diff --git a/llvm/include/llvm/IR/ModuleSummaryIndex.h b/llvm/include/llvm/IR/ModuleSummaryIndex.h
--- a/llvm/include/llvm/IR/ModuleSummaryIndex.h
+++ b/llvm/include/llvm/IR/ModuleSummaryIndex.h
@@ -635,7 +635,7 @@
 
   /// Get the SimilarityHash of this function.
   unsigned similarityHash() { return SimilarityHash; }
-  
+
   /// Get function summary flags.
   FFlags fflags() const { return FunFlags; }
 
@@ -1023,6 +1023,20 @@
   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>;
 
diff --git a/llvm/include/llvm/Transforms/IPO.h b/llvm/include/llvm/Transforms/IPO.h
--- a/llvm/include/llvm/Transforms/IPO.h
+++ b/llvm/include/llvm/Transforms/IPO.h
@@ -290,6 +290,9 @@
 ModulePass *createWriteThinLTOBitcodePass(raw_ostream &Str,
                                           raw_ostream *ThinLinkOS = nullptr);
 
+/// 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 llvm namespace
 
 #endif
diff --git a/llvm/include/llvm/Transforms/IPO/WholeProgramDevirt.h b/llvm/include/llvm/Transforms/IPO/WholeProgramDevirt.h
--- a/llvm/include/llvm/Transforms/IPO/WholeProgramDevirt.h
+++ b/llvm/include/llvm/Transforms/IPO/WholeProgramDevirt.h
@@ -258,7 +258,6 @@
     ModuleSummaryIndex &Summary,
     StringMap<FunctionImporter::ExportSetTy> &ExportLists,
     std::map<ValueInfo, std::vector<VTableSlotSummary>> &LocalWPDTargetsMap);
-
 } // end namespace llvm
 
 #endif // LLVM_TRANSFORMS_IPO_WHOLEPROGRAMDEVIRT_H
diff --git a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
--- a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
+++ b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
@@ -78,6 +78,8 @@
 extern cl::opt<std::string> RemarksFormat;
 }
 
+extern cl::opt<bool> EnableMergeSimilarFunctions;
+
 namespace {
 
 static cl::opt<int>
@@ -933,6 +935,8 @@
 
   // Sequential linking phase
   auto Index = linkCombinedIndex();
+  if (EnableMergeSimilarFunctions)
+    computeMergeSimilarFunctions(*Index);
 
   // Save temps: index.
   if (!SaveTempsDir.empty()) {
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -487,11 +487,27 @@
           NumImportedCriticalFunctionsThinLink++;
       }
 
-      // Any calls/references made by this function will be marked exported
-      // later, in ComputeCrossModuleImport, after import decisions are
-      // complete, which is more efficient than adding them here.
-      if (ExportLists)
-        (*ExportLists)[ExportModulePath].insert(VI);
+      // Make exports in the source module.
+      if (ExportLists) {
+        auto &ExportList = (*ExportLists)[ExportModulePath];
+        ExportList.insert(VI.getGUID());
+        if (!PreviouslyImported) {
+          // This is the first time this function was exported from its source
+          // module, so mark all functions and globals it references as exported
+          // to the outside if they are defined in the same source module.
+          // For efficiency, we unconditionally add all the referenced GUIDs
+          // to the ExportList for this module, and will prune out any not
+          // defined in the module later in a single pass.
+          for (auto &Edge : ResolvedCalleeSummary->calls()) {
+            auto CalleeGUID = Edge.first.getGUID();
+            ExportList.insert(CalleeGUID);
+          }
+          for (auto &Ref : ResolvedCalleeSummary->refs()) {
+            auto GUID = Ref.getGUID();
+            ExportList.insert(GUID);
+          }
+        }
+      }
     }
 
     auto GetAdjustedThreshold = [](unsigned Threshold, bool IsHotCallsite) {
diff --git a/llvm/lib/Transforms/IPO/MergeSimilarFunctions.cpp b/llvm/lib/Transforms/IPO/MergeSimilarFunctions.cpp
--- a/llvm/lib/Transforms/IPO/MergeSimilarFunctions.cpp
+++ b/llvm/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"
@@ -1301,6 +1302,42 @@
   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;
diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -149,7 +149,7 @@
     "enable-order-file-instrumentation", cl::init(false), cl::Hidden,
     cl::desc("Enable order file instrumentation (default = off)"));
 
-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)"));