swift-mirror/lib/SILGen/RValue.cpp

//===--- RValue.cpp - Exploded RValue Representation ----------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// A storage structure for holding a destructured rvalue with an optional
// cleanup(s).
// Ownership of the rvalue can be "forwarded" to disable the associated
// cleanup(s).
//
//===----------------------------------------------------------------------===//

#include "RValue.h"
#include "Initialization.h"
#include "SILGenFunction.h"
#include "swift/AST/CanTypeVisitor.h"
#include "swift/Basic/Assertions.h"
#include "swift/Basic/Defer.h"
#include "swift/Basic/STLExtras.h"
#include "swift/SIL/AbstractionPattern.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/TypeLowering.h"

using namespace swift;
using namespace Lowering;

//===----------------------------------------------------------------------===//
//                              Helper Routines
//===----------------------------------------------------------------------===//

unsigned RValue::getRValueSize(AbstractionPattern pattern, CanType formalType) {
  if (pattern.isTuple()) {
    if (pattern.doesTupleContainPackExpansionType())
      return 1;

    // We can use the naive parallel walk here because of the check above.
    unsigned count = 0;
    auto formalTupleType = cast<TupleType>(formalType);
    for (auto i : indices(formalTupleType.getElementTypes())) {
      count += getRValueSize(pattern.getTupleElementType(i),
                             formalTupleType.getElementType(i));
    }
    return count;
  }

  return 1;
}

/// Return the number of rvalue elements in the given canonical type.
unsigned RValue::getRValueSize(CanType type) {
  if (auto tupleType = dyn_cast<TupleType>(type)) {
    // Don't recursively expand tuples containing pack expansions.
    if (tupleType.containsPackExpansionType())
      return 1;

    unsigned count = 0;
    for (auto eltType : tupleType.getElementTypes())
      count += getRValueSize(eltType);
    return count;
  }

  return 1;
}

namespace {

class ExplodeTupleValue
  : public CanTypeVisitor<ExplodeTupleValue,
                          /*RetTy=*/ void,
                          /*Args...=*/ ManagedValue>
{
public:
  std::vector<ManagedValue> &values;
  SILGenFunction &SGF;
  SILLocation loc;

  ExplodeTupleValue(std::vector<ManagedValue> &values,
                    SILGenFunction &SGF, SILLocation loc)
    : values(values), SGF(SGF), loc(loc)
  {
  }

  void visitType(CanType formalType, ManagedValue v) {
    // If we have a loadable type that has not been loaded, actually load it.
    if (!v.getType().isObject()) {
      v = SGF.B.createLoadIfLoadable(loc, v);
    }

    values.push_back(v);
  }

  void visitObjectTupleType(CanTupleType tupleFormalType, ManagedValue tuple) {
    // If we have an object, destructure the object using ownership APIs to
    // propagate cleanups.
    SGF.B.emitDestructureValueOperation(
        loc, tuple, [&](unsigned index, ManagedValue elt) {
          CanType eltFormalType = tupleFormalType.getElementType(index);
          assert(eltFormalType->isMaterializable());

          auto eltTy = tuple.getType().getTupleElementType(index);
          assert(eltTy.isAddress() == tuple.getType().isAddress());
          auto &eltTI = SGF.getTypeLowering(eltTy);
          (void)eltTI;
          assert(eltTI.isLoadable() || !SGF.silConv.useLoweredAddresses());

          // Project the element.
          visit(eltFormalType, elt);
        });
  }

  void visitAddressTupleType(CanTupleType tupleFormalType, ManagedValue tuple) {
    bool isPlusOne = tuple.isPlusOne(SGF);

    for (unsigned i : indices(tupleFormalType->getElements())) {
      CanType eltFormalType = tupleFormalType.getElementType(i);
      assert(eltFormalType->isMaterializable());

      auto eltTy = tuple.getType().getTupleElementType(i);
      assert(eltTy.isAddress() == tuple.getType().isAddress());
      auto &eltTI = SGF.getTypeLowering(eltTy);

      // Project the element.
      ManagedValue elt = SGF.B.createTupleElementAddr(loc, tuple, i, eltTy);

      // RValue has an invariant that loadable values have been loaded. Except
      // it's not really an invariant, because argument emission likes to lie
      // sometimes.
      if (eltTI.isLoadable()) {
        if (isPlusOne) {
          elt = SGF.B.createLoadTake(loc, elt);
        } else {
          elt = SGF.B.createLoadBorrow(loc, elt);
        }
      } else {
        // In contrast if we have an address only type, we can not rely on
        // ownership APIs to help us. So, manually create a cleanup to make up
        // for the cleanup that we will forward on the tuple.
        if (isPlusOne)
          elt = SGF.emitManagedRValueWithCleanup(elt.getValue(), eltTI);
      }

      visit(eltFormalType, elt);
    }

    // Then forward the underlying tuple's cleanup since we have appropriately
    // pushed its cleanups onto its subcomponents.
    tuple.forward(SGF);
  }

  void visitTupleType(CanTupleType tupleFormalType, ManagedValue tuple) {
    // Don't recursively expand tuples containing pack expansions.
    if (tupleFormalType.containsPackExpansionType())
      return visitType(tupleFormalType, tuple);

    if (tuple.getType().isObject()) {
      return visitObjectTupleType(tupleFormalType, tuple);
    }

    visitAddressTupleType(tupleFormalType, tuple);
  }
};

enum class ImplodeKind { Unmanaged, Forward, Copy };

template <ImplodeKind KIND>
class ImplodeLoadableTupleValue
    : public CanTypeVisitor<ImplodeLoadableTupleValue<KIND>,
                            /*RetTy=*/ManagedValue,
                            /*Args...=*/SILLocation> {
public:
  ArrayRef<ManagedValue> values;
  SILGenFunction &SGF;

  static ManagedValue getValue(SILGenFunction &SGF, ManagedValue v,
                               SILLocation l) {
    switch (KIND) {
    case ImplodeKind::Unmanaged:
      assert(!v.hasCleanup());
      return v.unmanagedBorrow();
    case ImplodeKind::Forward:
      return v.ensurePlusOne(SGF, l);
    case ImplodeKind::Copy:
      return v.copy(SGF, l);
    }

    llvm_unreachable("Unhandled ImplodeKind in switch.");
  }

  ImplodeLoadableTupleValue(ArrayRef<ManagedValue> values,
                            SILGenFunction &SGF)
    : values(values), SGF(SGF)
  {}

  ManagedValue visitType(CanType t, SILLocation l) {
    ManagedValue result = getValue(SGF, values[0], l);
    values = values.slice(1);
    return result;
  }

  ManagedValue visitTupleType(CanTupleType t, SILLocation l) {
    // Tuples with pack expansions aren't exploded.
    if (t.containsPackExpansionType())
      return visitType(t, l);

    SmallVector<ManagedValue, 4> elts;
    for (auto fieldTy : t.getElementTypes())
      elts.push_back(this->visit(fieldTy, l));
    SILType ty = SGF.getLoweredLoadableType(t);
    return SGF.B.createTuple(l, ty, elts);
  }

  ~ImplodeLoadableTupleValue() {
  }
};

template <ImplodeKind KIND>
class ImplodeAddressOnlyTuple
    : public CanTypeVisitor<ImplodeAddressOnlyTuple<KIND>,
                            /*RetTy=*/void,
                            /*Args...=*/Initialization *, SILLocation> {
public:
  ArrayRef<ManagedValue> values;
  SILGenFunction &SGF;

  ImplodeAddressOnlyTuple(ArrayRef<ManagedValue> values,
                          SILGenFunction &SGF)
    : values(values), SGF(SGF)
  {}

  void visitType(CanType t, Initialization *address, SILLocation l) {
    ManagedValue v = values[0];
    switch (KIND) {
    case ImplodeKind::Unmanaged:
      llvm_unreachable("address-only types always managed!");

    case ImplodeKind::Forward:
      // If a value is forwarded into, we require the value to be at +1. If the
      // the value is already at +1, we just forward. Otherwise, we perform the
      // copy.
      address->copyOrInitValueInto(SGF, l, v.ensurePlusOne(SGF, l),
                                   true /*isInit*/);
      break;

    case ImplodeKind::Copy:
      address->copyOrInitValueInto(SGF, l, v, false /*isInit*/);
      break;
    }

    address->finishInitialization(SGF);
    values = values.slice(1);
  }

  void visitTupleType(CanTupleType t, Initialization *address, SILLocation l) {
    // Tuples containing pack expansions shouldn't be exploded.
    if (t.containsPackExpansionType())
      return visitType(t, address, l);

    assert(address->canSplitIntoTupleElements());
    llvm::SmallVector<InitializationPtr, 4> buf;
    auto bufResult = address->splitIntoTupleElements(SGF, l, t, buf);

    for (unsigned i : range(t->getNumElements())) {
      CanType fieldCanTy = t.getElementType(i);
      this->visit(fieldCanTy, bufResult[i].get(), l);
    }

    address->finishInitialization(SGF);
  }

  ~ImplodeAddressOnlyTuple() {
    assert(values.empty() && "values not exhausted imploding tuple?!");
  }
};

} // end anonymous namespace

template <ImplodeKind KIND>
static ManagedValue implodeTupleValues(ArrayRef<ManagedValue> values,
                                       SILGenFunction &SGF, CanType type,
                                       SILLocation l) {
  // Non-tuples don't need to be imploded.
  auto tupleType = dyn_cast<TupleType>(type);
  if (!tupleType || tupleType.containsPackExpansionType()) {
    assert(values.size() == 1 && "exploded non-tuple value?!");
    return ImplodeLoadableTupleValue<KIND>::getValue(SGF, values[0], l);
  }

  const auto &TL = SGF.getTypeLowering(tupleType);

  // To implode an address-only tuple, we need to create a buffer to hold the
  // result tuple.
  if (TL.isAddressOnly() && SGF.silConv.useLoweredAddresses()) {
    assert(KIND != ImplodeKind::Unmanaged &&
           "address-only values are always managed!");
    auto buffer = SGF.emitTemporary(l, TL);
    ImplodeAddressOnlyTuple<KIND>(values, SGF)
        .visitTupleType(tupleType, buffer.get(), l);
    return buffer->getManagedAddress();
  }

  // To implode loadable tuples, we just need to combine the elements with
  // TupleInsts.
  return ImplodeLoadableTupleValue<KIND>(values, SGF).visitTupleType(tupleType, l);
}

/// Perform a copy or init operation from an array of ManagedValue (from an
/// RValue) into an initialization. The RValue will have one scalar ManagedValue
/// for each exploded tuple element in the RValue, so this needs to make the
/// shape of the initialization match the available elements.  This can be done
/// one of two ways:
///
///  1) recursively scalarize down the initialization on demand if the type of
///     the RValue is tuple type and the initialization supports it.
///  2) implode the corresponding values in the RValue to a scalar value of
///     tuple type and process them as a unit.
///
/// We prefer to use approach #1 since it generates better code.
///
template <ImplodeKind KIND>
static void copyOrInitValuesInto(Initialization *init,
                                 ArrayRef<ManagedValue> &values, CanType type,
                                 SILLocation loc, SILGenFunction &SGF) {
  static_assert(KIND == ImplodeKind::Forward ||
                KIND == ImplodeKind::Copy, "Not handled by init");
  bool isInit = (KIND == ImplodeKind::Forward);

  // If the element has non-tuple type, just serve it up to the initialization.
  auto tupleType = dyn_cast<TupleType>(type);
  if (!tupleType || tupleType.containsPackExpansionType()) {
    // We take the first value.
    ManagedValue result = values[0];
    values = values.slice(1);
    init->copyOrInitValueInto(SGF, loc, result, isInit);
    init->finishInitialization(SGF);
    return;
  }
  
  bool implodeTuple = false;

  if (init->canPerformInPlaceInitialization() &&
      init->isInPlaceInitializationOfGlobal() &&
      SGF.getTypeLowering(type).isTrivial()) {
    // Implode tuples in initialization of globals if they are
    // of trivial types.
    implodeTuple = true;
  }
  
  // If we can satisfy the tuple type by breaking up the aggregate
  // initialization, do so.
  if (!implodeTuple && init->canSplitIntoTupleElements()) {
    SmallVector<InitializationPtr, 4> subInitBuf;
    auto subInits = init->splitIntoTupleElements(SGF, loc, type, subInitBuf);
    
    assert(subInits.size() == tupleType->getNumElements() &&
           "initialization does not match tuple?!");
    
    for (unsigned i = 0, e = subInits.size(); i < e; ++i)
      copyOrInitValuesInto<KIND>(subInits[i].get(), values,
                                 tupleType.getElementType(i), loc, SGF);

    init->finishInitialization(SGF);
    return;
  }
  
  // Otherwise, process this by turning the values corresponding to the tuple
  // into a single value (through an implosion) and then binding that value to
  // our initialization.
  ManagedValue scalar = implodeTupleValues<KIND>(values, SGF, type, loc);

  // This will have just used up the first values in the list, pop them off.
  values = values.slice(RValue::getRValueSize(type));

  init->copyOrInitValueInto(SGF, loc, scalar, isInit);
  init->finishInitialization(SGF);
}

LLVM_ATTRIBUTE_UNUSED
static unsigned
expectedExplosionSize(CanType type) {
  auto tuple = dyn_cast<TupleType>(type);
  if (!tuple || tuple.containsPackExpansionType())
    return 1;
  unsigned total = 0;
  for (unsigned i = 0; i < tuple->getNumElements(); ++i) {
    total += expectedExplosionSize(tuple.getElementType(i));
  }
  return total;
}

/// This is separate from the main verification routine, so I can minimize the
/// amount of places that need to use SILGenFunction &SGF.
static void verifyHelper(ArrayRef<ManagedValue> values,
                         NullablePtr<SILGenFunction> SGF = nullptr) {
// This is a no-op in non-assert builds.
#ifndef NDEBUG
  ValueOwnershipKind result = OwnershipKind::None;
  std::optional<bool> sameHaveCleanups;
  for (ManagedValue v : values) {
    ValueOwnershipKind kind = v.getOwnershipKind();
    if (kind == OwnershipKind::None)
      continue;

    // Merge together whether or not the RValue has cleanups.
    if (!sameHaveCleanups.has_value()) {
      sameHaveCleanups = v.hasCleanup();
    } else {
      assert(*sameHaveCleanups == v.hasCleanup());
    }

    // This variable is here so that if the assert below fires, the current
    // reduction value is still available.
    auto newResult = result.merge(kind);
    assert(newResult);
    result = newResult;
  }
#endif
}

//===----------------------------------------------------------------------===//
//                           RValue Implementation
//===----------------------------------------------------------------------===//

// Private helper constructor. Please see RValue.h for more information.
RValue::RValue(SILGenFunction *SGF, ArrayRef<ManagedValue> values, CanType type)
    : values(values.begin(), values.end()), type(type), elementsToBeAdded(0) {

  assert(values.size() == expectedExplosionSize(type)
         && "creating rvalue with wrong number of pre-exploded elements");
  
  if (values.size() == 1 && values[0].isInContext()) {
    values = ArrayRef<ManagedValue>();
    type = CanType();
    elementsToBeAdded = InContext;
    return;
  }

  verifyHelper(values, SGF);
}

RValue::RValue(SILGenFunction &SGF, SILLocation l, CanType formalType,
               ManagedValue v)
  : type(formalType), elementsToBeAdded(0)
{
  assert(v && "creating r-value with consumed value");

  if (v.isInContext()) {
    type = CanType();
    elementsToBeAdded = InContext;
    return;
  }

  ExplodeTupleValue(values, SGF, l).visit(formalType, v);
  assert(values.size() == getRValueSize(type));
  verify(SGF);
}

RValue::RValue(SILGenFunction &SGF, Expr *expr, ManagedValue v)
  : type(expr->getType()->getCanonicalType()), elementsToBeAdded(0) {

  if (v.isInContext()) {
    type = CanType();
    elementsToBeAdded = InContext;
    return;
  }

  assert(v && "creating r-value with consumed value");
  ExplodeTupleValue(values, SGF, expr).visit(type, v);
  assert(values.size() == getRValueSize(type));
  verify(SGF);
}

RValue::RValue(CanType type)
  : type(type), elementsToBeAdded(getRValueSize(type)) {
}

RValue::RValue(AbstractionPattern pattern, CanType type)
  : type(type), elementsToBeAdded(getRValueSize(pattern, type)) {
}

void RValue::addElement(RValue &&element) & {
  assert(!element.isUsed() && "adding consumed value to r-value");
  assert(!element.isInSpecialState() && "adding special value to r-value");
  assert(elementsToBeAdded >= element.values.size() && "rvalue too full");
  if (!element.values.empty()) {
    assert(!isInSpecialState() && "cannot add elements to a special r-value");
    elementsToBeAdded -= element.values.size();
    values.insert(values.end(),
                  element.values.begin(), element.values.end());
    element.makeUsed();
  }

  assert(!isComplete() || values.size() == getRValueSize(type));
  // Call into the verifier helper directly without an SGF since we know that
  // all of our loadable values are already loaded and thus we do not need to
  // recheck that. On the other hand, we need to check the consistency of
  // cleanups and ownership.
  verifyHelper(values);
}

void RValue::addElement(SILGenFunction &SGF, ManagedValue element,
                        CanType formalType, SILLocation l) & {
  assert(element && "adding consumed value to r-value");
  assert(!element.isInContext() && "adding in-context value to r-value");
  assert(!isComplete() && "rvalue already complete");
  assert(!isInSpecialState() && "cannot add elements to an in-context r-value");
  --elementsToBeAdded;

  ExplodeTupleValue(values, SGF, l).visit(formalType, element);

  assert(!isComplete() || values.size() == getRValueSize(type));
  verify(SGF);
}

SILValue RValue::forwardAsSingleValue(SILGenFunction &SGF, SILLocation l) && {
  assert(isComplete() && "rvalue is not complete");
  assert(!isUsed() && "rvalue was used?!");
  ManagedValue mv = std::move(*this).getAsSingleValue(SGF, l);
  makeUsed();
  return mv.forward(SGF);
}

SILValue RValue::forwardAsSingleStorageValue(SILGenFunction &SGF,
                                             SILType storageType,
                                             SILLocation l) && {
  assert(isComplete() && "rvalue is not complete");
  // Conversions must always be done at +1.
  SILValue result =
    std::move(*this).ensurePlusOne(SGF, l).forwardAsSingleValue(SGF, l);
  return SGF.emitConversionFromSemanticValue(l, result, storageType);
}

void RValue::forwardInto(SILGenFunction &SGF, SILLocation loc,
                         Initialization *I) && {
  assert(isComplete() && "rvalue is not complete");
  assert(isPlusOneOrTrivial(SGF) && "Can not forward borrowed RValues");
  ArrayRef<ManagedValue> elts = values;
  copyOrInitValuesInto<ImplodeKind::Forward>(I, elts, type, loc, SGF);
}

void RValue::copyInto(SILGenFunction &SGF, SILLocation loc,
                      Initialization *I) const & {
  assert(isComplete() && "rvalue is not complete");
  ArrayRef<ManagedValue> elts = values;
  copyOrInitValuesInto<ImplodeKind::Copy>(I, elts, type, loc, SGF);
}

void RValue::assignInto(SILGenFunction &SGF, SILLocation loc,
                        SILValue destAddr) && {
  assert(isComplete() && "rvalue is not complete");
  assert(isPlusOneOrTrivial(SGF) && "Can not assign borrowed RValues");
  ArrayRef<ManagedValue> srcMvValues = values;

  SWIFT_DEFER { assert(srcMvValues.empty() && "didn't claim all elements!"); };

  // If we do not have a tuple, just bail early.
  auto srcTupleType = dyn_cast<TupleType>(type);
  if (!srcTupleType || srcTupleType.containsPackExpansionType()) {
    // Otherwise, pull the front value off the list.
    auto srcValue = srcMvValues.front();
    srcMvValues = srcMvValues.slice(1);
    srcValue.assignInto(SGF, loc, destAddr);
    return;
  }

  assert(destAddr->getType().castTo<TupleType>()->getNumElements() ==
         srcTupleType->getNumElements());

  // If there are sourced managed values, initialize the address with a tuple.
  if (srcMvValues.size()) {
    if (SGF.useLoweredAddresses()) {
      // Without opaque values, a tuple_addr_constructor is used to initialize
      // the memory all at once.
      SGF.B.createTupleAddrConstructor(loc, destAddr, srcMvValues,
                                       IsNotInitialization);
    } else {
      // With opaque values, a tuple can always be formed and assigned to the
      // memory.
      auto tupleTy = destAddr->getType().getObjectType();
      auto tuple = SGF.B.createTuple(loc, tupleTy, srcMvValues);
      SGF.B.createAssign(loc, tuple.forward(SGF), destAddr,
                         AssignOwnershipQualifier::Unknown);
    }
  }
  srcMvValues = ArrayRef<ManagedValue>();
}

ManagedValue RValue::getAsSingleValue(SILGenFunction &SGF, SILLocation loc) && {
  assert(!isUsed() && "r-value already used");
  SWIFT_DEFER {
    makeUsed();
  };

  if (isInContext()) {
    return ManagedValue::forInContext();
  }

  // Avoid killing and re-emitting the cleanup if the enclosed value isn't a
  // tuple.
  if (!isa<TupleType>(type)) {
    assert(values.size() == 1 && "exploded non-tuple?!");
    return values[0];
  }

  // *NOTE* Inside implodeTupleValues, we copy our values if they are not at +1.
  return implodeTupleValues<ImplodeKind::Forward>(values, SGF, type, loc);
}

SILValue RValue::getUnmanagedSingleValue(SILGenFunction &SGF,
                                         SILLocation l) const & {
  assert(isComplete() && "rvalue is not complete");
  ManagedValue mv =
      implodeTupleValues<ImplodeKind::Unmanaged>(values, SGF, type, l);
  return mv.getValue();
}

void RValue::forwardAll(SILGenFunction &SGF,
                        SmallVectorImpl<SILValue> &dest) && {
  assert(isComplete() && "rvalue is not complete");

  for (auto value : values)
    dest.push_back(value.forward(SGF));

  makeUsed();
}

void RValue::getAll(SmallVectorImpl<ManagedValue> &dest) && {
  assert(isComplete() && "rvalue is not complete");

  dest.append(values.begin(), values.end());
  makeUsed();
}

void RValue::getAllUnmanaged(SmallVectorImpl<SILValue> &dest) const & {
  assert(isComplete() && "rvalue is not complete");

  for (auto value : values)
    dest.push_back(value.getUnmanagedValue());
}

/// Return the range of indexes for the given tuple type element.
static std::pair<unsigned,unsigned>
getElementRange(CanTupleType tupleType, unsigned eltIndex) {
  assert(eltIndex < tupleType->getNumElements());
  unsigned begin = 0;
  for (unsigned i = 0; i < eltIndex; ++i) {
    begin += RValue::getRValueSize(tupleType.getElementType(i));
  }
  unsigned end =
    begin + RValue::getRValueSize(tupleType.getElementType(eltIndex));
  return { begin, end };
}

RValue RValue::extractElement(unsigned n) && {
  assert(isComplete() && "rvalue is not complete");

  CanTupleType tupleTy = dyn_cast<TupleType>(type);
  if (!tupleTy) {
    assert(n == 0);
    unsigned to = getRValueSize(type);
    assert(to == values.size());
    RValue element(nullptr, llvm::ArrayRef(values).slice(0, to), type);
    makeUsed();
    return element;
  }

  // This is implementable, but we can do it lazily if we add that kind
  // of projection.
  assert(!tupleTy.containsPackExpansionType() &&
         "can't extract elements from tuples containing pack expansions "
         "right now");

  auto range = getElementRange(tupleTy, n);
  unsigned from = range.first, to = range.second;

  CanType eltType = tupleTy.getElementType(n);
  RValue element(nullptr, llvm::ArrayRef(values).slice(from, to - from),
                 eltType);
  makeUsed();
  return element;
}

void RValue::extractElements(SmallVectorImpl<RValue> &elements) && {
  assert(isComplete() && "rvalue is not complete");

  CanTupleType tupleTy = dyn_cast<TupleType>(type);
  if (!tupleTy) {
    unsigned to = getRValueSize(type);
    assert(to == values.size());
    // We use push_back instead of emplace_back since emplace_back can not
    // invoke the private constructor we are attempting to invoke.
    elements.push_back({nullptr, llvm::ArrayRef(values).slice(0, to), type});
    makeUsed();
    return;
  }

  // This is implementable, but we can do it lazily if we add that kind
  // of decomposition.
  assert(!tupleTy.containsPackExpansionType() &&
         "can't extract elements from tuples containing pack expansions "
         "right now");

  unsigned from = 0;
  for (auto eltType : tupleTy.getElementTypes()) {
    unsigned to = from + getRValueSize(eltType);
    // We use push_back instead of emplace_back since emplace_back can not
    // invoke the private constructor we are attempting to invoke.
    elements.push_back(
        {nullptr, llvm::ArrayRef(values).slice(from, to - from), eltType});
    from = to;
  }
  assert(from == values.size());

  makeUsed();
}

RValue RValue::copy(SILGenFunction &SGF, SILLocation loc) const & {
  assert((isComplete() || isInSpecialState()) &&
         "can't copy an incomplete rvalue");
  std::vector<ManagedValue> copiedValues;
  copiedValues.reserve(values.size());
  for (ManagedValue v : values) {
    copiedValues.emplace_back(v.copy(SGF, loc));
  }
  return RValue(SGF, std::move(copiedValues), type, elementsToBeAdded);
}

RValue RValue::ensurePlusOne(SILGenFunction &SGF, SILLocation loc) && {
  if (!isPlusOneOrTrivial(SGF))
    return copy(SGF, loc);
  return std::move(*this);
}

RValue RValue::borrow(SILGenFunction &SGF, SILLocation loc) const & {
  assert((isComplete() || isInSpecialState()) &&
         "can't borrow incomplete rvalue");
  std::vector<ManagedValue> borrowedValues;
  borrowedValues.reserve(values.size());
  for (ManagedValue v : values) {
    borrowedValues.emplace_back(v.borrow(SGF, loc));
  }
  return RValue(SGF, std::move(borrowedValues), type, elementsToBeAdded);
}

ManagedValue RValue::materialize(SILGenFunction &SGF, SILLocation loc) && {
  assert(isPlusOneOrTrivial(SGF) &&
         "Can not materialize a non-plus one RValue");
  auto &paramTL = SGF.getTypeLowering(getType());

  // If we're already materialized, we're done.
  if (values.size() == 1 &&
      values[0].getType() == paramTL.getLoweredType().getAddressType()) {
    auto value = values[0];
    makeUsed();
    return value;
  }

  // Otherwise, emit to a temporary.
  auto temp = SGF.emitTemporary(loc, paramTL);
  std::move(*this).forwardInto(SGF, loc, temp.get());
  return temp->getManagedAddress();
}

bool RValue::isObviouslyEqual(const RValue &rhs) const {
  assert(isComplete() && rhs.isComplete() && "Comparing incomplete rvalues");

  // Compare the count of elements instead of the type.
  if (values.size() != rhs.values.size())
    return false;

  return std::equal(values.begin(), values.end(), rhs.values.begin(),
                [](const ManagedValue &lhs, const ManagedValue &rhs) -> bool {
                  return areObviouslySameValue(lhs.getValue(), rhs.getValue());
                });
}

static SILValue getCanonicalValueSource(SILValue value) {
  while (true) {
    if (auto access = dyn_cast<BeginAccessInst>(value)) {
      value = access->getSource();
    } else {
      return value;
    }
  }
}

bool RValue::areObviouslySameValue(SILValue lhs, SILValue rhs) {
  return getCanonicalValueSource(lhs) == getCanonicalValueSource(rhs);
}

void RValue::dump() const {
  dump(llvm::errs());
}

void RValue::dump(raw_ostream &OS, unsigned indent) const {
  if (isInContext()) {
    OS.indent(indent) << "InContext\n";
    return;
  }

  getType().dump(OS, indent);
  for (auto &value : values) {
    value.dump(OS, indent + 2);
  }
}

void RValue::verify(SILGenFunction &SGF) const & {
// This is a no-op in non-assert builds.
#ifndef NDEBUG
  verifyHelper(values, &SGF);
#endif
}

bool RValue::isPlusOne(SILGenFunction &SGF) const & {
  return llvm::all_of(
      values, [&SGF](ManagedValue mv) -> bool { return mv.isPlusOne(SGF); });
}

bool RValue::isPlusOneOrTrivial(SILGenFunction &SGF) const & {
  return llvm::all_of(
      values, [&SGF](ManagedValue mv) -> bool {
        return mv.isPlusOneOrTrivial(SGF);
      });
}

bool RValue::isPlusZero(SILGenFunction &SGF) const & {
  return llvm::none_of(values,
                       [](ManagedValue mv) -> bool { return mv.isPlusZero(); });
}

const TypeLowering &RValue::getTypeLowering(SILGenFunction &SGF) const & {
  return SGF.getTypeLowering(getType());
}

SILType RValue::getLoweredType(SILGenFunction &SGF) const & {
  return getTypeLowering(SGF).getLoweredType();
}

SILType RValue::getLoweredImplodedTupleType(SILGenFunction &SGF) const & {
  SILType loweredType = getLoweredType(SGF);
  if (loweredType.isAddressOnly(SGF.F) &&
      SGF.silConv.useLoweredAddresses())
    return loweredType.getAddressType();
  return loweredType.getObjectType();
}

RValue RValue::copyForDiagnostics() const {
  assert(!isInSpecialState());
  assert(isComplete());
  RValue result(type);
  for (auto value : values)
    result.values.push_back(value);
  result.elementsToBeAdded = 0;
  return result;
}