[llvm] r255909 - [ThinLTO] Metadata linking for imported functions

Duncan P. N. Exon Smith via llvm-commits llvm-commits at lists.llvm.org
Wed Dec 23 08:34:23 PST 2015


> On 2015-Dec-17, at 12:14, Teresa Johnson via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: tejohnson
> Date: Thu Dec 17 11:14:09 2015
> New Revision: 255909
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=255909&view=rev
> Log:
> [ThinLTO] Metadata linking for imported functions
> 
> 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.
> 
> Reviewers: dexonsmith, joker.eph
> 
> Subscribers: davidxl, llvm-commits, joker.eph
> 
> Differential Revision: http://reviews.llvm.org/D14838
> 
> Added:
>    llvm/trunk/test/Linker/Inputs/thinlto_funcimport_debug.ll
>    llvm/trunk/test/Linker/thinlto_funcimport_debug.ll
>    llvm/trunk/test/Transforms/FunctionImport/Inputs/funcimport_debug.ll
>    llvm/trunk/test/Transforms/FunctionImport/funcimport_debug.ll
> Modified:
>    llvm/trunk/include/llvm/IR/GVMaterializer.h
>    llvm/trunk/include/llvm/IR/Metadata.h
>    llvm/trunk/include/llvm/IRReader/IRReader.h
>    llvm/trunk/include/llvm/Linker/IRMover.h
>    llvm/trunk/include/llvm/Linker/Linker.h
>    llvm/trunk/include/llvm/Transforms/Utils/ValueMapper.h
>    llvm/trunk/lib/Bitcode/Reader/BitcodeReader.cpp
>    llvm/trunk/lib/IR/Metadata.cpp
>    llvm/trunk/lib/IRReader/IRReader.cpp
>    llvm/trunk/lib/Linker/IRMover.cpp
>    llvm/trunk/lib/Linker/LinkModules.cpp
>    llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp
>    llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp
>    llvm/trunk/tools/llvm-link/llvm-link.cpp
> 
> Modified: llvm/trunk/include/llvm/IR/GVMaterializer.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/GVMaterializer.h?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/IR/GVMaterializer.h (original)
> +++ llvm/trunk/include/llvm/IR/GVMaterializer.h Thu Dec 17 11:14:09 2015
> @@ -18,12 +18,14 @@
> #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;
> 
> @@ -56,6 +58,14 @@ public:
>   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) {}

Please update callers to say `/* OnlyTempMD = */ true/false`.  I found
it hard to know which was which.

> +
>   virtual std::vector<StructType *> getIdentifiedStructTypes() const = 0;
> };
> 
> 
> Modified: llvm/trunk/include/llvm/IR/Metadata.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Metadata.h?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/IR/Metadata.h (original)
> +++ llvm/trunk/include/llvm/IR/Metadata.h Thu Dec 17 11:14:09 2015
> @@ -832,10 +832,11 @@ public:
>   /// \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.

Please expand a little on the semantic difference between `true` and
`false` here.  In the `false` case, you're blocking identical `MDNode`s
from getting uniqued together.

>   ///
>   /// \pre No operands (or operands' operands, etc.) have \a isTemporary().
> -  void resolveCycles();
> +  void resolveCycles(bool MDMaterialized = true);

I think this variable would be better named: "AllowTemps" (same comment
for the definition as well).

> 
>   /// \brief Replace a temporary node with a permanent one.
>   ///
> 
> Modified: llvm/trunk/include/llvm/IRReader/IRReader.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IRReader/IRReader.h?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/IRReader/IRReader.h (original)
> +++ llvm/trunk/include/llvm/IRReader/IRReader.h Thu Dec 17 11:14:09 2015
> @@ -27,10 +27,11 @@ 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
> 
> Modified: llvm/trunk/include/llvm/Linker/IRMover.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Linker/IRMover.h?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Linker/IRMover.h (original)
> +++ llvm/trunk/include/llvm/Linker/IRMover.h Thu Dec 17 11:14:09 2015
> @@ -16,6 +16,7 @@
> 
> namespace llvm {
> class GlobalValue;
> +class MDNode;
> class Module;
> class StructType;
> class Type;
> @@ -60,7 +61,9 @@ public:
>   /// Move in the provide values. The source is destroyed.
>   /// Returns true on error.
>   bool move(Module &Src, ArrayRef<GlobalValue *> ValuesToLink,
> -            std::function<void(GlobalValue &GV, ValueAdder Add)> AddLazyFor);
> +            std::function<void(GlobalValue &GV, ValueAdder Add)> AddLazyFor,
> +            DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr,
> +            bool IsMetadataLinkingPostpass = false);
>   Module &getModule() { return Composite; }
> 
> private:
> 
> Modified: llvm/trunk/include/llvm/Linker/Linker.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Linker/Linker.h?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Linker/Linker.h (original)
> +++ llvm/trunk/include/llvm/Linker/Linker.h Thu Dec 17 11:14:09 2015
> @@ -42,11 +42,14 @@ public:
>   /// For ThinLTO function importing/exporting the \p FunctionInfoIndex
>   /// is passed. If \p FunctionsToImport is provided, only the functions that
>   /// are part of the set will be imported from the source module.
> +  /// The \p ValIDToTempMDMap is populated by the linker when function
> +  /// importing is performed.
>   ///
>   /// Returns true on error.
>   bool linkInModule(std::unique_ptr<Module> Src, unsigned Flags = Flags::None,
>                     const FunctionInfoIndex *Index = nullptr,
> -                    DenseSet<const GlobalValue *> *FunctionsToImport = nullptr);
> +                    DenseSet<const GlobalValue *> *FunctionsToImport = nullptr,
> +                    DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr);
> 
>   /// This exists to implement the deprecated LLVMLinkModules C api. Don't use
>   /// for anything else.
> @@ -54,6 +57,14 @@ public:
> 
>   static bool linkModules(Module &Dest, std::unique_ptr<Module> Src,
>                           unsigned Flags = Flags::None);
> +
> +  /// \brief Link metadata from \p Src into the composite. The source is
> +  /// destroyed.
> +  ///
> +  /// The \p ValIDToTempMDMap sound have been populated earlier during function
> +  /// importing from \p Src.
> +  bool linkInMetadata(Module &Src,
> +                      DenseMap<unsigned, MDNode *> *ValIDToTempMDMap);
> };
> 
> /// Create a new module with exported local functions renamed and promoted
> 
> Modified: llvm/trunk/include/llvm/Transforms/Utils/ValueMapper.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Transforms/Utils/ValueMapper.h?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Transforms/Utils/ValueMapper.h (original)
> +++ llvm/trunk/include/llvm/Transforms/Utils/ValueMapper.h Thu Dec 17 11:14:09 2015
> @@ -55,6 +55,12 @@ namespace llvm {
>     /// It is called after the mapping is recorded, so it doesn't need to worry
>     /// about recursion.
>     virtual void materializeInitFor(GlobalValue *New, GlobalValue *Old);
> +
> +    /// 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.
> @@ -78,6 +84,11 @@ namespace llvm {
>     /// 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) {
> 
> Modified: llvm/trunk/lib/Bitcode/Reader/BitcodeReader.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Bitcode/Reader/BitcodeReader.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Bitcode/Reader/BitcodeReader.cpp (original)
> +++ llvm/trunk/lib/Bitcode/Reader/BitcodeReader.cpp Thu Dec 17 11:14:09 2015
> @@ -97,6 +97,7 @@ public:
> class BitcodeReaderMDValueList {
>   unsigned NumFwdRefs;
>   bool AnyFwdRefs;
> +  bool SavedFwdRefs;

Should have a Boolean verb, something like "HasSavedFwdRefs".

>   unsigned MinFwdRef;
>   unsigned MaxFwdRef;
>   std::vector<TrackingMDRef> MDValuePtrs;
> @@ -104,7 +105,12 @@ class BitcodeReaderMDValueList {
>   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(); }
> @@ -115,6 +121,8 @@ public:
>   void pop_back()             { MDValuePtrs.pop_back(); }
>   bool empty() const          { return MDValuePtrs.empty(); }
> 
> +  void savedFwdRefs() { SavedFwdRefs = true; }

Can you rename this something like "setHasSavedFwdRefs()"?  The first
few times I read this at the call site, I thought it was doing something,
and I couldn't figure out what.

> +
>   Metadata *operator[](unsigned i) const {
>     assert(i < MDValuePtrs.size());
>     return MDValuePtrs[i];
> @@ -274,6 +282,14 @@ public:
> 
>   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
> @@ -1069,6 +1085,9 @@ Metadata *BitcodeReaderMDValueList::getV
>     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();
> @@ -3062,6 +3081,30 @@ std::error_code BitcodeReader::materiali
> 
> void BitcodeReader::setStripDebugInfo() { StripDebugInfo = true; }
> 
> +void BitcodeReader::saveMDValueList(
> +    DenseMap<const Metadata *, unsigned> &MDValueToValIDMap, bool OnlyTempMD) {

"MDValue" is a horrible term that I wish I'd never come up with.
Please don't propagate it.  If you get a chance, please clean up
instances of it (even the ones you didn't add).

I think this should be called `saveMetadataList()`, and the map
should be `MetadataToIDs`.

> +  for (unsigned ValID = 0; ValID < MDValueList.size(); ++ValID) {

I suggest just using `ID` here instead of `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())) {

Why do you need to record metadata that isn't an MDNode here (i.e., why
not `continue` if `!N`)?

Also, it would be great to be able to assert:
--
assert(N->isResolved() || N->isTemporary());
--
This confirms that you've called `resolveCycles()`.  If you haven't
called `resolveCycles()` by now, then I don't think it's safe to put `N`
in the map.

Finally, there's an important missing assertion that I'm not sure how
to provide.  But it's important.

IIUC, it's vital that this *doesn't* happen:

 1. Temporary metadata T is put into MetadataToIDs.
 2. T.replaceAllUsesWith(MD) gets called.
 3. MetadataToIDs.count/find/etc.(T) or (MD).

First case (T): since `T` has been freed, some new temporary (or normal)
metadata might get allocated in the same spot.  Someone could be
checking for this new `N`, and get the ID for the old `T`.

Second case (MD): the map contains the ID for `MD`, but it's indexed
under the old pointer `T`.  Checking for `MD` will give the wrong answer.

As I mentioned, I don't know how to add an assertion or verification for
this, but I think it's really important.  Please look into how to
verify that these cases can't occur.

> +      // 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() {
> 
> Modified: llvm/trunk/lib/IR/Metadata.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Metadata.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/IR/Metadata.cpp (original)
> +++ llvm/trunk/lib/IR/Metadata.cpp Thu Dec 17 11:14:09 2015
> @@ -517,7 +517,7 @@ void MDNode::decrementUnresolvedOperandC
>     resolve();
> }
> 
> -void MDNode::resolveCycles() {
> +void MDNode::resolveCycles(bool MDMaterialized) {
>   if (isResolved())
>     return;
> 
> @@ -530,6 +530,8 @@ void MDNode::resolveCycles() {
>     if (!N)
>       continue;
> 
> +    if (N->isTemporary() && !MDMaterialized)
> +      continue;
>     assert(!N->isTemporary() &&
>            "Expected all forward declarations to be resolved");
>     if (!N->isResolved())
> 
> Modified: llvm/trunk/lib/IRReader/IRReader.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IRReader/IRReader.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/IRReader/IRReader.cpp (original)
> +++ llvm/trunk/lib/IRReader/IRReader.cpp Thu Dec 17 11:14:09 2015
> @@ -31,11 +31,11 @@ static const char *const TimeIRParsingNa
> 
> 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, ShouldLazyLoadMetadata);
>     if (std::error_code EC = ModuleOrErr.getError()) {
>       Err = SMDiagnostic(Buffer->getBufferIdentifier(), SourceMgr::DK_Error,
>                          EC.message());
> @@ -49,7 +49,8 @@ getLazyIRModule(std::unique_ptr<MemoryBu
> 
> 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()) {
> @@ -58,7 +59,8 @@ std::unique_ptr<Module> llvm::getLazyIRF
>     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,
> 
> Modified: llvm/trunk/lib/Linker/IRMover.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Linker/IRMover.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Linker/IRMover.cpp (original)
> +++ llvm/trunk/lib/Linker/IRMover.cpp Thu Dec 17 11:14:09 2015
> @@ -14,6 +14,7 @@
> #include "llvm/ADT/Triple.h"
> #include "llvm/IR/Constants.h"
> #include "llvm/IR/DiagnosticPrinter.h"
> +#include "llvm/IR/GVMaterializer.h"
> #include "llvm/IR/TypeFinder.h"
> #include "llvm/Transforms/Utils/Cloning.h"
> using namespace llvm;
> @@ -349,6 +350,9 @@ public:
>   GlobalValueMaterializer(IRLinker *ModLinker) : ModLinker(ModLinker) {}
>   Value *materializeDeclFor(Value *V) override;
>   void materializeInitFor(GlobalValue *New, GlobalValue *Old) override;
> +  Metadata *mapTemporaryMetadata(Metadata *MD) override;
> +  void replaceTemporaryMetadata(const Metadata *OrigMD,
> +                                Metadata *NewMD) override;
> };
> 
> class LocalValueMaterializer final : public ValueMaterializer {
> @@ -358,6 +362,9 @@ public:
>   LocalValueMaterializer(IRLinker *ModLinker) : ModLinker(ModLinker) {}
>   Value *materializeDeclFor(Value *V) override;
>   void materializeInitFor(GlobalValue *New, GlobalValue *Old) override;
> +  Metadata *mapTemporaryMetadata(Metadata *MD) override;
> +  void replaceTemporaryMetadata(const Metadata *OrigMD,
> +                                Metadata *NewMD) override;
> };
> 
> /// This is responsible for keeping track of the state used for moving data
> @@ -394,6 +401,24 @@ class IRLinker {
> 
>   bool HasError = false;
> 
> +  /// Flag indicating that we are just linking metadata (after function
> +  /// importing).
> +  bool IsMetadataLinkingPostpass;
> +
> +  /// Flags to pass to value mapper invocations.
> +  RemapFlags ValueMapperFlags = RF_MoveDistinctMDs;
> +
> +  /// 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;
> +
>   /// Handles cloning of a global values from the source module into
>   /// the destination module, including setting the attributes and visibility.
>   GlobalValue *copyGlobalValueProto(const GlobalValue *SGV, bool ForDefinition);
> @@ -409,6 +434,14 @@ class IRLinker {
>     SrcM.getContext().diagnose(LinkDiagnosticInfo(DS_Warning, Message));
>   }
> 
> +  /// Check whether we should be linking metadata from the source module.
> +  bool shouldLinkMetadata() {
> +    // ValIDToTempMDMap will be non-null when we are importing or otherwise want
> +    // to link metadata lazily, and then when linking the metadata.
> +    // We only want to return true for the former case.
> +    return ValIDToTempMDMap == nullptr || IsMetadataLinkingPostpass;
> +  }
> +
>   /// 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) {
> @@ -457,16 +490,35 @@ class IRLinker {
> public:
>   IRLinker(Module &DstM, IRMover::IdentifiedStructTypeSet &Set, Module &SrcM,
>            ArrayRef<GlobalValue *> ValuesToLink,
> -           std::function<void(GlobalValue &, IRMover::ValueAdder)> AddLazyFor)
> +           std::function<void(GlobalValue &, IRMover::ValueAdder)> AddLazyFor,
> +           DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr,
> +           bool IsMetadataLinkingPostpass = false)
>       : DstM(DstM), SrcM(SrcM), AddLazyFor(AddLazyFor), TypeMap(Set),
> -        GValMaterializer(this), LValMaterializer(this) {
> +        GValMaterializer(this), LValMaterializer(this),
> +        IsMetadataLinkingPostpass(IsMetadataLinkingPostpass),
> +        ValIDToTempMDMap(ValIDToTempMDMap) {
>     for (GlobalValue *GV : ValuesToLink)
>       maybeAdd(GV);
> +
> +    // If appropriate, tell the value mapper that it can expect to see
> +    // temporary metadata.
> +    if (!shouldLinkMetadata())
> +      ValueMapperFlags = ValueMapperFlags | RF_HaveUnmaterializedMetadata;
>   }
> 
>   bool run();
>   Value *materializeDeclFor(Value *V, bool ForAlias);
>   void materializeInitFor(GlobalValue *New, GlobalValue *Old, bool ForAlias);
> +
> +  /// 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);
> };
> }
> 
> @@ -500,6 +552,15 @@ void GlobalValueMaterializer::materializ
>   ModLinker->materializeInitFor(New, Old, false);
> }
> 
> +Metadata *GlobalValueMaterializer::mapTemporaryMetadata(Metadata *MD) {
> +  return ModLinker->mapTemporaryMetadata(MD);
> +}
> +
> +void GlobalValueMaterializer::replaceTemporaryMetadata(const Metadata *OrigMD,
> +                                                       Metadata *NewMD) {
> +  ModLinker->replaceTemporaryMetadata(OrigMD, NewMD);
> +}
> +
> Value *LocalValueMaterializer::materializeDeclFor(Value *V) {
>   return ModLinker->materializeDeclFor(V, true);
> }
> @@ -509,6 +570,15 @@ void LocalValueMaterializer::materialize
>   ModLinker->materializeInitFor(New, Old, true);
> }
> 
> +Metadata *LocalValueMaterializer::mapTemporaryMetadata(Metadata *MD) {
> +  return ModLinker->mapTemporaryMetadata(MD);
> +}
> +
> +void LocalValueMaterializer::replaceTemporaryMetadata(const Metadata *OrigMD,
> +                                                      Metadata *NewMD) {
> +  ModLinker->replaceTemporaryMetadata(OrigMD, NewMD);
> +}
> +
> Value *IRLinker::materializeDeclFor(Value *V, bool ForAlias) {
>   auto *SGV = dyn_cast<GlobalValue>(V);
>   if (!SGV)
> @@ -536,6 +606,51 @@ void IRLinker::materializeInitFor(Global
>     linkGlobalValueBody(*New, *Old);
> }
> 
> +Metadata *IRLinker::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 IRLinker::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);
> +  }
> +}
> +
> /// Loop through the global variables in the src module and merge them into the
> /// dest module.
> GlobalVariable *IRLinker::copyGlobalVariableProto(const GlobalVariable *SGVar) {
> @@ -793,16 +908,16 @@ Constant *IRLinker::linkAppendingVarProt
>     Constant *NewV;
>     if (IsOldStructor) {
>       auto *S = cast<ConstantStruct>(V);
> -      auto *E1 = MapValue(S->getOperand(0), ValueMap, RF_MoveDistinctMDs,
> +      auto *E1 = MapValue(S->getOperand(0), ValueMap, ValueMapperFlags,
>                           &TypeMap, &GValMaterializer);
> -      auto *E2 = MapValue(S->getOperand(1), ValueMap, RF_MoveDistinctMDs,
> +      auto *E2 = MapValue(S->getOperand(1), ValueMap, ValueMapperFlags,
>                           &TypeMap, &GValMaterializer);
>       Value *Null = Constant::getNullValue(VoidPtrTy);
>       NewV =
>           ConstantStruct::get(cast<StructType>(EltTy), E1, E2, Null, nullptr);
>     } else {
> -      NewV = MapValue(V, ValueMap, RF_MoveDistinctMDs, &TypeMap,
> -                      &GValMaterializer);
> +      NewV =
> +          MapValue(V, ValueMap, ValueMapperFlags, &TypeMap, &GValMaterializer);
>     }
>     DstElements.push_back(NewV);
>   }
> @@ -837,6 +952,14 @@ static bool useExistingDest(GlobalValue
> }
> 
> bool IRLinker::shouldLink(GlobalValue *DGV, GlobalValue &SGV) {
> +  // Already imported all the values. Just map to the Dest value
> +  // in case it is referenced in the metadata.
> +  if (IsMetadataLinkingPostpass) {
> +    assert(!ValuesToLink.count(&SGV) &&
> +           "Source value unexpectedly requested for link during metadata link");
> +    return false;
> +  }
> +
>   if (ValuesToLink.count(&SGV))
>     return true;
> 
> @@ -925,8 +1048,8 @@ Constant *IRLinker::linkGlobalValueProto
> /// referenced are in Dest.
> void IRLinker::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, &GValMaterializer));
> +  Dst.setInitializer(MapValue(Src.getInitializer(), ValueMap, ValueMapperFlags,
> +                              &TypeMap, &GValMaterializer));
> }
> 
> /// Copy the source function over into the dest function and fix up references
> @@ -939,22 +1062,28 @@ bool IRLinker::linkFunctionBody(Function
>   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,
> -                               &GValMaterializer));
> +    Dst.setPrefixData(MapValue(Src.getPrefixData(), ValueMap, ValueMapperFlags,
> +                               &TypeMap, &GValMaterializer));
> 
>   // Link in the prologue data.
>   if (Src.hasPrologueData())
>     Dst.setPrologueData(MapValue(Src.getPrologueData(), ValueMap,
> -                                 RF_MoveDistinctMDs, &TypeMap,
> +                                 ValueMapperFlags, &TypeMap,
>                                  &GValMaterializer));
> 
>   // Link in the personality function.
>   if (Src.hasPersonalityFn())
>     Dst.setPersonalityFn(MapValue(Src.getPersonalityFn(), ValueMap,
> -                                  RF_MoveDistinctMDs, &TypeMap,
> +                                  ValueMapperFlags, &TypeMap,
>                                   &GValMaterializer));
> 
>   // Go through and convert function arguments over, remembering the mapping.
> @@ -971,7 +1100,7 @@ bool IRLinker::linkFunctionBody(Function
>   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, &GValMaterializer));
> 
>   // Splice the body of the source function into the dest function.
> @@ -983,9 +1112,8 @@ bool IRLinker::linkFunctionBody(Function
>   // 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,
> -                       &GValMaterializer);
> +      RemapInstruction(&I, ValueMap, RF_IgnoreMissingEntries | ValueMapperFlags,
> +                       &TypeMap, &GValMaterializer);
> 
>   // There is no need to map the arguments anymore.
>   for (Argument &Arg : Src.args())
> @@ -997,7 +1125,7 @@ bool IRLinker::linkFunctionBody(Function
> 
> void IRLinker::linkAliasBody(GlobalAlias &Dst, GlobalAlias &Src) {
>   Constant *Aliasee = Src.getAliasee();
> -  Constant *Val = MapValue(Aliasee, AliasValueMap, RF_MoveDistinctMDs, &TypeMap,
> +  Constant *Val = MapValue(Aliasee, AliasValueMap, ValueMapperFlags, &TypeMap,
>                            &LValMaterializer);
>   Dst.setAliasee(Val);
> }
> @@ -1024,7 +1152,7 @@ void IRLinker::linkNamedMDNodes() {
>     // 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, &GValMaterializer));
>   }
> }
> @@ -1260,7 +1388,7 @@ bool IRLinker::run() {
>       continue;
> 
>     assert(!GV->isDeclaration());
> -    MapValue(GV, ValueMap, RF_MoveDistinctMDs, &TypeMap, &GValMaterializer);
> +    MapValue(GV, ValueMap, ValueMapperFlags, &TypeMap, &GValMaterializer);
>     if (HasError)
>       return true;
>   }
> @@ -1272,11 +1400,39 @@ bool IRLinker::run() {
>   // 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.
> -  linkNamedMDNodes();
> +  if (shouldLinkMetadata()) {
> +    // Even if just linking metadata we should link decls above in case
> +    // any are referenced by metadata. IRLinker::shouldLink ensures that
> +    // we don't actually link anything from source.
> +    if (IsMetadataLinkingPostpass) {
> +      // Ensure metadata materialized
> +      if (SrcM.getMaterializer()->materializeMetadata())
> +        return true;
> +      SrcM.getMaterializer()->saveMDValueList(MDValueToValIDMap, false);
> +    }
> +
> +    linkNamedMDNodes();
> +
> +    if (IsMetadataLinkingPostpass) {
> +      // 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,
> +                    &GValMaterializer);
> +      }
> +      assert(ValIDToTempMDMap->empty());
> +    }
> 
> -  // Merge the module flags into the DstM module.
> -  if (linkModuleFlagsMetadata())
> -    return true;
> +    // Merge the module flags into the DstM module.
> +    if (linkModuleFlagsMetadata())
> +      return true;
> +  }
> 
>   return false;
> }
> @@ -1384,9 +1540,11 @@ IRMover::IRMover(Module &M) : Composite(
> 
> bool IRMover::move(
>     Module &Src, ArrayRef<GlobalValue *> ValuesToLink,
> -    std::function<void(GlobalValue &, ValueAdder Add)> AddLazyFor) {
> +    std::function<void(GlobalValue &, ValueAdder Add)> AddLazyFor,
> +    DenseMap<unsigned, MDNode *> *ValIDToTempMDMap,
> +    bool IsMetadataLinkingPostpass) {
>   IRLinker TheLinker(Composite, IdentifiedStructTypes, Src, ValuesToLink,
> -                     AddLazyFor);
> +                     AddLazyFor, ValIDToTempMDMap, IsMetadataLinkingPostpass);
>   bool RetCode = TheLinker.run();
>   Composite.dropTriviallyDeadConstantArrays();
>   return RetCode;
> 
> Modified: llvm/trunk/lib/Linker/LinkModules.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Linker/LinkModules.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Linker/LinkModules.cpp (original)
> +++ llvm/trunk/lib/Linker/LinkModules.cpp Thu Dec 17 11:14:09 2015
> @@ -48,6 +48,11 @@ class ModuleLinker {
>   /// 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
> @@ -150,9 +155,10 @@ class ModuleLinker {
> public:
>   ModuleLinker(IRMover &Mover, Module &SrcM, unsigned Flags,
>                const FunctionInfoIndex *Index = nullptr,
> -               DenseSet<const GlobalValue *> *FunctionsToImport = nullptr)
> +               DenseSet<const GlobalValue *> *FunctionsToImport = nullptr,
> +               DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr)
>       : Mover(Mover), SrcM(SrcM), Flags(Flags), ImportIndex(Index),
> -        ImportFunction(FunctionsToImport) {
> +        ImportFunction(FunctionsToImport), ValIDToTempMDMap(ValIDToTempMDMap) {
>     assert((ImportIndex || !ImportFunction) &&
>            "Expect a FunctionInfoIndex when importing");
>     // If we have a FunctionInfoIndex but no function to import,
> @@ -161,6 +167,8 @@ public:
>     // may be exported to another backend compilation.
>     if (ImportIndex && !ImportFunction)
>       HasExportedFunctions = ImportIndex->hasExportedFunctions(SrcM);
> +    assert((ValIDToTempMDMap || !ImportFunction) &&
> +           "Function importing must provide a ValIDToTempMDMap");
>   }
> 
>   bool run();
> @@ -502,6 +510,7 @@ bool ModuleLinker::getComdatResult(const
> bool ModuleLinker::shouldLinkFromSource(bool &LinkFromSrc,
>                                         const GlobalValue &Dest,
>                                         const GlobalValue &Src) {
> +
>   // Should we unconditionally use the Src?
>   if (shouldOverrideFromSrc()) {
>     LinkFromSrc = true;
> @@ -776,7 +785,8 @@ bool ModuleLinker::run() {
>   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) {
> @@ -791,8 +801,10 @@ Linker::Linker(Module &M) : Mover(M) {}
> 
> bool Linker::linkInModule(std::unique_ptr<Module> Src, unsigned Flags,
>                           const FunctionInfoIndex *Index,
> -                          DenseSet<const GlobalValue *> *FunctionsToImport) {
> -  ModuleLinker TheLinker(Mover, *Src, Flags, Index, FunctionsToImport);
> +                          DenseSet<const GlobalValue *> *FunctionsToImport,
> +                          DenseMap<unsigned, MDNode *> *ValIDToTempMDMap) {
> +  ModuleLinker TheLinker(Mover, *Src, Flags, Index, FunctionsToImport,
> +                         ValIDToTempMDMap);
>   return TheLinker.run();
> }
> 
> @@ -801,6 +813,17 @@ bool Linker::linkInModuleForCAPI(Module
>   return TheLinker.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.
> //===----------------------------------------------------------------------===//
> 
> Modified: llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp (original)
> +++ llvm/trunk/lib/Transforms/IPO/FunctionImport.cpp Thu Dec 17 11:14:09 2015
> @@ -292,6 +292,9 @@ bool FunctionImporter::importFunctions(M
>                 ModuleToFunctionsToImportMap, Index, ModuleLoaderCache);
>   assert(Worklist.empty() && "Worklist hasn't been flushed in GetImportList");
> 
> +  StringMap<std::unique_ptr<DenseMap<unsigned, MDNode *>>>
> +      ModuleToTempMDValsMap;
> +
>   // Do the actual import of functions now, one Module at a time
>   for (auto &FunctionsToImportPerModule : ModuleToFunctionsToImportMap) {
>     // Get the module for the import
> @@ -301,13 +304,32 @@ bool FunctionImporter::importFunctions(M
>     assert(&DestModule.getContext() == &SrcModule->getContext() &&
>            "Context mismatch");
> 
> +    // 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[SrcModule->getModuleIdentifier()];
> +    if (!TempMDVals)
> +      TempMDVals = llvm::make_unique<DenseMap<unsigned, MDNode *>>();
> +
>     // Link in the specified functions.
>     if (TheLinker.linkInModule(std::move(SrcModule), Linker::Flags::None,
> -                               &Index, &FunctionsToImport))
> +                               &Index, &FunctionsToImport, TempMDVals.get()))
>       report_fatal_error("Function Import: link error");
> 
>     ImportedCount += FunctionsToImport.size();
>   }
> +
> +  // 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.
> +    auto &SrcModule = ModuleLoaderCache(SME.getKey());
> +
> +    // Link in all necessary metadata from this module.
> +    if (TheLinker.linkInMetadata(SrcModule, SME.getValue().get()))
> +      return false;
> +  }
> +
>   DEBUG(dbgs() << "Imported " << ImportedCount << " functions for Module "
>                << DestModule.getModuleIdentifier() << "\n");
>   return ImportedCount;
> 
> Modified: llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp (original)
> +++ llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp Thu Dec 17 11:14:09 2015
> @@ -167,13 +167,21 @@ Value *llvm::MapValue(const Value *V, Va
> }
> 
> 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,
> @@ -206,10 +214,13 @@ static Metadata *mapMetadataOp(Metadata
> }
> 
> /// 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.
> @@ -238,7 +249,7 @@ static bool remapOperands(MDNode &Node,
>       // 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));
>     }
>   }
> 
> @@ -266,7 +277,7 @@ static Metadata *mapDistinctNode(const M
> 
>   // Remap operands later.
>   DistinctWorklist.push_back(NewMD);
> -  return mapToMetadata(VM, Node, NewMD);
> +  return mapToMetadata(VM, Node, NewMD, Materializer, Flags);
> }
> 
> /// \brief Map a uniqued MDNode.
> @@ -277,22 +288,29 @@ static Metadata *mapUniquedNode(const MD
>                                 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,
> @@ -305,18 +323,18 @@ static Metadata *MapMetadataImpl(const M
>     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
> @@ -325,7 +343,8 @@ static Metadata *MapMetadataImpl(const M
>     //    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;
>   }
> 
> @@ -336,10 +355,25 @@ static Metadata *MapMetadataImpl(const M
>   // 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,
> @@ -363,7 +397,7 @@ Metadata *llvm::MapMetadata(const Metada
>     return NewMD;
> 
>   // Resolve cycles involving the entry metadata.
> -  resolveCycles(NewMD);
> +  resolveCycles(NewMD, !(Flags & RF_HaveUnmaterializedMetadata));
> 
>   // Remap the operands of distinct MDNodes.
>   while (!DistinctWorklist.empty())
> 
> Added: llvm/trunk/test/Linker/Inputs/thinlto_funcimport_debug.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Linker/Inputs/thinlto_funcimport_debug.ll?rev=255909&view=auto
> ==============================================================================
> --- llvm/trunk/test/Linker/Inputs/thinlto_funcimport_debug.ll (added)
> +++ llvm/trunk/test/Linker/Inputs/thinlto_funcimport_debug.ll Thu Dec 17 11:14:09 2015
> @@ -0,0 +1,38 @@
> +; 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: ".")
> +!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)
> 
> Added: llvm/trunk/test/Linker/thinlto_funcimport_debug.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Linker/thinlto_funcimport_debug.ll?rev=255909&view=auto
> ==============================================================================
> --- llvm/trunk/test/Linker/thinlto_funcimport_debug.ll (added)
> +++ llvm/trunk/test/Linker/thinlto_funcimport_debug.ll Thu Dec 17 11:14:09 2015
> @@ -0,0 +1,71 @@
> +; 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
> +
> +; Confirm that we link the metadata for the imported module.
> +; 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: ".")
> +!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)
> 
> Added: llvm/trunk/test/Transforms/FunctionImport/Inputs/funcimport_debug.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionImport/Inputs/funcimport_debug.ll?rev=255909&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/FunctionImport/Inputs/funcimport_debug.ll (added)
> +++ llvm/trunk/test/Transforms/FunctionImport/Inputs/funcimport_debug.ll Thu Dec 17 11:14:09 2015
> @@ -0,0 +1,27 @@
> +; ModuleID = 'funcimport_debug.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 void @func() #0 !dbg !4 {
> +entry:
> +    ret void, !dbg !10
> +}
> +
> +attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "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" }
> +
> +!llvm.dbg.cu = !{!0}
> +!llvm.module.flags = !{!7, !8}
> +!llvm.ident = !{!9}
> +
> +!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.8.0 (trunk 255685) (llvm/trunk 255682)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, subprograms: !3)
> +!1 = !DIFile(filename: "funcimport_debug.c", directory: ".")
> +!2 = !{}
> +!3 = !{!4}
> +!4 = distinct !DISubprogram(name: "func", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, variables: !2)
> +!5 = !DISubroutineType(types: !6)
> +!6 = !{null}
> +!7 = !{i32 2, !"Dwarf Version", i32 4}
> +!8 = !{i32 2, !"Debug Info Version", i32 3}
> +!9 = !{!"clang version 3.8.0 (trunk 255685) (llvm/trunk 255682)"}
> +!10 = !DILocation(line: 2, column: 1, scope: !4)
> 
> Added: llvm/trunk/test/Transforms/FunctionImport/funcimport_debug.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionImport/funcimport_debug.ll?rev=255909&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/FunctionImport/funcimport_debug.ll (added)
> +++ llvm/trunk/test/Transforms/FunctionImport/funcimport_debug.ll Thu Dec 17 11:14:09 2015
> @@ -0,0 +1,45 @@
> +; 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/funcimport_debug.ll -o %t2.bc
> +; RUN: llvm-lto -thinlto -o %t3 %t.bc %t2.bc
> +
> +; Do the import now and confirm that metadata is linked for imported function.
> +; RUN: opt -function-import -summary-file %t3.thinlto.bc %s -S | FileCheck %s
> +
> +; CHECK: define available_externally void @func()
> +; CHECK: distinct !DISubprogram(name: "main"
> +; CHECK: distinct !DISubprogram(name: "func"
> +
> +; ModuleID = 'funcimport_debug.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 void (...) @func(), !dbg !11
> +  ret i32 0, !dbg !12
> +}
> +
> +declare void @func(...) #1
> +
> +attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "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"="true" "no-frame-pointer-elim-non-leaf" "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" }
> +
> +!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 255685) (llvm/trunk 255682)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, subprograms: !3)
> +!1 = !DIFile(filename: "funcimport_debug.c", directory: ".")
> +!2 = !{}
> +!3 = !{!4}
> +!4 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 2, type: !5, isLocal: false, isDefinition: true, scopeLine: 2, isOptimized: false, 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 255685) (llvm/trunk 255682)"}
> +!11 = !DILocation(line: 3, column: 3, scope: !4)
> +!12 = !DILocation(line: 4, column: 1, scope: !4)
> 
> Modified: llvm/trunk/tools/llvm-link/llvm-link.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/tools/llvm-link/llvm-link.cpp?rev=255909&r1=255908&r2=255909&view=diff
> ==============================================================================
> --- llvm/trunk/tools/llvm-link/llvm-link.cpp (original)
> +++ llvm/trunk/tools/llvm-link/llvm-link.cpp Thu Dec 17 11:14:09 2015
> @@ -106,16 +106,21 @@ static cl::opt<bool> PreserveAssemblyUse
> // 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());
> 
> -  Result->materializeMetadata();
> -  UpgradeDebugInfo(*Result);
> +  if (MaterializeMetadata) {
> +    Result->materializeMetadata();
> +    UpgradeDebugInfo(*Result);
> +  }
> 
>   return Result;
> }
> @@ -148,6 +153,8 @@ static void diagnosticHandlerWithContext
> /// 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(':');
> @@ -159,7 +166,7 @@ static bool importFunctions(const char *
>     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;
> @@ -201,11 +208,39 @@ static bool importFunctions(const char *
>       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.
>     DenseSet<const GlobalValue *> FunctionsToImport;
>     FunctionsToImport.insert(F);
>     if (L.linkInModule(std::move(M), Linker::Flags::None, Index.get(),
> -                       &FunctionsToImport))
> +                       &FunctionsToImport, 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.linkInMetadata(*M, SME.getValue().get()))
>       return false;
>   }
>   return true;
> 
> 
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