diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
@@ -26,6 +26,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
+#include "llvm/Support/TargetSelect.h"
 
 #define DEBUG_TYPE "lld"
 
@@ -48,6 +49,17 @@ enum {
 #undef OPTION
 };
 
+// This function is called on startup. We need this for LTO since
+// LTO calls LLVM functions to compile bitcode files to native code.
+// Technically this can be delayed until we read bitcode files, but
+// we don't bother to do lazily because the initialization is fast.
+static void initLLVM() {
+  InitializeAllTargets();
+  InitializeAllTargetMCs();
+  InitializeAllAsmPrinters();
+  InitializeAllAsmParsers();
+}
+
 class LinkerDriver {
 public:
   void link(ArrayRef<const char *> ArgsArr);
@@ -72,6 +84,7 @@ bool lld::wasm::link(ArrayRef<const char *> Args, bool CanExitEarly,
   Config = make<Configuration>();
   Symtab = make<SymbolTable>();
 
+  initLLVM();
   LinkerDriver().link(Args);
 
   // Exit immediately if we don't need to return to the caller.
@@ -173,7 +186,8 @@ void LinkerDriver::addFile(StringRef Path) {
     return;
   MemoryBufferRef MBRef = *Buffer;
 
-  if (identify_magic(MBRef.getBuffer()) == file_magic::archive) {
+  switch (identify_magic(MBRef.getBuffer())) {
+  case file_magic::archive: {
     SmallString<128> ImportFile = Path;
     path::replace_extension(ImportFile, ".imports");
     if (fs::exists(ImportFile))
@@ -182,8 +196,12 @@ void LinkerDriver::addFile(StringRef Path) {
     Files.push_back(make<ArchiveFile>(MBRef));
     return;
   }
-
-  Files.push_back(make<ObjFile>(MBRef));
+  case file_magic::bitcode:
+    Files.push_back(make<BitcodeFile>(MBRef));
+    break;
+  default:
+    Files.push_back(make<ObjFile>(MBRef));
+  }
 }
 
 // Add a given library by searching it from input search paths.
@@ -261,6 +279,17 @@ static void handleWeakUndefines() {
   }
 }
 
+// Force Sym to be entered in the output. Used for -u or equivalent.
+static Symbol *addUndefined(StringRef Name) {
+  Symbol *S = Symtab->addUndefinedFunction(Name, 0, nullptr, nullptr);
+
+  // Since symbol S may not be used inside the program, LTO may
+  // eliminate it. Mark the symbol as "used" to prevent it.
+  S->IsUsedInRegularObj = true;
+
+  return S;
+}
+
 void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   WasmOptTable Parser;
   opt::InputArgList Args = Parser.parse(ArgsArr.slice(1));
@@ -288,12 +317,15 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
 
   Config->AllowUndefined = Args.hasArg(OPT_allow_undefined);
   Config->Demangle = Args.hasFlag(OPT_demangle, OPT_no_demangle, true);
+  Config->DisableVerify = Args.hasArg(OPT_disable_verify);
   Config->Entry = getEntry(Args, Args.hasArg(OPT_relocatable) ? "" : "_start");
   Config->ExportTable = Args.hasArg(OPT_export_table);
   errorHandler().FatalWarnings =
       Args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   Config->ImportMemory = Args.hasArg(OPT_import_memory);
   Config->ImportTable = Args.hasArg(OPT_import_table);
+  Config->LTOO = args::getInteger(Args, OPT_lto_O, 2);
+  Config->LTOPartitions = args::getInteger(Args, OPT_lto_partitions, 1);
   Config->Optimize = args::getInteger(Args, OPT_O, 0);
   Config->OutputFile = Args.getLastArgValue(OPT_o);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
@@ -304,10 +336,16 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
                    !Config->Relocatable);
   Config->PrintGcSections =
       Args.hasFlag(OPT_print_gc_sections, OPT_no_print_gc_sections, false);
+  Config->SaveTemps = Args.hasArg(OPT_save_temps);
   Config->SearchPaths = args::getStrings(Args, OPT_L);
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->StripDebug = Args.hasArg(OPT_strip_debug);
   Config->StackFirst = Args.hasArg(OPT_stack_first);
+  Config->ThinLTOCacheDir = Args.getLastArgValue(OPT_thinlto_cache_dir);
+  Config->ThinLTOCachePolicy = CHECK(
+      parseCachePruningPolicy(Args.getLastArgValue(OPT_thinlto_cache_policy)),
+      "--thinlto-cache-policy: invalid cache policy");
+  Config->ThinLTOJobs = args::getInteger(Args, OPT_thinlto_jobs, -1u);
   errorHandler().Verbose = Args.hasArg(OPT_verbose);
   ThreadsEnabled = Args.hasFlag(OPT_threads, OPT_no_threads, true);
 
@@ -319,6 +357,13 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
 
   Config->CompressRelocTargets = Config->Optimize > 0 && !Config->Relocatable;
 
+  if (Config->LTOO > 3)
+    error("invalid optimization level for LTO: " + Twine(Config->LTOO));
+  if (Config->LTOPartitions == 0)
+    error("--lto-partitions: number of threads must be > 0");
+  if (Config->ThinLTOJobs == 0)
+    error("--thinlto-jobs: number of threads must be > 0");
+
   if (auto *Arg = Args.getLastArg(OPT_allow_undefined_file))
     readImportFile(Arg->getValue());
 
@@ -372,12 +417,11 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
     // For now, since we don't actually use the start function as the
     // wasm start symbol, we don't need to care about it signature.
     if (!Config->Entry.empty())
-      EntrySym =
-          Symtab->addUndefinedFunction(Config->Entry, 0, nullptr, nullptr);
+      EntrySym = addUndefined(Config->Entry);
 
     // Handle the `--undefined <sym>` options.
     for (auto *Arg : Args.filtered(OPT_undefined))
-      Symtab->addUndefinedFunction(Arg->getValue(), 0, nullptr, nullptr);
+      addUndefined(Arg->getValue());
   }
 
   createFiles(Args);
@@ -388,11 +432,19 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // symbols that we need to the symbol table.
   for (InputFile *F : Files)
     Symtab->addFile(F);
+  if (errorCount())
+    return;
 
   // Add synthetic dummies for weak undefined functions.
   if (!Config->Relocatable)
     handleWeakUndefines();
 
+  // Do link-time optimization if given files are LLVM bitcode files.
+  // This compiles bitcode files into real object files.
+  Symtab->addCombinedLTOObject();
+  if (errorCount())
+    return;
+
   // Make sure we have resolved all symbols.
   if (!Config->Relocatable && !Config->AllowUndefined) {
     Symtab->reportRemainingUndefines();

diff --git a/lld/wasm/InputFiles.cpp b/lld/wasm/InputFiles.cpp
@@ -370,6 +370,48 @@ void ArchiveFile::addMember(const Archive::Symbol *Sym) {
   Symtab->addFile(Obj);
 }
 
+static uint8_t mapVisibility(GlobalValue::VisibilityTypes GvVisibility) {
+  switch (GvVisibility) {
+  case GlobalValue::DefaultVisibility:
+    return WASM_SYMBOL_VISIBILITY_DEFAULT;
+  case GlobalValue::HiddenVisibility:
+  case GlobalValue::ProtectedVisibility:
+    return WASM_SYMBOL_VISIBILITY_HIDDEN;
+  }
+  llvm_unreachable("unknown visibility");
+}
+
+static Symbol *createBitcodeSymbol(const lto::InputFile::Symbol &ObjSym,
+                                   BitcodeFile &F) {
+  StringRef Name = Saver.save(ObjSym.getName());
+
+  uint32_t Flags = ObjSym.isWeak() ? WASM_SYMBOL_BINDING_WEAK : 0;
+  Flags |= mapVisibility(ObjSym.getVisibility());
+
+  if (ObjSym.isUndefined()) {
+    if (ObjSym.isExecutable())
+      return Symtab->addUndefinedFunction(Name, Flags, &F, nullptr);
+    return Symtab->addUndefinedData(Name, Flags, &F);
+  }
+
+  if (ObjSym.isExecutable())
+    return Symtab->addDefinedFunction(Name, Flags, &F, nullptr);
+  return Symtab->addDefinedData(Name, Flags, &F, nullptr, 0, 0);
+}
+
+void BitcodeFile::parse() {
+  Obj = check(lto::InputFile::create(MemoryBufferRef(
+      MB.getBuffer(), Saver.save(ParentName + MB.getBufferIdentifier()))));
+  Triple T(Obj->getTargetTriple());
+  if (T.getArch() != Triple::wasm32) {
+    error(toString(MB.getBufferIdentifier()) + ": machine type must be wasm32");
+    return;
+  }
+
+  for (const lto::InputFile::Symbol &ObjSym : Obj->symbols())
+    Symbols.push_back(createBitcodeSymbol(ObjSym, *this));
+}
+
 // Returns a string in the format of "foo.o" or "foo.a(bar.o)".
 std::string lld::toString(const wasm::InputFile *File) {
   if (!File)

diff --git a/lld/wasm/InputFiles.h b/lld/wasm/InputFiles.h
@@ -14,6 +14,7 @@
 #include "lld/Common/LLVM.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/LTO/LTO.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/Wasm.h"
 #include "llvm/Support/MemoryBuffer.h"
@@ -28,6 +29,12 @@ using llvm::wasm::WasmImport;
 using llvm::wasm::WasmRelocation;
 using llvm::wasm::WasmSignature;
 
+namespace llvm {
+namespace lto {
+class InputFile;
+}
+} // namespace llvm
+
 namespace lld {
 namespace wasm {
 
@@ -42,6 +49,7 @@ class InputFile {
   enum Kind {
     ObjectKind,
     ArchiveKind,
+    BitcodeKind,
   };
 
   virtual ~InputFile() {}
@@ -57,10 +65,15 @@ class InputFile {
   // An archive file name if this file is created from an archive.
   StringRef ParentName;
 
+  ArrayRef<Symbol *> getSymbols() const { return Symbols; }
+
 protected:
   InputFile(Kind K, MemoryBufferRef M) : MB(M), FileKind(K) {}
   MemoryBufferRef MB;
 
+  // List of all symbols referenced or defined by this file.
+  std::vector<Symbol *> Symbols;
+
 private:
   const Kind FileKind;
 };
@@ -113,7 +126,6 @@ class ObjFile : public InputFile {
   std::vector<InputSection *> CustomSections;
   llvm::DenseMap<uint32_t, InputSection *> CustomSectionsByIndex;
 
-  ArrayRef<Symbol *> getSymbols() const { return Symbols; }
   Symbol *getSymbol(uint32_t Index) const { return Symbols[Index]; }
   FunctionSymbol *getFunctionSymbol(uint32_t Index) const;
   DataSymbol *getDataSymbol(uint32_t Index) const;
@@ -126,12 +138,18 @@ class ObjFile : public InputFile {
 
   bool isExcludedByComdat(InputChunk *Chunk) const;
 
-  // List of all symbols referenced or defined by this file.
-  std::vector<Symbol *> Symbols;
-
   std::unique_ptr<WasmObjectFile> WasmObj;
 };
 
+class BitcodeFile : public InputFile {
+public:
+  explicit BitcodeFile(MemoryBufferRef M) : InputFile(BitcodeKind, M) {}
+  static bool classof(const InputFile *F) { return F->kind() == BitcodeKind; }
+
+  void parse() override;
+  std::unique_ptr<llvm::lto::InputFile> Obj;
+};
+
 // Opens a given file.
 llvm::Optional<MemoryBufferRef> readFile(StringRef Path);
 

diff --git a/lld/wasm/LTO.cpp b/lld/wasm/LTO.cpp
@@ -0,0 +1,151 @@
+//===- LTO.cpp ------------------------------------------------------------===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "LTO.h"
+#include "Config.h"
+#include "InputFiles.h"
+#include "Symbols.h"
+#include "lld/Common/ErrorHandler.h"
+#include "lld/Common/Strings.h"
+#include "lld/Common/TargetOptionsCommandFlags.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/LTO/Caching.h"
+#include "llvm/LTO/Config.h"
+#include "llvm/LTO/LTO.h"
+#include "llvm/Object/SymbolicFile.h"
+#include "llvm/Support/CodeGen.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <system_error>
+#include <vector>
+
+using namespace llvm;
+using namespace llvm::object;
+
+using namespace lld;
+using namespace lld::wasm;
+
+static std::unique_ptr<lto::LTO> createLTO() {
+  lto::Config C;
+  C.Options = InitTargetOptionsFromCodeGenFlags();
+
+  // Always emit a section per function/datum with LTO.
+  C.Options.FunctionSections = true;
+  C.Options.DataSections = true;
+
+  C.DisableVerify = Config->DisableVerify;
+  C.DiagHandler = diagnosticHandler;
+  C.OptLevel = Config->LTOO;
+
+  if (Config->SaveTemps)
+    checkError(C.addSaveTemps(Config->OutputFile.str() + ".",
+                              /*UseInputModulePath*/ true));
+
+  lto::ThinBackend Backend;
+  if (Config->ThinLTOJobs != -1U)
+    Backend = lto::createInProcessThinBackend(Config->ThinLTOJobs);
+  return llvm::make_unique<lto::LTO>(std::move(C), Backend,
+                                     Config->LTOPartitions);
+}
+
+BitcodeCompiler::BitcodeCompiler() : LTOObj(createLTO()) {}
+
+BitcodeCompiler::~BitcodeCompiler() = default;
+
+static void undefine(Symbol *S) {
+  if (isa<DefinedFunction>(S))
+    replaceSymbol<UndefinedFunction>(S, S->getName(), 0);
+  else if (isa<DefinedData>(S))
+    replaceSymbol<UndefinedData>(S, S->getName(), 0);
+  else
+    llvm_unreachable("unexpected symbol kind");
+}
+
+void BitcodeCompiler::add(BitcodeFile &F) {
+  lto::InputFile &Obj = *F.Obj;
+  unsigned SymNum = 0;
+  ArrayRef<Symbol *> Syms = F.getSymbols();
+  std::vector<lto::SymbolResolution> Resols(Syms.size());
+
+  // Provide a resolution to the LTO API for each symbol.
+  for (const lto::InputFile::Symbol &ObjSym : Obj.symbols()) {
+    Symbol *Sym = Syms[SymNum];
+    lto::SymbolResolution &R = Resols[SymNum];
+    ++SymNum;
+
+    // Ideally we shouldn't check for SF_Undefined but currently IRObjectFile
+    // reports two symbols for module ASM defined. Without this check, lld
+    // flags an undefined in IR with a definition in ASM as prevailing.
+    // Once IRObjectFile is fixed to report only one symbol this hack can
+    // be removed.
+    R.Prevailing = !ObjSym.isUndefined() && Sym->getFile() == &F;
+    R.VisibleToRegularObj = Config->Relocatable || Sym->IsUsedInRegularObj;
+    if (R.Prevailing)
+      undefine(Sym);
+  }
+  checkError(LTOObj->add(std::move(F.Obj), Resols));
+}
+
+// Merge all the bitcode files we have seen, codegen the result
+// and return the resulting objects.
+std::vector<StringRef> BitcodeCompiler::compile() {
+  unsigned MaxTasks = LTOObj->getMaxTasks();
+  Buf.resize(MaxTasks);
+  Files.resize(MaxTasks);
+
+  // The --thinlto-cache-dir option specifies the path to a directory in which
+  // to cache native object files for ThinLTO incremental builds. If a path was
+  // specified, configure LTO to use it as the cache directory.
+  lto::NativeObjectCache Cache;
+  if (!Config->ThinLTOCacheDir.empty())
+    Cache = check(
+        lto::localCache(Config->ThinLTOCacheDir,
+                        [&](size_t Task, std::unique_ptr<MemoryBuffer> MB) {
+                          Files[Task] = std::move(MB);
+                        }));
+
+  checkError(LTOObj->run(
+      [&](size_t Task) {
+        return llvm::make_unique<lto::NativeObjectStream>(
+            llvm::make_unique<raw_svector_ostream>(Buf[Task]));
+      },
+      Cache));
+
+  if (!Config->ThinLTOCacheDir.empty())
+    pruneCache(Config->ThinLTOCacheDir, Config->ThinLTOCachePolicy);
+
+  std::vector<StringRef> Ret;
+  for (unsigned I = 0; I != MaxTasks; ++I) {
+    if (Buf[I].empty())
+      continue;
+    if (Config->SaveTemps) {
+      if (I == 0)
+        saveBuffer(Buf[I], Config->OutputFile + ".lto.o");
+      else
+        saveBuffer(Buf[I], Config->OutputFile + Twine(I) + ".lto.o");
+    }
+    Ret.emplace_back(Buf[I].data(), Buf[I].size());
+  }
+
+  for (std::unique_ptr<MemoryBuffer> &File : Files)
+    if (File)
+      Ret.push_back(File->getBuffer());
+
+  return Ret;
+}
diff --git a/lld/wasm/LTO.h b/lld/wasm/LTO.h
@@ -0,0 +1,57 @@
+//===- LTO.h ----------------------------------------------------*- C++ -*-===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides a way to combine bitcode files into one wasm
+// file by compiling them using LLVM.
+//
+// If LTO is in use, your input files are not in regular wasm files
+// but instead LLVM bitcode files. In that case, the linker has to
+// convert bitcode files into the native format so that we can create
+// a wasm file that contains native code. This file provides that
+// functionality.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_WASM_LTO_H
+#define LLD_WASM_LTO_H
+
+#include "lld/Common/LLVM.h"
+#include "llvm/ADT/SmallString.h"
+#include <memory>
+#include <vector>
+
+namespace llvm {
+namespace lto {
+class LTO;
+}
+} // namespace llvm
+
+namespace lld {
+namespace wasm {
+
+class BitcodeFile;
+class InputFile;
+
+class BitcodeCompiler {
+public:
+  BitcodeCompiler();
+  ~BitcodeCompiler();
+
+  void add(BitcodeFile &F);
+  std::vector<StringRef> compile();
+
+private:
+  std::unique_ptr<llvm::lto::LTO> LTOObj;
+  std::vector<SmallString<0>> Buf;
+  std::vector<std::unique_ptr<MemoryBuffer>> Files;
+};
+} // namespace wasm
+} // namespace lld
+
+#endif
diff --git a/lld/wasm/Options.td b/lld/wasm/Options.td
@@ -136,3 +136,16 @@ def alias_initial_memory_i: Flag<["-"], "i">, Alias<initial_memory>;
 def alias_max_memory_m: Flag<["-"], "m">, Alias<max_memory>;
 def alias_relocatable_r: Flag<["-"], "r">, Alias<relocatable>;
 def alias_undefined_u: JoinedOrSeparate<["-"], "u">, Alias<undefined>;
+
+// LTO-related options.
+def lto_O: J<"lto-O">, MetaVarName<"<opt-level>">,
+  HelpText<"Optimization level for LTO">;
+def lto_partitions: J<"lto-partitions=">,
+  HelpText<"Number of LTO codegen partitions">;
+def disable_verify: F<"disable-verify">;
+def save_temps: F<"save-temps">;
+def thinlto_cache_dir: J<"thinlto-cache-dir=">,
+  HelpText<"Path to ThinLTO cached object file directory">;
+defm thinlto_cache_policy: Eq<"thinlto-cache-policy">,
+  HelpText<"Pruning policy for the ThinLTO cache">;
+def thinlto_jobs: J<"thinlto-jobs=">, HelpText<"Number of ThinLTO jobs">;
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
@@ -29,17 +29,46 @@ void SymbolTable::addFile(InputFile *File) {
   log("Processing: " + toString(File));
   File->parse();
 
-  if (auto *F = dyn_cast<ObjFile>(File))
+  // LLVM bitcode file
+  if (auto *F = dyn_cast<BitcodeFile>(File))
+    BitcodeFiles.push_back(F);
+  else if (auto *F = dyn_cast<ObjFile>(File))
     ObjectFiles.push_back(F);
 }
 
+// This function is where all the optimizations of link-time
+// optimization happens. When LTO is in use, some input files are
+// not in native object file format but in the LLVM bitcode format.
+// This function compiles bitcode files into a few big native files
+// using LLVM functions and replaces bitcode symbols with the results.
+// Because all bitcode files that the program consists of are passed
+// to the compiler at once, it can do whole-program optimization.
+void SymbolTable::addCombinedLTOObject() {
+  if (BitcodeFiles.empty())
+    return;
+
+  // Compile bitcode files and replace bitcode symbols.
+  LTO.reset(new BitcodeCompiler);
+  for (BitcodeFile *F : BitcodeFiles)
+    LTO->add(*F);
+
+  for (StringRef Filename : LTO->compile()) {
+    auto *Obj = make<ObjFile>(MemoryBufferRef(Filename, "lto.tmp"));
+    Obj->parse();
+    ObjectFiles.push_back(Obj);
+  }
+}
+
 void SymbolTable::reportRemainingUndefines() {
   SetVector<Symbol *> Undefs;
   for (Symbol *Sym : SymVector) {
-    if (Sym->isUndefined() && !Sym->isWeak() &&
-        Config->AllowUndefinedSymbols.count(Sym->getName()) == 0) {
-      Undefs.insert(Sym);
-    }
+    if (!Sym->isUndefined() || Sym->isWeak())
+      continue;
+    if (Config->AllowUndefinedSymbols.count(Sym->getName()) != 0)
+      continue;
+    if (!Sym->IsUsedInRegularObj)
+      continue;
+    Undefs.insert(Sym);
   }
 
   if (Undefs.empty())
@@ -64,6 +93,7 @@ std::pair<Symbol *, bool> SymbolTable::insert(StringRef Name) {
   if (Sym)
     return {Sym, false};
   Sym = reinterpret_cast<Symbol *>(make<SymbolUnion>());
+  Sym->IsUsedInRegularObj = false;
   SymVector.emplace_back(Sym);
   return {Sym, true};
 }
@@ -178,12 +208,16 @@ Symbol *SymbolTable::addDefinedFunction(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name);
 
+  if (!File || File->kind() == InputFile::ObjectKind)
+    S->IsUsedInRegularObj = true;
+
   if (WasInserted || S->isLazy()) {
     replaceSymbol<DefinedFunction>(S, Name, Flags, File, Function);
     return S;
   }
 
-  checkFunctionType(S, File, &Function->Signature);
+  if (Function)
+    checkFunctionType(S, File, &Function->Signature);
 
   if (shouldReplace(S, File, Flags))
     replaceSymbol<DefinedFunction>(S, Name, Flags, File, Function);
@@ -199,6 +233,9 @@ Symbol *SymbolTable::addDefinedData(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name);
 
+  if (!File || File->kind() == InputFile::ObjectKind)
+    S->IsUsedInRegularObj = true;
+
   if (WasInserted || S->isLazy()) {
     replaceSymbol<DefinedData>(S, Name, Flags, File, Segment, Address, Size);
     return S;
@@ -218,6 +255,9 @@ Symbol *SymbolTable::addDefinedGlobal(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name);
 
+  if (!File || File->kind() == InputFile::ObjectKind)
+    S->IsUsedInRegularObj = true;
+
   if (WasInserted || S->isLazy()) {
     replaceSymbol<DefinedGlobal>(S, Name, Flags, File, Global);
     return S;
@@ -239,6 +279,9 @@ Symbol *SymbolTable::addUndefinedFunction(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name);
 
+  if (!File || File->kind() == InputFile::ObjectKind)
+    S->IsUsedInRegularObj = true;
+
   if (WasInserted)
     replaceSymbol<UndefinedFunction>(S, Name, Flags, File, Sig);
   else if (auto *Lazy = dyn_cast<LazySymbol>(S))
@@ -274,6 +317,9 @@ Symbol *SymbolTable::addUndefinedGlobal(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name);
 
+  if (!File || File->kind() == InputFile::ObjectKind)
+    S->IsUsedInRegularObj = true;
+
   if (WasInserted)
     replaceSymbol<UndefinedGlobal>(S, Name, Flags, File, Type);
   else if (auto *Lazy = dyn_cast<LazySymbol>(S))

diff --git a/lld/wasm/SymbolTable.h b/lld/wasm/SymbolTable.h
@@ -11,6 +11,7 @@
 #define LLD_WASM_SYMBOL_TABLE_H
 
 #include "InputFiles.h"
+#include "LTO.h"
 #include "Symbols.h"
 #include "llvm/ADT/CachedHashString.h"
 #include "llvm/ADT/DenseSet.h"
@@ -39,8 +40,10 @@ class InputSegment;
 class SymbolTable {
 public:
   void addFile(InputFile *File);
+  void addCombinedLTOObject();
 
   std::vector<ObjFile *> ObjectFiles;
+  std::vector<BitcodeFile *> BitcodeFiles;
   std::vector<InputFunction *> SyntheticFunctions;
   std::vector<InputGlobal *> SyntheticGlobals;
 
@@ -80,6 +83,9 @@ class SymbolTable {
   std::vector<Symbol *> SymVector;
 
   llvm::DenseSet<llvm::CachedHashStringRef> Comdats;
+
+  // For LTO.
+  std::unique_ptr<BitcodeCompiler> LTO;
 };
 
 extern SymbolTable *Symtab;

diff --git a/lld/wasm/Symbols.h b/lld/wasm/Symbols.h
@@ -91,10 +91,13 @@ class Symbol {
 
   WasmSymbolType getWasmType() const;
 
+  // True if this symbol was referenced by a regular (non-bitcode) object.
+  unsigned IsUsedInRegularObj : 1;
+
 protected:
   Symbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F)
-      : Name(Name), SymbolKind(K), Flags(Flags), File(F),
-        Referenced(!Config->GcSections) {}
+      : IsUsedInRegularObj(false), Name(Name), SymbolKind(K), Flags(Flags),
+        File(F), Referenced(!Config->GcSections) {}
 
   StringRef Name;
   Kind SymbolKind;
@@ -332,7 +335,12 @@ T *replaceSymbol(Symbol *S, ArgT &&... Arg) {
                 "SymbolUnion not aligned enough");
   assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
          "Not a Symbol");
-  return new (S) T(std::forward<ArgT>(Arg)...);
+
+  Symbol SymCopy = *S;
+
+  T *S2 = new (S) T(std::forward<ArgT>(Arg)...);
+  S2->IsUsedInRegularObj = SymCopy.IsUsedInRegularObj;
+  return S2;
 }
 
 } // namespace wasm

diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
@@ -715,6 +715,8 @@ void Writer::calculateImports() {
       continue;
     if (!Sym->isLive())
       continue;
+    if (!Sym->IsUsedInRegularObj)
+      continue;
 
     LLVM_DEBUG(dbgs() << "import: " << Sym->getName() << "\n");
     ImportedSymbols.emplace_back(Sym);