GenThunk.cpp

//===--- GenThunk.cpp - IR Generation for Method Dispatch Thunks ----------===//
//
// 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 file implements IR generation for class and protocol method dispatch
//  thunks, which are used in resilient builds to hide vtable and witness table
//  offsets from clients.
//
//===----------------------------------------------------------------------===//

#include "Callee.h"
#include "CallEmission.h"
#include "ClassMetadataVisitor.h"
#include "ConstantBuilder.h"
#include "Explosion.h"
#include "GenCall.h"
#include "GenClass.h"
#include "GenDecl.h"
#include "GenHeap.h"
#include "GenOpaque.h"
#include "GenPointerAuth.h"
#include "GenProto.h"
#include "GenType.h"
#include "IRGenFunction.h"
#include "IRGenModule.h"
#include "LoadableTypeInfo.h"
#include "MetadataLayout.h"
#include "NativeConventionSchema.h"
#include "ProtocolInfo.h"
#include "Signature.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/PrettyStackTrace.h"
#include "swift/IRGen/Linking.h"
#include "swift/SIL/SILDeclRef.h"
#include "llvm/IR/Function.h"

using namespace swift;
using namespace irgen;

/// Find the entry point for a method dispatch thunk.
llvm::Function *
IRGenModule::getAddrOfDispatchThunk(SILDeclRef declRef,
                                    ForDefinition_t forDefinition) {
  LinkEntity entity = LinkEntity::forDispatchThunk(declRef);

  llvm::Function *&entry = GlobalFuncs[entity];
  if (entry) {
    if (forDefinition) updateLinkageForDefinition(*this, entry, entity);
    return entry;
  }

  auto fnType = getSILModule().Types.getConstantFunctionType(
      getMaximalTypeExpansionContext(), declRef);
  Signature signature = getSignature(fnType);
  LinkInfo link = LinkInfo::get(*this, entity, forDefinition);

  entry = createFunction(*this, link, signature);
  return entry;
}

namespace {

class IRGenThunk {
  IRGenFunction &IGF;
  SILDeclRef declRef;
  TypeExpansionContext expansionContext;
  CanSILFunctionType origTy;
  CanSILFunctionType substTy;
  SubstitutionMap subMap;
  bool isAsync;
  bool isCoroutine;
  bool isWitnessMethod;

  llvm::Optional<AsyncContextLayout> asyncLayout;

  // Initialized by prepareArguments()
  llvm::Value *indirectReturnSlot = nullptr;
  llvm::Value *selfValue = nullptr;
  llvm::Value *errorResult = nullptr;
  WitnessMetadata witnessMetadata;
  Explosion params;

  void prepareArguments();
  Callee lookupMethod();

public:
  IRGenThunk(IRGenFunction &IGF, SILDeclRef declRef);
  void emit();
};

} // end namespace

IRGenThunk::IRGenThunk(IRGenFunction &IGF, SILDeclRef declRef)
  : IGF(IGF), declRef(declRef),
    expansionContext(IGF.IGM.getMaximalTypeExpansionContext()) {
  auto &Types = IGF.IGM.getSILModule().Types;
  origTy = Types.getConstantFunctionType(expansionContext, declRef);

  if (auto *genericEnv = Types.getConstantGenericEnvironment(declRef))
    subMap = genericEnv->getForwardingSubstitutionMap();

  substTy = origTy->substGenericArgs(
      IGF.IGM.getSILModule(), subMap, expansionContext);

  isAsync = origTy->isAsync();
  isCoroutine = origTy->isCoroutine();

  auto *decl = cast<AbstractFunctionDecl>(declRef.getDecl());
  isWitnessMethod = isa<ProtocolDecl>(decl->getDeclContext());

  if (isAsync) {
    asyncLayout.emplace(irgen::getAsyncContextLayout(
        IGF.IGM, origTy, substTy, subMap));
  }
}

// FIXME: This duplicates the structure of CallEmission. It should be
// possible to refactor some code and simplify this drastically, since
// conceptually all we're doing is forwarding the arguments verbatim
// using the sync or async calling convention.
void IRGenThunk::prepareArguments() {
  Explosion original = IGF.collectParameters();

  if (isWitnessMethod) {
    witnessMetadata.SelfWitnessTable = original.takeLast();
    witnessMetadata.SelfMetadata = original.takeLast();
  }

  SILFunctionConventions conv(origTy, IGF.getSILModule());

  if (origTy->hasErrorResult()) {
    if (isAsync) {
      // nothing to do.
    } else {
      errorResult = original.takeLast();
      auto errorType =
          conv.getSILErrorType(IGF.IGM.getMaximalTypeExpansionContext());
      auto &errorTI = cast<FixedTypeInfo>(IGF.getTypeInfo(errorType));

      IGF.setCallerErrorResultSlot(Address(errorResult,
                                           errorTI.getStorageType(),
                                           IGF.IGM.getPointerAlignment()));
    }
  }

  if (isCoroutine) {
    original.transferInto(params, 1);
  }

  selfValue = original.takeLast();

  // Prepare indirect results, if any.
  SILType directResultType = conv.getSILResultType(expansionContext);
  auto &directResultTL = IGF.IGM.getTypeInfo(directResultType);
  auto &schema = directResultTL.nativeReturnValueSchema(IGF.IGM);
  if (schema.requiresIndirect()) {
    indirectReturnSlot = original.claimNext();
  }

  original.transferInto(params, conv.getNumIndirectSILResults());

  // Chop off the async context parameters.
  if (isAsync) {
    unsigned numAsyncContextParams =
        (unsigned)AsyncFunctionArgumentIndex::Context + 1;
    (void)original.claim(numAsyncContextParams);
  }

  // Prepare each parameter.
  for (auto param : origTy->getParameters().drop_back()) {
    auto paramType = conv.getSILType(param, expansionContext);

    // If the SIL parameter isn't passed indirectly, we need to map it
    // to an explosion.
    if (paramType.isObject()) {
      auto &paramTI = IGF.getTypeInfo(paramType);
      auto &loadableParamTI = cast<LoadableTypeInfo>(paramTI);
      auto &nativeSchema = loadableParamTI.nativeParameterValueSchema(IGF.IGM);
      unsigned size = nativeSchema.size();

      Explosion nativeParam;
      if (nativeSchema.requiresIndirect()) {
        // If the explosion must be passed indirectly, load the value from the
        // indirect address.
        Address paramAddr =
            loadableParamTI.getAddressForPointer(original.claimNext());
        loadableParamTI.loadAsTake(IGF, paramAddr, nativeParam);
      } else {
        if (!nativeSchema.empty()) {
          // Otherwise, we map from the native convention to the type's
          // explosion schema.
          Explosion paramExplosion;
          original.transferInto(paramExplosion, size);
          nativeParam = nativeSchema.mapFromNative(IGF.IGM, IGF, paramExplosion,
                                                   paramType);
        }
      }

      nativeParam.transferInto(params, nativeParam.size());
    } else {
      params.add(original.claimNext());
    }
  }

  // Anything else, just pass along.  This will include things like
  // generic arguments.
  params.add(original.claimAll());
}

Callee IRGenThunk::lookupMethod() {
  CalleeInfo info(origTy, substTy, subMap);

  // Protocol case.
  if (isWitnessMethod) {
    // Find the witness we're interested in.
    auto *wtable = witnessMetadata.SelfWitnessTable;
    auto witness = emitWitnessMethodValue(IGF, wtable, declRef);

    return Callee(std::move(info), witness, selfValue);
  }

  // Class case.

  // Load the metadata, or use the 'self' value if we have a static method.
  auto selfTy = origTy->getSelfParameter().getSILStorageType(
      IGF.IGM.getSILModule(), origTy, expansionContext);

  // If 'self' is an instance, load the class metadata.
  llvm::Value *metadata;
  if (selfTy.is<MetatypeType>()) {
    metadata = selfValue;
  } else {
    auto &Types = IGF.IGM.getSILModule().Types;
    auto *env = Types.getConstantGenericEnvironment(declRef);
    auto sig = env ? env->getGenericSignature() : GenericSignature();
    metadata = emitHeapMetadataRefForHeapObject(IGF, selfValue, selfTy,
                                                sig, /*suppress cast*/ true);
  }

  // Find the method we're interested in.
  auto method = emitVirtualMethodValue(IGF, metadata, declRef, origTy);

  return Callee(std::move(info), method, selfValue);
}

void IRGenThunk::emit() {
  PrettyStackTraceDecl stackTraceRAII("emitting dispatch thunk for",
                                      declRef.getDecl());

  GenericContextScope scope(IGF.IGM, origTy->getInvocationGenericSignature());

  if (isAsync) {
    auto asyncContextIdx = Signature::forAsyncEntry(
                               IGF.IGM, origTy,
                               FunctionPointerKind::defaultAsync())
                               .getAsyncContextIndex();

    auto entity = LinkEntity::forDispatchThunk(declRef);
    emitAsyncFunctionEntry(IGF, *asyncLayout, entity, asyncContextIdx);
    emitAsyncFunctionPointer(IGF.IGM, IGF.CurFn, entity,
                             asyncLayout->getSize());
  }

  prepareArguments();

  auto callee = lookupMethod();

  std::unique_ptr<CallEmission> emission =
      getCallEmission(IGF, callee.getSwiftContext(), std::move(callee));

  emission->begin();

  emission->setArgs(params, /*isOutlined=*/false, &witnessMetadata);

  if (isCoroutine) {
    assert(!isAsync);

    auto *result = emission->emitCoroutineAsOrdinaryFunction();
    emission->end();

    IGF.Builder.CreateRet(result);
    return;
  }

  Explosion result;

  // Determine if the result is returned indirectly.
  SILFunctionConventions conv(origTy, IGF.getSILModule());
  SILType directResultType = conv.getSILResultType(expansionContext);
  auto &directResultTL = IGF.IGM.getTypeInfo(directResultType);
  auto &schema = directResultTL.nativeReturnValueSchema(IGF.IGM);
  if (schema.requiresIndirect()) {
    Address indirectReturnAddr(indirectReturnSlot,
                               IGF.IGM.getStorageType(directResultType),
                               directResultTL.getBestKnownAlignment());
    emission->emitToMemory(indirectReturnAddr,
                           cast<LoadableTypeInfo>(directResultTL), false);
  } else {
    emission->emitToExplosion(result, /*isOutlined=*/false);
  }

  llvm::Value *errorValue = nullptr;

  if (isAsync && origTy->hasErrorResult()) {
    SILType errorType = conv.getSILErrorType(expansionContext);
    Address calleeErrorSlot = emission->getCalleeErrorSlot(
        errorType, /*isCalleeAsync=*/origTy->isAsync());
    errorValue = IGF.Builder.CreateLoad(calleeErrorSlot);
  }

  emission->end();

  if (isAsync) {
    Explosion error;
    if (errorValue)
      error.add(errorValue);
    emitAsyncReturn(IGF, *asyncLayout, directResultType, origTy, result, error);
    return;
  }

  // Return the result.
  if (result.empty()) {
    IGF.Builder.CreateRetVoid();
    return;
  }

  auto resultTy = conv.getSILResultType(expansionContext);
  resultTy = resultTy.subst(IGF.getSILModule(), subMap);

  IGF.emitScalarReturn(resultTy, resultTy, result,
                       /*swiftCCReturn=*/false,
                       /*isOutlined=*/false);
}

void IRGenModule::emitDispatchThunk(SILDeclRef declRef) {
  auto *f = getAddrOfDispatchThunk(declRef, ForDefinition);
  if (!f->isDeclaration()) {
    return;
  }

  IRGenFunction IGF(*this, f);
  IRGenThunk(IGF, declRef).emit();
}

llvm::Constant *
IRGenModule::getAddrOfAsyncFunctionPointer(LinkEntity entity) {
  llvm::Constant *Pointer =
      getAddrOfLLVMVariable(LinkEntity::forAsyncFunctionPointer(entity),
                            NotForDefinition, DebugTypeInfo());
  if (!getOptions().IndirectAsyncFunctionPointer)
    return Pointer;

  // When the symbol does not have DLL Import storage, we must directly address
  // it. Otherwise, we will form an invalid reference.
  if (!Pointer->isDLLImportDependent())
    return Pointer;

  llvm::Constant *PointerPointer =
      getOrCreateGOTEquivalent(Pointer,
                               LinkEntity::forAsyncFunctionPointer(entity));
  llvm::Constant *PointerPointerConstant =
      llvm::ConstantExpr::getPtrToInt(PointerPointer, IntPtrTy);
  llvm::Constant *Marker =
      llvm::Constant::getIntegerValue(IntPtrTy, APInt(IntPtrTy->getBitWidth(),
                                                      1));
  // TODO(compnerd) ensure that the pointer alignment guarantees that bit-0 is
  // cleared. We cannot use an `getOr` here as it does not form a relocatable
  // expression.
  llvm::Constant *Address =
      llvm::ConstantExpr::getAdd(PointerPointerConstant, Marker);

  IndirectAsyncFunctionPointers[entity] = Address;
  return llvm::ConstantExpr::getIntToPtr(Address,
                                         AsyncFunctionPointerTy->getPointerTo());
}

llvm::Constant *
IRGenModule::getAddrOfAsyncFunctionPointer(SILFunction *function) {
  (void)getAddrOfSILFunction(function, NotForDefinition);
  return getAddrOfAsyncFunctionPointer(
      LinkEntity::forSILFunction(function));
}

llvm::Constant *IRGenModule::defineAsyncFunctionPointer(LinkEntity entity,
                                                        ConstantInit init) {
  auto asyncEntity = LinkEntity::forAsyncFunctionPointer(entity);
  auto *var = cast<llvm::GlobalVariable>(
      getAddrOfLLVMVariable(asyncEntity, init, DebugTypeInfo()));
  return var;
}

SILFunction *
IRGenModule::getSILFunctionForAsyncFunctionPointer(llvm::Constant *afp) {
  for (auto &entry : GlobalVars) {
    if (entry.getSecond() == afp) {
      auto entity = entry.getFirst();
      return entity.getSILFunction();
    }
  }
  for (auto &entry : IndirectAsyncFunctionPointers) {
    if (entry.getSecond() == afp) {
      auto entity = entry.getFirst();
      assert(getOptions().IndirectAsyncFunctionPointer &&
             "indirect async function found for non-indirect async function"
             " target?");
      return entity.getSILFunction();
    }
  }
  return nullptr;
}

llvm::GlobalValue *IRGenModule::defineMethodDescriptor(
    SILDeclRef declRef, NominalTypeDecl *nominalDecl,
    llvm::Constant *definition, llvm::Type *typeOfDefinitionValue) {
  auto entity = LinkEntity::forMethodDescriptor(declRef);
  return defineAlias(entity, definition, typeOfDefinitionValue);
}

/// Get or create a method descriptor variable.
llvm::Constant *
IRGenModule::getAddrOfMethodDescriptor(SILDeclRef declRef,
                                       ForDefinition_t forDefinition) {
  assert(forDefinition == NotForDefinition);
  assert(declRef.getOverriddenWitnessTableEntry() == declRef &&
         "Overriding protocol requirements do not have method descriptors");
  LinkEntity entity = LinkEntity::forMethodDescriptor(declRef);
  return getAddrOfLLVMVariable(entity, forDefinition, DebugTypeInfo());
}

/// Fetch the method lookup function for a resilient class.
llvm::Function *
IRGenModule::getAddrOfMethodLookupFunction(ClassDecl *classDecl,
                                           ForDefinition_t forDefinition) {
  IRGen.noteUseOfTypeMetadata(classDecl);

  LinkEntity entity = LinkEntity::forMethodLookupFunction(classDecl);
  llvm::Function *&entry = GlobalFuncs[entity];
  if (entry) {
    if (forDefinition) updateLinkageForDefinition(*this, entry, entity);
    return entry;
  }

  llvm::Type *params[] = {
    TypeMetadataPtrTy,
    MethodDescriptorStructTy->getPointerTo()
  };
  auto fnType = llvm::FunctionType::get(Int8PtrTy, params, false);
  Signature signature(fnType, llvm::AttributeList(), SwiftCC);
  LinkInfo link = LinkInfo::get(*this, entity, forDefinition);
  entry = createFunction(*this, link, signature);
  return entry;
}

void IRGenModule::emitMethodLookupFunction(ClassDecl *classDecl) {
  auto *f = getAddrOfMethodLookupFunction(classDecl, ForDefinition);
  if (!f->isDeclaration()) {
    assert(IRGen.isLazilyReemittingNominalTypeDescriptor(classDecl));
    return;
  }

  IRGenFunction IGF(*this, f);

  auto params = IGF.collectParameters();
  auto *metadata = params.claimNext();
  auto *method = params.claimNext();

  auto *description = getAddrOfTypeContextDescriptor(classDecl,
                                                     RequireMetadata);

  // Check for lookups of nonoverridden methods first.
  class LookUpNonoverriddenMethods
    : public ClassMetadataScanner<LookUpNonoverriddenMethods> {
  
    IRGenFunction &IGF;
    llvm::Value *methodArg;
      
  public:
    LookUpNonoverriddenMethods(IRGenFunction &IGF,
                               ClassDecl *classDecl,
                               llvm::Value *methodArg)
      : ClassMetadataScanner(IGF.IGM, classDecl), IGF(IGF),
        methodArg(methodArg) {}
      
    void noteNonoverriddenMethod(SILDeclRef method) {
      // The method lookup function would be used only for `super.` calls
      // from other modules, so we only need to look at public-visibility
      // methods here.
      if (!hasPublicVisibility(method.getLinkage(NotForDefinition))) {
        return;
      }
      
      auto methodDesc = IGM.getAddrOfMethodDescriptor(method, NotForDefinition);
      
      auto isMethod = IGF.Builder.CreateICmpEQ(methodArg, methodDesc);
      
      auto falseBB = IGF.createBasicBlock("");
      auto trueBB = IGF.createBasicBlock("");

      IGF.Builder.CreateCondBr(isMethod, trueBB, falseBB);
      
      IGF.Builder.emitBlock(trueBB);
      // Since this method is nonoverridden, we can produce a static result.
      auto entry = VTable->getEntry(IGM.getSILModule(), method);
      llvm::Value *impl = IGM.getAddrOfSILFunction(entry->getImplementation(),
                                                   NotForDefinition);
      // Sign using the discriminator we would include in the method
      // descriptor.
      if (auto &schema =
              entry->getImplementation()->getLoweredFunctionType()->isAsync()
                  ? IGM.getOptions().PointerAuth.AsyncSwiftClassMethods
                  : IGM.getOptions().PointerAuth.SwiftClassMethods) {
        auto discriminator =
          PointerAuthInfo::getOtherDiscriminator(IGM, schema, method);
        
        impl = emitPointerAuthSign(IGF, impl,
                            PointerAuthInfo(schema.getKey(), discriminator));
      }
      impl = IGF.Builder.CreateBitCast(impl, IGM.Int8PtrTy);
      IGF.Builder.CreateRet(impl);
      
      IGF.Builder.emitBlock(falseBB);
      // Continue emission on the false branch.
    }
      
    void noteResilientSuperclass() {}
    void noteStartOfImmediateMembers(ClassDecl *clazz) {}
  };
  
  LookUpNonoverriddenMethods(IGF, classDecl, method).layout();
  
  // Use the runtime to look up vtable entries.
  auto *result = IGF.Builder.CreateCall(getLookUpClassMethodFunctionPointer(),
                                        {metadata, method, description});
  IGF.Builder.CreateRet(result);
}