swift-mirror/lib/SILPasses/ClosureSpecializer.cpp

//===---- ClosureSpecializer.cpp ------ Performs Closure Specialization----===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
//  Closure Specialization
//  ----------------------
//
//  The purpose of the algorithm in this file is to perform the following
//  transformation: given a closure passed into a function which the closure is
//  then invoked in, clone the function and create a copy of the closure inside
//  the function. This closure will be able to be eliminated easily and the
//  overhead is gone. We then try to remove the original closure.
//
//  There are some complications. They are listed below and how we work around
//  them:
//
//  1. If we support the specialization of closures with multiple user callsites
//     that can be specialized, we need to ensure that any captured values have
//     their reference counts adjusted properly. This implies for every
//     specialized call site, we insert an additional retain for each captured
//     argument with reference semantics before the old partial apply. We will
//     pass them in as extra @owned to the specialized function. This @owned
//     will be consumed by the "copy" partial apply that is in the specialized
//     function. Now the partial apply will own those ref counts. This is
//     unapplicable to thin_to_thick_function since they do not have any
//     captured args.
//
//  2. If the closure was passed in @owned vs if the closure was passed in
//     @guaranteed. If the original closure was passed in @owned, then we know
//     that there is a balancing release for the new "copy" partial apply. But
//     since the original partial apply no longer will have that corresponding
//     -1, we need to insert a release for the old partial apply. We do this
//     right after the old call site where the original partial apply was
//     called. This ensures we do not shrink the lifetime of the old partial
//     apply. In the case where the old partial_apply was passed in at +0, we
//     know that the old partial_apply does not need to have any ref count
//     adjustments. On the other hand, the new "copy" partial apply in the
//     specialized function now needs to be balanced lest we leak. Thus we
//     insert a release right before any exit from the function. This ensures
//     that the release occurs in the epilog after any retains associated with
//     @owned return values.
//
//  3. Handling addresses. We currently do not handle address types. We can in
//     the future by introducing alloc_stacks.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "closure-specialization"
#include "swift/SILPasses/Passes.h"
#include "swift/SIL/Mangle.h"
#include "swift/SIL/SILCloner.h"
#include "swift/SIL/SILFunction.h"
#include "swift/SIL/SILInstruction.h"
#include "swift/SIL/SILModule.h"
#include "swift/SILAnalysis/CallGraphAnalysis.h"
#include "swift/SILAnalysis/ValueTracking.h"
#include "swift/SILAnalysis/CFG.h"
#include "swift/SILPasses/Transforms.h"
#include "swift/SILPasses/Utils/SILInliner.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"

using namespace swift;

STATISTIC(NumClosureSpecialized,
          "Number of functions with closures specialized");
STATISTIC(NumPropagatedClosuresEliminated,
          "Number of closures propagated and then eliminated");
STATISTIC(NumPropagatedClosuresNotEliminated,
          "Number of closures propagated but not eliminated");

llvm::cl::opt<bool> EliminateDeadClosures(
    "closure-specialize-eliminate-dead-closures", llvm::cl::init(true),
    llvm::cl::desc("Do not eliminate dead closures after closure "
                   "specialization. This is meant ot be used when testing."));

//===----------------------------------------------------------------------===//
//                                  Utility
//===----------------------------------------------------------------------===//

static bool isSupportedClosureKind(const SILInstruction *I) {
  return isa<ThinToThickFunctionInst>(I) || isa<PartialApplyInst>(I);
}

//===----------------------------------------------------------------------===//
//                       Closure Spec Cloner Interface
//===----------------------------------------------------------------------===//

namespace {

class CallSiteDescriptor;

/// \brief A SILCloner subclass which clones a function that takes a closure
/// argument. We update the parameter list to remove the parameter for the
/// closure argument and to append the variables captured in the closure.
/// We also need to replace the closure parameter with the partial apply
/// on the closure. We need to update the callsite to pass in the correct
/// arguments.
class ClosureSpecCloner : public SILClonerWithScopes<ClosureSpecCloner> {
public:
  using SuperTy = SILClonerWithScopes<ClosureSpecCloner>;
  friend class SILVisitor<ClosureSpecCloner>;
  friend class SILCloner<ClosureSpecCloner>;

  ClosureSpecCloner(const CallSiteDescriptor &CallSiteDesc,
                    StringRef ClonedName)
      : SuperTy(*initCloned(CallSiteDesc, ClonedName)),
        CallSiteDesc(CallSiteDesc) {}

  void populateCloned();

  SILFunction *getCloned() { return &getBuilder().getFunction(); }
  static SILFunction *cloneFunction(const CallSiteDescriptor &CallSiteDesc,
                                    StringRef NewName) {
    ClosureSpecCloner C(CallSiteDesc, NewName);
    C.populateCloned();
    ++NumClosureSpecialized;
    return C.getCloned();
  };

private:
  static SILFunction *initCloned(const CallSiteDescriptor &CallSiteDesc,
                                 StringRef ClonedName);
  const CallSiteDescriptor &CallSiteDesc;
};

} // end anonymous namespace

//===----------------------------------------------------------------------===//
//                            Call Site Descriptor
//===----------------------------------------------------------------------===//

namespace {

class CallSiteDescriptor {
  SILInstruction *Closure;
  ApplyInst *AI;
  unsigned ClosureIndex;
  SILParameterInfo ClosureParamInfo;

  // This is only needed if we have guaranteed parameters. In most cases it will
  // have only one element, a return inst.
  llvm::TinyPtrVector<SILBasicBlock *> NonFailureExitBBs;

public:
  CallSiteDescriptor(SILInstruction *ClosureInst, ApplyInst *AI,
                     unsigned ClosureIndex, SILParameterInfo ClosureParamInfo,
                     llvm::TinyPtrVector<SILBasicBlock *> &&NonFailureExitBBs)
      : Closure(ClosureInst), AI(AI), ClosureIndex(ClosureIndex),
        ClosureParamInfo(ClosureParamInfo),
        NonFailureExitBBs(NonFailureExitBBs) {}

  static bool isSupportedClosure(const SILInstruction *Closure);

  SILFunction *getApplyCallee() const {
    return cast<FunctionRefInst>(AI->getCallee())->getReferencedFunction();
  }

  SILFunction *getClosureCallee() const {
    if (auto *PAI = dyn_cast<PartialApplyInst>(Closure))
      return cast<FunctionRefInst>(PAI->getCallee())->getReferencedFunction();

    auto *TTTFI = cast<ThinToThickFunctionInst>(Closure);
    return cast<FunctionRefInst>(TTTFI->getCallee())->getReferencedFunction();
  }

  bool closureHasRefSemanticContext() const {
    return isa<PartialApplyInst>(Closure);
  }

  unsigned getClosureIndex() const { return ClosureIndex; }

  SILParameterInfo getClosureParameterInfo() const { return ClosureParamInfo; }

  SILInstruction *
  createNewClosure(SILBuilder &B, SILValue V,
                   llvm::SmallVectorImpl<SILValue> &Args) const {
    if (isa<PartialApplyInst>(Closure))
      return B.createPartialApply(Closure->getLoc(), V, V.getType(), {}, Args,
                                  Closure->getType(0));

    assert(isa<ThinToThickFunctionInst>(Closure) &&
           "We only support partial_apply and thin_to_thick_function");
    return B.createThinToThickFunction(Closure->getLoc(), V,
                                       Closure->getType(0));
  }

  ApplyInst *getApplyInst() const { return AI; }

  void specializeClosure() const;

  void createName(llvm::SmallString<64> &NewName) const;

  OperandValueArrayRef getArguments() const {
    if (auto *PAI = dyn_cast<PartialApplyInst>(Closure))
      return PAI->getArguments();

    // Thin to thick function has no non-callee arguments.
    assert(isa<ThinToThickFunctionInst>(Closure) &&
           "We only support partial_apply and thin_to_thick_function");
    return OperandValueArrayRef(ArrayRef<Operand>());
  }

  SILInstruction *getClosure() const { return Closure; }

  unsigned getNumArguments() const {
    if (auto *PAI = dyn_cast<PartialApplyInst>(Closure))
      return PAI->getNumArguments();

    // Thin to thick function has no non-callee arguments.
    assert(isa<ThinToThickFunctionInst>(Closure) &&
           "We only support partial_apply and thin_to_thick_function");
    return 0;
  }

  bool isClosureGuaranteed() const {
    return getClosureParameterInfo().isGuaranteed();
  }

  bool isClosureConsumed() const {
    return getClosureParameterInfo().isConsumed();
  }

  SILLocation getLoc() const { return Closure->getLoc(); }

  SILModule &getModule() const { return AI->getModule(); }

  ArrayRef<SILBasicBlock *> getNonFailureExitBBs() const {
    return NonFailureExitBBs;
  }
};

} // end anonymous namespace

/// Update the callsite to pass in the correct arguments.
static void rewriteApplyInst(const CallSiteDescriptor &CSDesc,
                             SILFunction *NewF) {
  ApplyInst *AI = CSDesc.getApplyInst();
  SILInstruction *Closure = CSDesc.getClosure();
  SILBuilderWithScope<2> Builder(Closure);
  FunctionRefInst *FRI = Builder.createFunctionRef(AI->getLoc(), NewF);

  // Create the args for the new apply by removing the closure argument...
  llvm::SmallVector<SILValue, 8> NewArgs;
  unsigned Index = 0;
  for (auto Arg : AI->getArguments()) {
    if (Index != CSDesc.getClosureIndex())
      NewArgs.push_back(Arg);
    Index++;
  }

  // ... and appending the captured arguments. We also insert retains here at
  // the location of the original closure. This is needed to balance the
  // implicit release of all captured arguments that occurs when the partial
  // apply is destroyed.
  SILModule &M = NewF->getModule();
  for (auto Arg : CSDesc.getArguments()) {
    NewArgs.push_back(Arg);

    SILType ArgTy = Arg.getType();

    // If our argument is of trivial type, continue...
    if (ArgTy.isTrivial(M))
      continue;

    // TODO: When we support address types, this code path will need to be
    // updated.
    Builder.createRetainValue(Closure->getLoc(), Arg);
  }

  SILType LoweredType = NewF->getLoweredType();
  SILType ResultType = LoweredType.castTo<SILFunctionType>()->getSILResult();
  Builder.setInsertionPoint(AI);
  ApplyInst *NewAI = Builder.createApply(AI->getLoc(), FRI, LoweredType,
                                         ResultType, ArrayRef<Substitution>(),
                                         NewArgs, NewF->isTransparent());

  // If we passed in the original closure as @owned, then insert a release right
  // after NewAI. This is to balance the +1 from being an @owned argument to AI.
  if (CSDesc.isClosureConsumed() && CSDesc.closureHasRefSemanticContext())
    Builder.createReleaseValue(Closure->getLoc(), Closure);

  // Replace all uses of the old apply with the new apply.
  AI->replaceAllUsesWith(NewAI);
  // Erase the old apply.
  AI->eraseFromParent();

  // TODO: Maybe include invalidation code for CallSiteDescriptor after we erase
  // AI from parent?
}

void CallSiteDescriptor::createName(llvm::SmallString<64> &NewName) const {
  llvm::raw_svector_ostream buffer(NewName);
  Mangle::Mangler M(buffer);
  auto P = Mangle::SpecializationPass::ClosureSpecializer;
  Mangle::FunctionSignatureSpecializationMangler FSSM(P, M, getApplyCallee());
  if (auto *PAI = dyn_cast<PartialApplyInst>(Closure)) {
    FSSM.setArgumentClosureProp(getClosureIndex(), PAI);
    FSSM.mangle();
    return;
  }

  auto *TTTFI = cast<ThinToThickFunctionInst>(Closure);
  FSSM.setArgumentClosureProp(getClosureIndex(), TTTFI);
  FSSM.mangle();
}

void CallSiteDescriptor::specializeClosure() const {
  llvm::SmallString<64> NewFName;
  createName(NewFName);
  DEBUG(llvm::dbgs() << "    Perform optimizations with new name " << NewFName
                     << '\n');

  // Then see if we already have a specialized version of this function in our
  // module.
  SILFunction *NewF = getModule().lookUpFunction(NewFName);

  // If not, create a specialized version of ApplyCallee calling the closure
  // directly.
  if (!NewF)
    NewF = ClosureSpecCloner::cloneFunction(*this, NewFName);

  // Rewrite the call
  rewriteApplyInst(*this, NewF);
}

bool CallSiteDescriptor::isSupportedClosure(const SILInstruction *Closure) {
  if (!isSupportedClosureKind(Closure))
    return false;

  // We only support simple closures where a partial_apply or
  // thin_to_thick_function is passed a function_ref. This will be stored here
  // so the checking of the Callee can use the same code in both cases.
  SILValue Callee;

  // If Closure is a partial apply...
  if (auto *PAI = dyn_cast<PartialApplyInst>(Closure)) {
    // And it has substitutions, return false.
    if (PAI->hasSubstitutions())
      return false;

    // If any arguments are not objects, return false. This is a temporary
    // limitation.
    for (SILValue Arg : PAI->getArguments())
      if (!Arg.getType().isObject())
        return false;

    // Ok, it is a closure we support, set Callee.
    Callee = PAI->getCallee();

  } else {
    // Otherwise closure must be a thin_to_thick_function.
    Callee = cast<ThinToThickFunctionInst>(Closure)->getCallee();
  }

  // Make sure that it is a simple partial apply (i.e. its callee is a
  // function_ref). We also do not handle indirect results currently in the
  // closure so make sure that does not happen at this point.
  //
  // TODO: We can probably handle other partial applies here.
  auto *FRI = dyn_cast<FunctionRefInst>(Callee);
  if (!FRI || FRI->getFunctionType()->hasIndirectResult())
    return false;

  // Otherwise, we do support specializing this closure.
  return true;
}

//===----------------------------------------------------------------------===//
//                     Closure Spec Cloner Implementation
//===----------------------------------------------------------------------===//

/// In this function we create the actual cloned function and its proper cloned
/// type. But we do not create any body. This implies that the creation of the
/// actual arguments in the function is in populateCloned.
///
/// \arg PAUser The function that is being passed the partial apply.
/// \arg PAI The partial apply that is being passed to PAUser.
/// \arg ClosureIndex The index of the partial apply in PAUser's function
///                   signature.
/// \arg ClonedName The name of the cloned function that we will create.
SILFunction *
ClosureSpecCloner::initCloned(const CallSiteDescriptor &CallSiteDesc,
                              StringRef ClonedName) {
  SILFunction *ClosureUser = CallSiteDesc.getApplyCallee();

  // This is the list of new interface parameters of the cloned function.
  llvm::SmallVector<SILParameterInfo, 4> NewParameterInfoList;

  // First add to NewParameterInfoList all of the SILParameterInfo in the
  // original function except for the closure.
  CanSILFunctionType ClosureUserFunTy = ClosureUser->getLoweredFunctionType();
  unsigned Index = 0;
  for (auto &param : ClosureUserFunTy->getParameters()) {
    if (Index != CallSiteDesc.getClosureIndex())
      NewParameterInfoList.push_back(param);
    ++Index;
  }

  // Then add any arguments that are captured in the closure to the function's
  // argument type. Since they are captured, we need to pass them directly into
  // the new specialized function.
  SILFunction *ClosedOverFun = CallSiteDesc.getClosureCallee();
  CanSILFunctionType ClosedOverFunTy = ClosedOverFun->getLoweredFunctionType();
  SILModule &M = ClosureUser->getModule();

  // Captured parameters are always appended to the function signature. If the
  // type of the captured argument is trivial, pass the argument as
  // Direct_Unowned. Otherwise pass it as Direct_Owned.
  //
  // We use the type of the closure here since we allow for the closure to be an
  // external declaration.
  unsigned NumTotalParams = ClosedOverFunTy->getParameters().size();
  unsigned NumNotCaptured = NumTotalParams - CallSiteDesc.getNumArguments();
  for (auto &PInfo : ClosedOverFunTy->getParameters().slice(NumNotCaptured)) {
    if (PInfo.getSILType().isTrivial(M)) {
      SILParameterInfo NewPInfo(PInfo.getType(),
                                ParameterConvention::Direct_Unowned);
      NewParameterInfoList.push_back(NewPInfo);
      continue;
    }

    SILParameterInfo NewPInfo(PInfo.getType(),
                              ParameterConvention::Direct_Owned);
    NewParameterInfoList.push_back(NewPInfo);
  }

  auto ClonedTy = SILFunctionType::get(
      ClosureUserFunTy->getGenericSignature(), ClosureUserFunTy->getExtInfo(),
      ClosureUserFunTy->getCalleeConvention(), NewParameterInfoList,
      ClosureUserFunTy->getResult(), M.getASTContext());

  // We make this function bare so we don't have to worry about decls in the
  // SILArgument.
  auto Fn = SILFunction::create(
      M, ClosureUser->getLinkage(), ClonedName, ClonedTy,
      ClosureUser->getContextGenericParams(), ClosureUser->getLocation(),
      IsBare, ClosureUser->isTransparent(), ClosureUser->isFragile(),
      ClosureUser->isThunk(), ClosureUser->getClassVisibility(),
      ClosureUser->getInlineStrategy(), ClosureUser->getEffectsKind(),
      ClosureUser, ClosureUser->getDebugScope());
  Fn->setSemanticsAttr(ClosureUser->getSemanticsAttr());
  return Fn;
}

/// \brief Populate the body of the cloned closure, modifying instructions as
/// necessary. This is where we create the actual specialized BB Arguments.
void ClosureSpecCloner::populateCloned() {
  SILFunction *Cloned = getCloned();
  SILModule &M = Cloned->getModule();

  SILFunction *ClosureUser = CallSiteDesc.getApplyCallee();

  // Create arguments for the entry block.
  SILBasicBlock *ClosureUserEntryBB = ClosureUser->begin();
  SILBasicBlock *ClonedEntryBB = new (M) SILBasicBlock(Cloned);

  // Remove the closure argument.
  SILArgument *ClosureArg = nullptr;
  for (size_t i = 0, e = ClosureUserEntryBB->bbarg_size(); i != e; ++i) {
    SILArgument *Arg = ClosureUserEntryBB->getBBArg(i);
    if (i == CallSiteDesc.getClosureIndex()) {
      ClosureArg = Arg;
      continue;
    }

    // Otherwise, create a new argument which copies the original argument
    SILValue MappedValue =
      new (M) SILArgument(ClonedEntryBB, Arg->getType(), Arg->getDecl());

    ValueMap.insert(std::make_pair(Arg, MappedValue));
  }

  // Next we need to add in any arguments that are not captured as arguments to
  // the cloned function.
  //
  // We do not insert the new mapped arugments into the value map since there by
  // definition is nothing in the partial apply user function that references
  // such arguments. After this pass is done the only thing that will reference
  // the arguments is the partial apply that we will create.
  SILFunction *ClosedOverFun = CallSiteDesc.getClosureCallee();
  CanSILFunctionType ClosedOverFunTy = ClosedOverFun->getLoweredFunctionType();
  unsigned NumTotalParams = ClosedOverFunTy->getParameters().size();
  unsigned NumNotCaptured = NumTotalParams - CallSiteDesc.getNumArguments();
  llvm::SmallVector<SILValue, 4> NewPAIArgs;
  for (auto &PInfo : ClosedOverFunTy->getParameters().slice(NumNotCaptured)) {
    SILValue MappedValue =
      new (M) SILArgument(ClonedEntryBB, PInfo.getSILType());
    NewPAIArgs.push_back(MappedValue);
  }

  SILBuilder &Builder = getBuilder();
  Builder.setInsertionPoint(ClonedEntryBB);

  // Clone FRI and PAI, and replace usage of the removed closure argument
  // with result of cloned PAI.
  SILValue FnVal =
      Builder.createFunctionRef(CallSiteDesc.getLoc(), ClosedOverFun);
  auto *NewClosure = CallSiteDesc.createNewClosure(Builder, FnVal, NewPAIArgs);
  ValueMap.insert(std::make_pair(ClosureArg, SILValue(NewClosure)));

  BBMap.insert(std::make_pair(ClosureUserEntryBB, ClonedEntryBB));
  // Recursively visit original BBs in depth-first preorder, starting with the
  // entry block, cloning all instructions other than terminators.
  visitSILBasicBlock(ClosureUserEntryBB);

  // Now iterate over the BBs and fix up the terminators.
  for (auto BI = BBMap.begin(), BE = BBMap.end(); BI != BE; ++BI) {
    Builder.setInsertionPoint(BI->second);
    visit(BI->first->getTerminator());
  }

  // Then insert a release in all non failure exit BBs if our partial apply was
  // guaranteed. This is b/c it was passed at +0 originally and we need to
  // balance the initial increment of the newly created closure.
  if (CallSiteDesc.isClosureGuaranteed() &&
      CallSiteDesc.closureHasRefSemanticContext()) {
    for (SILBasicBlock *BB : CallSiteDesc.getNonFailureExitBBs()) {
      SILBasicBlock *OpBB = BBMap[BB];

      TermInst *TI = OpBB->getTerminator();
      auto Loc = CleanupLocation::getCleanupLocation(NewClosure->getLoc());

      // If we have a return, we place the release right before it so we know
      // that it will be executed at the end of the epilogue.
      if (isa<ReturnInst>(TI)) {
        Builder.setInsertionPoint(TI);
        Builder.createReleaseValue(Loc, SILValue(NewClosure));
        continue;
      }

      // We use casts where findAllNonFailureExitBBs should have made sure that
      // this is true. This will ensure that the code is updated when we hit the
      // cast failure in debug builds.
      auto *Unreachable = cast<UnreachableInst>(TI);
      auto PrevIter = std::prev(SILBasicBlock::iterator(Unreachable));
      auto *NoReturnApply = cast<ApplyInst>(PrevIter);

      // We insert the release value right before the no return apply so that if
      // the partial apply is passed into the @noreturn function as an @owned
      // value, we will retain the partial apply before we release it and
      // potentially eliminate it.
      Builder.setInsertionPoint(NoReturnApply);
      Builder.createReleaseValue(Loc, SILValue(NewClosure));
    }
  }
}

//===----------------------------------------------------------------------===//
//                            Closure Specializer
//===----------------------------------------------------------------------===//

namespace {

class ClosureSpecializer {

  /// A vector consisting of closures that we propagated. After
  std::vector<SILInstruction *> PropagatedClosures;
  bool IsPropagatedClosuresUniqued = false;

public:
  ClosureSpecializer() = default;

  void gatherCallSites(SILFunction *Caller,
                       llvm::SmallVectorImpl<CallSiteDescriptor> &CallSites,
                       llvm::SmallPtrSet<ApplyInst *, 4> &MultipleClosureAI);
  bool specialize(SILFunction *Caller);

  ArrayRef<SILInstruction *> getPropagatedClosures() {
    if (IsPropagatedClosuresUniqued)
      return PropagatedClosures;

    sortUnique(PropagatedClosures);
    IsPropagatedClosuresUniqued = true;

    return PropagatedClosures;
  }
};

} // end anonymous namespace

void ClosureSpecializer::gatherCallSites(
    SILFunction *Caller, llvm::SmallVectorImpl<CallSiteDescriptor> &CallSites,
    llvm::SmallPtrSet<ApplyInst *, 4> &MultipleClosureAI) {

  // A set of apply inst that we have associated with a closure. We use this to
  // make sure that we do not handle call sites with multiple closure arguments.
  llvm::SmallPtrSet<ApplyInst *, 4> VisitedAI;

  // For each basic block BB in Caller...
  for (auto &BB : *Caller) {
    // For each instruction II in BB...
    for (auto &II : BB) {
      // If II is not a closure that we support specializing, skip it...
      if (!CallSiteDescriptor::isSupportedClosure(&II))
        continue;

      // Go through all uses of our closure.
      for (auto *Use : II.getUses()) {
        // If this use use is not an apply inst or an apply inst with
        // substitutions, there is nothing interesting for us to do, so
        // continue...
        auto *AI = dyn_cast<ApplyInst>(Use->getUser());
        if (!AI || AI->hasSubstitutions())
          continue;

        // Check if we have already associated this apply inst with a closure to
        // be specialized. We do not handle applies that take in multiple
        // closures at this time.
        if (!VisitedAI.insert(AI).second) {
          MultipleClosureAI.insert(AI);
          continue;
        }

        // If AI does not have a function_ref definition as its callee, we can
        // not do anything here... so continue...
        auto *ApplyCalleeFRI = dyn_cast<FunctionRefInst>(AI->getCallee());
        if (!ApplyCalleeFRI)
          continue;
        SILFunction *ApplyCallee = ApplyCalleeFRI->getReferencedFunction();
        if (ApplyCallee->isExternalDeclaration())
          continue;

        // Ok, we know that we can perform the optimization but not whether or not
        // the optimization is profitable. Find the index of the argument
        // corresponding to our partial apply.
        Optional<unsigned> ClosureIndex;
        for (unsigned i = 0, e = AI->getNumArguments(); i != e; ++i) {
          if (AI->getArgument(i) != SILValue(&II))
            continue;
          ClosureIndex = i;
          DEBUG(llvm::dbgs() << "    Found callsite with closure argument at "
                << i << ": " << *AI);
          break;
        }

        // If we did not find an index, there is nothing further to do, continue.
        if (!ClosureIndex.hasValue())
          continue;

        // Make sure that the Closure is invoked in the Apply's callee. We only
        // want to perform closure specialization if we know that we will be
        // able to change a partial_apply into an apply.
        //
        // TODO: Maybe just call the function directly instead of moving the
        // partial apply?
        SILValue Arg = ApplyCallee->getArgument(ClosureIndex.getValue());
        if (std::none_of(Arg->use_begin(), Arg->use_end(),
                         [&Arg](const Operand *Op) -> bool {
                           auto *UserAI = dyn_cast<ApplyInst>(Op->getUser());
                           return UserAI && UserAI->getCallee() == Arg;
                         })) {
          continue;
        }

        auto ParamInfo = AI->getSubstCalleeType()->getParameters();
        SILParameterInfo ClosureParamInfo = ParamInfo[ClosureIndex.getValue()];

        // Get all non-failure exit BBs in the Apply Callee if our partial apply
        // is guaranteed. If we do not understand one of the exit BBs, bail.
        //
        // We need this to make sure that we insert a release in the appropriate
        // locations to balance the +1 from the creation of the partial apply.
        llvm::TinyPtrVector<SILBasicBlock *> NonFailureExitBBs;
        if (ClosureParamInfo.isGuaranteed() &&
            !findAllNonFailureExitBBs(ApplyCallee, NonFailureExitBBs)) {
          continue;
        }

        // Now we know that CSDesc is profitable to specialize. Add it to our call
        // site list.
        CallSites.push_back(CallSiteDescriptor(&II, AI, ClosureIndex.getValue(),
                                               ClosureParamInfo,
                                               std::move(NonFailureExitBBs)));
      }
    }
  }
}

bool ClosureSpecializer::specialize(SILFunction *Caller) {
  DEBUG(llvm::dbgs() << "Optimizing callsites that take closure argument in "
                     << Caller->getName() << '\n');

  // Collect all of the PartialApplyInsts that are used as arguments to
  // ApplyInsts. Check the profitability of specializing the closure argument.
  llvm::SmallVector<CallSiteDescriptor, 8> CallSites;
  llvm::SmallPtrSet<ApplyInst *, 4> MultipleClosureAI;
  gatherCallSites(Caller, CallSites, MultipleClosureAI);

  bool Changed = false;
  for (auto &CSDesc : CallSites) {
    // Do not specialize apply insts that take in multiple closures. This pass
    // does not know how to do this yet.
    if (MultipleClosureAI.count(CSDesc.getApplyInst()))
      continue;

    CSDesc.specializeClosure();
    PropagatedClosures.push_back(CSDesc.getClosure());
    Changed = true;
  }

  return Changed;
}

//===----------------------------------------------------------------------===//
//                               Top Level Code
//===----------------------------------------------------------------------===//

namespace {

class SILClosureSpecializerTransform : public SILModuleTransform {
public:
  SILClosureSpecializerTransform() {}

  void run() override {
    auto *CGA = getAnalysis<CallGraphAnalysis>();

    bool Changed = false;
    ClosureSpecializer C;

    // Specialize going bottom-up in the call graph.
    for (auto *F : CGA->getCallGraph().getBottomUpFunctionOrder()) {
      // If F is an external declaration, attempt to link in its definition. If
      // we fail to do so, there is nothing further that we can do.
      if (F->isExternalDeclaration() &&
          !getModule()->linkFunction(F, SILModule::LinkingMode::LinkAll))
        continue;

      Changed |= C.specialize(F);
    }

    // Invalidate the call graph.
    if (Changed)
      invalidateAnalysis(SILAnalysis::PreserveKind::Nothing);

    // If for testing purposes we were asked to not eliminate dead closures,
    // return.
    if (!EliminateDeadClosures)
      return;

    // Otherwise, remove any local dead closures that are now dead since we
    // specialized all of their uses.
    DEBUG(llvm::dbgs() << "Trying to remove dead closures!\n");
    for (SILInstruction *Closure : C.getPropagatedClosures()) {
      DEBUG(llvm::dbgs() << "    Visiting: " << *Closure);
      if (!tryDeleteDeadClosure(Closure)) {
        DEBUG(llvm::dbgs() << "        Failed to delete closure!\n");
        NumPropagatedClosuresNotEliminated++;
        continue;
      }

      DEBUG(llvm::dbgs() << "        Deleted closure!\n");
      ++NumPropagatedClosuresEliminated;
    }
  }

  StringRef getName() override { return "Closure Specialization"; }
};

} // end anonymous namespace


SILTransform *swift::createClosureSpecializer() {
  return new SILClosureSpecializerTransform();
}