodeGenFunction &CGF) {
  E = E->IgnoreParens();

  // 0 and 0.0 won't require any non-zero stores!
  if (isSimpleZero(E, CGF)) return CharUnits::Zero();

  // If this is an initlist expr, sum up the size of sizes of the (present)
  // elements.  If this is something weird, assume the whole thing is non-zero.
  const InitListExpr *ILE = dyn_cast<InitListExpr>(E);
  if (!ILE || !CGF.getTypes().isZeroInitializable(ILE->getType()))
    return CGF.getContext().getTypeSizeInChars(E->getType());
  
  // InitListExprs for structs have to be handled carefully.  If there are
  // reference members, we need to consider the size of the reference, not the
  // referencee.  InitListExprs for unions and arrays can't have references.
  if (const RecordType *RT = E->getType()->getAs<RecordType>()) {
    if (!RT->isUnionType()) {
      RecordDecl *SD = E->getType()->getAs<RecordType>()->getDecl();
      CharUnits NumNonZeroBytes = CharUnits::Zero();
      
      unsigned ILEElement = 0;
      for (const auto *Field : SD->fields()) {
        // We're done once we hit the flexible array member or run out of
        // InitListExpr elements.
        if (Field->getType()->isIncompleteArrayType() ||
            ILEElement == ILE->getNumInits())
          break;
        if (Field->isUnnamedBitfield())
          continue;

        const Expr *E = ILE->getInit(ILEElement++);
        
        // Reference values are always non-null and have the width of a pointer.
        if (Field->getType()->isReferenceType())
          NumNonZeroBytes += CGF.getContext().toCharUnitsFromBits(
              CGF.getTarget().getPointerWidth(0));
        else
          NumNonZeroBytes += GetNumNonZeroBytesInInit(E, CGF);
      }
      
      return NumNonZeroBytes;
    }
  }
  
  
  CharUnits NumNonZeroBytes = CharUnits::Zero();
  for (unsigned i = 0, e = ILE->getNumInits(); i != e; ++i)
    NumNonZeroBytes += GetNumNonZeroBytesInInit(ILE->getInit(i), CGF);
  return NumNonZeroBytes;
}

/// CheckAggExprForMemSetUse - If the initializer is large and has a lot of
/// zeros in it, emit a memset and avoid storing the individual zeros.
///
static void CheckAggExprForMemSetUse(AggValueSlot &Slot, const Expr *E,
                                     CodeGenFunction &CGF) {
  // If the slot is already known to be zeroed, nothing to do.  Don't mess with
  // volatile stores.
  if (Slot.isZeroed() || Slot.isVolatile() || Slot.getAddr() == nullptr)
    return;

  // C++ objects with a user-declared constructor don't need zero'ing.
  if (CGF.getLangOpts().CPlusPlus)
    if (const RecordType *RT = CGF.getContext()
                       .getBaseElementType(E->getType())->getAs<RecordType>()) {
      const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
      if (RD->hasUserDeclaredConstructor())
        return;
    }

  // If the type is 16-bytes or smaller, prefer individual stores over memset.
  std::pair<CharUnits, CharUnits> TypeInfo =
    CGF.getContext().getTypeInfoInChars(E->getType());
  if (TypeInfo.first <= CharUnits::fromQuantity(16))
    return;

  // Check to see if over 3/4 of the initializer are known to be zero.  If so,
  // we prefer to emit memset + individual stores for the rest.
  CharUnits NumNonZeroBytes = GetNumNonZeroBytesInInit(E, CGF);
  if (NumNonZeroBytes*4 > TypeInfo.first)
    return;
  
  // Okay, it seems like a good idea to use an initial memset, emit the call.
  llvm::Constant *SizeVal = CGF.Builder.getInt64(TypeInfo.first.getQuantity());
  CharUnits Align = TypeInfo.second;

  llvm::Value *Loc = Slot.getAddr();
  
  Loc = CGF.Builder.CreateBitCast(Loc, CGF.Int8PtrTy);
  CGF.Builder.CreateMemSet(Loc, CGF.Builder.getInt8(0), SizeVal, 
                           Align.getQuantity(), false);
  
  // Tell the AggExprEmitter that the slot is known zero.
  Slot.setZeroed();
}




/// EmitAggExpr - Emit the computation of the specified expression of aggregate
/// type.  The result is computed into DestPtr.  Note that if DestPtr is null,
/// the value of the aggregate expression is not needed.  If VolatileDest is
/// true, DestPtr cannot be 0.
void CodeGenFunction::EmitAggExpr(const Expr *E, AggValueSlot Slot) {
  assert(E && hasAggregateEvaluationKind(E->getType()) &&
         "Invalid aggregate expression to emit");
  assert((Slot.getAddr() != nullptr || Slot.isIgnored()) &&
         "slot has bits but no address");

  // Optimize the slot if possible.
  CheckAggExprForMemSetUse(Slot, E, *this);
 
  AggExprEmitter(*this, Slot, Slot.isIgnored()).Visit(const_cast<Expr*>(E));
}

LValue CodeGenFunction::EmitAggExprToLValue(const Expr *E) {
  assert(hasAggregateEvaluationKind(E->getType()) && "Invalid argument!");
  llvm::Value *Temp = CreateMemTemp(E->getType());
  LValue LV = MakeAddrLValue(Temp, E->getType());
  EmitAggExpr(E, AggValueSlot::forLValue(LV, AggValueSlot::IsNotDestructed,
                                         AggValueSlot::DoesNotNeedGCBarriers,
                                         AggValueSlot::IsNotAliased));
  return LV;
}

void CodeGenFunction::EmitAggregateCopy(llvm::Value *DestPtr,
                                        llvm::Value *SrcPtr, QualType Ty,
                                        bool isVolatile,
                                        CharUnits alignment,
                                        bool isAssignment) {
  assert(!Ty->isAnyComplexType() && "Shouldn't happen for complex");

  if (getLangOpts().CPlusPlus) {
    if (const RecordType *RT = Ty->getAs<RecordType>()) {
      CXXRecordDecl *Record = cast<CXXRecordDecl>(RT->getDecl());
      assert((Record->hasTrivialCopyConstructor() || 
              Record->hasTrivialCopyAssignment() ||
              Record->hasTrivialMoveConstructor() ||
              Record->hasTrivialMoveAssignment() ||
              Record->isUnion()) &&
             "Trying to aggregate-copy a type without a trivial copy/move "
             "constructor or assignment operator");
      // Ignore empty classes in C++.
      if (Record->isEmpty())
        return;
    }
  }
  
  // Aggregate assignment turns into llvm.memcpy.  This is almost valid per
  // C99 6.5.16.1p3, which states "If the value being stored in an object is
  // read from another object that overlaps in anyway the storage of the first
  // object, then the overlap shall be exact and the two objects shall have
  // qualified or unqualified versions of a compatible type."
  //
  // memcpy is not defined if the source and destination pointers are exactly
  // equal, but other compilers do this optimization, and almost every memcpy
  // implementation handles this case safely.  If there is a libc that does not
  // safely handle this, we can add a target hook.

  // Get data size and alignment info for this aggregate. If this is an
  // assignment don't copy the tail padding. Otherwise copying it is fine.
  std::pair<CharUnits, CharUnits> TypeInfo;
  if (isAssignment)
    TypeInfo = getContext().getTypeInfoDataSizeInChars(Ty);
  else
    TypeInfo = getContext().getTypeInfoInChars(Ty);

  if (alignment.isZero())
    alignment = TypeInfo.second;

  llvm::Value *SizeVal = nullptr;
  if (TypeInfo.first.isZero()) {
    // But note that getTypeInfo returns 0 for a VLA.
    if (auto *VAT = dyn_cast_or_null<VariableArrayType>(
            getContext().getAsArrayType(Ty))) {
      QualType BaseEltTy;
      SizeVal = emitArrayLength(VAT, BaseEltTy, DestPtr);
      TypeInfo = getContext().getTypeInfoDataSizeInChars(BaseEltTy);
      std::pair<CharUnits, CharUnits> LastElementTypeInfo;
      if (!isAssignment)
        LastElementTypeInfo = getContext().getTypeInfoInChars(BaseEltTy);
      assert(!TypeInfo.first.isZero());
      SizeVal = Builder.CreateNUWMul(
          SizeVal,
          llvm::ConstantInt::get(SizeTy, TypeInfo.first.getQuantity()));
      if (!isAssignment) {
        SizeVal = Builder.CreateNUWSub(
            SizeVal,
            llvm::ConstantInt::get(SizeTy, TypeInfo.first.getQuantity()));
        SizeVal = Builder.CreateNUWAdd(
            SizeVal, llvm::ConstantInt::get(
                         SizeTy, LastElementTypeInfo.first.getQuantity()));
      }
    }
  }
  if (!SizeVal) {
    SizeVal = llvm::ConstantInt::get(SizeTy, TypeInfo.first.getQuantity());
  }

  // FIXME: If we have a volatile struct, the optimizer can remove what might
  // appear to be `extra' memory ops:
  //
  // volatile struct { int i; } a, b;
  //
  // int main() {
  //   a = b;
  //   a = b;
  // }
  //
  // we need to use a different call here.  We use isVolatile to indicate when
  // either the source or the destination is volatile.

  llvm::PointerType *DPT = cast<llvm::PointerType>(DestPtr->getType());
  llvm::Type *DBP =
    llvm::Type::getInt8PtrTy(getLLVMContext(), DPT->getAddressSpace());
  DestPtr = Builder.CreateBitCast(DestPtr, DBP);

  llvm::PointerType *SPT = cast<llvm::PointerType>(SrcPtr->getType());
  llvm::Type *SBP =
    llvm::Type::getInt8PtrTy(getLLVMContext(), SPT->getAddressSpace());
  SrcPtr = Builder.CreateBitCast(SrcPtr, SBP);

  // Don't do any of the memmove_collectable tests if GC isn't set.
  if (CGM.getLangOpts().getGC() == LangOptions::NonGC) {
    // fall through
  } else if (const RecordType *RecordTy = Ty->getAs<RecordType>()) {
    RecordDecl *Record = RecordTy->getDecl();
    if (Record->hasObjectMember()) {
      CGM.getObjCRuntime().EmitGCMemmoveCollectable(*this, DestPtr, SrcPtr, 
                                                    SizeVal);
      return;
    }
  } else if (Ty->isArrayType()) {
    QualType BaseType = getContext().getBaseElementType(Ty);
    if (const RecordType *RecordTy = BaseType->getAs<RecordType>()) {
      if (RecordTy->getDecl()->hasObjectMember()) {
        CGM.getObjCRuntime().EmitGCMemmoveCollectable(*this, DestPtr, SrcPtr, 
                                                      SizeVal);
        return;
      }
    }
  }

  // Determine the metadata to describe the position of any padding in this
  // memcpy, as well as the TBAA tags for the members of the struct, in case
  // the optimizer wishes to expand it in to scalar memory operations.
  llvm::MDNode *TBAAStructTag = CGM.getTBAAStructInfo(Ty);

  Builder.CreateMemCpy(DestPtr, SrcPtr, SizeVal, alignment.getQuantity(),
                       isVolatile, /*TBAATag=*/nullptr, TBAAStructTag);
}