swift-mirror/lib/SILGen/SILGenLValue.cpp

//===--- SILGenLValue.cpp - Constructs logical lvalues for SILGen ---------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// Emission of l-value expressions and basic operations on them.
//
//===----------------------------------------------------------------------===//


#include "SILGen.h"
#include "LValue.h"
#include "RValue.h"
#include "Scope.h"
#include "Initialization.h"
#include "swift/AST/AST.h"
#include "swift/AST/Decl.h"
#include "swift/AST/Types.h"
#include "swift/AST/DiagnosticsCommon.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/TypeLowering.h"
#include "llvm/Support/raw_ostream.h"
#include "ASTVisitor.h"
using namespace swift;
using namespace Lowering;

//===----------------------------------------------------------------------===//

/// A pending writeback.
namespace swift {
namespace Lowering {

  
/// Materialize - Represents a temporary allocation.
struct LLVM_LIBRARY_VISIBILITY Materialize {
  /// The address of the allocation.
  SILValue address;
  
  /// The cleanup to dispose of the value before deallocating the buffer.
  /// This cleanup can be killed by calling the consume method.
  CleanupHandle valueCleanup;
  
  /// Load and claim ownership of the value in the buffer. Does not deallocate
  /// the buffer.
  ManagedValue claim(SILGenFunction &gen, SILLocation loc);
};

  
struct LLVM_LIBRARY_VISIBILITY LValueWriteback {
  SILLocation loc;
  std::unique_ptr<LogicalPathComponent> component;
  ManagedValue base;
  Materialize temp;

  ~LValueWriteback() {}
  LValueWriteback(LValueWriteback&&) = default;
  LValueWriteback &operator=(LValueWriteback&&) = default;

  LValueWriteback() = default;
  LValueWriteback(SILLocation loc, std::unique_ptr<LogicalPathComponent> &&comp,
                  ManagedValue base, Materialize temp)
    : loc(loc), component(std::move(comp)), base(base), temp(temp) {
  }
};
}
}

std::vector<LValueWriteback> &SILGenFunction::getWritebackStack() {
  if (!WritebackStack)
    WritebackStack = new std::vector<LValueWriteback>();

  return *WritebackStack;
}

void SILGenFunction::freeWritebackStack() {
  delete WritebackStack;
}

Materialize SILGenFunction::emitMaterialize(SILLocation loc, ManagedValue v) {
  // Address-only values are already materialized.
  if (v.getType().isAddress()) {
    assert(v.getType().isAddressOnly(SGM.M) && "can't materialize an l-value");
    return Materialize{v.getValue(), v.getCleanup()};
  }
  
  assert(!v.isLValue() && "materializing a non-address-only lvalue?!");
  auto &lowering = getTypeLowering(v.getType().getSwiftType());
  
  // We don't use getBufferForExprResult here because the result of a
  // materialization is *not* the value, but an address of the value.
  SILValue tmpMem = emitTemporaryAllocation(loc, v.getType());
  v.forwardInto(*this, loc, tmpMem);
  
  CleanupHandle valueCleanup = CleanupHandle::invalid();
  if (!lowering.isTrivial())
    valueCleanup = enterDestroyCleanup(tmpMem);
  
  return Materialize{tmpMem, valueCleanup};
}

//===----------------------------------------------------------------------===//

static CanType getSubstFormalRValueType(Expr *expr) {
  return expr->getType()->getRValueType()->getCanonicalType();
}

static AbstractionPattern getOrigFormalRValueType(Type formalStorageType) {
  auto type = formalStorageType->getCanonicalType();
  if (auto ref = dyn_cast<ReferenceStorageType>(type)) {
    type = ref.getReferentType();
    if (isa<WeakStorageType>(ref))
      type = OptionalType::get(type)->getCanonicalType();
  }
  return AbstractionPattern(type);
}

/// Return the LValueTypeData for the formal type of a declaration
/// that needs no substitutions.
static LValueTypeData getUnsubstitutedTypeData(SILGenFunction &gen,
                                               CanType formalRValueType) {
  return {
    AbstractionPattern(formalRValueType),
    formalRValueType,
    gen.getLoweredType(formalRValueType),
  };
}

static LValueTypeData getMemberTypeData(SILGenFunction &gen,
                                        Type memberStorageType,
                                        Expr *lvalueExpr) {
  auto origFormalType = getOrigFormalRValueType(memberStorageType);
  auto substFormalType = getSubstFormalRValueType(lvalueExpr);
  return {
    origFormalType,
    substFormalType,
    gen.getLoweredType(origFormalType, substFormalType)
  };
}

/// SILGenLValue - An ASTVisitor for building logical lvalues.
class LLVM_LIBRARY_VISIBILITY SILGenLValue
  : public Lowering::ExprVisitor<SILGenLValue, LValue>
{
public:
  SILGenFunction &gen;
  SILGenLValue(SILGenFunction &gen) : gen(gen) {}
  
  LValue visitRec(Expr *e);
  
  /// Dummy handler to log unimplemented nodes.
  LValue visitExpr(Expr *e);

  // Nodes that form the root of lvalue paths

  LValue visitDeclRefExpr(DeclRefExpr *e);

  // Nodes that make up components of lvalue paths
  
  LValue visitMemberRefExpr(MemberRefExpr *e);
  LValue visitSubscriptExpr(SubscriptExpr *e);
  LValue visitTupleElementExpr(TupleElementExpr *e);
  LValue visitLValueConversionExpr(LValueConversionExpr *e);
  
  // Expressions that wrap lvalues
  
  LValue visitInOutExpr(InOutExpr *e);
  LValue visitDotSyntaxBaseIgnoredExpr(DotSyntaxBaseIgnoredExpr *e);
};

SILValue LogicalPathComponent::getMaterialized(SILGenFunction &gen,
                                               SILLocation loc,
                                               ManagedValue base) const {
  // If the writeback is disabled, just emit a load into a temporary memory
  // location.
  if (!gen.InWritebackScope) {
    ManagedValue value = get(gen, loc, base, SGFContext());
    return gen.emitMaterialize(loc, value).address;
  }

  // Otherwise, we need to emit a get and set.  The get operation will consume
  // the base's +1, so copy the base for the setter.
  ManagedValue getterBase = base;
  if (base && base.hasCleanup())
    getterBase = base.copy(gen, loc);

  ManagedValue value = get(gen, loc, getterBase, SGFContext());
  Materialize temp = gen.emitMaterialize(loc, value);
  
  gen.getWritebackStack().emplace_back(loc, clone(gen, loc), base, temp);
  return temp.address;
}

ManagedValue Materialize::claim(SILGenFunction &gen, SILLocation loc) {
  auto &addressTL = gen.getTypeLowering(address.getType());
  if (addressTL.isAddressOnly()) {
    // We can use the temporary as an address-only rvalue directly.
    return ManagedValue(address, valueCleanup);
  }

  // A materialized temporary is always its own type-of-rvalue because
  // we did a semantic load to produce it in the first place.

  if (valueCleanup.isValid())
    gen.Cleanups.setCleanupState(valueCleanup, CleanupState::Dead);
  return gen.emitLoad(loc, address, addressTL, SGFContext(), IsTake);
}

WritebackScope::WritebackScope(SILGenFunction &g)
  : gen(&g), wasInWritebackScope(g.InWritebackScope),
    savedDepth(g.getWritebackStack().size())
{
  // If we're in an inout conversion scope, disable nested writeback scopes.
  if (g.InInOutConversionScope) {
    gen = nullptr;
    return;
  }
  g.InWritebackScope = true;
}

WritebackScope::~WritebackScope() {
  if (!gen)
    return;
  
  gen->InWritebackScope = wasInWritebackScope;
  auto i = gen->getWritebackStack().end(),
       deepest = gen->getWritebackStack().begin() + savedDepth;
  while (i-- > deepest) {
    ManagedValue mv = i->temp.claim(*gen, i->loc);
    auto formalTy = i->component->getSubstFormalType();
    i->component->set(*gen, i->loc, RValue(*gen, i->loc, formalTy, mv),
                      i->base);
  }
  
  gen->getWritebackStack().erase(deepest, gen->getWritebackStack().end());
}

WritebackScope::WritebackScope(WritebackScope &&o)
  : gen(o.gen),
    wasInWritebackScope(o.wasInWritebackScope),
    savedDepth(o.savedDepth)
{
  o.gen = nullptr;
}

WritebackScope &WritebackScope::operator=(WritebackScope &&o) {
  gen = o.gen;
  wasInWritebackScope = o.wasInWritebackScope;
  savedDepth = o.savedDepth;
  o.gen = nullptr;
  return *this;
}

InOutConversionScope::InOutConversionScope(SILGenFunction &gen)
  : gen(gen)
{
  assert(gen.InWritebackScope
         && "inout conversions should happen in writeback scopes");
  assert(!gen.InInOutConversionScope
         && "inout conversions should not be nested");
  gen.InInOutConversionScope = true;
}

InOutConversionScope::~InOutConversionScope() {
  assert(gen.InInOutConversionScope && "already exited conversion scope?!");
  gen.InInOutConversionScope = false;
}

void PathComponent::_anchor() {}
void PhysicalPathComponent::_anchor() {}
void LogicalPathComponent::_anchor() {}

/// Return the LValueTypeData for a value whose type is its own
/// lowering.
static LValueTypeData getValueTypeData(SILValue value) {
  assert(value.getType().isObject() ||
         value.getType().getSwiftRValueType()->isExistentialType() ||
         value.getType().getSwiftRValueType()->is<ArchetypeType>());
  return {
    AbstractionPattern(value.getType().getSwiftRValueType()),
    value.getType().getSwiftRValueType(),
    value.getType()
  };
}

namespace {
  class RefElementComponent : public PhysicalPathComponent {
    VarDecl *Field;
    SILType SubstFieldType;
  public:
    RefElementComponent(VarDecl *field, SILType substFieldType,
                        LValueTypeData typeData)
      : PhysicalPathComponent(typeData),
        Field(field), SubstFieldType(substFieldType) {}
    
    ManagedValue offset(SILGenFunction &gen, SILLocation loc, ManagedValue base)
      const override
    {
      assert(base.getType().isObject() &&
             "base for ref element component must be an object");
      assert(base.getType().hasReferenceSemantics() &&
             "base for ref element component must be a reference type");
      auto Res = gen.B.createRefElementAddr(loc, base.getValue(), Field,
                                            SubstFieldType);
      return ManagedValue::forLValue(Res);
    }
  };
  
  class TupleElementComponent : public PhysicalPathComponent {
    unsigned ElementIndex;
  public:
    TupleElementComponent(unsigned elementIndex, LValueTypeData typeData)
      : PhysicalPathComponent(typeData), ElementIndex(elementIndex) {}
    
    ManagedValue offset(SILGenFunction &gen, SILLocation loc,
                        ManagedValue base) const override {
      assert(base && "invalid value for element base");
      auto Res = gen.B.createTupleElementAddr(loc, base.getUnmanagedValue(),
                                              ElementIndex,
                                              getTypeOfRValue().getAddressType());
      return ManagedValue::forLValue(Res);
    }
  };
  
  class StructElementComponent : public PhysicalPathComponent {
    VarDecl *Field;
    SILType SubstFieldType;
  public:
    StructElementComponent(VarDecl *field, SILType substFieldType,
                           LValueTypeData typeData)
      : PhysicalPathComponent(typeData),
        Field(field), SubstFieldType(substFieldType) {}
    
    ManagedValue offset(SILGenFunction &gen, SILLocation loc,
                        ManagedValue base) const override {
      assert(base && "invalid value for element base");
      auto Res = gen.B.createStructElementAddr(loc, base.getUnmanagedValue(),
                                               Field, SubstFieldType);
      return ManagedValue::forLValue(Res);
    }
  };

  class ValueComponent : public PhysicalPathComponent {
    ManagedValue Value;
  public:
    ValueComponent(ManagedValue value, LValueTypeData typeData) :
      PhysicalPathComponent(typeData),
      Value(value) {
    }

    ManagedValue offset(SILGenFunction &gen, SILLocation loc,
                        ManagedValue base) const override {
      assert(!base && "value component must be root of lvalue path");
      return Value;
    }
  };

  class GetterSetterComponent : public LogicalPathComponent {
    // The VarDecl or SubscriptDecl being get/set.
    AbstractStorageDecl *decl;
    bool IsSuper;
    std::vector<Substitution> substitutions;
    Expr *subscriptIndexExpr;
    mutable RValue origSubscripts;
    
    struct AccessorArgs {
      RValueSource base;
      RValue subscripts;
    };
    
    /// Returns a tuple of RValues holding the accessor value, base (retained if
    /// necessary), and subscript arguments, in that order.
    AccessorArgs
    prepareAccessorArgs(SILGenFunction &gen, SILLocation loc,
                        ManagedValue base, AbstractFunctionDecl *funcDecl) const
    {
      AccessorArgs result;
      if (base)
        result.base = gen.prepareAccessorBaseArg(loc, base, funcDecl);
      
      if (subscriptIndexExpr) {
        if (!origSubscripts)
          origSubscripts = gen.emitRValue(subscriptIndexExpr);
        // TODO: use the subscript expression as the source if we're
        // only using this l-value once.
        result.subscripts = origSubscripts.copy(gen, loc);
      }
      
      return result;
    }
    
  public:
    GetterSetterComponent(AbstractStorageDecl *decl,
                          bool isSuper,
                          ArrayRef<Substitution> substitutions,
                          LValueTypeData typeData,
                          Expr *subscriptIndexExpr = nullptr)
      : LogicalPathComponent(typeData),
        decl(decl),
        IsSuper(isSuper),
        substitutions(substitutions.begin(), substitutions.end()),
        subscriptIndexExpr(subscriptIndexExpr)
    {
    }
    
    GetterSetterComponent(const GetterSetterComponent &copied,
                          SILGenFunction &gen,
                          SILLocation loc)
      : LogicalPathComponent(copied.getTypeData()),
        decl(copied.decl),
        IsSuper(copied.IsSuper),
        substitutions(copied.substitutions),
        subscriptIndexExpr(copied.subscriptIndexExpr),
        origSubscripts(copied.origSubscripts.copy(gen, loc))
    {
    }
    
    void set(SILGenFunction &gen, SILLocation loc,
             RValue &&value, ManagedValue base) const override {
      // Pass in just the setter.
      auto args = prepareAccessorArgs(gen, loc, base, decl->getSetter());
      
      return gen.emitSetAccessor(loc, decl, substitutions,
                                 std::move(args.base), IsSuper,
                                 std::move(args.subscripts),
                                 std::move(value));
    }
    
    ManagedValue get(SILGenFunction &gen, SILLocation loc,
                     ManagedValue base, SGFContext c) const override {
      auto args = prepareAccessorArgs(gen, loc, base, decl->getGetter());
      
      return gen.emitGetAccessor(loc, decl, substitutions,
                                 std::move(args.base), IsSuper,
                                 std::move(args.subscripts), c);
    }
    
    std::unique_ptr<LogicalPathComponent>
    clone(SILGenFunction &gen, SILLocation loc) const override {
      LogicalPathComponent *clone = new GetterSetterComponent(*this, gen, loc);
      return std::unique_ptr<LogicalPathComponent>(clone);
    }
  };
  
  /// Convert an lvalue to a different type through a pair of conversion
  /// functions.
  class LValueConversionComponent : public LogicalPathComponent {
    LValueConversionExpr *Conversion;
    
  public:
    LValueConversionComponent(LValueConversionExpr *E,
                              LValueTypeData typeData)
      : LogicalPathComponent(typeData),
        Conversion(E)
    {}
    
    ManagedValue get(SILGenFunction &gen, SILLocation loc,
                     ManagedValue orig, SGFContext c) const override {
      // Load the original value.
      ManagedValue origVal = gen.emitLoad(loc, orig.getValue(),
                                          gen.getTypeLowering(orig.getType()),
                                          SGFContext(),
                                          IsNotTake);
      // Apply the "from" conversion.
      auto origTy = Conversion->getSubExpr()->getType()
        ->getLValueOrInOutObjectType()
        ->getCanonicalType();
      return gen.emitApplyConversionFunction(loc,
                     Conversion->getFromConversionFn(),
                     Conversion->getType()->getLValueOrInOutObjectType(),
                     RValue(gen, loc, origTy, origVal));
    }
    
    void set(SILGenFunction &gen, SILLocation loc,
             RValue &&value, ManagedValue origAddr) const override {
      // Apply the "to" conversion.
      ManagedValue convertedVal = gen.emitApplyConversionFunction(loc,
              Conversion->getToConversionFn(),
              Conversion->getSubExpr()->getType()->getLValueOrInOutObjectType(),
              std::move(value));
      
      // Store back to the original value.
      convertedVal.assignInto(gen, loc, origAddr.getValue());
    }
    
    std::unique_ptr<LogicalPathComponent>
    clone(SILGenFunction &gen, SILLocation loc) const override {
      LogicalPathComponent *clone
        = new LValueConversionComponent(Conversion, getTypeData());
      return std::unique_ptr<LogicalPathComponent>(clone);
    }
  };
  
  /// Remap an lvalue referencing a generic type to an lvalue of its substituted
  /// type in a concrete context.
  class OrigToSubstComponent : public LogicalPathComponent {
    AbstractionPattern origType;
    CanType substType;
    
  public:
    OrigToSubstComponent(SILGenFunction &gen,
                         AbstractionPattern origType, CanType substType)
      : LogicalPathComponent(getUnsubstitutedTypeData(gen, substType)),
        origType(origType), substType(substType)
    {}
    
    void set(SILGenFunction &gen, SILLocation loc,
             RValue &&value, ManagedValue base) const override {
      // Map the value to the original abstraction level.
      ManagedValue mv = std::move(value).getAsSingleValue(gen, loc);
      mv = gen.emitSubstToOrigValue(loc, mv, origType, substType);
      // Store to the base.
      mv.assignInto(gen, loc, base.getValue());
    }
    
    ManagedValue get(SILGenFunction &gen, SILLocation loc,
                     ManagedValue base, SGFContext c) const override {
      // Load the original value.
      ManagedValue baseVal = gen.emitLoad(loc, base.getValue(),
                                          gen.getTypeLowering(base.getType()),
                                          SGFContext(),
                                          IsNotTake);
      // Map the base value to its substituted representation.
      return gen.emitOrigToSubstValue(loc, baseVal,
                                      origType, substType, c);
    }
    
    std::unique_ptr<LogicalPathComponent>
    clone(SILGenFunction &gen, SILLocation loc) const override {
      LogicalPathComponent *clone
        = new OrigToSubstComponent(gen, origType, substType);
      return std::unique_ptr<LogicalPathComponent>(clone);
    }
  };
}

LValue SILGenFunction::emitLValue(Expr *e) {
  LValue r = SILGenLValue(*this).visit(e);
  // If the final component is physical with an abstraction change, introduce a
  // reabstraction component.
  if (r.isLastComponentPhysical()
      && getTypeLowering(r.getSubstFormalType()).getLoweredType()
            != r.getTypeOfRValue()) {
    r.add<OrigToSubstComponent>(*this, r.getOrigFormalType(),
                                r.getSubstFormalType());
  }
  return r;
}

LValue SILGenLValue::visitRec(Expr *e) {
  // Non-lvalue types (references, values, metatypes, etc) form the root of a
  // logical l-value.
  if (!e->getType()->is<LValueType>() && !e->getType()->is<InOutType>()) {
    // Calls through protocols can be done with +0 rvalues.  This allows us to
    // avoid materializing copies of existentials.
    SGFContext Ctx;
    if (e->getType()->isExistentialType() || e->getType()->is<ArchetypeType>())
      Ctx = SGFContext::AllowPlusZero;
    
    ManagedValue rv = gen.emitRValueAsSingleValue(e, Ctx);
    auto typeData = getValueTypeData(rv.getValue());
    LValue lv;
    lv.add<ValueComponent>(rv, typeData);
    return lv;
  }

  return visit(e);
}

LValue SILGenLValue::visitExpr(Expr *e) {
  e->dump(llvm::errs());
  llvm_unreachable("unimplemented lvalue expr");
}

static LValue emitLValueForNonMemberVarDecl(SILGenFunction &gen,
                                            SILLocation loc, VarDecl *var,
                                            CanType formalRValueType,
                                            bool isDirectPropertyAccess) {
  LValue lv;
  auto typeData = getUnsubstitutedTypeData(gen, formalRValueType);

  // If it's a computed variable, push a reference to the getter and setter.
  if (var->hasAccessorFunctions() && !isDirectPropertyAccess) {
    ArrayRef<Substitution> substitutions;
    if (auto genericParams
        = gen.SGM.Types.getEffectiveGenericParamsForContext(
                                                      var->getDeclContext()))
      substitutions = gen.buildForwardingSubstitutions(genericParams);

    lv.add<GetterSetterComponent>(var, /*isSuper=*/false, substitutions,
                                  typeData);
  } else {
    // If it's a physical value (e.g. a local variable in memory), push its
    // address.
    auto address = gen.emitLValueForDecl(loc, var, isDirectPropertyAccess);
    assert(address.isLValue() &&
           "physical lvalue decl ref must evaluate to an address");
    lv.add<ValueComponent>(address, typeData);
  }
  return std::move(lv);
}

LValue SILGenLValue::visitDeclRefExpr(DeclRefExpr *e) {
  // The only non-member decl that can be an lvalue is VarDecl.
  return emitLValueForNonMemberVarDecl(gen, e, cast<VarDecl>(e->getDecl()),
                                       getSubstFormalRValueType(e),
                                       e->isDirectPropertyAccess());
}

LValue SILGenLValue::visitDotSyntaxBaseIgnoredExpr(DotSyntaxBaseIgnoredExpr *e){
  // If it is convenient to avoid loading the base, don't bother loading it.
  gen.emitRValue(e->getLHS(), SGFContext::AllowPlusZero);
  return visitRec(e->getRHS());
}

LValue SILGenLValue::visitMemberRefExpr(MemberRefExpr *e) {
  LValue lv = visitRec(e->getBase());

  // MemberRefExpr can refer to type and function members, but the only case
  // that can be an lvalue is a VarDecl.
  VarDecl *var = cast<VarDecl>(e->getMember().getDecl());

  LValueTypeData typeData = getMemberTypeData(gen, var->getType(), e);

  // Use the property accessors if the variable has accessors and this isn't a
  // direct access to underlying storage.
  if (var->hasAccessorFunctions() && !e->isDirectPropertyAccess()) {
    lv.add<GetterSetterComponent>(var, e->isSuper(),
                                  e->getMember().getSubstitutions(), typeData);
    return std::move(lv);
  }

  // Otherwise, the lvalue access is performed with a fragile element reference.
  // Find the substituted storage type.
  SILType varStorageType =
    gen.SGM.Types.getSubstitutedStorageType(var, e->getType());
    
  // For static variables, emit a reference to the global variable backing
  // them.
  // FIXME: This has to be dynamically looked up for classes, and
  // dynamically instantiated for generics.
  if (var->isStatic()) {
    auto baseMeta = e->getBase()->getType()->castTo<MetatypeType>()
      ->getInstanceType();
    (void)baseMeta;
    assert(!baseMeta->is<BoundGenericType>() &&
           "generic static stored properties not implemented");
    assert((baseMeta->getStructOrBoundGenericStruct() ||
            baseMeta->getEnumOrBoundGenericEnum()) &&
           "static stored properties for classes/protocols not implemented");
    
    return emitLValueForNonMemberVarDecl(gen, e, var,
                                         getSubstFormalRValueType(e),
                                         e->isDirectPropertyAccess());
  }

  // For member variables, this access is done w.r.t. a base computation that
  // was already emitted.  This member is accessed off of it.
  if (!e->getBase()->getType()->is<LValueType>()) {
    assert(e->getBase()->getType()->hasReferenceSemantics());
    lv.add<RefElementComponent>(var, varStorageType, typeData);
  } else {
    lv.add<StructElementComponent>(var, varStorageType, typeData);
  }
  return std::move(lv);
}

LValue SILGenLValue::visitSubscriptExpr(SubscriptExpr *e) {
  auto decl = cast<SubscriptDecl>(e->getDecl().getDecl());
  auto typeData = getMemberTypeData(gen, decl->getElementType(), e);
  
  LValue lv = visitRec(e->getBase());
  lv.add<GetterSetterComponent>(decl, e->isSuper(),
                                e->getDecl().getSubstitutions(),
                                typeData, e->getIndex());
  return std::move(lv);
}

LValue SILGenLValue::visitTupleElementExpr(TupleElementExpr *e) {
  unsigned index = e->getFieldNumber();
  LValue lv = visitRec(e->getBase());

  auto baseTypeData = lv.getTypeData();
  LValueTypeData typeData = {
    baseTypeData.OrigFormalType.getTupleElementType(index),
    cast<TupleType>(baseTypeData.SubstFormalType).getElementType(index),
    baseTypeData.TypeOfRValue.getTupleElementType(index)
  };

  lv.add<TupleElementComponent>(index, typeData);
  return std::move(lv);
}

LValue SILGenLValue::visitLValueConversionExpr(LValueConversionExpr *e) {
  LValue lv = visitRec(e->getSubExpr());
  
  LValueTypeData typeData = getMemberTypeData(gen,
                                    e->getType()->getLValueOrInOutObjectType(),
                                    e);
  lv.add<LValueConversionComponent>(e, typeData);
  return std::move(lv);
}

LValue SILGenLValue::visitInOutExpr(InOutExpr *e) {
  return visitRec(e->getSubExpr());
}

LValue SILGenFunction::emitDirectIVarLValue(SILLocation loc, ManagedValue base,
                                            VarDecl *ivar) {
  SILGenLValue sgl(*this);
  LValue lv;
  
  auto baseType = base.getType().getSwiftRValueType();

  // Refer to 'self' as the base of the lvalue.
  lv.add<ValueComponent>(base, getUnsubstitutedTypeData(*this, baseType));

  auto origFormalType = getOrigFormalRValueType(ivar->getType());
  auto substFormalType = ivar->getType()->getCanonicalType();
  LValueTypeData typeData = { origFormalType, substFormalType,
                              getLoweredType(origFormalType, substFormalType) };

  // Find the substituted storage type.
  SILType varStorageType =
    SGM.Types.getSubstitutedStorageType(ivar, LValueType::get(ivar->getType()));

  if (baseType->hasReferenceSemantics())
    lv.add<RefElementComponent>(ivar, varStorageType, typeData);
  else
    lv.add<StructElementComponent>(ivar, varStorageType, typeData);

  return std::move(lv);
}

/// Load an r-value out of the given address.
///
/// \param rvalueTL - the type lowering for the type-of-rvalue
///   of the address
ManagedValue SILGenFunction::emitLoad(SILLocation loc, SILValue addr,
                                      const TypeLowering &rvalueTL,
                                      SGFContext C, IsTake_t isTake) {
  // Get the lowering for the address type.  We can avoid a re-lookup
  // in the very common case of this being equivalent to the r-value
  // type.
  auto &addrTL =
    (addr.getType() == rvalueTL.getLoweredType().getAddressType()
       ? rvalueTL : getTypeLowering(addr.getType()));

  if (rvalueTL.isAddressOnly()) {
    // If the client is cool with a +0 rvalue, the decl has an address-only
    // type, and there are no conversions, then we can return this as a +0
    // address RValue.
    if (C.isPlusZeroOk() && rvalueTL.getLoweredType() ==addrTL.getLoweredType())
      return ManagedValue::forUnmanaged(addr);
        
    // Copy the address-only value.
    SILValue copy = getBufferForExprResult(loc, rvalueTL.getLoweredType(), C);
    emitSemanticLoadInto(loc, addr, addrTL, copy, rvalueTL,
                         isTake, IsInitialization);
    return emitManagedRValueWithCleanup(copy, rvalueTL);
  }
  
  // Load the loadable value, and retain it if we aren't taking it.
  SILValue loadedV = emitSemanticLoad(loc, addr, addrTL, rvalueTL, isTake);
  return emitManagedRValueWithCleanup(loadedV, rvalueTL);
}

static void emitUnloweredStoreOfCopy(SILBuilder &B, SILLocation loc,
                                     SILValue value, SILValue addr,
                                     IsInitialization_t isInit) {
  if (isInit)
    B.createStore(loc, value, addr);
  else
    B.createAssign(loc, value, addr);
}

#ifndef NDEBUG
static bool hasDifferentTypeOfRValue(const TypeLowering &srcTL) {
  return srcTL.getLoweredType().is<ReferenceStorageType>();
}
#endif

static Substitution getSimpleSubstitution(GenericParamList &generics,
                                          CanType typeArg) {
  assert(generics.getParams().size() == 1);
  auto typeParamDecl = generics.getParams()[0].getAsTypeParam();
  return Substitution{typeParamDecl->getArchetype(), typeArg, {}};
}

/// Create the correct substitution for calling the given function at
/// the given type.
static Substitution getSimpleSubstitution(FuncDecl *fn, CanType typeArg) {
  auto polyFnType =
    cast<PolymorphicFunctionType>(fn->getType()->getCanonicalType());
  return getSimpleSubstitution(polyFnType->getGenericParams(), typeArg);
}

static CanType getOptionalValueType(SILType optType,
                                    OptionalTypeKind &optionalKind) {
  auto generic = cast<BoundGenericType>(optType.getSwiftRValueType());
  optionalKind = generic->getDecl()->classifyAsOptionalType();
  assert(optionalKind);
  return generic.getGenericArgs()[0];
}

/// Emit code to convert the given possibly-null reference value into
/// a value of the corresponding optional type.
static SILValue emitRefToOptional(SILGenFunction &gen, SILLocation loc,
                                  SILValue ref, const TypeLowering &optTL) {
  // TODO: we should probably emit this as a call to a helper function
  // that does this, because (1) this is a lot of code and (2) it's
  // dumb to redundantly emit and optimize it.
  auto isNullBB = gen.createBasicBlock();
  auto isNonNullBB = gen.createBasicBlock();
  auto contBB = gen.createBasicBlock();

  SILValue resultAddr =
    gen.emitTemporaryAllocation(loc, optTL.getLoweredType());

  CanType refType = ref.getType().getSwiftRValueType();
  assert(refType.hasReferenceSemantics());

  // Ask whether the value is null.
  auto isNonNull = gen.B.createIsNonnull(loc, ref);
  gen.B.createCondBranch(loc, isNonNull, isNonNullBB, isNullBB);

  // If it's non-null, use _injectValueIntoOptional.
  gen.B.emitBlock(isNonNullBB);
  {
    FullExpr scope(gen.Cleanups, CleanupLocation::getCleanupLocation(loc));
    RValueSource value(loc, RValue(ManagedValue::forUnmanaged(ref), refType));
    gen.emitInjectOptionalValueInto(loc, std::move(value), resultAddr, optTL);
  }
  gen.B.createBranch(loc, contBB);

  // If it's null, use _injectNothingIntoOptional.
  gen.B.emitBlock(isNullBB);
  gen.emitInjectOptionalNothingInto(loc, resultAddr, optTL);
  gen.B.createBranch(loc, contBB);

  // Continue.
  gen.B.emitBlock(contBB);
  auto result = gen.B.createLoad(loc, resultAddr);
  return result;
}

/// Emit code to convert the givenvalue of optional type into a
/// possibly-null reference value.
static SILValue emitOptionalToRef(SILGenFunction &gen, SILLocation loc,
                                  SILValue opt, const TypeLowering &optTL,
                                  SILType refType) {
  // TODO: we should probably emit this as a call to a helper function
  // that does this, because (1) this is a lot of code and (2) it's
  // dumb to redundantly emit and optimize it.
  auto isNotPresentBB = gen.createBasicBlock();
  auto isPresentBB = gen.createBasicBlock();
  auto contBB = gen.createBasicBlock();

  auto optType = opt.getType();
  assert(optType == optTL.getLoweredType());

  // This assertion might be unreasonable in the short term.
  assert(!optType.isAddress() &&
         "Optional<T> is address-only for reference type T?");

  assert(!refType.isAddress());
  assert(refType.hasReferenceSemantics());

  // Make an argument on contBB.
  auto result = new (gen.SGM.M) SILArgument(refType, contBB);

  // Materialize the optional value so we can pass it inout to
  // _doesOptionalHaveValue.  Really, we just want to pass it +0.
  auto allocation = gen.B.createAllocStack(loc, optType);

  // Note that our SIL-generation patterns here assume that these
  // library intrinsic functions won't throw.

  auto optAddr = allocation->getAddressResult();
  gen.B.createStore(loc, opt, optAddr);

  // Ask whether the value is present.
  auto isPresent = gen.emitDoesOptionalHaveValue(loc, optAddr);
  gen.B.createCondBranch(loc, isPresent, isPresentBB, isNotPresentBB);

  // If it's present, use _getOptionalValue.
  gen.B.emitBlock(isPresentBB);
  SILValue refValue;
  {
    FullExpr scope(gen.Cleanups, CleanupLocation::getCleanupLocation(loc));

    auto managedOptAddr = ManagedValue::forUnmanaged(optAddr);
    refValue = gen.emitGetOptionalValueFrom(loc, managedOptAddr, optTL,
                                            SGFContext()).forward(gen);
    assert(refValue.getType().isObject());
  }
  gen.B.createBranch(loc, contBB, refValue);

  // If it's not present, just create a null value.
  gen.B.emitBlock(isNotPresentBB);

  // %1 = integer_literal $Builtin.Word, 0
  auto WordTy = SILType::getBuiltinWordType(gen.getASTContext());
  SILValue null = gen.B.createIntegerLiteral(loc, WordTy, 0);

  // %2 = builtin_function_ref "inttoptr_Word" : $@thin (Word) -> RawPointer
  auto bfrInfo = SILFunctionType::ExtInfo(AbstractCC::Freestanding,
                                          FunctionType::Representation::Thin,
                                          /*noreturn*/ false,
                                          /*autoclosure*/ false);
  SILParameterInfo Param(WordTy.getSwiftRValueType(),
                         ParameterConvention::Direct_Unowned);
  SILResultInfo Result(gen.getASTContext().TheRawPointerType,
                       ResultConvention::Unowned);
  auto bfrFnType = SILFunctionType::get(nullptr, bfrInfo,
                                        ParameterConvention::Direct_Owned,
                                        Param, Result,
                                        gen.getASTContext());
  auto bfr = gen.B.createBuiltinFunctionRef(loc, "inttoptr_Word",
                                    SILType::getPrimitiveObjectType(bfrFnType));
  // %3 = apply %2(%1) : $@thin (Builtin.Word) -> Builtin.RawPointer
  null = gen.B.createApply(loc, bfr, null);

  null = gen.B.createRawPointerToRef(loc, null, refType);
  gen.B.emitReleaseValueOperation(loc, opt); // destroy the nothing value
  gen.B.createBranch(loc, contBB, null);

  // Continue.
  gen.B.emitBlock(contBB);
  gen.B.createDeallocStack(CleanupLocation::getCleanupLocation(loc),
                           allocation->getContainerResult());

  return result;
}

void SILGenFunction::emitInjectOptionalValueInto(SILLocation loc,
                                                 RValueSource &&value,
                                                 SILValue dest,
                                                 const TypeLowering &optTL) {
  SILType optType = optTL.getLoweredType();
  OptionalTypeKind optionalKind;
  CanType valueType = getOptionalValueType(optType, optionalKind);

  FuncDecl *fn =
    getASTContext().getInjectValueIntoOptionalDecl(nullptr, optionalKind);
  Substitution sub = getSimpleSubstitution(fn, valueType);

  // Materialize the r-value into a temporary.
  FullExpr scope(Cleanups, CleanupLocation::getCleanupLocation(loc));
  auto valueAddr = std::move(value).materialize(*this,
                                  AbstractionPattern(CanType(sub.Archetype)));

  TemporaryInitialization emitInto(dest, CleanupHandle::invalid());
  auto result = emitApplyOfLibraryIntrinsic(loc, fn, sub, valueAddr,
                                            SGFContext(&emitInto));
  assert(result.isInContext() && "didn't emit directly into buffer?");
  (void)result;
}

void SILGenFunction::emitInjectOptionalNothingInto(SILLocation loc, 
                                                   SILValue dest,
                                                   const TypeLowering &optTL) {
  SILType optType = optTL.getLoweredType();
  OptionalTypeKind optionalKind;
  CanType valueType = getOptionalValueType(optType, optionalKind);

  FuncDecl *fn =
    getASTContext().getInjectNothingIntoOptionalDecl(nullptr, optionalKind);
  Substitution sub = getSimpleSubstitution(fn, valueType);

  TemporaryInitialization emitInto(dest, CleanupHandle::invalid());
  auto result = emitApplyOfLibraryIntrinsic(loc, fn, sub, {},
                                            SGFContext(&emitInto));
  assert(result.isInContext() && "didn't emit directly into buffer?");
  (void)result;
}

SILValue SILGenFunction::emitDoesOptionalHaveValue(SILLocation loc, 
                                                   SILValue addr) {
  SILType optType = addr.getType().getObjectType();
  OptionalTypeKind optionalKind;
  CanType valueType = getOptionalValueType(optType, optionalKind);

  FuncDecl *fn =
    getASTContext().getDoesOptionalHaveValueDecl(nullptr, optionalKind);
  Substitution sub = getSimpleSubstitution(fn, valueType);

  // The argument to _doesOptionalHaveValue is passed by reference.
  return emitApplyOfLibraryIntrinsic(loc, fn, sub,
                                     ManagedValue::forUnmanaged(addr),
                                     SGFContext())
    .getUnmanagedValue();
}

ManagedValue SILGenFunction::emitGetOptionalValueFrom(SILLocation loc,
                                                      ManagedValue src,
                                                      const TypeLowering &optTL,
                                                      SGFContext C) {
  SILType optType = src.getType().getObjectType();
  OptionalTypeKind optionalKind;
  CanType valueType = getOptionalValueType(optType, optionalKind);

  FuncDecl *fn = getASTContext().getGetOptionalValueDecl(nullptr, optionalKind);
  Substitution sub = getSimpleSubstitution(fn, valueType);

  return emitApplyOfLibraryIntrinsic(loc, fn, sub, src, C);
}

SILValue SILGenFunction::emitConversionToSemanticRValue(SILLocation loc,
                                                        SILValue src,
                                                  const TypeLowering &valueTL) {
  // For @weak types, we need to create an Optional<T>.
  // Optional<T> is currently loadable, but it probably won't be forever.
  if (src.getType().is<WeakStorageType>())
    return emitRefToOptional(*this, loc, src, valueTL);

  // For @unowned(safe) types, we need to generate a strong retain and
  // strip the unowned box.
  if (auto unownedType = src.getType().getAs<UnownedStorageType>()) {
    B.createStrongRetainUnowned(loc, src);
    return B.createUnownedToRef(loc, src,
                SILType::getPrimitiveObjectType(unownedType.getReferentType()));
  }

  // For @unowned(unsafe) types, we need to strip the unmanaged box
  // and then do an (unsafe) retain.
  if (auto unmanagedType = src.getType().getAs<UnmanagedStorageType>()) {
    auto result = B.createUnmanagedToRef(loc, src,
              SILType::getPrimitiveObjectType(unmanagedType.getReferentType()));
    B.createStrongRetain(loc, result);
    return result;
  }

  llvm_unreachable("unexpected storage type that differs from type-of-rvalue");
}


/// Given that the type-of-rvalue differs from the type-of-storage,
/// and given that the type-of-rvalue is loadable, produce a +1 scalar
/// of the type-of-rvalue.
static SILValue emitLoadOfSemanticRValue(SILGenFunction &gen,
                                         SILLocation loc,
                                         SILValue src,
                                         const TypeLowering &valueTL,
                                         IsTake_t isTake) {
  SILType storageType = src.getType();

  // For @weak types, we need to create an Optional<T>.
  // Optional<T> is currently loadable, but it probably won't be forever.
  if (storageType.is<WeakStorageType>()) {
    auto refValue = gen.B.createLoadWeak(loc, src, isTake);
    return emitRefToOptional(gen, loc, refValue, valueTL);
  }

  // For @unowned(safe) types, we need to strip the unowned box.
  if (auto unownedType = storageType.getAs<UnownedStorageType>()) {
    auto unownedValue = gen.B.createLoad(loc, src);
    gen.B.createStrongRetainUnowned(loc, unownedValue);
    if (isTake) gen.B.createUnownedRelease(loc, unownedValue);
    return gen.B.createUnownedToRef(loc, unownedValue,
              SILType::getPrimitiveObjectType(unownedType.getReferentType()));
  }

  // For @unowned(unsafe) types, we need to strip the unmanaged box.
  if (auto unmanagedType = src.getType().getAs<UnmanagedStorageType>()) {
    auto value = gen.B.createLoad(loc, src);
    auto result = gen.B.createUnmanagedToRef(loc, value,
            SILType::getPrimitiveObjectType(unmanagedType.getReferentType()));
    gen.B.createStrongRetain(loc, result);
    return result;
  }

  llvm_unreachable("unexpected storage type that differs from type-of-rvalue");
}

/// Given that the type-of-rvalue differs from the type-of-storage,
/// store a +1 value (possibly not a scalar) of the type-of-rvalue
/// into the given address.
static void emitStoreOfSemanticRValue(SILGenFunction &gen,
                                      SILLocation loc,
                                      SILValue value,
                                      SILValue dest,
                                      const TypeLowering &valueTL,
                                      IsInitialization_t isInit) {
  auto storageType = dest.getType();

  // For @weak types, we need to break down an Optional<T> and then
  // emit the storeWeak ourselves.
  if (auto weakType = storageType.getAs<WeakStorageType>()) {
    auto refType = SILType::getPrimitiveObjectType(weakType.getReferentType());
    auto refValue = emitOptionalToRef(gen, loc, value, valueTL, refType);
    gen.B.createStoreWeak(loc, refValue, dest, isInit);

    // store_weak doesn't take ownership of the input, so cancel it out.
    gen.B.emitStrongRelease(loc, refValue);
    return;
  }

  // For @unowned(safe) types, we need to enter the unowned box by
  // turning the strong retain into an unowned retain.
  if (storageType.is<UnownedStorageType>()) {
    auto unownedValue =
      gen.B.createRefToUnowned(loc, value, storageType.getObjectType());
    gen.B.createUnownedRetain(loc, unownedValue);
    emitUnloweredStoreOfCopy(gen.B, loc, unownedValue, dest, isInit);
    gen.B.emitStrongRelease(loc, value);
    return;
  }

  // For @unowned(unsafe) types, we need to enter the unmanaged box and
  // release the strong retain.
  if (storageType.is<UnmanagedStorageType>()) {
    auto unmanagedValue =
      gen.B.createRefToUnmanaged(loc, value, storageType.getObjectType());
    emitUnloweredStoreOfCopy(gen.B, loc, unmanagedValue, dest, isInit);
    gen.B.emitStrongRelease(loc, value);
    return;
  }

  llvm_unreachable("unexpected storage type that differs from type-of-rvalue");
}

/// Load a value of the type-of-rvalue out of the given address as a
/// scalar.  The type-of-rvalue must be loadable.
SILValue SILGenFunction::emitSemanticLoad(SILLocation loc,
                                          SILValue src,
                                          const TypeLowering &srcTL,
                                          const TypeLowering &rvalueTL,
                                          IsTake_t isTake) {
  assert(srcTL.getLoweredType().getAddressType() == src.getType());
  assert(rvalueTL.isLoadable());

  // Easy case: the types match.
  if (srcTL.getLoweredType() == rvalueTL.getLoweredType()) {
    assert(!hasDifferentTypeOfRValue(srcTL));
    return srcTL.emitLoadOfCopy(B, loc, src, isTake);
  }

  return emitLoadOfSemanticRValue(*this, loc, src, rvalueTL, isTake);
}

/// Load a value of the type-of-reference out of the given address
/// and into the destination address.
void SILGenFunction::emitSemanticLoadInto(SILLocation loc,
                                          SILValue src,
                                          const TypeLowering &srcTL,
                                          SILValue dest,
                                          const TypeLowering &destTL,
                                          IsTake_t isTake,
                                          IsInitialization_t isInit) {
  assert(srcTL.getLoweredType().getAddressType() == src.getType());
  assert(destTL.getLoweredType().getAddressType() == dest.getType());

  // Easy case: the types match.
  if (srcTL.getLoweredType() == destTL.getLoweredType()) {
    assert(!hasDifferentTypeOfRValue(srcTL));
    B.createCopyAddr(loc, src, dest, isTake, isInit);
    return;
  }

  auto rvalue = emitLoadOfSemanticRValue(*this, loc, src, srcTL, isTake);
  emitUnloweredStoreOfCopy(B, loc, rvalue, dest, isInit);
}

/// Store an r-value into the given address as an initialization.
void SILGenFunction::emitSemanticStore(SILLocation loc,
                                       SILValue rvalue,
                                       SILValue dest,
                                       const TypeLowering &destTL,
                                       IsInitialization_t isInit) {
  assert(destTL.getLoweredType().getAddressType() == dest.getType());

  // Easy case: the types match.
  if (rvalue.getType() == destTL.getLoweredType()) {
    assert(!hasDifferentTypeOfRValue(destTL));
    assert(destTL.isAddressOnly() == rvalue.getType().isAddress());
    if (rvalue.getType().isAddress()) {
      B.createCopyAddr(loc, rvalue, dest, IsTake, isInit);
    } else {
      emitUnloweredStoreOfCopy(B, loc, rvalue, dest, isInit);
    }
    return;
  }

  auto &rvalueTL = getTypeLowering(rvalue.getType());
  emitStoreOfSemanticRValue(*this, loc, rvalue, dest, rvalueTL, isInit);
}

/// Convert a semantic rvalue to a value of storage type.
SILValue SILGenFunction::emitConversionFromSemanticValue(SILLocation loc,
                                                         SILValue semanticValue,
                                                         SILType storageType) {
  auto &destTL = getTypeLowering(storageType);
  (void)destTL;
  // Easy case: the types match.
  if (semanticValue.getType() == storageType) {
    assert(!hasDifferentTypeOfRValue(destTL));
    return semanticValue;
  }
  
  // @weak types are never loadable, so we don't need to handle them here.
  
  // For @unowned types, place into an unowned box.
  if (storageType.is<UnownedStorageType>()) {
    SILValue unowned = B.createRefToUnowned(loc, semanticValue, storageType);
    B.createUnownedRetain(loc, unowned);
    B.emitStrongRelease(loc, semanticValue);
    return unowned;
  }

  // For @unmanaged types, place into an unmanaged box.
  if (storageType.is<UnmanagedStorageType>()) {
    SILValue unmanaged =
      B.createRefToUnmanaged(loc, semanticValue, storageType);
    B.emitStrongRelease(loc, semanticValue);
    return unmanaged;
  }
  
  llvm_unreachable("unexpected storage type that differs from type-of-rvalue");
}

/// Produce a physical address that corresponds to the given l-value
/// component.
static ManagedValue drillIntoComponent(SILGenFunction &SGF,
                                       SILLocation loc,
                                       const PathComponent &component,
                                       ManagedValue base) {
  ManagedValue addr;
  if (component.isPhysical()) {
    addr = component.asPhysical().offset(SGF, loc, base);
  } else {
    auto &lcomponent = component.asLogical();
    addr = ManagedValue::forLValue(lcomponent.getMaterialized(SGF, loc, base));
  }

  return addr;
}

/// Find the last component of the given lvalue and derive a base
/// location for it.
static const PathComponent &drillToLastComponent(SILGenFunction &SGF,
                                                 SILLocation loc,
                                                 const LValue &lv,
                                                 ManagedValue &addr) {
  assert(lv.begin() != lv.end() &&
         "lvalue must have at least one component");

  auto component = lv.begin(), next = lv.begin(), end = lv.end();
  ++next;
  for (; next != end; component = next, ++next) {
    addr = drillIntoComponent(SGF, loc, *component, addr);
  }

  return *component;
}

ManagedValue SILGenFunction::emitLoadOfLValue(SILLocation loc,
                                              const LValue &src,
                                              SGFContext C) {
  // No need to write back to a loaded lvalue.
  DisableWritebackScope scope(*this);

  ManagedValue addr;
  auto &component = drillToLastComponent(*this, loc, src, addr);

  // If the last component is physical, just drill down and load from it.
  if (component.isPhysical()) {
    addr = component.asPhysical().offset(*this, loc, addr);
    return emitLoad(loc, addr.getValue(),
                    getTypeLowering(src.getTypeOfRValue()), C, IsNotTake);
  }

  // If the last component is logical, just emit a get.
  return component.asLogical().get(*this, loc, addr, C);
}

ManagedValue SILGenFunction::emitAddressOfLValue(SILLocation loc,
                                                 const LValue &src) {
  ManagedValue addr;
  auto &component = drillToLastComponent(*this, loc, src, addr);
  addr = drillIntoComponent(*this, loc, component, addr);
  assert(addr.getType().isAddress() &&
         "resolving lvalue did not give an address");
  return addr;
}

void SILGenFunction::emitAssignToLValue(SILLocation loc, RValue &&src,
                                        const LValue &dest) {
  WritebackScope scope(*this);
  
  // Resolve all components up to the last, keeping track of value-type logical
  // properties we need to write back to.
  ManagedValue destAddr;
  auto &component = drillToLastComponent(*this, loc, dest, destAddr);
  
  // Write to the tail component.
  if (component.isPhysical()) {
    auto finalDestAddr = component.asPhysical().offset(*this, loc, destAddr);
    
    std::move(src).getAsSingleValue(*this, loc)
      .assignInto(*this, loc, finalDestAddr.getValue());
  } else {
    component.asLogical().set(*this, loc, std::move(src), destAddr);
  }

  // The writeback scope closing will propagate the value back up through the
  // writeback chain.
}

void SILGenFunction::emitCopyLValueInto(SILLocation loc, const LValue &src,
                                        Initialization *dest) {
  auto skipPeephole = [&]{
    auto loaded = emitLoadOfLValue(loc, src, SGFContext(dest));
    if (!loaded.isInContext())
      RValue(*this, loc, src.getSubstFormalType(), loaded)
        .forwardInto(*this, dest, loc);
  };
  
  // If the source is a physical lvalue, the destination is a single address,
  // and there's no semantic conversion necessary, do a copy_addr from the
  // lvalue into the destination.
  if (!src.isPhysical() || dest->kind == Initialization::Kind::LetValue)
    return skipPeephole();
  auto destAddr = dest->getAddressOrNull();
  if (!destAddr)
    return skipPeephole();
  if (src.getTypeOfRValue().getSwiftRValueType()
        != destAddr.getType().getSwiftRValueType())
    return skipPeephole();
  
  auto srcAddr = emitAddressOfLValue(loc, src).getUnmanagedValue();
  B.createCopyAddr(loc, srcAddr, destAddr, IsNotTake, IsInitialization);
  dest->finishInitialization(*this);
}

void SILGenFunction::emitAssignLValueToLValue(SILLocation loc,
                                              const LValue &src,
                                              const LValue &dest) {
  auto skipPeephole = [&]{
    ManagedValue loaded = emitLoadOfLValue(loc, src, SGFContext());
    emitAssignToLValue(loc, RValue(*this, loc, src.getSubstFormalType(),
                                   loaded), dest);
  };
  
  // Only perform the peephole if both operands are physical and there's no
  // semantic conversion necessary.
  if (!src.isPhysical())
    return skipPeephole();
  if (!dest.isPhysical())
    return skipPeephole();
  
  auto srcAddr = emitAddressOfLValue(loc, src).getUnmanagedValue();
  auto destAddr = emitAddressOfLValue(loc, dest).getUnmanagedValue();

  if (srcAddr.getType() == destAddr.getType()) {
    B.createCopyAddr(loc, srcAddr, destAddr, IsNotTake, IsNotInitialization);
  } else {
    // If there's a semantic conversion necessary, do a load then assign.
    auto loaded = emitLoad(loc, srcAddr, getTypeLowering(src.getTypeOfRValue()),
                           SGFContext(),
                           IsNotTake);
    loaded.assignInto(*this, loc, destAddr);
  }
}