1 files changed, 908 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Linker/LinkModules.cpp b/contrib/llvm/lib/Linker/LinkModules.cpp
new file mode 100644
index 0000000..6ffa71e
--- /dev/null
+++ b/contrib/llvm/lib/Linker/LinkModules.cpp
@@ -0,0 +1,908 @@
+//===- lib/Linker/LinkModules.cpp - Module Linker Implementation ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the LLVM module linker.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Linker/Linker.h"
+#include "LinkDiagnosticInfo.h"
+#include "llvm-c/Linker.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/StringSet.h"
+#include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/IR/LLVMContext.h"
+using namespace llvm;
+
+namespace {
+
+/// This is an implementation class for the LinkModules function, which is the
+/// entrypoint for this file.
+class ModuleLinker {
+  IRMover &Mover;
+  Module &SrcM;
+
+  SetVector<GlobalValue *> ValuesToLink;
+  StringSet<> Internalize;
+
+  /// For symbol clashes, prefer those from Src.
+  unsigned Flags;
+
+  /// Function index passed into ModuleLinker for using in function
+  /// importing/exporting handling.
+  const FunctionInfoIndex *ImportIndex;
+
+  /// Functions to import from source module, all other functions are
+  /// imported as declarations instead of definitions.
+  DenseSet<const GlobalValue *> *FunctionsToImport;
+
+  /// Set to true if the given FunctionInfoIndex contains any functions
+  /// from this source module, in which case we must conservatively assume
+  /// that any of its functions may be imported into another module
+  /// as part of a different backend compilation process.
+  bool HasExportedFunctions = false;
+
+  /// Association between metadata value id and temporary metadata that
+  /// remains unmapped after function importing. Saved during function
+  /// importing and consumed during the metadata linking postpass.
+  DenseMap<unsigned, MDNode *> *ValIDToTempMDMap;
+
+  /// Used as the callback for lazy linking.
+  /// The mover has just hit GV and we have to decide if it, and other members
+  /// of the same comdat, should be linked. Every member to be linked is passed
+  /// to Add.
+  void addLazyFor(GlobalValue &GV, IRMover::ValueAdder Add);
+
+  bool shouldOverrideFromSrc() { return Flags & Linker::OverrideFromSrc; }
+  bool shouldLinkOnlyNeeded() { return Flags & Linker::LinkOnlyNeeded; }
+  bool shouldInternalizeLinkedSymbols() {
+    return Flags & Linker::InternalizeLinkedSymbols;
+  }
+
+  bool shouldLinkFromSource(bool &LinkFromSrc, const GlobalValue &Dest,
+                            const GlobalValue &Src);
+
+  /// Should we have mover and linker error diag info?
+  bool emitError(const Twine &Message) {
+    SrcM.getContext().diagnose(LinkDiagnosticInfo(DS_Error, Message));
+    return true;
+  }
+
+  bool getComdatLeader(Module &M, StringRef ComdatName,
+                       const GlobalVariable *&GVar);
+  bool computeResultingSelectionKind(StringRef ComdatName,
+                                     Comdat::SelectionKind Src,
+                                     Comdat::SelectionKind Dst,
+                                     Comdat::SelectionKind &Result,
+                                     bool &LinkFromSrc);
+  std::map<const Comdat *, std::pair<Comdat::SelectionKind, bool>>
+      ComdatsChosen;
+  bool getComdatResult(const Comdat *SrcC, Comdat::SelectionKind &SK,
+                       bool &LinkFromSrc);
+  // Keep track of the global value members of each comdat in source.
+  DenseMap<const Comdat *, std::vector<GlobalValue *>> ComdatMembers;
+
+  /// Given a global in the source module, return the global in the
+  /// destination module that is being linked to, if any.
+  GlobalValue *getLinkedToGlobal(const GlobalValue *SrcGV) {
+    Module &DstM = Mover.getModule();
+    // If the source has no name it can't link.  If it has local linkage,
+    // there is no name match-up going on.
+    if (!SrcGV->hasName() || GlobalValue::isLocalLinkage(SrcGV->getLinkage()))
+      return nullptr;
+
+    // Otherwise see if we have a match in the destination module's symtab.
+    GlobalValue *DGV = DstM.getNamedValue(SrcGV->getName());
+    if (!DGV)
+      return nullptr;
+
+    // If we found a global with the same name in the dest module, but it has
+    // internal linkage, we are really not doing any linkage here.
+    if (DGV->hasLocalLinkage())
+      return nullptr;
+
+    // Otherwise, we do in fact link to the destination global.
+    return DGV;
+  }
+
+  bool linkIfNeeded(GlobalValue &GV);
+
+  /// Helper method to check if we are importing from the current source
+  /// module.
+  bool isPerformingImport() const { return FunctionsToImport != nullptr; }
+
+  /// If we are importing from the source module, checks if we should
+  /// import SGV as a definition, otherwise import as a declaration.
+  bool doImportAsDefinition(const GlobalValue *SGV);
+
+public:
+  ModuleLinker(IRMover &Mover, Module &SrcM, unsigned Flags,
+               const FunctionInfoIndex *Index = nullptr,
+               DenseSet<const GlobalValue *> *FunctionsToImport = nullptr,
+               DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr)
+      : Mover(Mover), SrcM(SrcM), Flags(Flags), ImportIndex(Index),
+        FunctionsToImport(FunctionsToImport),
+        ValIDToTempMDMap(ValIDToTempMDMap) {
+    assert((ImportIndex || !FunctionsToImport) &&
+           "Expect a FunctionInfoIndex when importing");
+    // If we have a FunctionInfoIndex but no function to import,
+    // then this is the primary module being compiled in a ThinLTO
+    // backend compilation, and we need to see if it has functions that
+    // may be exported to another backend compilation.
+    if (ImportIndex && !FunctionsToImport)
+      HasExportedFunctions = ImportIndex->hasExportedFunctions(SrcM);
+    assert((ValIDToTempMDMap || !FunctionsToImport) &&
+           "Function importing must provide a ValIDToTempMDMap");
+  }
+
+  bool run();
+};
+
+/// Class to handle necessary GlobalValue changes required by ThinLTO including
+/// linkage changes and any necessary renaming.
+class ThinLTOGlobalProcessing {
+  /// The Module which we are exporting or importing functions from.
+  Module &M;
+
+  /// Function index passed in for function importing/exporting handling.
+  const FunctionInfoIndex *ImportIndex;
+
+  /// Functions to import from this module, all other functions will be
+  /// imported as declarations instead of definitions.
+  DenseSet<const GlobalValue *> *FunctionsToImport;
+
+  /// Set to true if the given FunctionInfoIndex contains any functions
+  /// from this source module, in which case we must conservatively assume
+  /// that any of its functions may be imported into another module
+  /// as part of a different backend compilation process.
+  bool HasExportedFunctions = false;
+
+  /// Populated during ThinLTO global processing with locals promoted
+  /// to global scope in an exporting module, which now need to be linked
+  /// in if calling from the ModuleLinker.
+  SetVector<GlobalValue *> NewExportedValues;
+
+  /// Check if we should promote the given local value to global scope.
+  bool doPromoteLocalToGlobal(const GlobalValue *SGV);
+
+  /// Helper methods to check if we are importing from or potentially
+  /// exporting from the current source module.
+  bool isPerformingImport() const { return FunctionsToImport != nullptr; }
+  bool isModuleExporting() const { return HasExportedFunctions; }
+
+  /// If we are importing from the source module, checks if we should
+  /// import SGV as a definition, otherwise import as a declaration.
+  bool doImportAsDefinition(const GlobalValue *SGV);
+
+  /// Get the name for SGV that should be used in the linked destination
+  /// module. Specifically, this handles the case where we need to rename
+  /// a local that is being promoted to global scope.
+  std::string getName(const GlobalValue *SGV);
+
+  /// Process globals so that they can be used in ThinLTO. This includes
+  /// promoting local variables so that they can be reference externally by
+  /// thin lto imported globals and converting strong external globals to
+  /// available_externally.
+  void processGlobalsForThinLTO();
+  void processGlobalForThinLTO(GlobalValue &GV);
+
+  /// Get the new linkage for SGV that should be used in the linked destination
+  /// module. Specifically, for ThinLTO importing or exporting it may need
+  /// to be adjusted.
+  GlobalValue::LinkageTypes getLinkage(const GlobalValue *SGV);
+
+public:
+  ThinLTOGlobalProcessing(
+      Module &M, const FunctionInfoIndex *Index,
+      DenseSet<const GlobalValue *> *FunctionsToImport = nullptr)
+      : M(M), ImportIndex(Index), FunctionsToImport(FunctionsToImport) {
+    // If we have a FunctionInfoIndex but no function to import,
+    // then this is the primary module being compiled in a ThinLTO
+    // backend compilation, and we need to see if it has functions that
+    // may be exported to another backend compilation.
+    if (!FunctionsToImport)
+      HasExportedFunctions = ImportIndex->hasExportedFunctions(M);
+  }
+
+  bool run();
+
+  /// Access the promoted globals that are now exported and need to be linked.
+  SetVector<GlobalValue *> &getNewExportedValues() { return NewExportedValues; }
+};
+}
+
+/// Checks if we should import SGV as a definition, otherwise import as a
+/// declaration.
+static bool
+doImportAsDefinitionImpl(const GlobalValue *SGV,
+                         DenseSet<const GlobalValue *> *FunctionsToImport) {
+  auto *GA = dyn_cast<GlobalAlias>(SGV);
+  if (GA) {
+    if (GA->hasWeakAnyLinkage())
+      return false;
+    const GlobalObject *GO = GA->getBaseObject();
+    if (!GO->hasLinkOnceODRLinkage())
+      return false;
+    return doImportAsDefinitionImpl(GO, FunctionsToImport);
+  }
+  // Always import GlobalVariable definitions, except for the special
+  // case of WeakAny which are imported as ExternalWeak declarations
+  // (see comments in ModuleLinker::getLinkage). The linkage changes
+  // described in ModuleLinker::getLinkage ensure the correct behavior (e.g.
+  // global variables with external linkage are transformed to
+  // available_externally definitions, which are ultimately turned into
+  // declarations after the EliminateAvailableExternally pass).
+  if (isa<GlobalVariable>(SGV) && !SGV->isDeclaration() &&
+      !SGV->hasWeakAnyLinkage())
+    return true;
+  // Only import the function requested for importing.
+  auto *SF = dyn_cast<Function>(SGV);
+  if (SF && FunctionsToImport->count(SF))
+    return true;
+  // Otherwise no.
+  return false;
+}
+
+bool ThinLTOGlobalProcessing::doImportAsDefinition(const GlobalValue *SGV) {
+  if (!isPerformingImport())
+    return false;
+  return doImportAsDefinitionImpl(SGV, FunctionsToImport);
+}
+
+bool ModuleLinker::doImportAsDefinition(const GlobalValue *SGV) {
+  if (!isPerformingImport())
+    return false;
+  return doImportAsDefinitionImpl(SGV, FunctionsToImport);
+}
+
+bool ThinLTOGlobalProcessing::doPromoteLocalToGlobal(const GlobalValue *SGV) {
+  assert(SGV->hasLocalLinkage());
+  // Both the imported references and the original local variable must
+  // be promoted.
+  if (!isPerformingImport() && !isModuleExporting())
+    return false;
+
+  // Local const variables never need to be promoted unless they are address
+  // taken. The imported uses can simply use the clone created in this module.
+  // For now we are conservative in determining which variables are not
+  // address taken by checking the unnamed addr flag. To be more aggressive,
+  // the address taken information must be checked earlier during parsing
+  // of the module and recorded in the function index for use when importing
+  // from that module.
+  auto *GVar = dyn_cast<GlobalVariable>(SGV);
+  if (GVar && GVar->isConstant() && GVar->hasUnnamedAddr())
+    return false;
+
+  // Eventually we only need to promote functions in the exporting module that
+  // are referenced by a potentially exported function (i.e. one that is in the
+  // function index).
+  return true;
+}
+
+std::string ThinLTOGlobalProcessing::getName(const GlobalValue *SGV) {
+  // For locals that must be promoted to global scope, ensure that
+  // the promoted name uniquely identifies the copy in the original module,
+  // using the ID assigned during combined index creation. When importing,
+  // we rename all locals (not just those that are promoted) in order to
+  // avoid naming conflicts between locals imported from different modules.
+  if (SGV->hasLocalLinkage() &&
+      (doPromoteLocalToGlobal(SGV) || isPerformingImport()))
+    return FunctionInfoIndex::getGlobalNameForLocal(
+        SGV->getName(),
+        ImportIndex->getModuleId(SGV->getParent()->getModuleIdentifier()));
+  return SGV->getName();
+}
+
+GlobalValue::LinkageTypes
+ThinLTOGlobalProcessing::getLinkage(const GlobalValue *SGV) {
+  // Any local variable that is referenced by an exported function needs
+  // to be promoted to global scope. Since we don't currently know which
+  // functions reference which local variables/functions, we must treat
+  // all as potentially exported if this module is exporting anything.
+  if (isModuleExporting()) {
+    if (SGV->hasLocalLinkage() && doPromoteLocalToGlobal(SGV))
+      return GlobalValue::ExternalLinkage;
+    return SGV->getLinkage();
+  }
+
+  // Otherwise, if we aren't importing, no linkage change is needed.
+  if (!isPerformingImport())
+    return SGV->getLinkage();
+
+  switch (SGV->getLinkage()) {
+  case GlobalValue::ExternalLinkage:
+    // External defnitions are converted to available_externally
+    // definitions upon import, so that they are available for inlining
+    // and/or optimization, but are turned into declarations later
+    // during the EliminateAvailableExternally pass.
+    if (doImportAsDefinition(SGV) && !dyn_cast<GlobalAlias>(SGV))
+      return GlobalValue::AvailableExternallyLinkage;
+    // An imported external declaration stays external.
+    return SGV->getLinkage();
+
+  case GlobalValue::AvailableExternallyLinkage:
+    // An imported available_externally definition converts
+    // to external if imported as a declaration.
+    if (!doImportAsDefinition(SGV))
+      return GlobalValue::ExternalLinkage;
+    // An imported available_externally declaration stays that way.
+    return SGV->getLinkage();
+
+  case GlobalValue::LinkOnceAnyLinkage:
+  case GlobalValue::LinkOnceODRLinkage:
+    // These both stay the same when importing the definition.
+    // The ThinLTO pass will eventually force-import their definitions.
+    return SGV->getLinkage();
+
+  case GlobalValue::WeakAnyLinkage:
+    // Can't import weak_any definitions correctly, or we might change the
+    // program semantics, since the linker will pick the first weak_any
+    // definition and importing would change the order they are seen by the
+    // linker. The module linking caller needs to enforce this.
+    assert(!doImportAsDefinition(SGV));
+    // If imported as a declaration, it becomes external_weak.
+    return GlobalValue::ExternalWeakLinkage;
+
+  case GlobalValue::WeakODRLinkage:
+    // For weak_odr linkage, there is a guarantee that all copies will be
+    // equivalent, so the issue described above for weak_any does not exist,
+    // and the definition can be imported. It can be treated similarly
+    // to an imported externally visible global value.
+    if (doImportAsDefinition(SGV) && !dyn_cast<GlobalAlias>(SGV))
+      return GlobalValue::AvailableExternallyLinkage;
+    else
+      return GlobalValue::ExternalLinkage;
+
+  case GlobalValue::AppendingLinkage:
+    // It would be incorrect to import an appending linkage variable,
+    // since it would cause global constructors/destructors to be
+    // executed multiple times. This should have already been handled
+    // by linkIfNeeded, and we will assert in shouldLinkFromSource
+    // if we try to import, so we simply return AppendingLinkage.
+    return GlobalValue::AppendingLinkage;
+
+  case GlobalValue::InternalLinkage:
+  case GlobalValue::PrivateLinkage:
+    // If we are promoting the local to global scope, it is handled
+    // similarly to a normal externally visible global.
+    if (doPromoteLocalToGlobal(SGV)) {
+      if (doImportAsDefinition(SGV) && !dyn_cast<GlobalAlias>(SGV))
+        return GlobalValue::AvailableExternallyLinkage;
+      else
+        return GlobalValue::ExternalLinkage;
+    }
+    // A non-promoted imported local definition stays local.
+    // The ThinLTO pass will eventually force-import their definitions.
+    return SGV->getLinkage();
+
+  case GlobalValue::ExternalWeakLinkage:
+    // External weak doesn't apply to definitions, must be a declaration.
+    assert(!doImportAsDefinition(SGV));
+    // Linkage stays external_weak.
+    return SGV->getLinkage();
+
+  case GlobalValue::CommonLinkage:
+    // Linkage stays common on definitions.
+    // The ThinLTO pass will eventually force-import their definitions.
+    return SGV->getLinkage();
+  }
+
+  llvm_unreachable("unknown linkage type");
+}
+
+static GlobalValue::VisibilityTypes
+getMinVisibility(GlobalValue::VisibilityTypes A,
+                 GlobalValue::VisibilityTypes B) {
+  if (A == GlobalValue::HiddenVisibility || B == GlobalValue::HiddenVisibility)
+    return GlobalValue::HiddenVisibility;
+  if (A == GlobalValue::ProtectedVisibility ||
+      B == GlobalValue::ProtectedVisibility)
+    return GlobalValue::ProtectedVisibility;
+  return GlobalValue::DefaultVisibility;
+}
+
+bool ModuleLinker::getComdatLeader(Module &M, StringRef ComdatName,
+                                   const GlobalVariable *&GVar) {
+  const GlobalValue *GVal = M.getNamedValue(ComdatName);
+  if (const auto *GA = dyn_cast_or_null<GlobalAlias>(GVal)) {
+    GVal = GA->getBaseObject();
+    if (!GVal)
+      // We cannot resolve the size of the aliasee yet.
+      return emitError("Linking COMDATs named '" + ComdatName +
+                       "': COMDAT key involves incomputable alias size.");
+  }
+
+  GVar = dyn_cast_or_null<GlobalVariable>(GVal);
+  if (!GVar)
+    return emitError(
+        "Linking COMDATs named '" + ComdatName +
+        "': GlobalVariable required for data dependent selection!");
+
+  return false;
+}
+
+bool ModuleLinker::computeResultingSelectionKind(StringRef ComdatName,
+                                                 Comdat::SelectionKind Src,
+                                                 Comdat::SelectionKind Dst,
+                                                 Comdat::SelectionKind &Result,
+                                                 bool &LinkFromSrc) {
+  Module &DstM = Mover.getModule();
+  // The ability to mix Comdat::SelectionKind::Any with
+  // Comdat::SelectionKind::Largest is a behavior that comes from COFF.
+  bool DstAnyOrLargest = Dst == Comdat::SelectionKind::Any ||
+                         Dst == Comdat::SelectionKind::Largest;
+  bool SrcAnyOrLargest = Src == Comdat::SelectionKind::Any ||
+                         Src == Comdat::SelectionKind::Largest;
+  if (DstAnyOrLargest && SrcAnyOrLargest) {
+    if (Dst == Comdat::SelectionKind::Largest ||
+        Src == Comdat::SelectionKind::Largest)
+      Result = Comdat::SelectionKind::Largest;
+    else
+      Result = Comdat::SelectionKind::Any;
+  } else if (Src == Dst) {
+    Result = Dst;
+  } else {
+    return emitError("Linking COMDATs named '" + ComdatName +
+                     "': invalid selection kinds!");
+  }
+
+  switch (Result) {
+  case Comdat::SelectionKind::Any:
+    // Go with Dst.
+    LinkFromSrc = false;
+    break;
+  case Comdat::SelectionKind::NoDuplicates:
+    return emitError("Linking COMDATs named '" + ComdatName +
+                     "': noduplicates has been violated!");
+  case Comdat::SelectionKind::ExactMatch:
+  case Comdat::SelectionKind::Largest:
+  case Comdat::SelectionKind::SameSize: {
+    const GlobalVariable *DstGV;
+    const GlobalVariable *SrcGV;
+    if (getComdatLeader(DstM, ComdatName, DstGV) ||
+        getComdatLeader(SrcM, ComdatName, SrcGV))
+      return true;
+
+    const DataLayout &DstDL = DstM.getDataLayout();
+    const DataLayout &SrcDL = SrcM.getDataLayout();
+    uint64_t DstSize =
+        DstDL.getTypeAllocSize(DstGV->getType()->getPointerElementType());
+    uint64_t SrcSize =
+        SrcDL.getTypeAllocSize(SrcGV->getType()->getPointerElementType());
+    if (Result == Comdat::SelectionKind::ExactMatch) {
+      if (SrcGV->getInitializer() != DstGV->getInitializer())
+        return emitError("Linking COMDATs named '" + ComdatName +
+                         "': ExactMatch violated!");
+      LinkFromSrc = false;
+    } else if (Result == Comdat::SelectionKind::Largest) {
+      LinkFromSrc = SrcSize > DstSize;
+    } else if (Result == Comdat::SelectionKind::SameSize) {
+      if (SrcSize != DstSize)
+        return emitError("Linking COMDATs named '" + ComdatName +
+                         "': SameSize violated!");
+      LinkFromSrc = false;
+    } else {
+      llvm_unreachable("unknown selection kind");
+    }
+    break;
+  }
+  }
+
+  return false;
+}
+
+bool ModuleLinker::getComdatResult(const Comdat *SrcC,
+                                   Comdat::SelectionKind &Result,
+                                   bool &LinkFromSrc) {
+  Module &DstM = Mover.getModule();
+  Comdat::SelectionKind SSK = SrcC->getSelectionKind();
+  StringRef ComdatName = SrcC->getName();
+  Module::ComdatSymTabType &ComdatSymTab = DstM.getComdatSymbolTable();
+  Module::ComdatSymTabType::iterator DstCI = ComdatSymTab.find(ComdatName);
+
+  if (DstCI == ComdatSymTab.end()) {
+    // Use the comdat if it is only available in one of the modules.
+    LinkFromSrc = true;
+    Result = SSK;
+    return false;
+  }
+
+  const Comdat *DstC = &DstCI->second;
+  Comdat::SelectionKind DSK = DstC->getSelectionKind();
+  return computeResultingSelectionKind(ComdatName, SSK, DSK, Result,
+                                       LinkFromSrc);
+}
+
+bool ModuleLinker::shouldLinkFromSource(bool &LinkFromSrc,
+                                        const GlobalValue &Dest,
+                                        const GlobalValue &Src) {
+
+  // Should we unconditionally use the Src?
+  if (shouldOverrideFromSrc()) {
+    LinkFromSrc = true;
+    return false;
+  }
+
+  // We always have to add Src if it has appending linkage.
+  if (Src.hasAppendingLinkage()) {
+    // Should have prevented importing for appending linkage in linkIfNeeded.
+    assert(!isPerformingImport());
+    LinkFromSrc = true;
+    return false;
+  }
+
+  bool SrcIsDeclaration = Src.isDeclarationForLinker();
+  bool DestIsDeclaration = Dest.isDeclarationForLinker();
+
+  if (isPerformingImport()) {
+    if (isa<Function>(&Src)) {
+      // For functions, LinkFromSrc iff this is a function requested
+      // for importing. For variables, decide below normally.
+      LinkFromSrc = FunctionsToImport->count(&Src);
+      return false;
+    }
+
+    // Check if this is an alias with an already existing definition
+    // in Dest, which must have come from a prior importing pass from
+    // the same Src module. Unlike imported function and variable
+    // definitions, which are imported as available_externally and are
+    // not definitions for the linker, that is not a valid linkage for
+    // imported aliases which must be definitions. Simply use the existing
+    // Dest copy.
+    if (isa<GlobalAlias>(&Src) && !DestIsDeclaration) {
+      assert(isa<GlobalAlias>(&Dest));
+      LinkFromSrc = false;
+      return false;
+    }
+  }
+
+  if (SrcIsDeclaration) {
+    // If Src is external or if both Src & Dest are external..  Just link the
+    // external globals, we aren't adding anything.
+    if (Src.hasDLLImportStorageClass()) {
+      // If one of GVs is marked as DLLImport, result should be dllimport'ed.
+      LinkFromSrc = DestIsDeclaration;
+      return false;
+    }
+    // If the Dest is weak, use the source linkage.
+    if (Dest.hasExternalWeakLinkage()) {
+      LinkFromSrc = true;
+      return false;
+    }
+    // Link an available_externally over a declaration.
+    LinkFromSrc = !Src.isDeclaration() && Dest.isDeclaration();
+    return false;
+  }
+
+  if (DestIsDeclaration) {
+    // If Dest is external but Src is not:
+    LinkFromSrc = true;
+    return false;
+  }
+
+  if (Src.hasCommonLinkage()) {
+    if (Dest.hasLinkOnceLinkage() || Dest.hasWeakLinkage()) {
+      LinkFromSrc = true;
+      return false;
+    }
+
+    if (!Dest.hasCommonLinkage()) {
+      LinkFromSrc = false;
+      return false;
+    }
+
+    const DataLayout &DL = Dest.getParent()->getDataLayout();
+    uint64_t DestSize = DL.getTypeAllocSize(Dest.getType()->getElementType());
+    uint64_t SrcSize = DL.getTypeAllocSize(Src.getType()->getElementType());
+    LinkFromSrc = SrcSize > DestSize;
+    return false;
+  }
+
+  if (Src.isWeakForLinker()) {
+    assert(!Dest.hasExternalWeakLinkage());
+    assert(!Dest.hasAvailableExternallyLinkage());
+
+    if (Dest.hasLinkOnceLinkage() && Src.hasWeakLinkage()) {
+      LinkFromSrc = true;
+      return false;
+    }
+
+    LinkFromSrc = false;
+    return false;
+  }
+
+  if (Dest.isWeakForLinker()) {
+    assert(Src.hasExternalLinkage());
+    LinkFromSrc = true;
+    return false;
+  }
+
+  assert(!Src.hasExternalWeakLinkage());
+  assert(!Dest.hasExternalWeakLinkage());
+  assert(Dest.hasExternalLinkage() && Src.hasExternalLinkage() &&
+         "Unexpected linkage type!");
+  return emitError("Linking globals named '" + Src.getName() +
+                   "': symbol multiply defined!");
+}
+
+bool ModuleLinker::linkIfNeeded(GlobalValue &GV) {
+  GlobalValue *DGV = getLinkedToGlobal(&GV);
+
+  if (shouldLinkOnlyNeeded() && !(DGV && DGV->isDeclaration()))
+    return false;
+
+  if (DGV && !GV.hasLocalLinkage() && !GV.hasAppendingLinkage()) {
+    auto *DGVar = dyn_cast<GlobalVariable>(DGV);
+    auto *SGVar = dyn_cast<GlobalVariable>(&GV);
+    if (DGVar && SGVar) {
+      if (DGVar->isDeclaration() && SGVar->isDeclaration() &&
+          (!DGVar->isConstant() || !SGVar->isConstant())) {
+        DGVar->setConstant(false);
+        SGVar->setConstant(false);
+      }
+      if (DGVar->hasCommonLinkage() && SGVar->hasCommonLinkage()) {
+        unsigned Align = std::max(DGVar->getAlignment(), SGVar->getAlignment());
+        SGVar->setAlignment(Align);
+        DGVar->setAlignment(Align);
+      }
+    }
+
+    GlobalValue::VisibilityTypes Visibility =
+        getMinVisibility(DGV->getVisibility(), GV.getVisibility());
+    DGV->setVisibility(Visibility);
+    GV.setVisibility(Visibility);
+
+    bool HasUnnamedAddr = GV.hasUnnamedAddr() && DGV->hasUnnamedAddr();
+    DGV->setUnnamedAddr(HasUnnamedAddr);
+    GV.setUnnamedAddr(HasUnnamedAddr);
+  }
+
+  // Don't want to append to global_ctors list, for example, when we
+  // are importing for ThinLTO, otherwise the global ctors and dtors
+  // get executed multiple times for local variables (the latter causing
+  // double frees).
+  if (GV.hasAppendingLinkage() && isPerformingImport())
+    return false;
+
+  if (isPerformingImport() && !doImportAsDefinition(&GV))
+    return false;
+
+  if (!DGV && !shouldOverrideFromSrc() &&
+      (GV.hasLocalLinkage() || GV.hasLinkOnceLinkage() ||
+       GV.hasAvailableExternallyLinkage()))
+    return false;
+
+  if (GV.isDeclaration())
+    return false;
+
+  if (const Comdat *SC = GV.getComdat()) {
+    bool LinkFromSrc;
+    Comdat::SelectionKind SK;
+    std::tie(SK, LinkFromSrc) = ComdatsChosen[SC];
+    if (LinkFromSrc)
+      ValuesToLink.insert(&GV);
+    return false;
+  }
+
+  bool LinkFromSrc = true;
+  if (DGV && shouldLinkFromSource(LinkFromSrc, *DGV, GV))
+    return true;
+  if (LinkFromSrc)
+    ValuesToLink.insert(&GV);
+  return false;
+}
+
+void ModuleLinker::addLazyFor(GlobalValue &GV, IRMover::ValueAdder Add) {
+  // Add these to the internalize list
+  if (!GV.hasLinkOnceLinkage())
+    return;
+
+  if (shouldInternalizeLinkedSymbols())
+    Internalize.insert(GV.getName());
+  Add(GV);
+
+  const Comdat *SC = GV.getComdat();
+  if (!SC)
+    return;
+  for (GlobalValue *GV2 : ComdatMembers[SC]) {
+    if (!GV2->hasLocalLinkage() && shouldInternalizeLinkedSymbols())
+      Internalize.insert(GV2->getName());
+    Add(*GV2);
+  }
+}
+
+void ThinLTOGlobalProcessing::processGlobalForThinLTO(GlobalValue &GV) {
+  if (GV.hasLocalLinkage() &&
+      (doPromoteLocalToGlobal(&GV) || isPerformingImport())) {
+    GV.setName(getName(&GV));
+    GV.setLinkage(getLinkage(&GV));
+    if (!GV.hasLocalLinkage())
+      GV.setVisibility(GlobalValue::HiddenVisibility);
+    if (isModuleExporting())
+      NewExportedValues.insert(&GV);
+    return;
+  }
+  GV.setLinkage(getLinkage(&GV));
+}
+
+void ThinLTOGlobalProcessing::processGlobalsForThinLTO() {
+  for (GlobalVariable &GV : M.globals())
+    processGlobalForThinLTO(GV);
+  for (Function &SF : M)
+    processGlobalForThinLTO(SF);
+  for (GlobalAlias &GA : M.aliases())
+    processGlobalForThinLTO(GA);
+}
+
+bool ThinLTOGlobalProcessing::run() {
+  processGlobalsForThinLTO();
+  return false;
+}
+
+bool ModuleLinker::run() {
+  for (const auto &SMEC : SrcM.getComdatSymbolTable()) {
+    const Comdat &C = SMEC.getValue();
+    if (ComdatsChosen.count(&C))
+      continue;
+    Comdat::SelectionKind SK;
+    bool LinkFromSrc;
+    if (getComdatResult(&C, SK, LinkFromSrc))
+      return true;
+    ComdatsChosen[&C] = std::make_pair(SK, LinkFromSrc);
+  }
+
+  for (GlobalVariable &GV : SrcM.globals())
+    if (const Comdat *SC = GV.getComdat())
+      ComdatMembers[SC].push_back(&GV);
+
+  for (Function &SF : SrcM)
+    if (const Comdat *SC = SF.getComdat())
+      ComdatMembers[SC].push_back(&SF);
+
+  for (GlobalAlias &GA : SrcM.aliases())
+    if (const Comdat *SC = GA.getComdat())
+      ComdatMembers[SC].push_back(&GA);
+
+  // Insert all of the globals in src into the DstM module... without linking
+  // initializers (which could refer to functions not yet mapped over).
+  for (GlobalVariable &GV : SrcM.globals())
+    if (linkIfNeeded(GV))
+      return true;
+
+  for (Function &SF : SrcM)
+    if (linkIfNeeded(SF))
+      return true;
+
+  for (GlobalAlias &GA : SrcM.aliases())
+    if (linkIfNeeded(GA))
+      return true;
+
+  if (ImportIndex) {
+    ThinLTOGlobalProcessing ThinLTOProcessing(SrcM, ImportIndex,
+                                              FunctionsToImport);
+    if (ThinLTOProcessing.run())
+      return true;
+    for (auto *GV : ThinLTOProcessing.getNewExportedValues())
+      ValuesToLink.insert(GV);
+  }
+
+  for (unsigned I = 0; I < ValuesToLink.size(); ++I) {
+    GlobalValue *GV = ValuesToLink[I];
+    const Comdat *SC = GV->getComdat();
+    if (!SC)
+      continue;
+    for (GlobalValue *GV2 : ComdatMembers[SC])
+      ValuesToLink.insert(GV2);
+  }
+
+  if (shouldInternalizeLinkedSymbols()) {
+    for (GlobalValue *GV : ValuesToLink)
+      Internalize.insert(GV->getName());
+  }
+
+  if (Mover.move(SrcM, ValuesToLink.getArrayRef(),
+                 [this](GlobalValue &GV, IRMover::ValueAdder Add) {
+                   addLazyFor(GV, Add);
+                 },
+                 ValIDToTempMDMap, false))
+    return true;
+  Module &DstM = Mover.getModule();
+  for (auto &P : Internalize) {
+    GlobalValue *GV = DstM.getNamedValue(P.first());
+    GV->setLinkage(GlobalValue::InternalLinkage);
+  }
+
+  return false;
+}
+
+Linker::Linker(Module &M) : Mover(M) {}
+
+bool Linker::linkInModule(std::unique_ptr<Module> Src, unsigned Flags,
+                          const FunctionInfoIndex *Index,
+                          DenseSet<const GlobalValue *> *FunctionsToImport,
+                          DenseMap<unsigned, MDNode *> *ValIDToTempMDMap) {
+  ModuleLinker ModLinker(Mover, *Src, Flags, Index, FunctionsToImport,
+                         ValIDToTempMDMap);
+  return ModLinker.run();
+}
+
+bool Linker::linkInModuleForCAPI(Module &Src) {
+  ModuleLinker ModLinker(Mover, Src, 0, nullptr, nullptr);
+  return ModLinker.run();
+}
+
+bool Linker::linkInMetadata(Module &Src,
+                            DenseMap<unsigned, MDNode *> *ValIDToTempMDMap) {
+  SetVector<GlobalValue *> ValuesToLink;
+  if (Mover.move(
+          Src, ValuesToLink.getArrayRef(),
+          [this](GlobalValue &GV, IRMover::ValueAdder Add) { assert(false); },
+          ValIDToTempMDMap, true))
+    return true;
+  return false;
+}
+
+//===----------------------------------------------------------------------===//
+// LinkModules entrypoint.
+//===----------------------------------------------------------------------===//
+
+/// This function links two modules together, with the resulting Dest module
+/// modified to be the composite of the two input modules. If an error occurs,
+/// true is returned and ErrorMsg (if not null) is set to indicate the problem.
+/// Upon failure, the Dest module could be in a modified state, and shouldn't be
+/// relied on to be consistent.
+bool Linker::linkModules(Module &Dest, std::unique_ptr<Module> Src,
+                         unsigned Flags) {
+  Linker L(Dest);
+  return L.linkInModule(std::move(Src), Flags);
+}
+
+bool llvm::renameModuleForThinLTO(Module &M, const FunctionInfoIndex *Index) {
+  ThinLTOGlobalProcessing ThinLTOProcessing(M, Index);
+  return ThinLTOProcessing.run();
+}
+
+//===----------------------------------------------------------------------===//
+// C API.
+//===----------------------------------------------------------------------===//
+
+static void diagnosticHandler(const DiagnosticInfo &DI, void *C) {
+  auto *Message = reinterpret_cast<std::string *>(C);
+  raw_string_ostream Stream(*Message);
+  DiagnosticPrinterRawOStream DP(Stream);
+  DI.print(DP);
+}
+
+LLVMBool LLVMLinkModules(LLVMModuleRef Dest, LLVMModuleRef Src,
+                         LLVMLinkerMode Unused, char **OutMessages) {
+  Module *D = unwrap(Dest);
+  LLVMContext &Ctx = D->getContext();
+
+  LLVMContext::DiagnosticHandlerTy OldDiagnosticHandler =
+      Ctx.getDiagnosticHandler();
+  void *OldDiagnosticContext = Ctx.getDiagnosticContext();
+  std::string Message;
+  Ctx.setDiagnosticHandler(diagnosticHandler, &Message, true);
+
+  Linker L(*D);
+  Module *M = unwrap(Src);
+  LLVMBool Result = L.linkInModuleForCAPI(*M);
+
+  Ctx.setDiagnosticHandler(OldDiagnosticHandler, OldDiagnosticContext, true);
+
+  if (OutMessages && Result)
+    *OutMessages = strdup(Message.c_str());
+  return Result;
+}
+
+LLVMBool LLVMLinkModules2(LLVMModuleRef Dest, LLVMModuleRef Src) {
+  Module *D = unwrap(Dest);
+  std::unique_ptr<Module> M(unwrap(Src));
+  return Linker::linkModules(*D, std::move(M));
+}