Index: include/llvm/IR/ModuleSummaryIndex.h
===================================================================
--- include/llvm/IR/ModuleSummaryIndex.h
+++ include/llvm/IR/ModuleSummaryIndex.h
@@ -814,6 +814,20 @@
   StringSaver Saver;
   BumpPtrAllocator Alloc;
 
+  /// Functions with hash code as a measure of structural similarity.
+  /// Used by MergeSimilarFunctions. The value_type is the hash code
+  /// computed by profileFunction.
+  std::map<GlobalValue::GUID, unsigned> FunctionSimilarityHashes;
+  /// Reverse map of @var SimilarFunctionsHash to be used during thin-lto.
+  /// Each GUID in the vector corresponds to functions with same hash (id).
+  std::map<unsigned, std::vector<GlobalValue::GUID>> SimilarFunctions;
+  /// Functions having multiple entries in ModuleSummaryIndex.
+  std::set<GlobalValue::GUID> DuplicateFunctions;
+  /// All similar functions w.r.t the one in this set will be imported into
+  /// the module of these functions during thin-lto stage.
+  /// FIXME: Maybe use a vector?
+  std::set<GlobalValue::GUID> HostSimilarFunction;
+
   // YAML I/O support.
   friend yaml::MappingTraits<ModuleSummaryIndex>;
 
@@ -898,6 +912,76 @@
     return CallGraphRoot;
   }
 
+  bool hasSimilarFunction(unsigned id) { return SimilarFunctions.count(id); }
+
+  void addToSimilarFunctions(unsigned id, GlobalValue::GUID GUID) {
+    if (id == 0)
+      return; // invalid id.
+    if (DuplicateFunctions.count(GUID))
+      return;
+
+    if (FunctionSimilarityHashes.count(GUID)) {
+      // Erase the GUID having multiple visits in the ModuleSummaryIndex.
+      FunctionSimilarityHashes.erase(GUID);
+      DuplicateFunctions.insert(GUID);
+      return;
+    }
+
+    FunctionSimilarityHashes[GUID] = id;
+  }
+
+  void populateReverseSimilarityHashMap() {
+    for (auto &p : FunctionSimilarityHashes)
+      SimilarFunctions[p.second].push_back(p.first);
+  }
+
+  void removeSingleEntriesFromSimHashMaps() {
+    // Iterate over the hash to remove entries with no duplicates.
+    for (auto I = SimilarFunctions.begin(), E = SimilarFunctions.end();
+         I != E;) {
+      auto Next = std::next(I);
+      assert(I->second.size() && "Empty Entry!");
+      if (I->second.size() == 1) {
+        FunctionSimilarityHashes.erase(I->second[0]);
+        SimilarFunctions.erase(I);
+      }
+      I = Next;
+    }
+  }
+
+  std::map<unsigned, std::vector<GlobalValue::GUID>> &getSimilarFunctions() {
+    return SimilarFunctions;
+  }
+
+  const std::map<unsigned, std::vector<GlobalValue::GUID>> &
+  getSimilarFunctions() const {
+    return SimilarFunctions;
+  }
+
+  unsigned getSimilarityHash(GlobalValue::GUID ID) const {
+    return FunctionSimilarityHashes.find(ID)->second;
+  }
+
+  std::map<GlobalValue::GUID, unsigned> &getSimilarFunctionsHash() {
+    return FunctionSimilarityHashes;
+  }
+
+  const std::map<GlobalValue::GUID, unsigned> &getSimilarFunctionsHash() const {
+    return FunctionSimilarityHashes;
+  }
+
+  void addToHostSimilarFunction(GlobalValue::GUID ID) {
+    HostSimilarFunction.insert(ID);
+  }
+
+  std::set<GlobalValue::GUID> &getHostSimilarFunction() {
+    return HostSimilarFunction;
+  }
+
+  const std::set<GlobalValue::GUID> &getHostSimilarFunction() const {
+    return HostSimilarFunction;
+  }
+
   bool withGlobalValueDeadStripping() const {
     return WithGlobalValueDeadStripping;
   }
Index: include/llvm/Transforms/IPO/WholeProgramDevirt.h
===================================================================
--- include/llvm/Transforms/IPO/WholeProgramDevirt.h
+++ include/llvm/Transforms/IPO/WholeProgramDevirt.h
@@ -229,6 +229,10 @@
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &);
 };
 
+/// Decide which functions to import for function merging during the thinlto.
+/// Populates information from each function summary to module summary index.
+void computeMergeSimilarFunctions(ModuleSummaryIndex &Index);
+
 } // end namespace llvm
 
 #endif // LLVM_TRANSFORMS_IPO_WHOLEPROGRAMDEVIRT_H
Index: lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- lib/LTO/ThinLTOCodeGenerator.cpp
+++ lib/LTO/ThinLTOCodeGenerator.cpp
@@ -72,6 +72,8 @@
 extern cl::opt<bool> LTOPassRemarksWithHotness;
 }
 
+extern cl::opt<bool> EnableMergeSimilarFunctions;
+
 namespace {
 
 static cl::opt<int>
@@ -870,6 +872,9 @@
     ProducedBinaryFiles.resize(Modules.size());
   }
 
+  if (EnableMergeSimilarFunctions)
+    computeMergeSimilarFunctions(*Index);
+
   if (CodeGenOnly) {
     // Perform only parallel codegen and return.
     ThreadPool Pool;
Index: lib/Transforms/IPO/MergeSimilarFunctions.cpp
===================================================================
--- lib/Transforms/IPO/MergeSimilarFunctions.cpp
+++ lib/Transforms/IPO/MergeSimilarFunctions.cpp
@@ -40,6 +40,7 @@
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/IPO/WholeProgramDevirt.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/ValueMapper.h"
@@ -1295,6 +1296,42 @@
   return new MergeSimilarFunctions(S);
 }
 
+void llvm::computeMergeSimilarFunctions(ModuleSummaryIndex &Index) {
+  assert(Index.getSimilarFunctions().empty() && "Inserting SimHash twice");
+  for (auto &P : Index) {
+    for (auto &S : P.second.SummaryList) {
+      FunctionSummary *FS = dyn_cast<FunctionSummary>(S.get());
+      if (!FS)
+        continue;
+      LLVM_DEBUG(llvm::errs() << "\nSimilarity hash: " << FS->similarityHash());
+      Index.addToSimilarFunctions(FS->similarityHash(), FS->getOriginalName());
+    }
+  }
+  Index.populateReverseSimilarityHashMap();
+  llvm::errs() << "\nSize SimilarFunctionsHash: "
+               << Index.getSimilarFunctionsHash().size();
+  llvm::errs() << "\nSize SimilarFunctions: "
+               << Index.getSimilarFunctions().size();
+  Index.removeSingleEntriesFromSimHashMaps();
+
+  LLVM_DEBUG(llvm::errs() << "\nSize SimilarFunctionsHash: "
+                     << Index.getSimilarFunctionsHash().size());
+  LLVM_DEBUG(llvm::errs() << "\nSize SimilarFunctions: "
+                     << Index.getSimilarFunctions().size());
+
+  auto &SimFunctions = Index.getSimilarFunctions();
+  // Shouldn't have entries with hash of 0, because those are placeholders.
+  assert(!SimFunctions.count(0));
+  auto &HostSimFunction = Index.getHostSimilarFunction();
+  for (auto I = SimFunctions.begin(), E = SimFunctions.end(); I != E; ++I) {
+    // Make the first of all similar functions as host.
+    HostSimFunction.insert(I->second[0]);
+  }
+  LLVM_DEBUG(llvm::errs() << "\nSize getHostSimilarFunction: "
+                     << Index.getHostSimilarFunction().size());
+  return;
+}
+
 bool MergeSimilarFunctions::runOnModule(Module &M) {
   if (Opt::MergeLevel == Opt::none)
     return false;
Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -159,7 +159,7 @@
     EnableCHR("enable-chr", cl::init(true), cl::Hidden,
               cl::desc("Enable control height reduction optimization (CHR)"));
 
-static cl::opt<bool> EnableMergeSimilarFunctions(
+cl::opt<bool> EnableMergeSimilarFunctions(
     "enable-merge-sim-functions", cl::init(false), cl::Hidden,
     cl::desc("Enable the Function merging pass (default = on)"));