//===--- AccessEnforcementSelection.cpp - Select access enforcement -------===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// This pass eliminates 'unknown' access enforcement by selecting either
/// static or dynamic enforcement.
///
/// TODO: This is currently a module transform so that it can process closures
/// after analyzing their parent scope. This isn't a big problem now because
/// AccessMarkerElimination is also a module pass that follows this pass, so all
/// markers will still be present when this pass runs. However, we would like to
/// mostly eliminate module transforms. This could be done by changing the
/// PassManager to follow ClosureScopeAnalysis. A new ClosureTransform type
/// would be pipelined just like FunctionTransform, but would have an entry
/// point that handled a parent closure scope and all its children in one
/// invocation. For function pipelining to be upheld, we would need to verify
/// that BasicCalleeAnalysis never conflicts with ClosureScopeAnalysis. i.e. we
/// could never create a caller->callee edge when the callee is passed as a
/// function argument. Normal FunctionTransforms would then be called on each
/// closure function and its parent scope before calling the ClosureTransform.
///
/// FIXME: handle boxes used by copy_value when neither copy is captured.
///
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "access-enforcement-selection"
#include "swift/SIL/ApplySite.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/SILFunction.h"
#include "swift/SIL/SILUndef.h"
#include "swift/SILOptimizer/Analysis/ClosureScope.h"
#include "swift/SILOptimizer/Analysis/PostOrderAnalysis.h"
#include "swift/SILOptimizer/PassManager/Transforms.h"

using namespace swift;

static void setStaticEnforcement(BeginAccessInst *access) {
  // TODO: delete if we're not using static enforcement?
  access->setEnforcement(SILAccessEnforcement::Static);

  LLVM_DEBUG(llvm::dbgs() << "Static Access: " << *access);
}

static void setDynamicEnforcement(BeginAccessInst *access) {
  // TODO: delete if we're not using dynamic enforcement?
  access->setEnforcement(SILAccessEnforcement::Dynamic);

  LLVM_DEBUG(llvm::dbgs() << "Dynamic Access: " << *access);
}

namespace {
// Information about an address-type closure capture.
// This is only valid for inout_aliasable parameters.
//
// TODO: Verify somewhere that we properly handle any non-inout_aliasable
// partial apply captures or that they never happen. Eventually @inout_aliasable
// should be simply replaced by @in or @out, once we don't have special aliasing
// rules.
struct AddressCapture {
  ApplySite site;
  unsigned calleeArgIdx;

  AddressCapture(Operand &oper)
      : site(oper.getUser()), calleeArgIdx(site.getCalleeArgIndex(oper)) {
    if (site.getOrigCalleeConv().getSILArgumentConvention(calleeArgIdx)
        != SILArgumentConvention::Indirect_InoutAliasable) {
      site = ApplySite();
      calleeArgIdx = ~0U;
      return;
    }
    assert(oper.get()->getType().isAddress());
  }

  bool isValid() const { return bool(site); }
};

LLVM_ATTRIBUTE_UNUSED
raw_ostream &operator<<(raw_ostream &os, const AddressCapture &capture) {
  os << *capture.site.getInstruction() << " captures Arg #"
     << capture.calleeArgIdx;
  auto *F = capture.site.getCalleeFunction();
  if (F)
    os << " of " << F->getName();
  os << '\n';
  return os;
}

// For each non-escaping closure, record the indices of arguments that
// require dynamic enforcement.
class DynamicCaptures {
  llvm::DenseMap<SILFunction *, SmallVector<unsigned, 4>> dynamicCaptureMap;

  DynamicCaptures(DynamicCaptures &) = delete;

public:
  DynamicCaptures() = default;

  void recordCapture(AddressCapture capture) {
    LLVM_DEBUG(llvm::dbgs() << "Dynamic Capture: " << capture);

    auto callee = capture.site.getCalleeFunction();
    assert(callee && "cannot locate function ref for nonescaping closure");

    auto &dynamicArgs = dynamicCaptureMap[callee];
    if (!llvm::is_contained(dynamicArgs, capture.calleeArgIdx))
      dynamicArgs.push_back(capture.calleeArgIdx);
  }

  bool isDynamic(SILFunctionArgument *arg) const {
    auto pos = dynamicCaptureMap.find(arg->getFunction());
    if (pos == dynamicCaptureMap.end())
      return false;

    auto &dynamicArgs = pos->second;
    return llvm::is_contained(dynamicArgs, arg->getIndex());
  }
};
} // anonymous namespace

namespace {
class SelectEnforcement {
  // Reference back to the known dynamically enforced non-escaping closure
  // arguments in this module. Parent scopes are processed before the closures
  // they reference.
  DynamicCaptures &dynamicCaptures;

  AllocBoxInst *Box;

  /// A state for tracking escape information about a variable.
  /// StateMap only has entries for blocks for which the variable
  /// has potentially escaped at exit.
  struct State {
    bool IsInWorklist = false;

    // At least one of the following must be true.
    bool HasEscape = false;
    bool HasPotentiallyEscapedAtEntry = false;

    // In a more advanced problem, this could easily be passed a State.
    bool adjustForEscapeInPredecessor() {
      bool updateSuccessors = false;

      if (!HasPotentiallyEscapedAtEntry) {
        HasPotentiallyEscapedAtEntry = true;
        updateSuccessors = !HasEscape;
      }

      return updateSuccessors;
    }
  };
  llvm::DenseMap<SILBasicBlock*, State> StateMap;

  /// All the accesses of Box in the function.
  SmallVector<BeginAccessInst*, 8> Accesses;

  /// All the non-escaping closure captures of the Boxed value in this function.
  SmallVector<AddressCapture, 8> Captures;

  /// All the escapes in the function.
  SmallPtrSet<SILInstruction*, 8> Escapes;

  /// A worklist we use for various purposes.
  SmallVector<SILBasicBlock*, 8> Worklist;

public:
  SelectEnforcement(DynamicCaptures &dc, AllocBoxInst *box)
      : dynamicCaptures(dc), Box(box) {}

  void run();

private:
  void analyzeUsesOfBox(SingleValueInstruction *source);
  void analyzeProjection(ProjectBoxInst *projection);

  /// Note that the given instruction is a use of the box (or a use of
  /// a projection from it) in which the address escapes.
  void noteEscapingUse(SILInstruction *inst);

  void propagateEscapes();
  void propagateEscapesFrom(SILBasicBlock *bb);

  bool hasPotentiallyEscapedAt(SILInstruction *inst);

  typedef llvm::SmallSetVector<SILBasicBlock*, 8> BlockSetVector;
  void findBlocksAccessedAcross(EndAccessInst *endAccess,
                                BlockSetVector &blocksAccessedAcross);
  bool hasPotentiallyEscapedAtAnyReachableBlock(
    BeginAccessInst *access, BlockSetVector &blocksAccessedAcross);

  void updateAccesses();
  void updateAccess(BeginAccessInst *access);
  void updateCapture(AddressCapture capture);
};
} // end anonymous namespace

void SelectEnforcement::run() {
  LLVM_DEBUG(llvm::dbgs() << "  Box: " << *Box);

  // Set up the data-flow problem.
  analyzeUsesOfBox(Box);

  // Run the data-flow problem.
  propagateEscapes();

  // Update all the accesses.
  updateAccesses();
}

// FIXME: This should cover a superset of AllocBoxToStack's findUnexpectedBoxUse
// to avoid perturbing codegen. They should be sharing the same analysis.
void SelectEnforcement::analyzeUsesOfBox(SingleValueInstruction *source) {
  // Collect accesses rooted off of projections.
  for (auto use : source->getUses()) {
    auto user = use->getUser();

    if (auto MUI = dyn_cast<MarkUninitializedInst>(user)) {
      analyzeUsesOfBox(MUI);
      continue;
    }

    if (auto projection = dyn_cast<ProjectBoxInst>(user)) {
      analyzeProjection(projection);
      continue;
    }
      
    // Ignore certain other uses that do not capture the value.
    if (isa<StrongRetainInst>(user) ||
        isa<StrongReleaseInst>(user) ||
        isa<DestroyValueInst>(user) ||
        isa<DeallocBoxInst>(user))
      continue;

    // Treat everything else as an escape.
    // A Box typically escapes via copy_value.
    noteEscapingUse(user);
  }
  // Accesses may still be empty if the user of the Box is a partial apply
  // capture and, for some reason, the closure is dead.
}

// Verify that accesses are not nested before mandatory inlining.
// Closure captures should also not be nested within an access.
static void checkUsesOfAccess(BeginAccessInst *access) {
#ifndef NDEBUG
  // These conditions are only true prior to mandatory inlining.
  assert(!access->getFunction()->wasDeserializedCanonical());
  for (auto *use : access->getUses()) {
    auto user = use->getUser();
    assert(!isa<BeginAccessInst>(user));
    assert(!isa<PartialApplyInst>(user));
  }
#endif
}

void SelectEnforcement::analyzeProjection(ProjectBoxInst *projection) {
  for (auto *use : projection->getUses()) {
    auto user = use->getUser();

    // Collect accesses.
    if (auto *access = dyn_cast<BeginAccessInst>(user)) {
      if (access->getEnforcement() == SILAccessEnforcement::Unknown)
        Accesses.push_back(access);

      checkUsesOfAccess(access);

      continue;
    }
    // Handle both partial applies and directly applied non-escaping closures.
    if (ApplySite::isa(user)) {
      AddressCapture capture(*use);
      if (capture.isValid())
        Captures.emplace_back(capture);
      else
        // Only full apply sites can have non-inout_aliasable address arguments,
        // but those aren't actually captures.
        assert(FullApplySite::isa(user));
    }
  }
}

void SelectEnforcement::noteEscapingUse(SILInstruction *inst) {
  LLVM_DEBUG(llvm::dbgs() << "    Escape: " << *inst);

  // Add it to the escapes set.
  Escapes.insert(inst);

  // Record this point as escaping.
  auto userBB = inst->getParent();
  auto &state = StateMap[userBB];
  if (!state.IsInWorklist) {
    state.HasEscape = true;
    state.IsInWorklist = true;
    Worklist.push_back(userBB);
  }
  assert(state.HasEscape);
  assert(state.IsInWorklist);
}

void SelectEnforcement::propagateEscapes() {
  while (!Worklist.empty()) {
    auto bb = Worklist.pop_back_val();
    auto it = StateMap.find(bb);
    assert(it != StateMap.end() &&
           "block was in worklist but doesn't have a tracking state");
    auto &state = it->second;
    assert(state.HasEscape || state.HasPotentiallyEscapedAtEntry);
    state.IsInWorklist = false;
    propagateEscapesFrom(bb);
  }
}

/// Given that the box potentially escaped before we exited the
/// given block, propagate that information to all of its successors.
void SelectEnforcement::propagateEscapesFrom(SILBasicBlock *bb) {
  assert(StateMap.count(bb));

  // Iterate over the successors of the block.
  for (SILBasicBlock *succ : bb->getSuccessors()) {
    auto &succState = StateMap[succ];

    // If updating the successor changes it in a way that will
    // require us to update its successors, add it to the worklist.
    if (succState.adjustForEscapeInPredecessor()) {
      if (!succState.IsInWorklist) {
        succState.IsInWorklist = true;
        Worklist.push_back(succ);
      }
    }
  }
}

bool SelectEnforcement::hasPotentiallyEscapedAt(SILInstruction *point) {
  auto bb = point->getParent();

  // If we're not tracking anything for the whole block containing
  // the instruction, we're done; it hasn't escaped here.
  auto it = StateMap.find(bb);
  if (it == StateMap.end())
    return false;

  // If the tracking information says there are escapes before entry,
  // we're done; it has potentially escaped.
  const auto &state = it->second;
  if (state.HasPotentiallyEscapedAtEntry)
    return true;

  // Okay, there must be an escape within this block.
  assert(state.HasEscape);
  for (auto ii = point->getIterator(), ie = bb->begin(); ii != ie; ) {
    auto inst = &*--ii;

    // Maybe just record the first escape in the block and see if we
    // come after it?
    if (Escapes.count(inst))
      return true;
  }

  return false;
}

/// Add all blocks to `Worklist` between the given `endAccess` and its
/// `begin_access` in which the access is active at the end of the block.
void SelectEnforcement::findBlocksAccessedAcross(
  EndAccessInst *endAccess, BlockSetVector &blocksAccessedAcross) {

  // Fast path: we're not tracking any escapes.  (But the box should
  // probably have been promoted to the stack in this case.)
  if (StateMap.empty())
    return;

  SILBasicBlock *beginBB = endAccess->getBeginAccess()->getParent();
  if (endAccess->getParent() == beginBB)
    return;

  assert(Worklist.empty());
  Worklist.push_back(endAccess->getParent());
  while (!Worklist.empty()) {
    SILBasicBlock *bb = Worklist.pop_back_val();
    for (auto *predBB : bb->getPredecessorBlocks()) {
      if (!blocksAccessedAcross.insert(predBB)) continue;
      if (predBB == beginBB) continue;
      Worklist.push_back(predBB);
    }
  }
}

bool SelectEnforcement::hasPotentiallyEscapedAtAnyReachableBlock(
  BeginAccessInst *access, BlockSetVector &blocksAccessedAcross) {

  assert(Worklist.empty());
  SmallPtrSet<SILBasicBlock*, 8> visited;

  // Don't follow any paths that lead to an end_access.
  for (auto endAccess : access->getEndAccesses())
    visited.insert(endAccess->getParent());

  /// Initialize the worklist with all blocks that exit the access path.
  for (SILBasicBlock *bb : blocksAccessedAcross) {
    for (SILBasicBlock *succBB : bb->getSuccessorBlocks()) {
      if (blocksAccessedAcross.count(succBB)) continue;
      if (visited.insert(succBB).second)
        Worklist.push_back(succBB);
    }
  }

  while (!Worklist.empty()) {
    SILBasicBlock *bb = Worklist.pop_back_val();
    assert(visited.count(bb));

    // If we're tracking information for this block, there's an escape.
    if (StateMap.count(bb))
      return true;

    // Add all reachable successors.
    for (SILBasicBlock *succ : bb->getSuccessors()) {
      if (visited.insert(succ).second)
        Worklist.push_back(succ);
    }
  }

  // No reachable block has an escape.
  return false;
}

void SelectEnforcement::updateAccesses() {
  for (auto *access : Accesses) {
    LLVM_DEBUG(llvm::dbgs() << "    Access: " << *access);
    updateAccess(access);
  }
  for (AddressCapture &capture : Captures) {
    LLVM_DEBUG(llvm::dbgs() << "    Capture: " << capture);
    updateCapture(capture);
  }
}

void SelectEnforcement::updateAccess(BeginAccessInst *access) {
  assert(access->getEnforcement() == SILAccessEnforcement::Unknown);

  // Check whether the variable escaped before any of the end_accesses.
  BlockSetVector blocksAccessedAcross;
  for (auto endAccess : access->getEndAccesses()) {
    if (hasPotentiallyEscapedAt(endAccess))
      return setDynamicEnforcement(access);

    // Add all blocks to blocksAccessedAcross between begin_access and this
    // end_access.
    findBlocksAccessedAcross(endAccess, blocksAccessedAcross);
  }
  assert(blocksAccessedAcross.empty()
         || blocksAccessedAcross.count(access->getParent()));

  // For every path through this access that doesn't reach an end_access, check
  // if any block reachable from that path can see an escaped value.
  if (hasPotentiallyEscapedAtAnyReachableBlock(access, blocksAccessedAcross)) {
    setDynamicEnforcement(access);
    return;
  }
  // Otherwise, use static enforcement.
  setStaticEnforcement(access);
}

void SelectEnforcement::updateCapture(AddressCapture capture) {
  auto captureIfEscaped = [&](SILInstruction *user) {
    if (hasPotentiallyEscapedAt(user))
      dynamicCaptures.recordCapture(capture);
  };
  SingleValueInstruction *PAIUser = dyn_cast<PartialApplyInst>(capture.site);
  if (!PAIUser) {
    // This is a full apply site. Immediately record the capture and return.
    captureIfEscaped(capture.site.getInstruction());
    return;
  }
  // For partial applies, check all use points of the closure.
  llvm::SmallSetVector<SingleValueInstruction *, 8> worklist;
  auto visitUse = [&](Operand *oper) {
    auto *user = oper->getUser();
    if (FullApplySite::isa(user)) {
      // A call is considered a closure access regardless of whether it calls
      // the closure or accepts the closure as an argument.
      captureIfEscaped(user);
      return;
    }
    switch (user->getKind()) {
    case SILInstructionKind::ConvertEscapeToNoEscapeInst:
    case SILInstructionKind::MarkDependenceInst:
    case SILInstructionKind::ConvertFunctionInst:
    case SILInstructionKind::BeginBorrowInst:
    case SILInstructionKind::CopyValueInst:
    case SILInstructionKind::EnumInst:
    case SILInstructionKind::StructInst:
    case SILInstructionKind::TupleInst:
    case SILInstructionKind::PartialApplyInst:
      // Propagate the closure.
      worklist.insert(cast<SingleValueInstruction>(user));
      return;
    case SILInstructionKind::StrongRetainInst:
    case SILInstructionKind::StrongReleaseInst:
    case SILInstructionKind::DebugValueInst:
    case SILInstructionKind::DestroyValueInst:
    case SILInstructionKind::RetainValueInst:
    case SILInstructionKind::ReleaseValueInst:
    case SILInstructionKind::EndBorrowInst:
    // partial_apply [stack] is matched with dealloc_stack.
    case SILInstructionKind::DeallocStackInst:
      // Benign use.
      return;
    case SILInstructionKind::TupleExtractInst:
    case SILInstructionKind::StructExtractInst:
    case SILInstructionKind::AssignInst:
    case SILInstructionKind::BranchInst:
    case SILInstructionKind::CondBranchInst:
    case SILInstructionKind::ReturnInst:
    case SILInstructionKind::StoreInst:
      // These are all valid partial_apply users, however we don't expect them
      // to occur with non-escaping closures. Handle them conservatively just in
      // case they occur.
      LLVM_FALLTHROUGH;
    default:
      LLVM_DEBUG(llvm::dbgs() << "    Unrecognized partial_apply user: "
                              << *user);

      // Handle unknown uses conservatively by assuming a capture.
      captureIfEscaped(user);
    }
  };
  while (true) {
    for (auto *oper : PAIUser->getUses())
      visitUse(oper);
    if (worklist.empty())
      break;
    PAIUser = worklist.pop_back_val();
  }
}

namespace {

// Model the kind of access needed based on analyzing the access's source.
// This is either determined to be static or dynamic, or requires further
// analysis of a boxed variable.
struct SourceAccess {
  enum { StaticAccess, DynamicAccess, BoxAccess } kind;
  AllocBoxInst *allocBox;

  static SourceAccess getStaticAccess() { return {StaticAccess, nullptr}; }
  static SourceAccess getDynamicAccess() { return {DynamicAccess, nullptr}; }

  static SourceAccess getBoxedAccess(AllocBoxInst *inst) {
    return {BoxAccess, inst};
  }
};

/// The pass.
///
/// This can't be a SILFunctionTransform because DynamicCaptures need to be
/// recorded while analyzing a closure's parent scopes before processing the
/// closures.
///
/// TODO: Make this a "ClosureTransform". See the file-level comments above.
class AccessEnforcementSelection : public SILModuleTransform {
  // Reference back to the known dynamically enforced non-escaping closure
  // arguments in this module. Parent scopes are processed before the closures
  // they reference.
  DynamicCaptures dynamicCaptures;

#ifndef NDEBUG
  // Per-function book-keeping to verify that a box is processed before all of
  // its accesses and captures are seen.
  llvm::DenseSet<AllocBoxInst *> handledBoxes;
  llvm::DenseSet<SILFunction *> visited;
#endif

public:
  void run() override;

protected:
  void processFunction(SILFunction *F);
  SourceAccess getAccessKindForBox(ProjectBoxInst *projection);
  SourceAccess getSourceAccess(SILValue address);
  void handleApply(ApplySite apply);
  void handleAccess(BeginAccessInst *access);
};

void AccessEnforcementSelection::run() {
  auto *CSA = getAnalysis<ClosureScopeAnalysis>();
  TopDownClosureFunctionOrder closureOrder(CSA);
  closureOrder.visitFunctions(
      [this](SILFunction *F) { this->processFunction(F); });
}

void AccessEnforcementSelection::processFunction(SILFunction *F) {
  if (F->isExternalDeclaration())
    return;

  LLVM_DEBUG(llvm::dbgs() << "Access Enforcement Selection in " << F->getName()
                          << "\n");

  // This ModuleTransform needs to analyze closures and their parent scopes in
  // the same pass, and the parent needs to be analyzed before the closure.
#ifndef NDEBUG
  auto *CSA = getAnalysis<ClosureScopeAnalysis>();
  if (isNonEscapingClosure(F->getLoweredFunctionType())) {
    for (auto *scopeF : CSA->getClosureScopes(F)) {
      LLVM_DEBUG(llvm::dbgs() << "  Parent scope: " << scopeF->getName()
                              << "\n");
      assert(visited.count(scopeF));
      // Closures must be defined in the same module as their parent scope.
      assert(scopeF->wasDeserializedCanonical()
             == F->wasDeserializedCanonical());
    }
  }
  visited.insert(F);
#endif

  // Deserialized functions, which have been mandatory inlined, no longer meet
  // the structural requirements on access markers required by this pass.
  if (F->wasDeserializedCanonical())
    return;

  // Perform an RPO walk so that boxes are always processed before their access.
  auto *PO = getAnalysis<PostOrderAnalysis>()->get(F);
  for (SILBasicBlock *bb : PO->getReversePostOrder()) {
    for (auto ii = bb->begin(), ie = bb->end(); ii != ie;) {
      SILInstruction *inst = &*ii;
      ++ii;

      // Analyze all boxes. Even if they aren't accessed in this function, they
      // may still have captures that require dynamic enforcement because the
      // box has escaped prior to the capture.
      if (auto box = dyn_cast<AllocBoxInst>(inst)) {
        SelectEnforcement(dynamicCaptures, box).run();
        assert(handledBoxes.insert(box).second);

      } else if (auto access = dyn_cast<BeginAccessInst>(inst))
        handleAccess(access);

      else if (auto access = dyn_cast<BeginUnpairedAccessInst>(inst))
        assert(access->getEnforcement() == SILAccessEnforcement::Dynamic);

      // Check for unboxed captures in both partial_applies and direct
      // applications of non-escaping closures.
      else if (auto apply = ApplySite::isa(inst))
        handleApply(apply);
    }
  }
  invalidateAnalysis(F, SILAnalysis::InvalidationKind::Instructions);
#ifndef NDEBUG
  // There's no need to track handled boxes across functions.
  handledBoxes.clear();
#endif
}

SourceAccess
AccessEnforcementSelection::getAccessKindForBox(ProjectBoxInst *projection) {
  SILValue source = projection->getOperand();
  if (auto *MUI = dyn_cast<MarkUninitializedInst>(source))
    source = MUI->getOperand();

  // If we didn't allocate the box, assume that we need to use
  // dynamic enforcement.
  // TODO: use static enforcement in certain provable cases.
  auto box = dyn_cast<AllocBoxInst>(source);
  if (!box)
    return SourceAccess::getDynamicAccess();

  return SourceAccess::getBoxedAccess(box);
}

SourceAccess AccessEnforcementSelection::getSourceAccess(SILValue address) {
  // Recurse through MarkUninitializedInst.
  if (auto *MUI = dyn_cast<MarkUninitializedInst>(address))
    return getSourceAccess(MUI->getOperand());

  if (auto box = dyn_cast<ProjectBoxInst>(address))
    return getAccessKindForBox(box);

  if (auto arg = dyn_cast<SILFunctionArgument>(address)) {
    switch (arg->getArgumentConvention()) {
    case SILArgumentConvention::Indirect_Inout:
      // `inout` arguments are checked on the caller side, either statically
      // or dynamically if necessary. The @inout does not alias and cannot
      // escape within the callee, so static enforcement is always sufficient.
      return SourceAccess::getStaticAccess();

    case SILArgumentConvention::Indirect_InoutAliasable:
      if (dynamicCaptures.isDynamic(arg))
        return SourceAccess::getDynamicAccess();

      return SourceAccess::getStaticAccess();
    case SILArgumentConvention::Indirect_In:
    case SILArgumentConvention::Indirect_In_Guaranteed:
      // @in/@in_guaranteed cannot be mutably accessed, mutably captured, or
      // passed as inout. @in/@in_guaranteed may be captured @inout_aliasable.
      // (This is fairly horrible, but presumably Sema/SILGen made sure a copy
      // wasn't needed?)
      //  
      // FIXME: When we have borrowed arguments, a "read" needs to be enforced
      // on the caller side.
      return SourceAccess::getStaticAccess();

    case SILArgumentConvention::Indirect_Out:
      // We use an initialized 'out' argument as a parameter.
      return SourceAccess::getStaticAccess();

    default:
      llvm_unreachable("Expecting an inout argument.");
    }
  }
  // If we're not accessing a box or argument, we must've lowered to a stack
  // element. Other sources of access are either outright dynamic (GlobalAddr,
  // RefElementAddr), or only exposed after mandatory inlining (nested
  // dependent BeginAccess).
  //
  // Running before diagnostic constant propagation requires handling 'undef'.
  assert(isa<AllocStackInst>(address) || isa<SILUndef>(address));
  return SourceAccess::getStaticAccess();
}

void AccessEnforcementSelection::handleApply(ApplySite apply) {
  auto calleeTy = apply.getOrigCalleeType();
  SILFunctionConventions calleeConv(calleeTy, *getModule());

  for (Operand &oper : apply.getArgumentOperands()) {
    AddressCapture capture(oper);
    if (!capture.isValid())
      continue;

    // This is a non-escaping closure argument. If the argument requires dynamic
    // access, record that in dynamicCaptures.
    auto sourceAccess = getSourceAccess(oper.get());
    switch (sourceAccess.kind) {
    case SourceAccess::StaticAccess:
      // If the captured variable does not require dynamic enforcement, then
      // there's no need to track it.
      break;
    case SourceAccess::DynamicAccess: {
      dynamicCaptures.recordCapture(capture);
      break;
    }
    case SourceAccess::BoxAccess:
      // Captures of box projections are handled during SelectEnforcement, which
      // determines the access enforcement for all users of a box. Within
      // SelectEnforcement, we know whether the box has escaped before the
      // capture. Here there's just nothing to do.
      assert(handledBoxes.count(sourceAccess.allocBox));
      break;
    }
  }
}

void AccessEnforcementSelection::handleAccess(BeginAccessInst *access) {
  if (access->getEnforcement() != SILAccessEnforcement::Unknown)
    return;

  auto sourceAccess = getSourceAccess(access->getOperand());
  switch (sourceAccess.kind) {
  case SourceAccess::StaticAccess:
    setStaticEnforcement(access);
    break;
  case SourceAccess::DynamicAccess:
    setDynamicEnforcement(access);
    break;
  case SourceAccess::BoxAccess:
    llvm_unreachable("All boxes must have already been selected.");
  }
}

} // end anonymous namespace

SILTransform *swift::createAccessEnforcementSelection() {
  return new AccessEnforcementSelection();
}