swift-mirror/lib/SILPasses/Devirtualizer.cpp

//===-- Devirtualizer.cpp ------ Devirtualize virtual calls ---------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Devirtualizes virtual function calls into direct function calls.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sil-devirtualizer"
#include "swift/Basic/DemangleWrappers.h"
#include "swift/Basic/Fallthrough.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/SILBuilder.h"
#include "swift/SIL/SILFunction.h"
#include "swift/SIL/SILInstruction.h"
#include "swift/SIL/SILModule.h"
#include "swift/SILAnalysis/ClassHierarchyAnalysis.h"
#include "swift/SILPasses/Passes.h"
#include "swift/SILPasses/Utils/Local.h"
#include "swift/SILPasses/PassManager.h"
#include "swift/SILPasses/Transforms.h"
#include "swift/SILPasses/Utils/SILInliner.h"
#include "swift/AST/ASTContext.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringSet.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/CommandLine.h"
using namespace swift;

STATISTIC(NumInlineCaches, "Number of monomorphic inline caches inserted");
STATISTIC(NumDevirtualized, "Number of calls devirtualized");
STATISTIC(NumAMI, "Number of witness_method devirtualized");

// The number of subclasses to allow when placing polymorphic inline caches.
static const int MaxNumPolymorphicInlineCaches = 6;

//===----------------------------------------------------------------------===//
//                         Class Method Optimization
//===----------------------------------------------------------------------===//

/// Is this an instruction kind which allows us to conclude definitively what
/// the class decls of its results are.
///
/// FIXME: We can expand this to use typed GEPs.
static bool isClassDeclOracle(ValueKind Kind) {
  switch (Kind) {
  case ValueKind::AllocRefInst:
  case ValueKind::AllocRefDynamicInst:
  case ValueKind::MetatypeInst:
    return true;
  default:
    return false;
  }
}

/// \brief Recursively searches the ClassDecl for a class_method operand.
/// Return the ClassDecl from the point of construction of \p S or null.
static ClassDecl *getClassFromConstructor(SILValue S) {
  // First strip off casts.
  S = S.stripCasts();

  // Then if S is not a class decl oracle, we can not ascertain what its results
  // "true" type is.
  if (!isClassDeclOracle(S->getKind()))
    return nullptr;

  // Look for a a static ClassTypes in AllocRefInst or MetatypeInst.
  if (AllocRefInst *ARI = dyn_cast<AllocRefInst>(S))
    return ARI->getType().getClassOrBoundGenericClass();

  auto *MTI = dyn_cast<MetatypeInst>(S);
  if (!MTI)
    return nullptr;

  CanType instTy = MTI->getType().castTo<MetatypeType>().getInstanceType();
  return instTy.getClassOrBoundGenericClass();
}

/// Return bound generic type for the unbound type Superclass,
/// which is a superclass of a bound generic type BoundDerived
/// (Base may be also the same as BoundDerived).
static SILType bindSuperclass(Module *Module,
                              CanType Superclass,
                              SILType BoundDerived,
                              ArrayRef<Substitution>& Subs) {
  SILType BoundSuperclass = BoundDerived;

  while (true) {
    auto CanBoundSuperclass = BoundSuperclass.getSwiftRValueType();
    // Get declaration of the superclass.
    auto *Decl = CanBoundSuperclass.getNominalOrBoundGenericNominal();
    // Obtain the unbound variant of the current superclass
    CanType UnboundSuperclass = Decl->getDeclaredType()->getCanonicalType();
    // Check if we found a superclass we are looking for.
    if (UnboundSuperclass == Superclass) {
      auto BoundBaseType = dyn_cast<BoundGenericType>(CanBoundSuperclass);
      if (BoundBaseType)
        // If it is a bound generic type, look up its substitutions
        Subs = BoundBaseType->getSubstitutions(Module,
                                               nullptr);
      else
        // If it is a non-bound type, there are no substitutions.
        Subs.empty();
      return BoundSuperclass;
    }
    // Get the superclass of current one
    BoundSuperclass = BoundSuperclass.getSuperclass(nullptr);
    // Stop iteration if there is no superclass.
    if (!BoundSuperclass)
      return SILType();
  }

  return SILType();
}

// Returns true if any generic types parameters of the class are
// unbound.
static bool isClassWithUnboundGenericParameters(SILType C, SILModule &M) {
  auto *CD = C.getClassOrBoundGenericClass();
  if (CD && CD->getGenericSignature()) {
    auto InstanceTypeSubsts =
        C.gatherAllSubstitutions(M);

    if (!InstanceTypeSubsts.empty()) {
      if (hasUnboundGenericTypes(InstanceTypeSubsts))
        return true;
    }
  }
  return false;
}

/// \brief Devirtualize an Apply instruction and a class member obtained
/// using the class_method instruction into a direct call to a specific
/// member of a specific class.
///
/// \p AI is the apply to devirtualize.
/// \p Member is the class member to devirtualize.
/// \p ClassInstance is the operand for the ClassMethodInst or an alternative
///    reference (such as downcasted class reference).
/// \p KnownClass (can be null) is a specific class type to devirtualize to.
static bool devirtMethod(ApplyInst *AI, SILDeclRef Member,
                         SILValue ClassInstance, ClassDecl *CD) {
  DEBUG(llvm::dbgs() << "    Trying to devirtualize : " << *AI);

  // First attempt to lookup the origin for our class method. The origin should
  // either be a metatype or an alloc_ref.
  DEBUG(llvm::dbgs() << "        Origin: " << ClassInstance);

  assert(CD && "Invalid class type");

  // Otherwise lookup from the module the least derived implementing method from
  // the module vtables.
  SILModule &Mod = AI->getModule();
  // Find the implementation of the member which should be invoked.
  SILFunction *F = Mod.lookUpFunctionFromVTable(CD, Member);

  // If we do not find any such function, we have no function to devirtualize
  // to... so bail.
  if (!F) {
    DEBUG(llvm::dbgs() << "        FAIL: Could not find matching VTable or "
                          "vtable method for this class.\n");
    return false;
  }

  // Ok, we found a function F that we can devirtualize our class method
  // to. We want to do everything on the substituted type in the case of
  // generics. Thus construct our subst callee type for F.
  SILModule &M = F->getModule();
  CanSILFunctionType GenCalleeType = F->getLoweredFunctionType();
  unsigned CalleeGenericParamsNum = 0;
  if (GenCalleeType->isPolymorphic())
    CalleeGenericParamsNum = GenCalleeType->getGenericSignature()
                                          ->getGenericParams().size();
  // Class F belongs to.
  CanType FSelfClass = GenCalleeType->getSelfParameter().getType();
  SILType ClassInstanceType = ClassInstance.getType();

  // Bail if any generic types parameters of the class instance type are
  // unbound.
  // We cannot devirtualize unbound generic calls yet.
  if (isClassWithUnboundGenericParameters(ClassInstanceType, AI->getModule()))
    return false;

  // *NOTE*:
  // Apply instruction substitutions are for the Member from a protocol or
  // class B, where this member was first defined, before it got overridden by
  // derived classes.
  //
  // The implementation F (the implementing method) which was found may have
  // a different set of generic parameters, e.g. because it is implemented by a
  // class D1 derived from B.
  //
  // ClassInstanceType may have a type different from both the type B
  // the Member belongs to and from the ClassInstanceType, e.g. if
  // ClassInstance is of a class D2, which is derived from D1, but does not
  // override the Member.
  //
  // As a result, substitutions provided by AI are for Member, whereas
  // substitutions in ClassInstanceType are for D2. And substitutions for D1
  // are not available directly in a general case. Therefore, they have to
  // be computed.
  //
  // What we know for sure:
  //   B is a superclass of D1
  //   D1 is a superclass of D2.
  // D1 can be the same as D2. D1 can be the same as B.
  //
  // So, substitutions from AI are for class B.
  // Substitutions for class D1 by means of bindSuperclass(), which starts
  // with a bound type ClassInstanceType and checks its superclasses until it
  // finds a bound superclass matching D1 and returns its substitutions.

  ArrayRef<Substitution> Substitutions;
  SILType FSelfSubstType;

  if (GenCalleeType->isPolymorphic()) {
    // Declaration of the class F belongs to.
    if (auto *FSelfTypeDecl = FSelfClass.getNominalOrBoundGenericNominal()) {
      // Get the unbound generic type F belongs to.
      CanType FSelfGenericType =
          FSelfTypeDecl->getDeclaredType()->getCanonicalType();

      assert((isa<BoundGenericType>(ClassInstanceType.getSwiftRValueType()) ||
              isa<NominalType>(ClassInstanceType.getSwiftRValueType())) &&
              "Self type should be either a bound generic type"
              "or a non-generic type");

      assert((isa<UnboundGenericType>(FSelfGenericType) ||
              isa<NominalType>(FSelfGenericType)) &&
              "Method implementation self type should be generic");

      // We know that ClassInstanceType is derived from FSelfGenericType.
      // We need to determine the proper substitutions for FGenericSILClass
      // based on the bound generic type of ClassInstanceType.
      FSelfSubstType = bindSuperclass(AI->getModule().getSwiftModule(),
                                      FSelfGenericType,
                                      ClassInstanceType,
                                      Substitutions);

      // Bail if it was not possible to determine the bound generic class.
      if (FSelfSubstType == SILType()) {
        return false;
      }

      if (!isa<BoundGenericType>(ClassInstanceType.getSwiftRValueType()) &&
          CalleeGenericParamsNum &&
          Substitutions.empty())
        // If ClassInstance is not a bound generic type, try to derive
        // substitutions from the apply instruction.
        Substitutions = AI->getSubstitutions();
    } else {
      // It is not a type or bound type.
      // It could be that GenCalleeType is generic, but its arguments cannot
      // be derived from the type of self. In this case, we can try to
      // approach it from another end and take the AI substitutions.
      Substitutions = AI->getSubstitutions();
    }
  }

  // If implementing method is not polymorphic, there is no need to
  // use any substitutions.
  if (CalleeGenericParamsNum == 0 && !Substitutions.empty())
    Substitutions = {};
  else if (CalleeGenericParamsNum != Substitutions.size())
    // Bail if the number of generic parameters of the callee does not match
    // the number of substitutions, because we don't know how to handle this.
    return false;

  CanSILFunctionType SubstCalleeType =
    GenCalleeType->substGenericArgs(M, M.getSwiftModule(), Substitutions);


  // If F's this pointer has a different type from CMI's operand and the
  // "this" pointer type is a super class of the CMI's operand, insert an
  // upcast.
  auto paramTypes =
    SubstCalleeType->getParameterSILTypesWithoutIndirectResult();

  // We should always have a this pointer. Assert on debug builds, return
  // nullptr on release builds.
  assert(!paramTypes.empty() &&
         "Must have a this pointer when calling a class method inst.");
  if (paramTypes.empty())
    return false;

  // Grab the self type from the function ref and the self type from the class
  // method inst.
  SILType FuncSelfTy = paramTypes[paramTypes.size() - 1];
  SILType OriginTy = ClassInstance.getType();
  SILBuilderWithScope<16> B(AI);

  // Then compare the two types and if they are unequal...
  if (FuncSelfTy != OriginTy) {
    if (ClassInstance.stripUpCasts().getType().getAs<MetatypeType>()) {
      auto &Module = AI->getModule();
      (void) Module;
      assert(FuncSelfTy.getMetatypeInstanceType(Module).
             isSuperclassOf(OriginTy.getMetatypeInstanceType(Module)) &&
             "Can not call a class method"
             " on a non-subclass of the class_methods class.");
    } else {
      assert(FuncSelfTy.isSuperclassOf(OriginTy) &&
             "Can not call a class method"
             " on a non-subclass of the class_methods class.");
    }
    // Otherwise, upcast origin to the appropriate type.
    ClassInstance = B.createUpcast(AI->getLoc(), ClassInstance, FuncSelfTy);
  }

  // Success! Perform the devirtualization.
  FunctionRefInst *FRI = B.createFunctionRef(AI->getLoc(), F);

  // Construct a new arg list. First process all non-self operands, ref, addr
  // casting them to the appropriate types for F so that we allow for covariant
  // indirect return types and contravariant arguments.
  llvm::SmallVector<SILValue, 8> NewArgs;
  auto Args = AI->getArguments();
  auto allParamTypes = SubstCalleeType->getParameterSILTypes();

  // For each old argument Op...
  for (unsigned i = 0, e = Args.size() - 1; i != e; ++i) {
    SILValue Op = Args[i];
    SILType OpTy = Op.getType();
    SILType FOpTy = allParamTypes[i];

    // If the type matches the type for the given parameter in F, just add it to
    // our arg list and continue.
    if (OpTy == FOpTy) {
      NewArgs.push_back(Op);
      continue;
    }

    // Otherwise we have either a covariant return type or a contravariant
    // argument type. Cast it appropriately.
    assert((OpTy.isAddress() || OpTy.isHeapObjectReferenceType()) &&
           "Only addresses and refs can have their types changed due to "
           "covariant return types or contravariant argument types.");

    // If OpTy is an address, perform an unchecked_addr_cast.
    if (OpTy.isAddress()) {
      NewArgs.push_back(B.createUncheckedAddrCast(AI->getLoc(), Op, FOpTy));
    } else {
      // Otherwise perform a ref cast.
      NewArgs.push_back(B.createUncheckedRefCast(AI->getLoc(), Op, FOpTy));
    }
  }

  // Add in self to the end.
  NewArgs.push_back(ClassInstance);

  // If we have a direct return type, make sure we use the subst callee return
  // type. If we have an indirect return type, AI's return type of the empty
  // tuple should be ok.
  SILType ReturnType = AI->getType();
  if (!SubstCalleeType->hasIndirectResult()) {
    ReturnType = SubstCalleeType->getSILResult();
  }

  SILType SubstCalleeSILType = SILType::getPrimitiveObjectType(SubstCalleeType);
  ApplyInst *NewAI =
      B.createApply(AI->getLoc(), FRI, SubstCalleeSILType, ReturnType,
                    Substitutions, NewArgs,
                    FRI->getReferencedFunction()->isTransparent());

  // If our return type differs from AI's return type, then we know that we have
  // a covariant return type. Cast it before we RAUW. This can not happen 
  if (ReturnType != AI->getType()) {

    // Check if the return type is an optional of the apply_inst type
    // or the other way around
    bool UnwrapOptionalResult = false;
    OptionalTypeKind OTK;

    auto OptionalReturnType = ReturnType.getSwiftRValueType()
                                        .getAnyOptionalObjectType();
    if (OptionalReturnType == AI->getType().getSwiftRValueType()) {
      ReturnType.getSwiftRValueType().getAnyOptionalObjectType(OTK);
      UnwrapOptionalResult = true;
    }

    assert((ReturnType.isAddress() ||
            ReturnType.isHeapObjectReferenceType() ||
            UnwrapOptionalResult) &&
           "Only addresses and refs can have their types changed due to "
           "covariant return types or contravariant argument types.");

    SILValue CastedAI = NewAI;
    if (UnwrapOptionalResult) {
      // The devirtualized method returns an optional result.
      // We need to extract the actual result from the optional.
      auto *SomeDecl = B.getASTContext().getOptionalSomeDecl(OTK);
      CastedAI = B.createUncheckedEnumData(AI->getLoc(), NewAI, SomeDecl);
    } else if (ReturnType.isAddress()) {
      CastedAI = B.createUncheckedAddrCast(AI->getLoc(), NewAI, AI->getType());
    } else {
      CastedAI = B.createUncheckedRefCast(AI->getLoc(), NewAI, AI->getType());
    }
    SILValue(AI).replaceAllUsesWith(CastedAI);
  } else {
    AI->replaceAllUsesWith(NewAI);
  }

  AI->eraseFromParent();

  DEBUG(llvm::dbgs() << "        SUCCESS: " << F->getName() << "\n");
  NumDevirtualized++;
  return true;
}

//===----------------------------------------------------------------------===//
//                        Witness Method Optimization
//===----------------------------------------------------------------------===//

static SILValue upcastArgument(SILValue Arg, SILType SuperTy, ApplyInst *AI) {
  SILType ArgTy = Arg.getType();
  if (dyn_cast<MetatypeInst>(Arg) || dyn_cast<ValueMetatypeInst>(Arg)) {
    // In case of metatypes passed as parameters, we need to upcast to a
    // metatype of a superclass.
    auto &Module = AI->getModule();
    (void) Module;
    assert (SuperTy.getSwiftRValueType()->is<AnyMetatypeType>() &&
           "super type should be a metatype");
    assert(SuperTy.getMetatypeInstanceType(Module)
           .isSuperclassOf(ArgTy.getMetatypeInstanceType(Module)) &&
           "Should only create upcasts for sub class devirtualization.");
  } else {
    assert(SuperTy.isSuperclassOf(ArgTy) &&
           "Should only create upcasts for sub class devirtualization.");
    (void)ArgTy;
  }

  Arg = SILBuilderWithScope<1>(AI).createUpcast(AI->getLoc(), Arg, SuperTy);
  return Arg;
}

/// Devirtualize the application of a witness_method. Replace this application
/// by invocation of a witness thunk which was found by findFuncInWitnessTable.
bool devirtualizeWitness(ApplyInst *AI,
                        SILFunction *F,
                        ArrayRef<Substitution> Subs) {
  // We know the witness thunk and the corresponding set of substitutions
  // required to invoke the protocol method at this point.
  auto &Module = AI->getModule();

  // Collect all the required substitutions.
  //
  // The complete set of substitutions may be different, e.g. because the found
  // witness thunk F may have been created by a  specialization pass and have
  // additional generic parameters.
  SmallVector<Substitution, 16> NewSubstList(Subs.begin(),
                                             Subs.end());

  // Add the non-self-derived substitutions from the original application.
  for (auto &origSub : AI->getSubstitutionsWithoutSelfSubstitution()) {
    if (!origSub.getArchetype()->isSelfDerived())
      NewSubstList.push_back(origSub);
  }

  // Figure out the exact bound type of the function to be called by
  // applying all substitutions.
  auto CalleeCanType = F->getLoweredFunctionType();
  auto SubstCalleeCanType = CalleeCanType->substGenericArgs(
      Module, Module.getSwiftModule(), NewSubstList);

  // Collect arguments from the apply instruction.
  auto Arguments = SmallVector<SILValue, 4>();

  auto ParamTypes = SubstCalleeCanType->getParameterSILTypes();
  // Type of the current parameter being processed
  auto ParamType = ParamTypes.begin();
  // Iterate over the non self arguments and add them to the
  // new argument list, upcasting when required.
  for (SILValue A : AI->getArguments()) {
    if (A.getType() != *ParamType) {
      // Upcast argument
      A = upcastArgument(A, *ParamType, AI);
    }
    Arguments.push_back(A);
    ++ParamType;
  }

  // Replace old apply instruction by a new apply instruction that invokes
  // the witness thunk.
  SILBuilderWithScope<2> Builder(AI);
  SILLocation Loc = AI->getLoc();
  FunctionRefInst *FRI = Builder.createFunctionRef(Loc, F);

  auto SubstCalleeSILType = SILType::getPrimitiveObjectType(SubstCalleeCanType);
  auto ResultSILType = SubstCalleeCanType->getSILResult();
  auto *SAI = Builder.createApply(Loc, FRI, SubstCalleeSILType,
                                  ResultSILType, NewSubstList, Arguments,
                                 FRI->getReferencedFunction()->isTransparent());
  AI->replaceAllUsesWith(SAI);
  AI->eraseFromParent();
  NumAMI++;
  return true;
}

/// Devirtualize apply instructions that call witness_method instructions:
///
///   %8 = witness_method $Optional<UInt16>, #LogicValue.boolValue!getter.1
///   %9 = apply %8<Self = CodeUnit?>(%6#1) : ...
///
static bool optimizeWitnessMethod(ApplyInst *AI, WitnessMethodInst *WMI) {
  // Use findFuncInWitnessTable to walk the inherited/specialized conformances
  // chain until it finds a NormalProtocolConformance. If such a conformance
  // is found, it would return the witness thunk as F, the corresponding
  // WitnessTable and a set of substitutions.
  //
  // Once we have this information, we essentially only need to replace the
  // current apply instruction by a new apply instruction that would invoke the
  // witness thunk and use proper substitutions. Creation of the new apply
  // instruction may also involve upcasting the arguments to the types required
  // by the witness thunk.
  //
  // Later on, the invocation of this witness thunk will be inlined by the
  // compiler, which would create an opportunity for a speculative
  // devirtualization of class_method invocations inside the body of this thunk.

  ProtocolConformance *C = WMI->getConformance();
  if (!C) {
    DEBUG(llvm::dbgs() << "        FAIL: Null conformance.\n");
    return false;
  }

  // Lookup the witness method in the witness tables.

  SILFunction *F;
  ArrayRef<Substitution> Subs;
  SILWitnessTable *WT;
  std::tie(F, WT, Subs) =
      AI->getModule().findFuncInWitnessTable(C, WMI->getMember());

  if (!F) {
    assert(!WT && "WitnessTable should always be null if F is.");
    DEBUG(llvm::dbgs() << "        FAIL: Did not find a matching witness "
                          "table or witness method.\n");
    return false;
  }
  assert(WT && "WitnessTable should never be null if F is not.");

  return devirtualizeWitness(AI, F, Subs);
}

//===----------------------------------------------------------------------===//
//                              Top Level Driver
//===----------------------------------------------------------------------===//

/// Return the final class decl based on access control information.
static ClassDecl *getClassFromAccessControl(ClassMethodInst *CMI) {
  const DeclContext *associatedDC = CMI->getModule().getAssociatedContext();
  if (!associatedDC) {
    // Without an associated context, we can't perform any access-based
    // optimizations.
    return nullptr;
  }

  SILDeclRef Member = CMI->getMember();
  FuncDecl *FD = Member.getFuncDecl();
  SILType ClassType = CMI->getOperand().stripUpCasts().getType();
  ClassDecl *CD = ClassType.getClassOrBoundGenericClass();

  // Only handle valid non-dynamic non-overridden members.
  if (!CD || !FD || FD->isInvalid() || FD->isDynamic() || FD->isOverridden())
    return nullptr;

  // Only handle members defined within the SILModule's associated context.
  if (!FD->isChildContextOf(associatedDC))
    return nullptr;

  if (!FD->hasAccessibility())
    return nullptr;

  // Only consider 'private' members, unless we are in whole-module compilation.
  switch (FD->getAccessibility()) {
  case Accessibility::Public:
    return nullptr;
  case Accessibility::Internal:
    if (!CMI->getModule().isWholeModule())
      return nullptr;
    break;
  case Accessibility::Private:
    break;
  }

  Type selfTypeInMember = FD->getDeclContext()->getDeclaredTypeInContext();
  return selfTypeInMember->getClassOrBoundGenericClass();
}

static bool optimizeApplyInst(ApplyInst *AI) {
  DEBUG(llvm::dbgs() << "    Trying to optimize ApplyInst : " << *AI);

  // Devirtualize apply instructions that call witness_method instructions:
  //
  //   %8 = witness_method $Optional<UInt16>, #LogicValue.boolValue!getter.1
  //   %9 = apply %8<Self = CodeUnit?>(%6#1) : ...
  //
  if (auto *AMI = dyn_cast<WitnessMethodInst>(AI->getCallee()))
    return optimizeWitnessMethod(AI, AMI);

  /// Optimize a class_method and alloc_ref pair into a direct function
  /// reference:
  ///
  /// \code
  /// %XX = alloc_ref $Foo
  /// %YY = class_method %XX : $Foo, #Foo.get!1 : $@cc(method) @thin ...
  /// \endcode
  ///
  ///  or
  ///
  /// %XX = metatype $...
  /// %YY = class_method %XX : ...
  ///
  ///  into
  ///
  /// %YY = function_ref @...
  if (auto *CMI = dyn_cast<ClassMethodInst>(AI->getCallee())) {
    // Check if the class member is known to be final.
    if (ClassDecl *C = getClassFromAccessControl(CMI))
      return devirtMethod(AI, CMI->getMember(), CMI->getOperand(), C);

    // Try to search for the point of construction.
    if (ClassDecl *C = getClassFromConstructor(CMI->getOperand()))
      return devirtMethod(AI, CMI->getMember(),
                          CMI->getOperand().stripUpCasts(), C);
  }

  return false;
}

namespace {

class SILDevirtualizationPass : public SILModuleTransform {
public:
  virtual ~SILDevirtualizationPass() {}

  /// The entry point to the transformation.
  void run() override {

    bool Changed = false;

    // Perform devirtualization locally and compute potential polymorphic
    // arguments for all existing functions.
    for (auto &F : *getModule()) {
      DEBUG(llvm::dbgs() << "*** Devirtualizing Function: "
              << demangle_wrappers::demangleSymbolAsString(F.getName())
              << "\n");
      for (auto &BB : F) {
        for (auto II = BB.begin(), IE = BB.end(); II != IE;) {
          ApplyInst *AI = dyn_cast<ApplyInst>(&*II);
          ++II;

          if (!AI)
            continue;

          Changed |= optimizeApplyInst(AI);
        }
      }
      DEBUG(llvm::dbgs() << "\n");
    }

    if (Changed) {
      PM->scheduleAnotherIteration();
      invalidateAnalysis(SILAnalysis::InvalidationKind::CallGraph);
    }
  }

  StringRef getName() override { return "Devirtualization"; }
};

} // end anonymous namespace

SILTransform *swift::createDevirtualizer() {
  return new SILDevirtualizationPass();
}

// A utility function for cloning the apply instruction.
static ApplyInst *CloneApply(ApplyInst *AI, SILBuilder &Builder) {
  // Clone the Apply.
  auto Args = AI->getArguments();
  SmallVector<SILValue, 8> Ret(Args.size());
  for (unsigned i = 0, e = Args.size(); i != e; ++i)
    Ret[i] = Args[i];

  auto NAI = Builder.createApply(AI->getLoc(), AI->getCallee(),
                                 AI->getSubstCalleeSILType(),
                                 AI->getType(),
                                 AI->getSubstitutions(),
                                 Ret, AI->isTransparent());
  NAI->setDebugScope(AI->getDebugScope());
  return NAI;
}

/// Insert monomorphic inline caches for a specific class type \p SubClassTy.
static ApplyInst* insertMonomorphicInlineCaches(ApplyInst *AI,
                                                SILType SubClassTy) {
  ClassMethodInst *CMI = cast<ClassMethodInst>(AI->getCallee());
  SILValue ClassInstance = CMI->getOperand();
  ClassDecl *CD = SubClassTy.getClassOrBoundGenericClass();

  // If method implementation for a SubClassTy is not known,
  // bail out early as we won't be able to devirtualize it.
  // This may happen e.g in case of a -primary-file
  // compilations, where information about methods implemented
  // in other files is unavailable.
  // Early exit guarantees that we do not create two
  // basic blocks which both perform virtual calls of a method.
  auto &Mod = AI->getModule();
  auto *Method = Mod.lookUpFunctionFromVTable(CD, CMI->getMember());
  if (!Method)
    return nullptr;

  bool IsValueMetatype = false;
  SILType RealSubClassTy = SubClassTy;

  if (isa<ValueMetatypeInst>(ClassInstance.stripUpCasts())) {
    // Convert this type to its metatype type.
    Lowering::TypeConverter TC(AI->getModule());
    RealSubClassTy = TC.getLoweredLoadableType(CD->getType());
    IsValueMetatype = true;
  }

  // Create a diamond shaped control flow and a checked_cast_branch
  // instruction that checks the exact type of the object.
  // This cast selects between two paths: one that calls the slow dynamic
  // dispatch and one that calls the specific method.
  SILBasicBlock::iterator It = AI;
  SILFunction *F = AI->getFunction();
  SILBasicBlock *Entry = AI->getParent();

  // Iden is the basic block containing the direct call.
  SILBasicBlock *Iden = F->createBasicBlock();
  // Virt is the block containing the slow virtual call.
  SILBasicBlock *Virt = F->createBasicBlock();
  Iden->createBBArg(RealSubClassTy);

  SILBasicBlock *Continue = Entry->splitBasicBlock(It);

  SILBuilderWithScope<> Builder(Entry, AI->getDebugScope());
  // Create the checked_cast_branch instruction that checks at runtime if the
  // class instance is identical to the SILType.
  if (!IsValueMetatype)
    assert(SubClassTy.getClassOrBoundGenericClass() &&
           "Dest type must be a class type");

  It = Builder.createCheckedCastBranch(AI->getLoc(), /*exact*/ true,
                                       ClassInstance, RealSubClassTy, Iden,
                                       Virt);

  SILBuilder VirtBuilder(Virt);
  SILBuilder IdenBuilder(Iden);
  // This is the class reference downcasted into subclass SubClassTy.
  SILValue DownCastedClassInstance = Iden->getBBArg(0);

  // Try sinking the retain of the class instance into the diamond. This may
  // allow additional ARC optimizations on the fast path.
  if (It != Entry->begin()) {
    StrongRetainInst *SRI = dyn_cast<StrongRetainInst>(--It);
    // Try to skip another instruction, in case the class_method came first.
    if (!SRI && It != Entry->begin())
      SRI = dyn_cast<StrongRetainInst>(--It);
    if (SRI && SRI->getOperand() == ClassInstance) {
      VirtBuilder.createStrongRetain(SRI->getLoc(), ClassInstance)
        ->setDebugScope(SRI->getDebugScope());
      IdenBuilder.createStrongRetain(SRI->getLoc(), DownCastedClassInstance)
        ->setDebugScope(SRI->getDebugScope());
      SRI->eraseFromParent();
    }
  }

  // Copy the two apply instructions into the two blocks.
  ApplyInst *IdenAI = CloneApply(AI, IdenBuilder);
  ApplyInst *VirtAI = CloneApply(AI, VirtBuilder);

  // Create a PHInode for returning the return value from both apply
  // instructions.
  SILArgument *Arg = Continue->createBBArg(AI->getType());
  IdenBuilder.createBranch(AI->getLoc(), Continue, ArrayRef<SILValue>(IdenAI))
    ->setDebugScope(AI->getDebugScope());
  VirtBuilder.createBranch(AI->getLoc(), Continue, ArrayRef<SILValue>(VirtAI))
    ->setDebugScope(AI->getDebugScope());

  // Remove the old Apply instruction.
  AI->replaceAllUsesWith(Arg);
  AI->eraseFromParent();

  // Update the stats.
  NumInlineCaches++;

  // Devirtualize the apply instruction on the identical path.
  devirtMethod(IdenAI, CMI->getMember(), DownCastedClassInstance, CD);

  // Sink class_method instructions down to their single user.
  if (CMI->hasOneUse())
    CMI->moveBefore(CMI->use_begin()->getUser());

  return VirtAI;
}

/// \brief Returns true, if a method implementation to be called by the
/// default case handler of a speculative devirtualization is statically
/// known. This happens if it can be proven that generated
/// checked_cast_br instructions cover all other possible cases.
///
/// \p CHA class hierarchy analysis to be used
/// \p AI  invocation instruction
/// \p CD  static class of the instance whose method is being invoked
/// \p Subs set of direct subclasses of this class
static bool isDefaultCaseKnown(ClassHierarchyAnalysis *CHA,
                               ApplyInst *AI,
                               ClassDecl *CD,
                               ClassHierarchyAnalysis::ClassList &Subs) {
  ClassMethodInst *CMI = cast<ClassMethodInst>(AI->getCallee());
  auto *Method = CMI->getMember().getFuncDecl();
  const DeclContext *DC = AI->getModule().getAssociatedContext();

  // Without an associated context we cannot perform any
  // access-based optimizations.
  if (!DC)
    return false;

  // Only handle classes defined within the SILModule's associated context.
  if (!CD->isChildContextOf(DC))
    return false;

  if (!CD->hasAccessibility())
    return false;

  // Only consider 'private' members, unless we are in whole-module compilation.
  switch (CD->getAccessibility()) {
  case Accessibility::Public:
    return false;
  case Accessibility::Internal:
    if (!AI->getModule().isWholeModule())
      return false;
    break;
  case Accessibility::Private:
    break;
  }

  // This is a private or a module internal class.
  //
  // We can analyze the class hierarchy rooted at it and
  // eventually devirtualize a method call more efficiently.

  // First, analyze all direct subclasses.
  // We know that a dedicated checked_cast_br check is
  // generated for each direct subclass by insertInlineCaches.
  for (auto S : Subs) {
    // Check if the subclass overrides a method
    auto *FD = S->findOverridingDecl(Method);
    if (!FD)
      continue;
    if (CHA->hasKnownDirectSubclasses(S)) {
      // This subclass has its own subclasses and
      // they will use this implementation or provide
      // their own. In either case it is not covered by
      // checked_cast_br instructions generated by
      // insertInlineCaches. Therefore it increases
      // the number of remaining cases to be handled
      // by the default case handler.
      return false;
    }
  }

  // Then, analyze indirect subclasses.

  // Set of indirect subclasses for the class.
  auto &IndirectSubs = CHA->getIndirectSubClasses(CD);

  // Check if any indirect subclasses use an implementation
  // of the method different from the implementation in
  // the current class. If this is the case, then such
  // an indirect subclass would need a dedicated
  // checked_cast_br check to be devirtualized. But this is
  // not done by insertInlineCaches yet and therefore
  // such a subclass should be handled by the "default"
  // case handler, which essentially means that "default"
  // case cannot be devirtualized since it covers more
  // then one alternative.
  for (auto S : IndirectSubs) {
    auto *ImplFD = S->findImplementingMethod(Method);
    if (ImplFD != Method) {
      // Different implementation is used by a subclass.
      // Therefore, the default case is not known.
      return false;
    }
  }

  return true;
}

/// \brief Try to insert inline cahces for the call \p AI. This function
/// returns true if a change was made.
static bool insertInlineCaches(ApplyInst *AI, ClassHierarchyAnalysis *CHA) {
  ClassMethodInst *CMI = cast<ClassMethodInst>(AI->getCallee());
  assert(CMI && "Invalid class method instruction");

  SILValue ClassInstance = CMI->getOperand();
  // The static type used by the class_method instruction
  // is the class which a given method belongs to.
  // Is either the class of the instance itself or one of its superclasses.
  // Therefore, strip all upcasts to get the real static type
  // of the instance.
  // Specifically, only the upcast to the static class which method belongs to
  // should be stripped.
  SILType InstanceType = ClassInstance.stripUpCasts().getType();
  ClassDecl *CD = InstanceType.getClassOrBoundGenericClass();

  if (auto *VMTI = dyn_cast<ValueMetatypeInst>(ClassInstance.stripUpCasts())) {
    CanType InstTy = VMTI->getType().castTo<MetatypeType>().getInstanceType();
    CD = InstTy.getClassOrBoundGenericClass();
  }

  // Check if it is legal to insert inline caches.
  if (!CD || CMI->isVolatile())
    return false;

  if (ClassInstance.getType() != InstanceType) {
    // The implementation of a method to be invoked may actually
    // be defined by one of the superclasses.
    if (!ClassInstance.getType().isSuperclassOf(InstanceType))
      return false;
    // ClassInstance and InstanceType should match for devirtMethod to work.
    ClassInstance = ClassInstance.stripUpCasts();
  }

  // Bail if any generic types parameters of the class instance type are
  // unbound.
  // We cannot devirtualize unbound generic calls yet.
  if (isClassWithUnboundGenericParameters(InstanceType, AI->getModule()))
    return false;

  if (!CHA->hasKnownDirectSubclasses(CD)) {
    DEBUG(llvm::dbgs() << "Inserting monomorphic inline caches for class " <<
          CD->getName() << "\n");
    return insertMonomorphicInlineCaches(AI, InstanceType);
  }

  // Collect the direct subclasses for the class.
  auto &Subs = CHA->getDirectSubClasses(CD);

  if (Subs.size() > MaxNumPolymorphicInlineCaches) {
    DEBUG(llvm::dbgs() << "Class " << CD->getName() << " has too many (" <<
          Subs.size() << ") subclasses. Not inserting inline caches.\n");
    return false;
  }

  DEBUG(llvm::dbgs() << "Class " << CD->getName() << " is a superclass. "
        "Inserting polymorphic inline caches.\n");

  // Perform a speculative devirtualization of a method invocation.
  // It replaces an indirect class_method-based call by a code to perform
  // a direct call of the method implementation based on the dynamic class
  // of the instance.
  //
  // The code is generated according to the following principles:
  //
  // - For each direct subclass, a dedicated checked_cast_br instruction
  // is generated to check if a dynamic class of the instance is exactly
  // this subclass.
  //
  // - If this check succeeds, then it jumps to the code which performs a
  // direct call of a method implementation specific to this subclass.
  //
  // - If this check fails, then a different subclass is checked by means of
  // checked_cast_br in a similar way.
  //
  // - Finally, if the instance does not exactly match any of the direct
  // subclasses, the "default" case code is generated, which should handle
  // all remaining alternatives, i.e. it should be able to dispatch to any
  // possible remaining method implementations. Typically this is achieved by
  // using a class_method instruction, which performs an indirect invocation.
  // But if it can be proven that only one specific implementation of
  // a method will be always invoked by this code, then a class_method-based
  // call can be devirtualized and replaced by a more efficient direct
  // invocation of this specific method implementation.
  //
  // Remark: With the current implementation of a speculative devirtualization,
  // if devirtualization of the "default" case is possible, then it would
  // by construction directly invoke the implementation of the method
  // corresponding to the static type of the instance. This may change
  // in the future, if we start using PGO for ordering of checked_cast_br
  // checks.

  // TODO: The ordering of checks may benefit from using a PGO, because
  // the most probable alternatives could be checked first.

  // Number of subclasses which cannot be handled by checked_cast_br checks.
  int NotHandledSubsNum = 0;
  // True if any instructions were changed or generated.
  bool Changed = false;

  for (auto S : Subs) {
    DEBUG(llvm::dbgs() << "Inserting a cache for class " << CD->getName() <<
          " and subclass " << S->getName() << "\n");

    CanType CanClassType = S->getDeclaredType()->getCanonicalType();
    SILType InstanceType = SILType::getPrimitiveObjectType(CanClassType);
    if (!InstanceType.getClassOrBoundGenericClass()) {
      // This subclass cannot be handled. This happens e.g. if it is
      // a generic class.
      NotHandledSubsNum++;
      continue;
    }

    AI = insertMonomorphicInlineCaches(AI, InstanceType);
    assert(AI && "Unable to insert inline caches!");
    Changed = true;
  }

  // Check if there is only a single statically known implementation
  // of the method which can be called by the default case handler.
  if (NotHandledSubsNum || !isDefaultCaseKnown(CHA, AI, CD, Subs)) {
    // Devirtualization of remaining cases is not possible,
    // because more than one implementation of the method
    // needs to be handled here. Thus, an indirect call through
    // the class_method cannot be eliminated completely.
    //
    // But we can still try to devirtualize the static class of instance
    // if it is possible.
    return insertMonomorphicInlineCaches(AI, InstanceType);
  }

  // At this point it is known that there is only one remaining method
  // implementation which is not covered by checked_cast_br checks yet.
  // So, it is safe to replace a class_method invocation by
  // a direct call of this remaining implementation.
  devirtMethod(AI, CMI->getMember(), ClassInstance, CD);

  return true;
}

namespace {
  /// Generate inline caches of virtual calls by speculating that the requested
  /// class is at the bottom of the class hierarchy.
  class SILInlineCaches : public SILFunctionTransform {
  public:
    virtual ~SILInlineCaches() {}

    void run() override {
      ClassHierarchyAnalysis *CHA = PM->getAnalysis<ClassHierarchyAnalysis>();

      bool Changed = false;

      // Collect virtual calls that may be specialized.
      SmallVector<ApplyInst *, 16> ToSpecialize;
      for (auto &BB : *getFunction()) {
        for (auto II = BB.begin(), IE = BB.end(); II != IE; ++II) {
          ApplyInst *AI = dyn_cast<ApplyInst>(&*II);
          if (AI && isa<ClassMethodInst>(AI->getCallee()))
            ToSpecialize.push_back(AI);
        }
      }

      // Create the inline caches.
      for (auto AI : ToSpecialize)
        Changed |= insertInlineCaches(AI, CHA);

      if (Changed) {
        invalidateAnalysis(SILAnalysis::InvalidationKind::CallGraph);
      }
    }

    StringRef getName() override { return "Inline Caches"; }
  };

} // end anonymous namespace

SILTransform *swift::createInlineCaches() {
  return new SILInlineCaches();
}