[ThinLTO] Metadata linking for imported functions

[tejohnson]

Summary

Second patch split out from http://reviews.llvm.org/D14752.

Maps metadata as a post-pass from each module when importing complete,
suturing up final metadata to the temporary metadata left on the
imported instructions.

This entails saving the mapping from bitcode value id to temporary
metadata in the importing pass, and from bitcode value id to final
metadata during the metadata linking postpass.

Depends on D14825.

[tejohnson]

• Remove a change that shouldn't be in this patch.

[tejohnson]

Ping.
Thanks, Teresa

[mehdi_amini]

It looks good to me, with a few minor comments.
I'd rather have another pair of eyes on it since I'm not sure I understand everything though.

Thanks!

include/llvm/IR/Metadata.h

833–834

I think we are usually using \p instead of \a when referring to parameters

lib/Bitcode/Reader/BitcodeReader.cpp

3119

I'm reluctant to i as a variable name, maybe unsigned CurrentValueID instead?

3131

I'm not sure about this early exit?

lib/Linker/LinkModules.cpp

506

Suggestion: ValueMapperFlags |= RF_HaveUnmaterializedMetadata;

994

early return:

if (!ValIDToTempMDMap) return nullptr;

1013

same early return: if (!ValIDToTempMDMap) return;

2254–2256

Comment?

tools/llvm-link/llvm-link.cpp

169

StringMap<std::unique_ptr<DenseMap<unsigned, MDNode *>>> ModuleToTempMDValsMap;

230

May I suggest:

auto &TempMDVals = ModuleToTempMDValsMap[FileName];
if (!TempMDVals)
  TempMDVals = new DenseMap<unsigned, MDNode *>();

(one query to the map)

[tejohnson]

Thanks for the comments. Will upload new patch after testing.

include/llvm/IR/Metadata.h

833–834

Fixed.

lib/Bitcode/Reader/BitcodeReader.cpp

3119

Renamed to ValID.

3131

Good catch, should be a continue. This isn't triggered with current HEAD since it only would affect the case where we import >1 function at a time, which is obviously in progress.

I added another change to set the SavedFwdRefs flag to true whenever we add a forward ref which should allow us to catch any case where we miss saving off any forward references (e.g. if this bug wasn't fixed).

lib/Linker/LinkModules.cpp

506

I did that initially because the |= form gives me:

error: assigning to 'llvm::RemapFlags' from incompatible type 'int'

No amount of casting the RF_ value to the enum type gets me past this error. So I have left it as is for now. Suggestions?

994

Done

1013

Done

2254–2256

Done

tools/llvm-link/llvm-link.cpp

169

Good idea, done

230

Done

[tejohnson]

• Address comments from Mehdi and Duncan.

[mehdi_amini]

LGTM

lib/Linker/LinkModules.cpp

625

commit separately if possible

include/llvm/IR/DebugInfoMetadata.h

static DILocation *getImpl(LLVMContext &Context, unsigned Line,
                              unsigned Column, DILocalScope *Scope,
                              DILocation *InlinedAt, StorageType Storage,
                              bool ShouldCreate = true) {
     return getImpl(Context, Line, Column, static_cast<Metadata *>(Scope),
                    static_cast<Metadata *>(InlinedAt), Storage, ShouldCreate);
   }
 
   TempDILocation cloneImpl() const {
-    return getTemporary(getContext(), getLine(), getColumn(), getScope(),
-                        getInlinedAt());
+    // Get the raw scope/inlinedAt since it is possible to invoke this on
+    // a DILocation containing temporary metadata.
+    return getTemporary(getContext(), getLine(), getColumn(), getRawScope(),
+                        getRawInlinedAt());
   }
 
   // Disallow replacing operands.
   void replaceOperandWith(unsigned I, Metadata *New) = delete;
 
 public:
   DEFINE_MDNODE_GET(DILocation,
                     (unsigned Line, unsigned Column, Metadata *Scope,

include/llvm/IR/GVMaterializer.h

 // or interprocedural optimizers that do not need the entire program in memory
 // at the same time.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_IR_GVMATERIALIZER_H
 #define LLVM_IR_GVMATERIALIZER_H
 
+#include "llvm/ADT/DenseMap.h"
 #include <system_error>
 #include <vector>
 
 namespace llvm {
 class Function;
 class GlobalValue;
+class Metadata;
 class Module;
 class StructType;
 
 class GVMaterializer {
 protected:
   GVMaterializer() {}
 
 public:
public:
 
   /// Make sure the entire Module has been completely read.
   ///
   virtual std::error_code materializeModule(Module *M) = 0;
 
   virtual std::error_code materializeMetadata() = 0;
   virtual void setStripDebugInfo() = 0;
 
+  /// Client should define this interface if the mapping between metadata
+  /// values and value ids needs to be preserved, e.g. across materializer
+  /// instantiations. If OnlyTempMD is true, only those that have remained
+  /// temporary metadata are recorded in the map.
+  virtual void
+  saveMDValueList(DenseMap<const Metadata *, unsigned> &MDValueToValIDMap,
+                  bool OnlyTempMD) {}
+
   virtual std::vector<StructType *> getIdentifiedStructTypes() const = 0;
 };
 
 } // End llvm namespace
 
 #endif

include/llvm/IR/Metadata.h

void replaceAllUsesWith(Metadata *MD) {
     assert(isTemporary() && "Expected temporary node");
     assert(!isResolved() && "Expected RAUW support");
     Context.getReplaceableUses()->replaceAllUsesWith(MD);
   }
 
   /// \brief Resolve cycles.
   ///
   /// Once all forward declarations have been resolved, force cycles to be
-  /// resolved.
+  /// resolved. If \p MDMaterialized is true, then any temporary metadata
+  /// is ignored, otherwise it asserts when encountering temporary metadata.
   ///
   /// \pre No operands (or operands' operands, etc.) have \a isTemporary().
-  void resolveCycles();
+  void resolveCycles(bool MDMaterialized = true);
 
   /// \brief Replace a temporary node with a permanent one.
   ///
   /// Try to create a uniqued version of \c N -- in place, if possible -- and
   /// return it.  If \c N cannot be uniqued, return a distinct node instead.
   template <class T>
   static typename std::enable_if<std::is_base_of<MDNode, T>::value, T *>::type
   replaceWithPermanent(std::unique_ptr<T, TempMDNodeDeleter> N) {

include/llvm/IRReader/IRReader.h

 
 class Module;
 class SMDiagnostic;
 class LLVMContext;
 
 /// If the given file holds a bitcode image, return a Module
 /// for it which does lazy deserialization of function bodies.  Otherwise,
 /// attempt to parse it as LLVM Assembly and return a fully populated
-/// Module.
-std::unique_ptr<Module> getLazyIRFileModule(StringRef Filename,
-                                            SMDiagnostic &Err,
-                                            LLVMContext &Context);
+/// Module. The ShouldLazyLoadMetadata flag is passed down to the bitcode
+/// reader to optionally enable lazy metadata loading.
+std::unique_ptr<Module>
+getLazyIRFileModule(StringRef Filename, SMDiagnostic &Err, LLVMContext &Context,
+                    bool ShouldLazyLoadMetadata = false);
 
 /// If the given MemoryBuffer holds a bitcode image, return a Module
 /// for it.  Otherwise, attempt to parse it as LLVM Assembly and return
 /// a Module for it.
 std::unique_ptr<Module> parseIR(MemoryBufferRef Buffer, SMDiagnostic &Err,
                                 LLVMContext &Context);
 
 /// If the given file holds a bitcode image, return a Module for it.
 /// Otherwise, attempt to parse it as LLVM Assembly and return a Module
 /// for it.
 std::unique_ptr<Module> parseIRFile(StringRef Filename, SMDiagnostic &Err,
                                     LLVMContext &Context);
 }
 
 #endif

include/llvm/Linker/Linker.h

struct IdentifiedStructTypeSet {
     StructType *findNonOpaque(ArrayRef<Type *> ETypes, bool IsPacked);
     bool hasType(StructType *Ty);
   };
 
   enum Flags {
     None = 0,
     OverrideFromSrc = (1 << 0),
     LinkOnlyNeeded = (1 << 1),
-    InternalizeLinkedSymbols = (1 << 2)
+    InternalizeLinkedSymbols = (1 << 2),
+    /// Flag indicating whether this is simply a metadata linking postpass
+    /// when the functions and other global values were parsed and linked
+    /// by an earlier module link (e.g. during function importing).
+    MetadataLinkingPostpass = (1 << 3)
   };
 
   Linker(Module *M, DiagnosticHandlerFunction DiagnosticHandler);
   Linker(Module *M);
 
   Module *getModule() const { return Composite; }
   void deleteModule();
 
   /// \brief Link \p Src into the composite. The source is destroyed.
   /// Passing OverrideSymbols as true will have symbols from Src
   /// shadow those in the Dest.
   /// For ThinLTO function importing/exporting the \p FunctionInfoIndex
   /// is passed. If a \p FuncToImport is provided, only that single
-  /// function is imported from the source module.
+  /// function is imported from the source module. The ValIDToTempMDMap
+  /// is populated by the linker when function importing is performed, and
+  /// is passed as input along with the MetadataLinkingPostpass flag.
   /// Returns true on error.
   bool linkInModule(Module *Src, unsigned Flags = Flags::None,
                     FunctionInfoIndex *Index = nullptr,
-                    Function *FuncToImport = nullptr);
+                    Function *FuncToImport = nullptr,
+                    DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr);
 
   /// \brief Set the composite to the passed-in module.
   void setModule(Module *Dst);
 
   static bool LinkModules(Module *Dest, Module *Src,
                           DiagnosticHandlerFunction DiagnosticHandler,
                           unsigned Flags = Flags::None);
 

include/llvm/Transforms/Utils/ValueMapper.h

protected:
     ValueMaterializer(const ValueMaterializer&) = default;
     ValueMaterializer &operator=(const ValueMaterializer&) = default;
 
   public:
     /// materializeValueFor - The client should implement this method if they
     /// want to generate a mapped Value on demand. For example, if linking
     /// lazily.
     virtual Value *materializeValueFor(Value *V) = 0;
+
+    /// If the client needs to handle temporary metadata it must implement
+    /// these methods.
+    virtual Metadata *mapTemporaryMetadata(Metadata *MD) { return nullptr; }
+    virtual void replaceTemporaryMetadata(const Metadata *OrigMD,
+                                          Metadata *NewMD) {}
   };
 
   /// RemapFlags - These are flags that the value mapping APIs allow.
   enum RemapFlags {
     RF_None = 0,
 
     /// RF_NoModuleLevelChanges - If this flag is set, the remapper knows that
     /// only local values within a function (such as an instruction or argument)
     /// are mapped, not global values like functions and global metadata.
     RF_NoModuleLevelChanges = 1,
 
     /// RF_IgnoreMissingEntries - If this flag is set, the remapper ignores
     /// entries that are not in the value map.  If it is unset, it aborts if an
     /// operand is asked to be remapped which doesn't exist in the mapping.
     RF_IgnoreMissingEntries = 2,
 
     /// Instruct the remapper to move distinct metadata instead of duplicating
     /// it when there are module-level changes.
     RF_MoveDistinctMDs = 4,
 
     /// Any global values not in value map are mapped to null instead of
     /// mapping to self. Illegal if RF_IgnoreMissingEntries is also set.
     RF_NullMapMissingGlobalValues = 8,
+
+    /// Set when there is still temporary metadata that must be handled,
+    /// such as when we are doing function importing and will materialize
+    /// and link metadata as a postpass.
+    RF_HaveUnmaterializedMetadata = 16,
   };
 
   static inline RemapFlags operator|(RemapFlags LHS, RemapFlags RHS) {
     return RemapFlags(unsigned(LHS)|unsigned(RHS));
   }
 
   Value *MapValue(const Value *V, ValueToValueMapTy &VM,
                   RemapFlags Flags = RF_None,

lib/Bitcode/Reader/BitcodeReader.cpp

public:
   /// Once all constants are read, this method bulk resolves any forward
   /// references.
   void resolveConstantForwardRefs();
 };
 
 class BitcodeReaderMDValueList {
   unsigned NumFwdRefs;
   bool AnyFwdRefs;
+  bool SavedFwdRefs;
   unsigned MinFwdRef;
   unsigned MaxFwdRef;
   std::vector<TrackingMDRef> MDValuePtrs;
 
   LLVMContext &Context;
 public:
   BitcodeReaderMDValueList(LLVMContext &C)
-      : NumFwdRefs(0), AnyFwdRefs(false), Context(C) {}
+      : NumFwdRefs(0), AnyFwdRefs(false), SavedFwdRefs(false), Context(C) {}
+  ~BitcodeReaderMDValueList() {
+    // Assert that we either replaced all forward references, or saved
+    // them for later replacement.
+    assert(!NumFwdRefs || SavedFwdRefs);
+  }
 
   // vector compatibility methods
   unsigned size() const       { return MDValuePtrs.size(); }
   void resize(unsigned N)     { MDValuePtrs.resize(N); }
   void push_back(Metadata *MD) { MDValuePtrs.emplace_back(MD); }
   void clear()                { MDValuePtrs.clear();  }
   Metadata *back() const      { return MDValuePtrs.back(); }
   void pop_back()             { MDValuePtrs.pop_back(); }
   bool empty() const          { return MDValuePtrs.empty(); }
 
+  void savedFwdRefs() { SavedFwdRefs = true; }
+
   Metadata *operator[](unsigned i) const {
     assert(i < MDValuePtrs.size());
     return MDValuePtrs[i];
   }
 
   void shrinkTo(unsigned N) {
     assert(N <= size() && "Invalid shrinkTo request!");
     MDValuePtrs.resize(N);
public:
 
   static uint64_t decodeSignRotatedValue(uint64_t V);
 
   /// Materialize any deferred Metadata block.
   std::error_code materializeMetadata() override;
 
   void setStripDebugInfo() override;
 
+  /// Save the mapping between the metadata values and the corresponding
+  /// value id that were recorded in the MDValueList during parsing. If
+  /// OnlyTempMD is true, then only record those entries that are still
+  /// temporary metadata. This interface is used when metadata linking is
+  /// performed as a postpass, such as during function importing.
+  void saveMDValueList(DenseMap<const Metadata *, unsigned> &MDValueToValIDMap,
+                       bool OnlyTempMD) override;
+
 private:
   /// Parse the "IDENTIFICATION_BLOCK_ID" block, populate the
   // ProducerIdentification data member, and do some basic enforcement on the
   // "epoch" encoded in the bitcode.
   std::error_code parseBitcodeVersion();
 
   std::vector<StructType *> IdentifiedStructTypes;
   StructType *createIdentifiedStructType(LLVMContext &Context, StringRef Name);
Metadata *BitcodeReaderMDValueList::getValueFwdRef(unsigned Idx) {
   if (AnyFwdRefs) {
     MinFwdRef = std::min(MinFwdRef, Idx);
     MaxFwdRef = std::max(MaxFwdRef, Idx);
   } else {
     AnyFwdRefs = true;
     MinFwdRef = MaxFwdRef = Idx;
   }
   ++NumFwdRefs;
+  // Reset flag to ensure that we save this forward reference if we
+  // are delaying metadata mapping (e.g. for function importing).
+  SavedFwdRefs = false;
 
   // Create and return a placeholder, which will later be RAUW'd.
   Metadata *MD = MDNode::getTemporary(Context, None).release();
   MDValuePtrs[Idx].reset(MD);
   return MD;
 }
 
 void BitcodeReaderMDValueList::tryToResolveCycles() {
if (std::error_code EC = parseMetadata(true))
       return EC;
   }
   DeferredMetadataInfo.clear();
   return std::error_code();
 }
 
 void BitcodeReader::setStripDebugInfo() { StripDebugInfo = true; }
 
+void BitcodeReader::saveMDValueList(
+    DenseMap<const Metadata *, unsigned> &MDValueToValIDMap, bool OnlyTempMD) {
+  for (unsigned ValID = 0; ValID < MDValueList.size(); ++ValID) {
+    Metadata *MD = MDValueList[ValID];
+    auto *N = dyn_cast_or_null<MDNode>(MD);
+    // Save all values if !OnlyTempMD, otherwise just the temporary metadata.
+    if (!OnlyTempMD || (N && N->isTemporary())) {
+      // Will call this after materializing each function, in order to
+      // handle remapping of the function's instructions/metadata.
+      // See if we already have an entry in that case.
+      if (OnlyTempMD && MDValueToValIDMap.count(MD)) {
+        assert(MDValueToValIDMap[MD] == ValID &&
+               "Inconsistent metadata value id");
+        continue;
+      }
+      MDValueToValIDMap[MD] = ValID;
+      // Flag that we saved the forward refs (temporary metadata) for error
+      // checking during MDValueList destruction.
+      if (OnlyTempMD)
+        MDValueList.savedFwdRefs();
+    }
+  }
+}
+
 /// When we see the block for a function body, remember where it is and then
 /// skip it.  This lets us lazily deserialize the functions.
 std::error_code BitcodeReader::rememberAndSkipFunctionBody() {
   // Get the function we are talking about.
   if (FunctionsWithBodies.empty())
     return error("Insufficient function protos");
 
   Function *Fn = FunctionsWithBodies.back();

lib/IR/Metadata.cpp

 }
 
 void MDNode::decrementUnresolvedOperandCount() {
   if (!--NumUnresolved)
     // Last unresolved operand has just been resolved.
     resolve();
 }
 
-void MDNode::resolveCycles() {
+void MDNode::resolveCycles(bool MDMaterialized) {
   if (isResolved())
     return;
 
   // Resolve this node immediately.
   resolve();
 
   // Resolve all operands.
   for (const auto &Op : operands()) {
     auto *N = dyn_cast_or_null<MDNode>(Op);
     if (!N)
       continue;
 
+    if (N->isTemporary() && !MDMaterialized)
+      continue;
     assert(!N->isTemporary() &&
            "Expected all forward declarations to be resolved");
     if (!N->isResolved())
       N->resolveCycles();
   }
 }
 
 static bool hasSelfReference(MDNode *N) {

lib/IRReader/IRReader.cpp

namespace llvm {
   extern bool TimePassesIsEnabled;
 }
 
 static const char *const TimeIRParsingGroupName = "LLVM IR Parsing";
 static const char *const TimeIRParsingName = "Parse IR";
 
 static std::unique_ptr<Module>
 getLazyIRModule(std::unique_ptr<MemoryBuffer> Buffer, SMDiagnostic &Err,
-                LLVMContext &Context) {
+                LLVMContext &Context, bool ShouldLazyLoadMetadata) {
   if (isBitcode((const unsigned char *)Buffer->getBufferStart(),
                 (const unsigned char *)Buffer->getBufferEnd())) {
-    ErrorOr<std::unique_ptr<Module>> ModuleOrErr =
-        getLazyBitcodeModule(std::move(Buffer), Context);
+    ErrorOr<std::unique_ptr<Module>> ModuleOrErr = getLazyBitcodeModule(
+        std::move(Buffer), Context, nullptr, ShouldLazyLoadMetadata);
     if (std::error_code EC = ModuleOrErr.getError()) {
       Err = SMDiagnostic(Buffer->getBufferIdentifier(), SourceMgr::DK_Error,
                          EC.message());
       return nullptr;
     }
     return std::move(ModuleOrErr.get());
   }
 
   return parseAssembly(Buffer->getMemBufferRef(), Err, Context);
 }
 
 std::unique_ptr<Module> llvm::getLazyIRFileModule(StringRef Filename,
                                                   SMDiagnostic &Err,
-                                                  LLVMContext &Context) {
+                                                  LLVMContext &Context,
+                                                  bool ShouldLazyLoadMetadata) {
   ErrorOr<std::unique_ptr<MemoryBuffer>> FileOrErr =
       MemoryBuffer::getFileOrSTDIN(Filename);
   if (std::error_code EC = FileOrErr.getError()) {
     Err = SMDiagnostic(Filename, SourceMgr::DK_Error,
                        "Could not open input file: " + EC.message());
     return nullptr;
   }
 
-  return getLazyIRModule(std::move(FileOrErr.get()), Err, Context);
+  return getLazyIRModule(std::move(FileOrErr.get()), Err, Context,
+                         ShouldLazyLoadMetadata);
 }
 
 std::unique_ptr<Module> llvm::parseIR(MemoryBufferRef Buffer, SMDiagnostic &Err,
                                       LLVMContext &Context) {
   NamedRegionTimer T(TimeIRParsingName, TimeIRParsingGroupName,
                      TimePassesIsEnabled);
   if (isBitcode((const unsigned char *)Buffer.getBufferStart(),
                 (const unsigned char *)Buffer.getBufferEnd())) {

lib/Linker/LinkModules.cpp

 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/IR/GVMaterializer.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/TypeFinder.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include <cctype>

 public:
   ValueMaterializerTy(TypeMapTy &TypeMap, Module *DstM,
                       std::vector<GlobalValue *> &LazilyLinkGlobalValues,
                       ModuleLinker *ModLinker)
       : ValueMaterializer(), TypeMap(TypeMap), DstM(DstM),
         LazilyLinkGlobalValues(LazilyLinkGlobalValues), ModLinker(ModLinker) {}
 
   Value *materializeValueFor(Value *V) override;
+
+  /// Invoke ModuleLinker callback to save the mapping between the
+  /// given temporary metadata and its metadata value id. Used to support
+  /// metadata linking as a postpass for function importing.
+  Metadata *mapTemporaryMetadata(Metadata *MD) override;
+
+  /// Invoke ModuleLinker callback to replace any temporary metadata saved
+  /// for the source metadata's id with the new non-temporary metadata.
+  /// Used when metadata linking as a postpass for function importing.
+  void replaceTemporaryMetadata(const Metadata *OrigMD,
+                                Metadata *NewMD) override;
 };
 
 class LinkDiagnosticInfo : public DiagnosticInfo {
   const Twine &Msg;
 
 public:
   LinkDiagnosticInfo(DiagnosticSeverity Severity, const Twine &Msg);
   void print(DiagnosticPrinter &DP) const override;
class ModuleLinker {
   // Vector of GlobalValues to lazily link in.
   std::vector<GlobalValue *> LazilyLinkGlobalValues;
 
   DiagnosticHandlerFunction DiagnosticHandler;
 
   /// For symbol clashes, prefer those from Src.
   unsigned Flags;
 
+  /// Flags to pass to value mapper invocations.
+  RemapFlags ValueMapperFlags;
+
   /// Function index passed into ModuleLinker for using in function
   /// importing/exporting handling.
   FunctionInfoIndex *ImportIndex;
 
   /// Function to import from source module, all other functions are
   /// imported as declarations instead of definitions.
   Function *ImportFunction;
 
   /// 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;
 
   /// Set to true when all global value body linking is complete (including
   /// lazy linking). Used to prevent metadata linking from creating new
   /// references.
   bool DoneLinkingBodies;
 
+  /// Association between metadata values created during bitcode parsing and
+  /// the value id. Used to correlate temporary metadata created during
+  /// function importing with the final metadata parsed during the subsequent
+  /// metadata linking postpass.
+  DenseMap<const Metadata *, unsigned> MDValueToValIDMap;
+
+  /// 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;
+
 public:
   ModuleLinker(Module *dstM, Linker::IdentifiedStructTypeSet &Set, Module *srcM,
                DiagnosticHandlerFunction DiagnosticHandler, unsigned Flags,
                FunctionInfoIndex *Index = nullptr,
-               Function *FuncToImport = nullptr)
+               Function *FuncToImport = nullptr,
+               DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr)
       : DstM(dstM), SrcM(srcM), TypeMap(Set),
         ValMaterializer(TypeMap, DstM, LazilyLinkGlobalValues, this),
-        DiagnosticHandler(DiagnosticHandler), Flags(Flags), ImportIndex(Index),
+        DiagnosticHandler(DiagnosticHandler), Flags(Flags),
+        ValueMapperFlags(RF_MoveDistinctMDs), ImportIndex(Index),
         ImportFunction(FuncToImport), HasExportedFunctions(false),
-        DoneLinkingBodies(false) {
+        DoneLinkingBodies(false), ValIDToTempMDMap(ValIDToTempMDMap) {
     assert((ImportIndex || !ImportFunction) &&
            "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 && !ImportFunction)
       HasExportedFunctions = ImportIndex->hasExportedFunctions(SrcM);
+
+    assert(!(isMetadataLinkingPostpass() && (ImportIndex || ImportFunction)) &&
+           "Illegal to pass a function index or function to import into "
+           "metadata linking postpass");
+    assert((ValIDToTempMDMap ||
+            (!ImportFunction && !isMetadataLinkingPostpass())) &&
+           "Function importing and metadata postpass linking must both provide "
+           "a ValIDToTempMDMap");
+
+    // If appropriate, tell the value mapper that it can expect to see
+    // temporary metadata.
+    if (!shouldLinkMetadata())
+      ValueMapperFlags = ValueMapperFlags | RF_HaveUnmaterializedMetadata;
   }
 
   bool run();
 
   bool shouldOverrideFromSrc() { return Flags & Linker::OverrideFromSrc; }
   bool shouldLinkOnlyNeeded() { return Flags & Linker::LinkOnlyNeeded; }
   bool shouldInternalizeLinkedSymbols() {
     return Flags & Linker::InternalizeLinkedSymbols;
   }
+  bool isMetadataLinkingPostpass() {
+    return Flags & Linker::MetadataLinkingPostpass;
+  }
+
+  bool shouldLinkMetadata() {
+    // This will be non-null when we are importing or otherwise want to link
+    // metadata lazily (LTO), and when linking the metadata.
+    return ValIDToTempMDMap == nullptr || isMetadataLinkingPostpass();
+  }
 
   /// Handles cloning of a global values from the source module into
   /// the destination module, including setting the attributes and visibility.
   GlobalValue *copyGlobalValueProto(TypeMapTy &TypeMap, const GlobalValue *SGV,
                                     const GlobalValue *DGV = nullptr);
 
   /// Check if we should promote the given local value to global scope.
   bool doPromoteLocalToGlobal(const GlobalValue *SGV);
 
   /// Check if all global value body linking is complete.
   bool doneLinkingBodies() { return DoneLinkingBodies; }
 
+  /// Save the mapping between the given temporary metadata and its metadata
+  /// value id. Used to support metadata linking as a postpass for function
+  /// importing.
+  Metadata *mapTemporaryMetadata(Metadata *MD);
+
+  /// Replace any temporary metadata saved for the source metadata's id with
+  /// the new non-temporary metadata. Used when metadata linking as a postpass
+  /// for function importing.
+  void replaceTemporaryMetadata(const Metadata *OrigMD, Metadata *NewMD);
+
 private:
   bool shouldLinkFromSource(bool &LinkFromSrc, const GlobalValue &Dest,
                             const GlobalValue &Src);
 
   /// Helper method for setting a message and returning an error code.
   bool emitError(const Twine &Message) {
     DiagnosticHandler(LinkDiagnosticInfo(DS_Error, Message));
     return true;
private:
   /// into the destination module.
   GlobalVariable *copyGlobalVariableProto(TypeMapTy &TypeMap,
                                           const GlobalVariable *SGVar);
   Function *copyFunctionProto(TypeMapTy &TypeMap, const Function *SF);
   GlobalValue *copyGlobalAliasProto(TypeMapTy &TypeMap, const GlobalAlias *SGA);
 
   /// Helper methods to check if we are importing from or potentially
   /// exporting from the current source module.
-  bool isPerformingImport() { return ImportFunction != nullptr; }
-  bool isModuleExporting() { return HasExportedFunctions; }
+  bool isPerformingImport() const { return ImportFunction != 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.
if (auto *DGO = dyn_cast<GlobalObject>(DGV)) {
       DGO->setComdat(DC);
     }
   }
 
   LazilyLinkGlobalValues.push_back(SGV);
   return DGV;
 }
 
+Metadata *ValueMaterializerTy::mapTemporaryMetadata(Metadata *MD) {
+  return ModLinker->mapTemporaryMetadata(MD);
+}
+
+void ValueMaterializerTy::replaceTemporaryMetadata(const Metadata *OrigMD,
+                                                   Metadata *NewMD) {
+  ModLinker->replaceTemporaryMetadata(OrigMD, NewMD);
+}
+
+Metadata *ModuleLinker::mapTemporaryMetadata(Metadata *MD) {
+  if (!ValIDToTempMDMap)
+    return nullptr;
+  // If this temporary metadata has a value id recorded during function
+  // parsing, record that in the ValIDToTempMDMap if one was provided.
+  if (MDValueToValIDMap.count(MD)) {
+    unsigned Idx = MDValueToValIDMap[MD];
+    // Check if we created a temp MD when importing a different function from
+    // this module. If so, reuse it the same temporary metadata, otherwise
+    // add this temporary metadata to the map.
+    if (!ValIDToTempMDMap->count(Idx)) {
+      MDNode *Node = cast<MDNode>(MD);
+      assert(Node->isTemporary());
+      (*ValIDToTempMDMap)[Idx] = Node;
+    }
+    return (*ValIDToTempMDMap)[Idx];
+  }
+  return nullptr;
+}
+
+void ModuleLinker::replaceTemporaryMetadata(const Metadata *OrigMD,
+                                            Metadata *NewMD) {
+  if (!ValIDToTempMDMap)
+    return;
+#ifndef NDEBUG
+  auto *N = dyn_cast_or_null<MDNode>(NewMD);
+  assert(!N || !N->isTemporary());
+#endif
+  // If a mapping between metadata value ids and temporary metadata
+  // created during function importing was provided, and the source
+  // metadata has a value id recorded during metadata parsing, replace
+  // the temporary metadata with the final mapped metadata now.
+  if (MDValueToValIDMap.count(OrigMD)) {
+    unsigned Idx = MDValueToValIDMap[OrigMD];
+    // Nothing to do if we didn't need to create a temporary metadata during
+    // function importing.
+    if (!ValIDToTempMDMap->count(Idx))
+      return;
+    MDNode *TempMD = (*ValIDToTempMDMap)[Idx];
+    TempMD->replaceAllUsesWith(NewMD);
+    MDNode::deleteTemporary(TempMD);
+    ValIDToTempMDMap->erase(Idx);
+  }
+}
+
 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 +
bool ModuleLinker::getComdatResult(const Comdat *SrcC,
   Comdat::SelectionKind DSK = DstC->getSelectionKind();
   return computeResultingSelectionKind(ComdatName, SSK, DSK, Result,
                                        LinkFromSrc);
 }
 
 bool ModuleLinker::shouldLinkFromSource(bool &LinkFromSrc,
                                         const GlobalValue &Dest,
                                         const GlobalValue &Src) {
+  // Already imported all the values. Just map to the Dest value
+  // in case it is referenced in the metadata.
+  if (isMetadataLinkingPostpass()) {
+    LinkFromSrc = false;
+    return false;
+  }
+
   // 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()) {
void ModuleLinker::linkAppendingVarInit(const AppendingVarInfo &AVI) {
 
   for (auto *V : SrcElements) {
     if (IsNewStructor) {
       Constant *Key = V->getAggregateElement(2);
       if (DoNotLinkFromSource.count(Key))
         continue;
     }
     DstElements.push_back(
-        MapValue(V, ValueMap, RF_MoveDistinctMDs, &TypeMap, &ValMaterializer));
+        MapValue(V, ValueMap, ValueMapperFlags, &TypeMap, &ValMaterializer));
   }
   if (IsNewStructor) {
     NewType = ArrayType::get(NewType->getElementType(), DstElements.size());
     AVI.NewGV->mutateType(PointerType::get(NewType, 0));
   }
 
   AVI.NewGV->setInitializer(ConstantArray::get(NewType, DstElements));
 }
 
 /// Update the initializers in the Dest module now that all globals that may be
 /// referenced are in Dest.
 void ModuleLinker::linkGlobalInit(GlobalVariable &Dst, GlobalVariable &Src) {
   // Figure out what the initializer looks like in the dest module.
-  Dst.setInitializer(MapValue(Src.getInitializer(), ValueMap,
-                              RF_MoveDistinctMDs, &TypeMap, &ValMaterializer));
+  Dst.setInitializer(MapValue(Src.getInitializer(), ValueMap, ValueMapperFlags,
+                              &TypeMap, &ValMaterializer));
 }
 
 /// Copy the source function over into the dest function and fix up references
 /// to values. At this point we know that Dest is an external function, and
 /// that Src is not.
 bool ModuleLinker::linkFunctionBody(Function &Dst, Function &Src) {
   assert(Dst.isDeclaration() && !Src.isDeclaration());
 
   // Materialize if needed.
   if (std::error_code EC = Src.materialize())
     return emitError(EC.message());
 
+  if (!shouldLinkMetadata())
+    // This is only supported for lazy links. Do after materialization of
+    // a function and before remapping metadata on instructions below
+    // in RemapInstruction, as the saved mapping is used to handle
+    // the temporary metadata hanging off instructions.
+    SrcM->getMaterializer()->saveMDValueList(MDValueToValIDMap, true);
+
   // Link in the prefix data.
   if (Src.hasPrefixData())
-    Dst.setPrefixData(MapValue(Src.getPrefixData(), ValueMap,
-                               RF_MoveDistinctMDs, &TypeMap, &ValMaterializer));
+    Dst.setPrefixData(MapValue(Src.getPrefixData(), ValueMap, ValueMapperFlags,
+                               &TypeMap, &ValMaterializer));
 
   // Link in the prologue data.
   if (Src.hasPrologueData())
     Dst.setPrologueData(MapValue(Src.getPrologueData(), ValueMap,
-                                 RF_MoveDistinctMDs, &TypeMap,
-                                 &ValMaterializer));
+                                 ValueMapperFlags, &TypeMap, &ValMaterializer));
 
   // Link in the personality function.
   if (Src.hasPersonalityFn())
     Dst.setPersonalityFn(MapValue(Src.getPersonalityFn(), ValueMap,
-                                  RF_MoveDistinctMDs, &TypeMap,
+                                  ValueMapperFlags, &TypeMap,
                                   &ValMaterializer));
 
   // Go through and convert function arguments over, remembering the mapping.
   Function::arg_iterator DI = Dst.arg_begin();
   for (Argument &Arg : Src.args()) {
     DI->setName(Arg.getName());  // Copy the name over.
 
     // Add a mapping to our mapping.
     ValueMap[&Arg] = &*DI;
     ++DI;
   }
 
   // Copy over the metadata attachments.
   SmallVector<std::pair<unsigned, MDNode *>, 8> MDs;
   Src.getAllMetadata(MDs);
   for (const auto &I : MDs)
-    Dst.setMetadata(I.first, MapMetadata(I.second, ValueMap, RF_MoveDistinctMDs,
+    Dst.setMetadata(I.first, MapMetadata(I.second, ValueMap, ValueMapperFlags,
                                          &TypeMap, &ValMaterializer));
 
   // Splice the body of the source function into the dest function.
   Dst.getBasicBlockList().splice(Dst.end(), Src.getBasicBlockList());
 
   // At this point, all of the instructions and values of the function are now
   // copied over.  The only problem is that they are still referencing values in
   // the Source function as operands.  Loop through all of the operands of the
   // functions and patch them up to point to the local versions.
   for (BasicBlock &BB : Dst)
     for (Instruction &I : BB)
-      RemapInstruction(&I, ValueMap,
-                       RF_IgnoreMissingEntries | RF_MoveDistinctMDs, &TypeMap,
-                       &ValMaterializer);
+      RemapInstruction(&I, ValueMap, RF_IgnoreMissingEntries | ValueMapperFlags,
+                       &TypeMap, &ValMaterializer);
 
   // There is no need to map the arguments anymore.
   for (Argument &Arg : Src.args())
     ValueMap.erase(&Arg);
 
   Src.dematerialize();
   return false;
 }
 
 void ModuleLinker::linkAliasBody(GlobalAlias &Dst, GlobalAlias &Src) {
   Constant *Aliasee = Src.getAliasee();
-  Constant *Val = MapValue(Aliasee, ValueMap, RF_MoveDistinctMDs, &TypeMap,
-                           &ValMaterializer);
+  Constant *Val =
+      MapValue(Aliasee, ValueMap, ValueMapperFlags, &TypeMap, &ValMaterializer);
   Dst.setAliasee(Val);
 }
 
 bool ModuleLinker::linkGlobalValueBody(GlobalValue &Src) {
   Value *Dst = ValueMap[&Src];
   assert(Dst);
   if (const Comdat *SC = Src.getComdat()) {
     // To ensure that we don't generate an incomplete comdat group,
void ModuleLinker::linkNamedMDNodes() {
   for (const NamedMDNode &NMD : SrcM->named_metadata()) {
     // Don't link module flags here. Do them separately.
     if (&NMD == SrcModFlags)
       continue;
     NamedMDNode *DestNMD = DstM->getOrInsertNamedMetadata(NMD.getName());
     // Add Src elements into Dest node.
     for (const MDNode *op : NMD.operands())
       DestNMD->addOperand(MapMetadata(
-          op, ValueMap, RF_MoveDistinctMDs | RF_NullMapMissingGlobalValues,
+          op, ValueMap, ValueMapperFlags | RF_NullMapMissingGlobalValues,
           &TypeMap, &ValMaterializer));
   }
 }
 
 /// Merge the linker flags in Src into the Dest module.
 bool ModuleLinker::linkModuleFlagsMetadata() {
   // If the source module has no module flags, we are done.
   const NamedMDNode *SrcModFlags = SrcM->getModuleFlagsMetadata();
for (Function &F :*SrcM)
     if (linkGlobalValueProto(&F))
       return true;
 
   // If there were any aliases, link them now.
   for (GlobalAlias &GA : SrcM->aliases())
     if (linkGlobalValueProto(&GA))
       return true;
 
+  if (isMetadataLinkingPostpass()) {
+    // Ensure metadata materialized
+    if (SrcM->getMaterializer()->materializeMetadata())
+      return true;
+    SrcM->getMaterializer()->saveMDValueList(MDValueToValIDMap, false);
+
+    linkNamedMDNodes();
+
+    // Handle anything left in the ValIDToTempMDMap, such as metadata nodes
+    // not reached by the dbg.cu NamedMD (i.e. only reached from instructions).
+    // Walk the MDValueToValIDMap once to find the set of new (imported) MD
+    // that still has corresponding temporary metadata, and invoke metadata
+    // mapping on each one.
+    for (auto MDI : MDValueToValIDMap) {
+      if (!ValIDToTempMDMap->count(MDI.second))
+        continue;
+      MapMetadata(MDI.first, ValueMap, ValueMapperFlags, &TypeMap,
+                  &ValMaterializer);
+    }
+    assert(ValIDToTempMDMap->empty());
+
+    // Merge the module flags into the DstM module.
+    if (linkModuleFlagsMetadata())
+      return true;
+
+    return false;
+  }
+
   for (const AppendingVarInfo &AppendingVar : AppendingVars)
     linkAppendingVarInit(AppendingVar);
 
   for (const auto &Entry : DstM->getComdatSymbolTable()) {
     const Comdat &C = Entry.getValue();
     if (C.getSelectionKind() == Comdat::Any)
       continue;
     const GlobalValue *GV = SrcM->getNamedValue(C.getName());
     if (GV)
-      MapValue(GV, ValueMap, RF_MoveDistinctMDs, &TypeMap, &ValMaterializer);
+      MapValue(GV, ValueMap, ValueMapperFlags, &TypeMap, &ValMaterializer);
   }
 
   // Link in the function bodies that are defined in the source module into
   // DstM.
   for (Function &SF : *SrcM) {
     // Skip if no body (function is external).
     if (SF.isDeclaration())
       continue;
bool ModuleLinker::run() {
 
   // Note that we are done linking global value bodies. This prevents
   // metadata linking from creating new references.
   DoneLinkingBodies = true;
 
   // Remap all of the named MDNodes in Src into the DstM module. We do this
   // after linking GlobalValues so that MDNodes that reference GlobalValues
   // are properly remapped.
+  if (shouldLinkMetadata()) {
     linkNamedMDNodes();
 
     // Merge the module flags into the DstM module.
     if (linkModuleFlagsMetadata())
       return true;
+  }
 
   return false;
 }
 
 Linker::StructTypeKeyInfo::KeyTy::KeyTy(ArrayRef<Type *> E, bool P)
     : ETypes(E), IsPacked(P) {}
 
 Linker::StructTypeKeyInfo::KeyTy::KeyTy(const StructType *ST)

 }
 
 void Linker::deleteModule() {
   delete Composite;
   Composite = nullptr;
 }
 
 bool Linker::linkInModule(Module *Src, unsigned Flags, FunctionInfoIndex *Index,
-                          Function *FuncToImport) {
+                          Function *FuncToImport,
+                          DenseMap<unsigned, MDNode *> *ValIDToTempMDMap) {
   ModuleLinker TheLinker(Composite, IdentifiedStructTypes, Src,
-                         DiagnosticHandler, Flags, Index, FuncToImport);
+                         DiagnosticHandler, Flags, Index, FuncToImport,
+                         ValIDToTempMDMap);
   bool RetCode = TheLinker.run();
+  // No need to do this if we were just linking in metadata.
+  if (!(Flags & Linker::MetadataLinkingPostpass))
     Composite->dropTriviallyDeadConstantArrays();
   return RetCode;
 }
 
 void Linker::setModule(Module *Dst) {
   init(Dst, DiagnosticHandler);
 }
 
 //===----------------------------------------------------------------------===//

lib/Transforms/Utils/ValueMapper.cpp

if (isa<UndefValue>(C))
     return VM[V] = UndefValue::get(NewTy);
   if (isa<ConstantAggregateZero>(C))
     return VM[V] = ConstantAggregateZero::get(NewTy);
   assert(isa<ConstantPointerNull>(C));
   return VM[V] = ConstantPointerNull::get(cast<PointerType>(NewTy));
 }
 
 static Metadata *mapToMetadata(ValueToValueMapTy &VM, const Metadata *Key,
-                     Metadata *Val) {
+                               Metadata *Val, ValueMaterializer *Materializer,
+                               RemapFlags Flags) {
   VM.MD()[Key].reset(Val);
+  if (Materializer && !(Flags & RF_HaveUnmaterializedMetadata)) {
+    auto *N = dyn_cast_or_null<MDNode>(Val);
+    // Need to invoke this once we have non-temporary MD.
+    if (!N || !N->isTemporary())
+      Materializer->replaceTemporaryMetadata(Key, Val);
+  }
   return Val;
 }
 
-static Metadata *mapToSelf(ValueToValueMapTy &VM, const Metadata *MD) {
-  return mapToMetadata(VM, MD, const_cast<Metadata *>(MD));
+static Metadata *mapToSelf(ValueToValueMapTy &VM, const Metadata *MD,
+                           ValueMaterializer *Materializer, RemapFlags Flags) {
+  return mapToMetadata(VM, MD, const_cast<Metadata *>(MD), Materializer, Flags);
 }
 
 static Metadata *MapMetadataImpl(const Metadata *MD,
                                  SmallVectorImpl<MDNode *> &DistinctWorklist,
                                  ValueToValueMapTy &VM, RemapFlags Flags,
                                  ValueMapTypeRemapper *TypeMapper,
                                  ValueMaterializer *Materializer);
 
static Metadata *mapMetadataOp(Metadata *Op,
   // split.
   //
   //    assert((Flags & RF_NullMapMissingGlobalValues) &&
   //           "Referenced metadata not in value map!");
   return nullptr;
 }
 
 /// Resolve uniquing cycles involving the given metadata.
-static void resolveCycles(Metadata *MD) {
-  if (auto *N = dyn_cast_or_null<MDNode>(MD))
+static void resolveCycles(Metadata *MD, bool MDMaterialized) {
+  if (auto *N = dyn_cast_or_null<MDNode>(MD)) {
+    if (!MDMaterialized && N->isTemporary())
+      return;
     if (!N->isResolved())
-      N->resolveCycles();
+      N->resolveCycles(MDMaterialized);
+  }
 }
 
 /// Remap the operands of an MDNode.
 ///
 /// If \c Node is temporary, uniquing cycles are ignored.  If \c Node is
 /// distinct, uniquing cycles are resolved as they're found.
 ///
 /// \pre \c Node.isDistinct() or \c Node.isTemporary().
Metadata *New = mapMetadataOp(Old, DistinctWorklist, VM, Flags, TypeMapper,
                                   Materializer);
     if (Old != New) {
       AnyChanged = true;
       Node.replaceOperandWith(I, New);
 
       // Resolve uniquing cycles underneath distinct nodes on the fly so they
       // don't infect later operands.
       if (IsDistinct)
-        resolveCycles(New);
+        resolveCycles(New, !(Flags & RF_HaveUnmaterializedMetadata));
     }
   }
 
   return AnyChanged;
 }
 
 /// Map a distinct MDNode.
 ///
static Metadata *mapDistinctNode(const MDNode *Node,
   MDNode *NewMD;
   if (Flags & RF_MoveDistinctMDs)
     NewMD = const_cast<MDNode *>(Node);
   else
     NewMD = MDNode::replaceWithDistinct(Node->clone());
 
   // Remap operands later.
   DistinctWorklist.push_back(NewMD);
-  return mapToMetadata(VM, Node, NewMD);
+  return mapToMetadata(VM, Node, NewMD, Materializer, Flags);
 }
 
 /// \brief Map a uniqued MDNode.
 ///
 /// Uniqued nodes may not need to be recreated (they may map to themselves).
 static Metadata *mapUniquedNode(const MDNode *Node,
                                 SmallVectorImpl<MDNode *> &DistinctWorklist,
                                 ValueToValueMapTy &VM, RemapFlags Flags,
                                 ValueMapTypeRemapper *TypeMapper,
                                 ValueMaterializer *Materializer) {
-  assert(Node->isUniqued() && "Expected uniqued node");
+  assert(((Flags & RF_HaveUnmaterializedMetadata) || Node->isUniqued()) &&
+         "Expected uniqued node");
 
   // Create a temporary node and map it upfront in case we have a uniquing
   // cycle.  If necessary, this mapping will get updated by RAUW logic before
   // returning.
   auto ClonedMD = Node->clone();
-  mapToMetadata(VM, Node, ClonedMD.get());
+  mapToMetadata(VM, Node, ClonedMD.get(), Materializer, Flags);
   if (!remapOperands(*ClonedMD, DistinctWorklist, VM, Flags, TypeMapper,
                      Materializer)) {
     // No operands changed, so use the original.
     ClonedMD->replaceAllUsesWith(const_cast<MDNode *>(Node));
-    return const_cast<MDNode *>(Node);
+    // Even though replaceAllUsesWith would have replaced the value map
+    // entry, we need to explictly map with the final non-temporary node
+    // to replace any temporary metadata via the callback.
+    return mapToSelf(VM, Node, Materializer, Flags);
   }
 
-  // Uniquify the cloned node.
-  return MDNode::replaceWithUniqued(std::move(ClonedMD));
+  // Uniquify the cloned node. Explicitly map it with the final non-temporary
+  // node so that replacement of temporary metadata via the callback occurs.
+  return mapToMetadata(VM, Node,
+                       MDNode::replaceWithUniqued(std::move(ClonedMD)),
+                       Materializer, Flags);
 }
 
 static Metadata *MapMetadataImpl(const Metadata *MD,
                                  SmallVectorImpl<MDNode *> &DistinctWorklist,
                                  ValueToValueMapTy &VM, RemapFlags Flags,
                                  ValueMapTypeRemapper *TypeMapper,
                                  ValueMaterializer *Materializer) {
   // If the value already exists in the map, use it.
   if (Metadata *NewMD = VM.MD().lookup(MD).get())
     return NewMD;
 
   if (isa<MDString>(MD))
-    return mapToSelf(VM, MD);
+    return mapToSelf(VM, MD, Materializer, Flags);
 
   if (isa<ConstantAsMetadata>(MD))
     if ((Flags & RF_NoModuleLevelChanges))
-      return mapToSelf(VM, MD);
+      return mapToSelf(VM, MD, Materializer, Flags);
 
   if (const auto *VMD = dyn_cast<ValueAsMetadata>(MD)) {
     Value *MappedV =
         MapValue(VMD->getValue(), VM, Flags, TypeMapper, Materializer);
     if (VMD->getValue() == MappedV ||
         (!MappedV && (Flags & RF_IgnoreMissingEntries)))
-      return mapToSelf(VM, MD);
+      return mapToSelf(VM, MD, Materializer, Flags);
 
     // FIXME: This assert crashes during bootstrap, but I think it should be
     // correct.  For now, just match behaviour from before the metadata/value
     // split.
     //
     //    assert((MappedV || (Flags & RF_NullMapMissingGlobalValues)) &&
     //           "Referenced metadata not in value map!");
     if (MappedV)
-      return mapToMetadata(VM, MD, ValueAsMetadata::get(MappedV));
+      return mapToMetadata(VM, MD, ValueAsMetadata::get(MappedV), Materializer,
+                           Flags);
     return nullptr;
   }
 
   // Note: this cast precedes the Flags check so we always get its associated
   // assertion.
   const MDNode *Node = cast<MDNode>(MD);
 
   // If this is a module-level metadata and we know that nothing at the
   // module level is changing, then use an identity mapping.
   if (Flags & RF_NoModuleLevelChanges)
-    return mapToSelf(VM, MD);
+    return mapToSelf(VM, MD, Materializer, Flags);
 
   // Require resolved nodes whenever metadata might be remapped.
-  assert(Node->isResolved() && "Unexpected unresolved node");
+  assert(((Flags & RF_HaveUnmaterializedMetadata) || Node->isResolved()) &&
+         "Unexpected unresolved node");
+
+  if (Materializer && Node->isTemporary()) {
+    assert(Flags & RF_HaveUnmaterializedMetadata);
+    Metadata *TempMD =
+        Materializer->mapTemporaryMetadata(const_cast<Metadata *>(MD));
+    // If the above callback returned an existing temporary node, use it
+    // instead of the current temporary node. This happens when earlier
+    // function importing passes already created and saved a temporary
+    // metadata node for the same value id.
+    if (TempMD) {
+      mapToMetadata(VM, MD, TempMD, Materializer, Flags);
+      return TempMD;
+    }
+  }
 
   if (Node->isDistinct())
     return mapDistinctNode(Node, DistinctWorklist, VM, Flags, TypeMapper,
                            Materializer);
 
   return mapUniquedNode(Node, DistinctWorklist, VM, Flags, TypeMapper,
                         Materializer);
 }
 
 Metadata *llvm::MapMetadata(const Metadata *MD, ValueToValueMapTy &VM,
                             RemapFlags Flags, ValueMapTypeRemapper *TypeMapper,
                             ValueMaterializer *Materializer) {
   SmallVector<MDNode *, 8> DistinctWorklist;
   Metadata *NewMD = MapMetadataImpl(MD, DistinctWorklist, VM, Flags, TypeMapper,
                                     Materializer);
 
   // When there are no module-level changes, it's possible that the metadata
   // graph has temporaries.  Skip the logic to resolve cycles, since it's
   // unnecessary (and invalid) in that case.
   if (Flags & RF_NoModuleLevelChanges)
     return NewMD;
 
   // Resolve cycles involving the entry metadata.
-  resolveCycles(NewMD);
+  resolveCycles(NewMD, !(Flags & RF_HaveUnmaterializedMetadata));
 
   // Remap the operands of distinct MDNodes.
   while (!DistinctWorklist.empty())
     remapOperands(*DistinctWorklist.pop_back_val(), DistinctWorklist, VM, Flags,
                   TypeMapper, Materializer);
 
   return NewMD;
 }

test/Linker/Inputs/thinlto_funcimport_debug.ll

+; ModuleID = 'dbg_main.o'
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind uwtable
+define i32 @main() #0 !dbg !4 {
+entry:
+  %call = tail call i32 @func1(i32 10) #2, !dbg !11
+  %call1 = tail call i32 @func2(i32 10) #2, !dbg !12
+  ret i32 0, !dbg !13
+}
+
+declare i32 @func1(i32) #1
+
+declare i32 @func2(i32) #1
+
+attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!8, !9}
+!llvm.ident = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.8.0 (trunk 251407) (llvm/trunk 251401)", isOptimized: true, runtimeVersion: 0, emissionKind: 1, enums: !2, subprograms: !3)
+!1 = !DIFile(filename: "dbg_main.c", directory: "/usr/local/google/home/tejohnson/llvm/tmp")
+!2 = !{}
+!3 = !{!4}
+!4 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 3, type: !5, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true, variables: !2)
+!5 = !DISubroutineType(types: !6)
+!6 = !{!7}
+!7 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!8 = !{i32 2, !"Dwarf Version", i32 4}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{!"clang version 3.8.0 (trunk 251407) (llvm/trunk 251401)"}
+!11 = !DILocation(line: 4, column: 3, scope: !4)
+!12 = !DILocation(line: 5, column: 3, scope: !4)
+!13 = !DILocation(line: 6, column: 1, scope: !4)

test/Linker/thinlto_funcimport_debug.ll

+; Do setup work for all below tests: generate bitcode and combined index
+; RUN: llvm-as -function-summary %s -o %t.bc
+; RUN: llvm-as -function-summary %p/Inputs/thinlto_funcimport_debug.ll -o %t2.bc
+; RUN: llvm-lto -thinlto -o %t3 %t.bc %t2.bc
+
+; If we import func1 and not func2 we should only link DISubprogram for func1
+; RUN: llvm-link %t2.bc -functionindex=%t3.thinlto.bc -import=func1:%t.bc -S | FileCheck %s
+
+; CHECK: declare i32 @func2
+; CHECK: define available_externally i32 @func1
+; CHECK: distinct !DISubprogram(name: "func1"
+; CHECK: distinct !DISubprogram(name: "func2"
+
+; ModuleID = 'dbg.o'
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind readnone uwtable
+define i32 @func1(i32 %n) #0 !dbg !4 {
+entry:
+  tail call void @llvm.dbg.value(metadata i32 %n, i64 0, metadata !9, metadata !17), !dbg !18
+  tail call void @llvm.dbg.value(metadata i32 5, i64 0, metadata !10, metadata !17), !dbg !19
+  %cmp = icmp sgt i32 %n, 10, !dbg !20
+  %. = select i1 %cmp, i32 10, i32 5, !dbg !22
+  tail call void @llvm.dbg.value(metadata i32 %., i64 0, metadata !10, metadata !17), !dbg !19
+  ret i32 %., !dbg !23
+}
+
+; Function Attrs: nounwind readnone uwtable
+define i32 @func2(i32 %n) #0 !dbg !11 {
+entry:
+  tail call void @llvm.dbg.value(metadata i32 %n, i64 0, metadata !13, metadata !17), !dbg !24
+  ret i32 %n, !dbg !25
+}
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata) #1
+
+attributes #0 = { nounwind readnone uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!14, !15}
+!llvm.ident = !{!16}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.8.0 (trunk 251407) (llvm/trunk 251401)", isOptimized: true, runtimeVersion: 0, emissionKind: 1, enums: !2, subprograms: !3)
+!1 = !DIFile(filename: "dbg.c", directory: "/usr/local/google/home/tejohnson/llvm/tmp")
+!2 = !{}
+!3 = !{!4, !11}
+!4 = distinct !DISubprogram(name: "func1", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, variables: !8)
+!5 = !DISubroutineType(types: !6)
+!6 = !{!7, !7}
+!7 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!8 = !{!9, !10}
+!9 = !DILocalVariable(name: "n", arg: 1, scope: !4, file: !1, line: 1, type: !7)
+!10 = !DILocalVariable(name: "x", scope: !4, file: !1, line: 2, type: !7)
+!11 = distinct !DISubprogram(name: "func2", scope: !1, file: !1, line: 8, type: !5, isLocal: false, isDefinition: true, scopeLine: 8, flags: DIFlagPrototyped, isOptimized: true, variables: !12)
+!12 = !{!13}
+!13 = !DILocalVariable(name: "n", arg: 1, scope: !11, file: !1, line: 8, type: !7)
+!14 = !{i32 2, !"Dwarf Version", i32 4}
+!15 = !{i32 2, !"Debug Info Version", i32 3}
+!16 = !{!"clang version 3.8.0 (trunk 251407) (llvm/trunk 251401)"}
+!17 = !DIExpression()
+!18 = !DILocation(line: 1, column: 15, scope: !4)
+!19 = !DILocation(line: 2, column: 7, scope: !4)
+!20 = !DILocation(line: 3, column: 9, scope: !21)
+!21 = distinct !DILexicalBlock(scope: !4, file: !1, line: 3, column: 7)
+!22 = !DILocation(line: 3, column: 7, scope: !4)
+!23 = !DILocation(line: 5, column: 3, scope: !4)
+!24 = !DILocation(line: 8, column: 15, scope: !11)
+!25 = !DILocation(line: 9, column: 3, scope: !11)

tools/llvm-link/llvm-link.cpp

 static cl::opt<bool> PreserveAssemblyUseListOrder(
     "preserve-ll-uselistorder",
     cl::desc("Preserve use-list order when writing LLVM assembly."),
     cl::init(false), cl::Hidden);
 
 // Read the specified bitcode file in and return it. This routine searches the
 // link path for the specified file to try to find it...
 //
-static std::unique_ptr<Module>
-loadFile(const char *argv0, const std::string &FN, LLVMContext &Context) {
+static std::unique_ptr<Module> loadFile(const char *argv0,
+                                        const std::string &FN,
+                                        LLVMContext &Context,
+                                        bool MaterializeMetadata = true) {
   SMDiagnostic Err;
   if (Verbose) errs() << "Loading '" << FN << "'\n";
-  std::unique_ptr<Module> Result = getLazyIRFileModule(FN, Err, Context);
+  std::unique_ptr<Module> Result =
+      getLazyIRFileModule(FN, Err, Context, !MaterializeMetadata);
   if (!Result)
     Err.print(argv0, errs());
 
+  if (MaterializeMetadata) {
     Result->materializeMetadata();
     UpgradeDebugInfo(*Result);
+  }
 
   return Result;
 }
 
 static void diagnosticHandler(const DiagnosticInfo &DI) {
   unsigned Severity = DI.getSeverity();
   switch (Severity) {
   case DS_Error:
loadIndex(LLVMContext &Context, const Module *ExportingModule = nullptr) {
 
   object::FunctionIndexObjectFile &Obj = **ObjOrErr;
   return Obj.takeIndex();
 }
 
 /// Import any functions requested via the -import option.
 static bool importFunctions(const char *argv0, LLVMContext &Context,
                             Linker &L) {
+  StringMap<std::unique_ptr<DenseMap<unsigned, MDNode *>>>
+      ModuleToTempMDValsMap;
   for (const auto &Import : Imports) {
     // Identify the requested function and its bitcode source file.
     size_t Idx = Import.find(':');
     if (Idx == std::string::npos) {
       errs() << "Import parameter bad format: " << Import << "\n";
       return false;
     }
     std::string FunctionName = Import.substr(0, Idx);
     std::string FileName = Import.substr(Idx + 1, std::string::npos);
 
     // Load the specified source module.
-    std::unique_ptr<Module> M = loadFile(argv0, FileName, Context);
+    std::unique_ptr<Module> M = loadFile(argv0, FileName, Context, false);
     if (!M.get()) {
       errs() << argv0 << ": error loading file '" << FileName << "'\n";
       return false;
     }
 
     if (verifyModule(*M, &errs())) {
       errs() << argv0 << ": " << FileName
              << ": error: input module is broken!\n";
if (!FunctionIndex.empty()) {
       std::error_code EC = IndexOrErr.getError();
       if (EC) {
         errs() << EC.message() << '\n';
         return false;
       }
       Index = std::move(IndexOrErr.get());
     }
 
+    // Save the mapping of value ids to temporary metadata created when
+    // importing this function. If we have already imported from this module,
+    // add new temporary metadata to the existing mapping.
+    auto &TempMDVals = ModuleToTempMDValsMap[FileName];
+    if (!TempMDVals)
+      TempMDVals = llvm::make_unique<DenseMap<unsigned, MDNode *>>();
+
     // Link in the specified function.
-    if (L.linkInModule(M.get(), Linker::Flags::None, Index.get(), F))
+    if (L.linkInModule(M.get(), Linker::Flags::None, Index.get(), F,
+                       TempMDVals.get()))
+      return false;
+  }
+
+  // Now link in metadata for all modules from which we imported functions.
+  for (StringMapEntry<std::unique_ptr<DenseMap<unsigned, MDNode *>>> &SME :
+       ModuleToTempMDValsMap) {
+    // Load the specified source module.
+    std::unique_ptr<Module> M = loadFile(argv0, SME.getKey(), Context, true);
+    if (!M.get()) {
+      errs() << argv0 << ": error loading file '" << SME.getKey() << "'\n";
+      return false;
+    }
+
+    if (verifyModule(*M, &errs())) {
+      errs() << argv0 << ": " << SME.getKey()
+             << ": error: input module is broken!\n";
+      return false;
+    }
+
+    // Link in all necessary metadata from this module.
+    if (L.linkInModule(M.get(), Linker::Flags::MetadataLinkingPostpass, nullptr,
+                       nullptr, SME.getValue().get()))
       return false;
   }
   return true;
 }
 
 static bool linkFiles(const char *argv0, LLVMContext &Context, Linker &L,
                       const cl::list<std::string> &Files,
                       unsigned Flags) {