Linker.cpp

//===--- Linker.cpp -------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// \file
///
/// The SIL linker walks the call graph beginning at a starting function,
/// deserializing functions, vtables and witness tables.
///
/// The behavior of the linker is controlled by a LinkMode value. The LinkMode
/// has two possible values:
///
/// - LinkNormal: The linker deserializes bodies for declarations that must be
///   emitted into the client because they do not have definitions available
///   externally. This includes:
///
///   - witness tables for imported conformances
///
///   - functions with shared linkage
///
/// - LinkAll: All reachable functions (including public functions) are
///   deserialized, including public functions.
///
/// The primary entry point into the linker is the SILModule::linkFunction()
/// function, which recursively walks the call graph starting from the given
/// function.
///
/// In the mandatory pipeline (-Onone), the linker is invoked from the mandatory
/// SIL linker pass, which pulls in just enough to allow us to emit code, using
/// LinkNormal mode.
///
/// In the performance pipeline, after guaranteed optimizations but before
/// performance optimizations, the 'performance SILLinker' pass links
/// transitively all reachable functions, to uncover optimization opportunities
/// that might be missed from deserializing late. The performance pipeline uses
/// LinkAll mode.
///
/// *NOTE*: In LinkAll mode, we deserialize all vtables and witness tables,
/// even those with public linkage. This is not strictly necessary, since the
/// devirtualizer deserializes vtables and witness tables as needed. However,
/// doing so early creates more opportunities for optimization.
///
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sil-linker"
#include "Linker.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/FoldingSet.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/Debug.h"
#include "swift/AST/DiagnosticsSIL.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/SubstitutionMap.h"
#include "swift/Basic/Assertions.h"
#include "swift/ClangImporter/ClangModule.h"
#include "swift/SIL/FormalLinkage.h"
#include "swift/Serialization/SerializedSILLoader.h"
#include <functional>

using namespace swift;
using namespace Lowering;

STATISTIC(NumFuncLinked, "Number of SIL functions linked");

//===----------------------------------------------------------------------===//
//                               Linker Helpers
//===----------------------------------------------------------------------===//

void SILLinkerVisitor::deserializeAndPushToWorklist(SILFunction *F) {
  ASSERT(F->isExternalDeclaration());

  LLVM_DEBUG(llvm::dbgs() << "Imported function: "
                          << F->getName() << "\n");
  SILFunction *NewF =
    Mod.getSILLoader()->lookupSILFunction(F, /*onlyUpdateLinkage*/ false);
  ASSERT(!NewF || NewF == F);
  if (!NewF || F->isExternalDeclaration()) {
    ASSERT((!hasSharedVisibility(F->getLinkage()) || F->hasForeignBody()) &&
           "cannot deserialize shared function");
    return;
  }

  ASSERT(!F->isAnySerialized() == Mod.isSerialized() &&
         "the de-serializer did set the wrong serialized flag");
  
  F->setBare(IsBare);
  toVerify.push_back(F);
  Worklist.push_back(F);
  Changed = true;
  ++NumFuncLinked;
}

/// Deserialize a function and add it to the worklist for processing.
void SILLinkerVisitor::maybeAddFunctionToWorklist(
    SILFunction *F, SerializedKind_t callerSerializedKind, SILFunction *caller) {
  SILLinkage linkage = F->getLinkage();

  // Originally this was an assert. But it can happen if the user "re-defines"
  // an existing function with a wrong linkage, e.g. using `@_cdecl`.
  if(!(callerSerializedKind == IsNotSerialized ||
            F->hasValidLinkageForFragileRef(callerSerializedKind) ||
            hasSharedVisibility(linkage) || F->isExternForwardDeclaration())) {
    StringRef name = "a serialized function";
    llvm::SmallVector<char> scratch;

    if (caller) {
      name = caller->getName();
      if (SILDeclRef declRef = caller->getDeclRef()) {
        if (auto *decl = declRef.getDecl()) {
          name = decl->getName().getString(scratch);
        }
      }
    }
    F->getModule().getASTContext().Diags.diagnose(
      F->getLocation().getSourceLoc(),
      diag::wrong_linkage_for_serialized_function, name);
    hasError = true;
    return;
  }

  if (!F->isExternalDeclaration()) {
    // The function is already in the module, so no need to de-serialized it.
    // But check if we need to set the IsSerialized flag.
    // See the top-level comment for SILLinkerVisitor for details.
    if (callerSerializedKind == IsSerialized &&
        hasSharedVisibility(linkage) &&
        !Mod.isSerialized() &&
        !F->isAnySerialized()) {
      F->setSerializedKind(IsSerialized);

      // Push the function to the worklist so that all referenced shared functions
      // are also set to IsSerialized.
      Worklist.push_back(F);
    }
    return;
  }

  // In the performance pipeline or embedded mode, we deserialize all reachable
  // functions.
  if (isLinkAll())
    return deserializeAndPushToWorklist(F);

  // Otherwise, make sure to deserialize shared functions; we need to
  // emit them into the client binary since they're not available
  // externally.
  if (hasSharedVisibility(linkage))
    return deserializeAndPushToWorklist(F);

  // Functions with PublicNonABI linkage are deserialized as having
  // HiddenExternal linkage when they are declarations, then they
  // become Shared after the body has been deserialized.
  // So try deserializing HiddenExternal functions too.
  if (linkage == SILLinkage::HiddenExternal)
    return deserializeAndPushToWorklist(F);
  
  // Update the linkage of the function in case it's different in the serialized
  // SIL than derived from the AST. This can be the case with cross-module-
  // optimizations.
  Mod.updateFunctionLinkage(F);
}

/// Process F, recursively deserializing any thing F may reference.
bool SILLinkerVisitor::processFunction(SILFunction *F) {
  // If F is a declaration, first deserialize it.
  if (F->isExternalDeclaration()) {
    maybeAddFunctionToWorklist(F, /*serializedKind*/ IsNotSerialized);
  } else {
    Worklist.push_back(F);
  }

  process();
  return Changed;
}

bool SILLinkerVisitor::processConformance(ProtocolConformanceRef conformanceRef) {
  visitProtocolConformance(conformanceRef, false);
  process();
  return Changed;
}


/// Deserialize the given VTable all SIL the VTable transitively references.
void SILLinkerVisitor::linkInVTable(ClassDecl *D) {
  // Devirtualization already deserializes vtables as needed in both the
  // mandatory and performance pipelines, and we don't support specialized
  // vtables that might have shared linkage yet, so this is only needed in
  // the performance pipeline to deserialize more functions early, and expose
  // optimization opportunities.
  ASSERT(isLinkAll());

  // Attempt to lookup the Vtbl from the SILModule.
  SILVTable *Vtbl = Mod.lookUpVTable(D);
  if (!Vtbl)
    return;

  // Ok we found our VTable. Visit each function referenced by the VTable. If
  // any of the functions are external declarations, add them to the worklist
  // for processing.
  for (auto &entry : Vtbl->getEntries()) {
    SILFunction *impl = entry.getImplementation();
    if (!Vtbl->isAnySerialized() ||
        impl->hasValidLinkageForFragileRef(Vtbl->getSerializedKind())) {
      // Deserialize and recursively walk any vtable entries that do not have
      // bodies yet.
      maybeAddFunctionToWorklist(impl, 
                                 Vtbl->getSerializedKind());
    }
  }

  if (auto *S = D->getSuperclassDecl()) {
    linkInVTable(S);
  }
}

//===----------------------------------------------------------------------===//
//                                  Visitors
//===----------------------------------------------------------------------===//

void SILLinkerVisitor::visitApplyInst(ApplyInst *AI) {
  visitApplySubstitutions(AI->getSubstitutionMap());
}

void SILLinkerVisitor::visitTryApplyInst(TryApplyInst *TAI) {
  visitApplySubstitutions(TAI->getSubstitutionMap());
}

void SILLinkerVisitor::visitPartialApplyInst(PartialApplyInst *PAI) {
  visitApplySubstitutions(PAI->getSubstitutionMap());
}

void SILLinkerVisitor::visitFunctionRefInst(FunctionRefInst *FRI) {
  maybeAddFunctionToWorklist(FRI->getReferencedFunction(),
                             FRI->getFunction()->getSerializedKind(),
                             FRI->getFunction());
}

void SILLinkerVisitor::visitDynamicFunctionRefInst(
    DynamicFunctionRefInst *FRI) {
  maybeAddFunctionToWorklist(FRI->getInitiallyReferencedFunction(),
                             FRI->getFunction()->getSerializedKind(),
                             FRI->getFunction());
}

void SILLinkerVisitor::visitPreviousDynamicFunctionRefInst(
    PreviousDynamicFunctionRefInst *FRI) {
  maybeAddFunctionToWorklist(FRI->getInitiallyReferencedFunction(),
                             FRI->getFunction()->getSerializedKind(),
                             FRI->getFunction());
}

// Eagerly visiting all used conformances leads to a large blowup
// in the amount of SIL we read in. For optimization purposes we can defer
// reading in most conformances until we need them for devirtualization.
// However, we *must* pull in shared clang-importer-derived conformances
// we potentially use, since we may not otherwise have a local definition.
static bool mustDeserializeProtocolConformance(SILModule &M,
                                               ProtocolConformanceRef c) {
  if (!c.isConcrete())
    return false;
  auto conformance = c.getConcrete()->getRootConformance();
  return M.Types.protocolRequiresWitnessTable(conformance->getProtocol())
    && conformance->isSynthesized();
}

void SILLinkerVisitor::visitProtocolConformance(
    ProtocolConformanceRef ref, bool referencedFromInitExistential) {
  // If an abstract protocol conformance was passed in, do nothing.
  if (ref.isAbstract())
    return;
  
  bool isEmbedded = Mod.getOptions().EmbeddedSwift;
  bool mustDeserialize = (isEmbedded && referencedFromInitExistential) ||
                         mustDeserializeProtocolConformance(Mod, ref);

  // Otherwise try and lookup a witness table for C.
  ProtocolConformance *C = ref.getConcrete();
  
  if (!VisitedConformances.insert(C).second)
    return;

  RootProtocolConformance *rootC = C->getRootConformance();
  auto *WT = Mod.lookUpWitnessTable(rootC);

  if ((!WT || WT->isDeclaration()) &&
      (mustDeserialize || Mode == SILModule::LinkingMode::LinkAll)) {
    if (!WT) {
      // Marker protocols should never have witness tables.
      if (C->getProtocol()->isMarkerProtocol())
        return;

      SILLinkage linkage = getLinkageForProtocolConformance(rootC, NotForDefinition);
      WT = SILWitnessTable::create(Mod, linkage,
                                   const_cast<RootProtocolConformance *>(rootC),
                                   /*specialized=*/ false);
    }
    // If the module is at or past the Lowered stage, then we can't do any
    // further deserialization, since pre-IRGen SIL lowering changes the types
    // of definitions to make them incompatible with canonical serialized SIL.
    if (Mod.getStage() == SILStage::Lowered)
      return;
  
    WT = Mod.getSILLoader()->lookupWitnessTable(WT);
  }

  // If the looked up witness table is a declaration, there is nothing we can
  // do here.
  if (WT == nullptr || WT->isDeclaration()) {
#ifndef NDEBUG
    if (mustDeserialize) {
      llvm::errs() << "SILGen failed to emit required conformance:\n";
      ref.dump(llvm::errs());
      llvm::errs() << "\n";
      abort();
    }
#endif
    return;
  }

  if (Mod.getASTContext().LangOpts.hasFeature(Feature::Embedded) &&
      isAvailableExternally(WT->getLinkage()) &&
      WT->getProtocol()->requiresClass()) {
    // In embedded swift all the code is generated in the top-level module.
    // De-serialized tables (= public_external) must be code-gen'd and
    // therefore made non-external.
    // Note: for functions we do that at the end of the pipeline in the
    // IRGenPrepare pass to be able to eliminate dead functions.
    WT->setLinkage(SILLinkage::Hidden);
  }

  auto maybeVisitRelatedConformance = [&](ProtocolConformanceRef c) {
    // Formally all conformances referenced by a used conformance are used.
    // However, eagerly visiting them all at this point leads to a large blowup
    // in the amount of SIL we read in. For optimization purposes we can defer
    // reading in most conformances until we need them for devirtualization.
    // However, we *must* pull in shared clang-importer-derived conformances
    // we potentially use, since we may not otherwise have a local definition.
    if ((isEmbedded && referencedFromInitExistential) ||
        mustDeserializeProtocolConformance(Mod, c)) {
      visitProtocolConformance(c, referencedFromInitExistential);
    }
  };
  
  // For each entry in the witness table...
  for (auto &E : WT->getEntries()) {
    switch (E.getKind()) {
    // If the entry is a witness method...
    case SILWitnessTable::WitnessKind::Method: {
      // The witness could be removed by dead function elimination.
      if (!E.getMethodWitness().Witness)
        continue;

      // Otherwise, deserialize the witness if it has shared linkage, or if
      // we were asked to deserialize everything.
      maybeAddFunctionToWorklist(E.getMethodWitness().Witness,
        (WT->isSerialized() || isAvailableExternally(WT->getLinkage()) ?
         IsSerialized : WT->getSerializedKind()));
      break;
    }
    
    // If the entry is a related witness table, see whether we need to
    // eagerly deserialize it.
    case SILWitnessTable::WitnessKind::BaseProtocol: {
      auto baseConformance = E.getBaseProtocolWitness().Witness;
      maybeVisitRelatedConformance(ProtocolConformanceRef(baseConformance));
      break;
    }
    case SILWitnessTable::WitnessKind::AssociatedConformance: {
      auto assocConformance = E.getAssociatedConformanceWitness().Witness;
      maybeVisitRelatedConformance(assocConformance);
      break;
    }
    
    case SILWitnessTable::WitnessKind::AssociatedType:
    case SILWitnessTable::WitnessKind::Invalid:
      break;
    }
  }
}

void SILLinkerVisitor::visitApplySubstitutions(SubstitutionMap subs) {
  for (auto conformance : subs.getConformances()) {
    // Formally all conformances referenced in a function application are
    // used. However, eagerly visiting them all at this point leads to a
    // large blowup in the amount of SIL we read in, and we aren't very
    // systematic about laziness. For optimization purposes we can defer
    // reading in most conformances until we need them for devirtualization.
    // However, we *must* pull in shared clang-importer-derived conformances
    // we potentially use, since we may not otherwise have a local definition.
    if (mustDeserializeProtocolConformance(Mod, conformance)) {
      visitProtocolConformance(conformance, false);
    }
  }
}

void SILLinkerVisitor::visitInitExistentialAddrInst(
    InitExistentialAddrInst *IEI) {
  // Link in all protocol conformances that this touches.
  //
  // TODO: There might be a two step solution where the init_existential_inst
  // causes the witness table to be brought in as a declaration and then the
  // protocol method inst causes the actual deserialization. For now we are
  // not going to be smart about this to enable avoiding any issues with
  // visiting the open_existential_addr/witness_method before the
  // init_existential_inst.
  for (ProtocolConformanceRef C : IEI->getConformances()) {
    visitProtocolConformance(C, true);
  }
}

void SILLinkerVisitor::visitInitExistentialRefInst(
    InitExistentialRefInst *IERI) {
  // Link in all protocol conformances that this touches.
  //
  // TODO: There might be a two step solution where the init_existential_inst
  // causes the witness table to be brought in as a declaration and then the
  // protocol method inst causes the actual deserialization. For now we are
  // not going to be smart about this to enable avoiding any issues with
  // visiting the protocol_method before the init_existential_inst.
  for (ProtocolConformanceRef C : IERI->getConformances()) {
    visitProtocolConformance(C, true);
  }
}

void SILLinkerVisitor::visitBuiltinInst(BuiltinInst *bi) {
  switch (bi->getBuiltinInfo().ID) {
    case BuiltinValueKind::BuildOrdinaryTaskExecutorRef:
    case BuiltinValueKind::BuildOrdinarySerialExecutorRef:
    case BuiltinValueKind::BuildComplexEqualitySerialExecutorRef:
      if (Mod.getOptions().EmbeddedSwift) {
        // Those builtins act like init_existential_ref instructions and therefore
        // it's important to have the Executor witness tables available in embedded
        // mode.
        auto executorConf = bi->getSubstitutions().getConformances()[0];
        visitProtocolConformance(executorConf, true);
      }
      break;
    default:
      break;
  }
}

void SILLinkerVisitor::visitAllocRefDynamicInst(AllocRefDynamicInst *ARI) {
  if (!isLinkAll())
    return;

  if (!ARI->isDynamicTypeDeinitAndSizeKnownEquivalentToBaseType())
    return;

  // Grab the class decl from the alloc ref inst.
  ClassDecl *D = ARI->getType().getClassOrBoundGenericClass();
  if (!D)
    return;

  linkInVTable(D);
}

void SILLinkerVisitor::visitAllocRefInst(AllocRefInst *ARI) {
  if (!isLinkAll())
    return;

  // Grab the class decl from the alloc ref inst.
  ClassDecl *D = ARI->getType().getClassOrBoundGenericClass();
  if (!D)
    return;

  linkInVTable(D);
}

void SILLinkerVisitor::visitMetatypeInst(MetatypeInst *MI) {
  if (!isLinkAll())
    return;

  CanType instTy = MI->getType().castTo<MetatypeType>().getInstanceType();
  ClassDecl *C = instTy.getClassOrBoundGenericClass();
  if (!C)
    return;

  linkInVTable(C);
}

void SILLinkerVisitor::visitGlobalAddrInst(GlobalAddrInst *GAI) {
  if (!Mod.getOptions().EmbeddedSwift)
    return;

  // In Embedded Swift, we want to actually link globals from other modules too,
  // so strip "external" from the linkage.
  SILGlobalVariable *G = GAI->getReferencedGlobal();
  G->setLinkage(stripExternalFromLinkage(G->getLinkage()));
}

//===----------------------------------------------------------------------===//
//                             Top Level Routine
//===----------------------------------------------------------------------===//

// Main loop of the visitor. Called by one of the other *visit* methods.
void SILLinkerVisitor::process() {
  // Process everything transitively referenced by one of the functions in the
  // worklist.
  while (!Worklist.empty()) {
    auto *Fn = Worklist.pop_back_val();

    if (Fn->getModule().isSerialized()) {
      // If the containing module has been serialized,
      // Remove The Serialized state (if any)
      //  This allows for more optimizations
      Fn->setSerializedKind(SerializedKind_t::IsNotSerialized);
    }

    if (Fn->getModule().getOptions().EmbeddedSwift &&
        Fn->getModule().getASTContext().LangOpts.DebuggerSupport) {
      // LLDB requires that functions with bodies are not external.
      Fn->setLinkage(stripExternalFromLinkage(Fn->getLinkage()));
    }

    LLVM_DEBUG(llvm::dbgs() << "Process imports in function: "
                            << Fn->getName() << "\n");

    for (auto &BB : *Fn) {
      for (auto &I : BB) {
        visit(&I);

        if (hasError)
          return;
      }
    }
  }

  while (!toVerify.empty()) {
    auto *fn = toVerify.pop_back_val();
    fn->verify();
  }
}