Index: include/llvm/InitializePasses.h
===================================================================
--- include/llvm/InitializePasses.h
+++ include/llvm/InitializePasses.h
@@ -302,6 +302,7 @@
 void initializeDemandedBitsPass(PassRegistry&);
 void initializeFuncletLayoutPass(PassRegistry &);
 void initializeLoopLoadEliminationPass(PassRegistry&);
+void initializeFunctionImportPassPass(PassRegistry&);
 }
 
 #endif
Index: include/llvm/Transforms/IPO/FunctionImport.h
===================================================================
--- /dev/null
+++ include/llvm/Transforms/IPO/FunctionImport.h
@@ -0,0 +1,51 @@
+//===- llvm/Transforms/IPO/FunctionImport.h - ThinLTO importing -*- C++ -*-===//
+//
+//                      The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUNCTIONIMPORT_H
+#define LLVM_FUNCTIONIMPORT_H
+
+#include "llvm/IR/DiagnosticInfo.h"
+#include "llvm/ADT/StringMap.h"
+
+namespace llvm {
+class LLVMContext;
+class Module;
+class FunctionInfoIndex;
+
+/// The function importer is automatically importing function from other modules
+/// based on the provided summary informations.
+class FunctionImporter {
+
+  /// Cache of lazily loaded module for import.
+  StringMap<std::unique_ptr<Module>> ModuleMap;
+
+  /// The context that will be used for importing.
+  LLVMContext &Context;
+
+  /// The summaries index used to trigger importing.
+  const FunctionInfoIndex &Index;
+
+  /// Diagnostic will be sent to this handler.
+  DiagnosticHandlerFunction DiagnosticHandler;
+
+  /// Retrieve a Module from the cache or lazily load it on demand.
+  Module &getOrLoadModule(StringRef FileName);
+
+public:
+  /// Create a Function Importer.
+  FunctionImporter(LLVMContext &Context, const FunctionInfoIndex &Index,
+                   DiagnosticHandlerFunction DiagnosticHandler)
+      : Context(Context), Index(Index), DiagnosticHandler(DiagnosticHandler) {}
+
+  /// Import functions in Module \p M based on the summary informations.
+  bool importFunctions(Module &M);
+};
+}
+
+#endif // LLVM_FUNCTIONIMPORT_H
Index: lib/Transforms/IPO/CMakeLists.txt
===================================================================
--- lib/Transforms/IPO/CMakeLists.txt
+++ lib/Transforms/IPO/CMakeLists.txt
@@ -6,6 +6,7 @@
   ElimAvailExtern.cpp
   ExtractGV.cpp
   FunctionAttrs.cpp
+  FunctionImport.cpp
   GlobalDCE.cpp
   GlobalOpt.cpp
   IPConstantPropagation.cpp
Index: lib/Transforms/IPO/FunctionImport.cpp
===================================================================
--- /dev/null
+++ lib/Transforms/IPO/FunctionImport.cpp
@@ -0,0 +1,243 @@
+//===- FunctionImport.cpp - ThinLTO Summary-based Function Import ---------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements Function import based on summaries.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO/FunctionImport.h"
+
+#include "llvm/ADT/StringSet.h"
+#include "llvm/IR/AutoUpgrade.h"
+#include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IRReader/IRReader.h"
+#include "llvm/Linker/Linker.h"
+#include "llvm/Object/FunctionIndexObjectFile.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/SourceMgr.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "function-import"
+
+// Load lazily a module from \p FileName in \p Context.
+static std::unique_ptr<Module> loadFile(const std::string &FileName,
+                                        LLVMContext &Context) {
+  SMDiagnostic Err;
+  DEBUG(dbgs() << "Loading '" << FileName << "'\n");
+  std::unique_ptr<Module> Result = getLazyIRFileModule(FileName, Err, Context);
+  if (!Result) {
+    Err.print("function-import", errs());
+    return nullptr;
+  }
+
+  Result->materializeMetadata();
+  UpgradeDebugInfo(*Result);
+
+  return Result;
+}
+
+// Get a Module for \p FileName from the cache, or load it lazily.
+Module &FunctionImporter::getOrLoadModule(StringRef FileName) {
+  auto &Module = ModuleMap[FileName];
+  if (!Module)
+    Module = loadFile(FileName, Context);
+  return *Module;
+}
+
+// Automatically import functions in Module \p M based on the summaries index.
+//
+// The current implementation imports every called functions that exists in the
+// summaries index.
+bool FunctionImporter::importFunctions(Module &M) {
+  assert(&Context == &M.getContext());
+
+  bool Changed = false;
+
+  /// First step is collecting the called functions and the one defined in this
+  /// module.
+  StringSet<> CalledFunctions;
+  StringSet<> DefinedFunctions;
+  for (auto &F : M) {
+    if (F.isDeclaration() || F.hasFnAttribute(Attribute::OptimizeNone))
+      continue;
+    // Keep definition around so that we differentiate calls to external
+    // functions from the ones to internally defined functions.
+    DefinedFunctions.insert(F.getName());
+    for (auto &BB : F) {
+      for (auto &I : BB) {
+        if (isa<CallInst>(I)) {
+          DEBUG(dbgs() << "Found a call: '" << I << "'\n");
+          if (cast<CallInst>(I).getCalledFunction() && cast<CallInst>(I).getCalledFunction()->hasName())
+            CalledFunctions.insert(
+                cast<CallInst>(I).getCalledFunction()->getName());
+        }
+      }
+    }
+  }
+
+  /// Second step: for every call to an external function, try to import it.
+
+  // Linker that will be used for importing function
+  Linker L(&M, DiagnosticHandler);
+
+  for (auto &CalledFunction : CalledFunctions) {
+    auto CalledFunctionName = CalledFunction.first();
+    // Do not try to import locally defined function.
+    if (DefinedFunctions.count(CalledFunctionName))
+      continue;
+
+    // Try to get a summary for this function call.
+    auto InfoList = Index.findFunctionInfoList(CalledFunctionName);
+    if (InfoList == Index.end()) {
+      DEBUG(dbgs() << "No summary for " << CalledFunctionName
+                   << " Ignoring.\n");
+      continue;
+    }
+    // This should not happen, could we assert here?
+    if (InfoList->second.empty()) {
+      DEBUG(dbgs() << "No summary for " << CalledFunctionName
+                   << " Ignoring.\n");
+      continue;
+    }
+    // Comdat can have multiple entries, FIXME: what do we do with them?
+    auto &Info = InfoList->second[0];
+    if (!Info) {
+      DEBUG(dbgs() << "Missing summary list for " << CalledFunctionName
+                   << ", error at import?\n");
+      continue;
+    }
+
+    auto *Summary = Info->functionSummary();
+    if (!Summary) {
+      dbgs() << "Missing summary for  " << CalledFunctionName
+             << ", error at import?\n";
+      llvm_unreachable("Missing summary");
+    }
+
+    //
+    // No profitability notion right now, just import all the time...
+    //
+
+    // Get the module path from the summary.
+    auto FileName = Summary->modulePath();
+    DEBUG(dbgs() << "Importing " << CalledFunctionName << " from " << FileName
+                 << "\n");
+
+    // Get the module for the import (potentially from the cache).
+    auto &Module = getOrLoadModule(FileName);
+
+    // The function that we will import!
+    Function *F = Module.getFunction(CalledFunctionName);
+    if (!F) {
+      errs() << "Can't load function '" << CalledFunctionName << "' in Module '"
+             << FileName << "', error in the summary?\n";
+
+      //FIXME this can happens with alias for instance, to be implemented...
+      continue;
+      // llvm_unreachable("Can't load function in Module");
+    }
+
+    // We cannot import weak_any functions without possibly affecting the
+    // order they are seen and selected by the linker, changing program
+    // semantics.
+    if (F->hasWeakAnyLinkage()) {
+      DEBUG(dbgs() << "Ignoring import request for weak-any function "
+                   << CalledFunctionName << " from " << FileName << "\n");
+      continue;
+    }
+
+    // Link in the specified function.
+    if (L.linkInModule(&Module, Linker::Flags::None, &Index, F))
+      report_fatal_error("Function Import: link error");
+
+    Changed = true;
+  }
+  return Changed;
+}
+
+
+/// Summary file to use for function importing when using -function-import from
+/// the command line.
+static cl::opt<std::string>
+    SummaryFile("summary-file",
+                cl::desc("The summary file to use for function importing."));
+
+static void diagnosticHandler(const DiagnosticInfo &DI) {
+  raw_ostream &OS = errs();
+  DiagnosticPrinterRawOStream DP(OS);
+  DI.print(DP);
+  OS << '\n';
+}
+
+/// Parse the function index out of an IR file and return the function
+/// index object if found, or nullptr if not.
+static std::unique_ptr<FunctionInfoIndex>
+getFunctionIndexForFile(StringRef Path, std::string &Error,
+                        DiagnosticHandlerFunction DiagnosticHandler) {
+  std::unique_ptr<MemoryBuffer> Buffer;
+  ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr =
+      MemoryBuffer::getFile(Path);
+  if (std::error_code EC = BufferOrErr.getError()) {
+    Error = EC.message();
+    return nullptr;
+  }
+  Buffer = std::move(BufferOrErr.get());
+  ErrorOr<std::unique_ptr<object::FunctionIndexObjectFile>> ObjOrErr =
+      object::FunctionIndexObjectFile::create(Buffer->getMemBufferRef(),
+                                              DiagnosticHandler);
+  if (std::error_code EC = ObjOrErr.getError()) {
+    Error = EC.message();
+    return nullptr;
+  }
+  return (*ObjOrErr)->takeIndex();
+}
+
+
+/// Pass that performs cross-module function import provided a summary file.
+class FunctionImportPass : public ModulePass {
+
+public:
+  /// Pass identification, replacement for typeid
+  static char ID;
+
+  explicit FunctionImportPass() : ModulePass(ID) {}
+
+  bool runOnModule(Module &M) override {
+    if (SummaryFile.empty()) {
+      report_fatal_error("error: -function-import requires -summary-file\n");
+    }
+    std::string Error;
+    std::unique_ptr<FunctionInfoIndex> Index =
+        getFunctionIndexForFile(SummaryFile, Error, diagnosticHandler);
+    if (!Index) {
+      errs() << "Error loading file '" << SummaryFile << "': " << Error
+                   << "\n";
+      return false;
+    }
+
+    // Perform the import now.
+    FunctionImporter Importer(M.getContext(), *Index, diagnosticHandler);
+    return Importer.importFunctions(M);
+
+    return false;
+  }
+};
+
+char FunctionImportPass::ID = 0;
+INITIALIZE_PASS_BEGIN(FunctionImportPass, "function-import",
+                      "Summary Based Function Import", false, false)
+INITIALIZE_PASS_END(FunctionImportPass, "function-import",
+                    "Summary Based Function Import", false, false)
+
+namespace llvm {
+Pass *createFunctionImportPass() { return new FunctionImportPass(); }
+}
Index: lib/Transforms/IPO/IPO.cpp
===================================================================
--- lib/Transforms/IPO/IPO.cpp
+++ lib/Transforms/IPO/IPO.cpp
@@ -48,6 +48,7 @@
   initializeBarrierNoopPass(Registry);
   initializeEliminateAvailableExternallyPass(Registry);
   initializeSampleProfileLoaderPass(Registry);
+  initializeFunctionImportPassPass(Registry);
 }
 
 void LLVMInitializeIPO(LLVMPassRegistryRef R) {
Index: lib/Transforms/IPO/LLVMBuild.txt
===================================================================
--- lib/Transforms/IPO/LLVMBuild.txt
+++ lib/Transforms/IPO/LLVMBuild.txt
@@ -20,4 +20,4 @@
 name = IPO
 parent = Transforms
 library_name = ipo
-required_libraries = Analysis Core InstCombine ProfileData Scalar Support TransformUtils Vectorize
+required_libraries = Analysis Core InstCombine Linker ProfileData Scalar Support TransformUtils Vectorize
Index: test/Transforms/FunctionImport/Inputs/funcimport.ll
===================================================================
--- /dev/null
+++ test/Transforms/FunctionImport/Inputs/funcimport.ll
@@ -0,0 +1,93 @@
+@globalvar = global i32 1, align 4
+@staticvar = internal global i32 1, align 4
+@staticconstvar = internal unnamed_addr constant [2 x i32] [i32 10, i32 20], align 4
+@commonvar = common global i32 0, align 4
+@P = internal global void ()* null, align 8
+
+@weakalias = weak alias void (...), bitcast (void ()* @globalfunc1 to void (...)*)
+@analias = alias void (...), bitcast (void ()* @globalfunc2 to void (...)*)
+@linkoncealias = alias void (...), bitcast (void ()* @linkoncefunc to void (...)*)
+
+define void @globalfunc1() #0 {
+entry:
+  ret void
+}
+
+define void @globalfunc2() #0 {
+entry:
+  ret void
+}
+
+define linkonce_odr void @linkoncefunc() #0 {
+entry:
+  ret void
+}
+
+define i32 @referencestatics(i32 %i) #0 {
+entry:
+  %i.addr = alloca i32, align 4
+  store i32 %i, i32* %i.addr, align 4
+  %call = call i32 @staticfunc()
+  %0 = load i32, i32* @staticvar, align 4
+  %add = add nsw i32 %call, %0
+  %1 = load i32, i32* %i.addr, align 4
+  %idxprom = sext i32 %1 to i64
+  %arrayidx = getelementptr inbounds [2 x i32], [2 x i32]* @staticconstvar, i64 0, i64 %idxprom
+  %2 = load i32, i32* %arrayidx, align 4
+  %add1 = add nsw i32 %add, %2
+  ret i32 %add1
+}
+
+define i32 @referenceglobals(i32 %i) #0 {
+entry:
+  %i.addr = alloca i32, align 4
+  store i32 %i, i32* %i.addr, align 4
+  call void @globalfunc1()
+  %0 = load i32, i32* @globalvar, align 4
+  ret i32 %0
+}
+
+define i32 @referencecommon(i32 %i) #0 {
+entry:
+  %i.addr = alloca i32, align 4
+  store i32 %i, i32* %i.addr, align 4
+  %0 = load i32, i32* @commonvar, align 4
+  ret i32 %0
+}
+
+define void @setfuncptr() #0 {
+entry:
+  store void ()* @staticfunc2, void ()** @P, align 8
+  ret void
+}
+
+define void @callfuncptr() #0 {
+entry:
+  %0 = load void ()*, void ()** @P, align 8
+  call void %0()
+  ret void
+}
+
+@weakvar = weak global i32 1, align 4
+define weak void @weakfunc() #0 {
+entry:
+  ret void
+}
+
+define void @callweakfunc() #0 {
+entry:
+  call void @weakfunc()
+  ret void
+}
+
+define internal i32 @staticfunc() #0 {
+entry:
+  ret i32 1
+}
+
+define internal void @staticfunc2() #0 {
+entry:
+  ret void
+}
+
+
Index: test/Transforms/FunctionImport/funcimport.ll
===================================================================
--- /dev/null
+++ test/Transforms/FunctionImport/funcimport.ll
@@ -0,0 +1,42 @@
+; 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.ll -o %t2.bc
+; RUN: llvm-lto -thinlto -o %t3 %t.bc %t2.bc
+
+; Do the import now
+; RUN: opt -function-import -summary-file %t3.thinlto.bc %s -S | FileCheck %s
+
+define i32 @main() #0 {
+entry:
+  call void (...) @weakalias()
+  call void (...) @analias()
+  %call = call i32 (...) @referencestatics()
+  %call1 = call i32 (...) @referenceglobals()
+  %call2 = call i32 (...) @referencecommon()
+  call void (...) @setfuncptr()
+  call void (...) @callfuncptr()
+  call void (...) @callweakfunc()
+  ret i32 0
+}
+
+; Won't import alias
+declare void @weakalias(...) #1
+declare void @analias(...) #1
+
+; CHECK-DAG: define available_externally i32 @referencestatics(i32 %i)
+declare i32 @referencestatics(...) #1
+
+; CHECK-DAG: define available_externally i32 @referenceglobals(i32 %i)
+declare i32 @referenceglobals(...) #1
+
+; CHECK-DAG: define available_externally i32 @referencecommon(i32 %i)
+declare i32 @referencecommon(...) #1
+
+; CHECK-DAG: define available_externally void @setfuncptr()
+declare void @setfuncptr(...) #1
+
+; CHECK-DAG: define available_externally void @callfuncptr()
+declare void @callfuncptr(...) #1
+
+; Won't import weak func
+declare void @callweakfunc(...) #1