Vendor import of lld trunk r290819:vendor/lld/lld-trunk-r290819

https://llvm.org/svn/llvm-project/lld/trunk@290819

28 files changed, 1694 insertions, 1107 deletions

diff --git a/COFF/CMakeLists.txt b/COFF/CMakeLists.txt
index ad5b6fda1693..70a33b9fdd81 100644
--- a/COFF/CMakeLists.txt
+++ b/COFF/CMakeLists.txt
@@ -2,6 +2,10 @@ set(LLVM_TARGET_DEFINITIONS Options.td)
 tablegen(LLVM Options.inc -gen-opt-parser-defs)
 add_public_tablegen_target(COFFOptionsTableGen)
 
+if(NOT LLD_BUILT_STANDALONE)
+  set(tablegen_deps intrinsics_gen)
+endif()
+
 add_lld_library(lldCOFF
   Chunks.cpp
   DLL.cpp
@@ -14,6 +18,7 @@ add_lld_library(lldCOFF
   MarkLive.cpp
   ModuleDef.cpp
   PDB.cpp
+  Strings.cpp
   SymbolTable.cpp
   Symbols.cpp
   Writer.cpp
@@ -21,6 +26,9 @@ add_lld_library(lldCOFF
   LINK_COMPONENTS
   ${LLVM_TARGETS_TO_BUILD}
   Core
+  DebugInfoCodeView
+  DebugInfoMSF
+  DebugInfoPDB
   LTO
   LibDriver
   Object
@@ -30,7 +38,11 @@ add_lld_library(lldCOFF
   Option
   Support
 
-  LINK_LIBS ${PTHREAD_LIB}
-  )
+  LINK_LIBS
+  lldCore
+  ${PTHREAD_LIB}
 
-add_dependencies(lldCOFF COFFOptionsTableGen intrinsics_gen)
+  DEPENDS
+  COFFOptionsTableGen
+  ${tablegen_deps}
+  )
diff --git a/COFF/Chunks.cpp b/COFF/Chunks.cpp
index 1c1b18176aa2..7f0dfa92ec10 100644
--- a/COFF/Chunks.cpp
+++ b/COFF/Chunks.cpp
@@ -28,7 +28,7 @@ namespace lld {
 namespace coff {
 
 SectionChunk::SectionChunk(ObjectFile *F, const coff_section *H)
-    : Chunk(SectionKind), Repl(this), File(F), Header(H),
+    : Chunk(SectionKind), Repl(this), Header(H), File(F),
       Relocs(File->getCOFFObj()->getRelocations(Header)),
       NumRelocs(std::distance(Relocs.begin(), Relocs.end())) {
   // Initialize SectionName.
@@ -81,11 +81,23 @@ void SectionChunk::applyRelX86(uint8_t *Off, uint16_t Type, Defined *Sym,
 }
 
 static void applyMOV(uint8_t *Off, uint16_t V) {
-  or16(Off, ((V & 0x800) >> 1) | ((V >> 12) & 0xf));
-  or16(Off + 2, ((V & 0x700) << 4) | (V & 0xff));
+  write16le(Off, (read16le(Off) & 0xfbf0) | ((V & 0x800) >> 1) | ((V >> 12) & 0xf));
+  write16le(Off + 2, (read16le(Off + 2) & 0x8f00) | ((V & 0x700) << 4) | (V & 0xff));
+}
+
+static uint16_t readMOV(uint8_t *Off) {
+  uint16_t Opcode1 = read16le(Off);
+  uint16_t Opcode2 = read16le(Off + 2);
+  uint16_t Imm = (Opcode2 & 0x00ff) | ((Opcode2 >> 4) & 0x0700);
+  Imm |= ((Opcode1 << 1) & 0x0800) | ((Opcode1 & 0x000f) << 12);
+  return Imm;
 }
 
 static void applyMOV32T(uint8_t *Off, uint32_t V) {
+  uint16_t ImmW = readMOV(Off);     // read MOVW operand
+  uint16_t ImmT = readMOV(Off + 4); // read MOVT operand
+  uint32_t Imm = ImmW | (ImmT << 16);
+  V += Imm;                         // add the immediate offset
   applyMOV(Off, V);           // set MOVW operand
   applyMOV(Off + 4, V >> 16); // set MOVT operand
 }
@@ -99,11 +111,14 @@ static void applyBranch20T(uint8_t *Off, int32_t V) {
 }
 
 static void applyBranch24T(uint8_t *Off, int32_t V) {
+  if (!isInt<25>(V))
+    fatal("relocation out of range");
   uint32_t S = V < 0 ? 1 : 0;
   uint32_t J1 = ((~V >> 23) & 1) ^ S;
   uint32_t J2 = ((~V >> 22) & 1) ^ S;
   or16(Off, (S << 10) | ((V >> 12) & 0x3ff));
-  or16(Off + 2, (J1 << 13) | (J2 << 11) | ((V >> 1) & 0x7ff));
+  // Clear out the J1 and J2 bits which may be set.
+  write16le(Off + 2, (read16le(Off + 2) & 0xd000) | (J1 << 13) | (J2 << 11) | ((V >> 1) & 0x7ff));
 }
 
 void SectionChunk::applyRelARM(uint8_t *Off, uint16_t Type, Defined *Sym,
@@ -119,6 +134,7 @@ void SectionChunk::applyRelARM(uint8_t *Off, uint16_t Type, Defined *Sym,
   case IMAGE_REL_ARM_BRANCH20T: applyBranch20T(Off, S - P - 4); break;
   case IMAGE_REL_ARM_BRANCH24T: applyBranch24T(Off, S - P - 4); break;
   case IMAGE_REL_ARM_BLX23T:    applyBranch24T(Off, S - P - 4); break;
+  case IMAGE_REL_ARM_SECREL:    add32(Off, Sym->getSecrel()); break;
   default:
     fatal("unsupported relocation type");
   }
@@ -134,7 +150,7 @@ void SectionChunk::writeTo(uint8_t *Buf) const {
   // Apply relocations.
   for (const coff_relocation &Rel : Relocs) {
     uint8_t *Off = Buf + OutputSectionOff + Rel.VirtualAddress;
-    SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex)->repl();
+    SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex);
     Defined *Sym = cast<Defined>(Body);
     uint64_t P = RVA + Rel.VirtualAddress;
     switch (Config->Machine) {
@@ -187,7 +203,7 @@ void SectionChunk::getBaserels(std::vector<Baserel> *Res) {
     uint8_t Ty = getBaserelType(Rel);
     if (Ty == IMAGE_REL_BASED_ABSOLUTE)
       continue;
-    SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex)->repl();
+    SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex);
     if (isa<DefinedAbsolute>(Body))
       continue;
     Res->emplace_back(RVA + Rel.VirtualAddress, Ty);
@@ -210,7 +226,7 @@ void SectionChunk::printDiscardedMessage() const {
   // Removed by dead-stripping. If it's removed by ICF, ICF already
   // printed out the name, so don't repeat that here.
   if (Sym && this == Repl)
-    llvm::outs() << "Discarded " << Sym->getName() << "\n";
+    outs() << "Discarded " << Sym->getName() << "\n";
 }
 
 StringRef SectionChunk::getDebugName() {
@@ -233,7 +249,7 @@ void SectionChunk::replace(SectionChunk *Other) {
 CommonChunk::CommonChunk(const COFFSymbolRef S) : Sym(S) {
   // Common symbols are aligned on natural boundaries up to 32 bytes.
   // This is what MSVC link.exe does.
-  Align = std::min(uint64_t(32), NextPowerOf2(Sym.getValue()));
+  Align = std::min(uint64_t(32), PowerOf2Ceil(Sym.getValue()));
 }
 
 uint32_t CommonChunk::getPermissions() const {
diff --git a/COFF/Chunks.h b/COFF/Chunks.h
index cd0e2e69ef5d..59e36b84c9b0 100644
--- a/COFF/Chunks.h
+++ b/COFF/Chunks.h
@@ -17,7 +17,6 @@
 #include "llvm/ADT/iterator.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Object/COFF.h"
-#include <atomic>
 #include <utility>
 #include <vector>
 
@@ -29,7 +28,6 @@ using llvm::object::COFFSymbolRef;
 using llvm::object::SectionRef;
 using llvm::object::coff_relocation;
 using llvm::object::coff_section;
-using llvm::sys::fs::file_magic;
 
 class Baserel;
 class Defined;
@@ -187,11 +185,12 @@ public:
   // Auxiliary Format 5: Section Definitions. Used for ICF.
   uint32_t Checksum = 0;
 
+  const coff_section *Header;
+
 private:
   // A file this chunk was created from.
   ObjectFile *File;
 
-  const coff_section *Header;
   StringRef SectionName;
   std::vector<SectionChunk *> AssocChildren;
   llvm::iterator_range<const coff_relocation *> Relocs;
@@ -202,7 +201,7 @@ private:
 
   // Used for ICF (Identical COMDAT Folding)
   void replace(SectionChunk *Other);
-  std::atomic<uint64_t> GroupID = { 0 };
+  uint32_t Color[2] = {0, 0};
 
   // Sym points to a section symbol if this is a COMDAT chunk.
   DefinedRegular *Sym = nullptr;
diff --git a/COFF/Config.h b/COFF/Config.h
index a5472e937fa1..0fa3338aa28c 100644
--- a/COFF/Config.h
+++ b/COFF/Config.h
@@ -26,7 +26,8 @@ using llvm::StringRef;
 class DefinedAbsolute;
 class DefinedRelative;
 class StringChunk;
-class Undefined;
+struct Symbol;
+class SymbolBody;
 
 // Short aliases.
 static const auto AMD64 = llvm::COFF::IMAGE_FILE_MACHINE_AMD64;
@@ -37,7 +38,7 @@ static const auto I386 = llvm::COFF::IMAGE_FILE_MACHINE_I386;
 struct Export {
   StringRef Name;       // N in /export:N or /export:E=N
   StringRef ExtName;    // E in /export:E=N
-  Undefined *Sym = nullptr;
+  SymbolBody *Sym = nullptr;
   uint16_t Ordinal = 0;
   bool Noname = false;
   bool Data = false;
@@ -61,6 +62,13 @@ struct Export {
   }
 };
 
+enum class DebugType {
+  None  = 0x0,
+  CV    = 0x1,  /// CodeView
+  PData = 0x2,  /// Procedure Data
+  Fixup = 0x4,  /// Relocation Table
+};
+
 // Global configuration.
 struct Configuration {
   enum ManifestKind { SideBySide, Embed, No };
@@ -69,7 +77,7 @@ struct Configuration {
   llvm::COFF::MachineTypes Machine = IMAGE_FILE_MACHINE_UNKNOWN;
   bool Verbose = false;
   WindowsSubsystem Subsystem = llvm::COFF::IMAGE_SUBSYSTEM_UNKNOWN;
-  Undefined *Entry = nullptr;
+  SymbolBody *Entry = nullptr;
   bool NoEntry = false;
   std::string OutputFile;
   bool DoGC = true;
@@ -78,9 +86,11 @@ struct Configuration {
   bool Force = false;
   bool Debug = false;
   bool WriteSymtab = true;
+  unsigned DebugTypes = static_cast<unsigned>(DebugType::None);
+  StringRef PDBPath;
 
   // Symbols in this set are considered as live by the garbage collector.
-  std::set<Undefined *> GCRoot;
+  std::set<SymbolBody *> GCRoot;
 
   std::set<StringRef> NoDefaultLibs;
   bool NoDefaultLibAll = false;
@@ -91,11 +101,11 @@ struct Configuration {
   std::vector<Export> Exports;
   std::set<std::string> DelayLoads;
   std::map<std::string, int> DLLOrder;
-  Undefined *DelayLoadHelper = nullptr;
+  SymbolBody *DelayLoadHelper = nullptr;
 
   // Used for SafeSEH.
-  DefinedRelative *SEHTable = nullptr;
-  DefinedAbsolute *SEHCount = nullptr;
+  Symbol *SEHTable = nullptr;
+  Symbol *SEHCount = nullptr;
 
   // Used for /opt:lldlto=N
   unsigned LTOOptLevel = 2;
@@ -141,6 +151,10 @@ struct Configuration {
   bool TerminalServerAware = true;
   bool LargeAddressAware = false;
   bool HighEntropyVA = false;
+
+  // This is for debugging.
+  bool DebugPdb = false;
+  bool DumpPdb = false;
 };
 
 extern Configuration *Config;
diff --git a/COFF/DLL.cpp b/COFF/DLL.cpp
index 9ac370c11d59..f93dc5cde44c 100644
--- a/COFF/DLL.cpp
+++ b/COFF/DLL.cpp
@@ -324,7 +324,7 @@ public:
       if (E.ForwardChunk) {
         write32le(P, E.ForwardChunk->getRVA());
       } else {
-        write32le(P, cast<Defined>(E.Sym->repl())->getRVA());
+        write32le(P, cast<Defined>(E.Sym)->getRVA());
       }
     }
   }
diff --git a/COFF/Driver.cpp b/COFF/Driver.cpp
index bb6a60e4fc4c..dc3a00ba55ed 100644
--- a/COFF/Driver.cpp
+++ b/COFF/Driver.cpp
@@ -7,15 +7,17 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "Config.h"
 #include "Driver.h"
+#include "Config.h"
 #include "Error.h"
 #include "InputFiles.h"
+#include "Memory.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "Writer.h"
 #include "lld/Driver/Driver.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringSwitch.h"
 #include "llvm/LibDriver/LibDriver.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
@@ -28,6 +30,13 @@
 #include <algorithm>
 #include <memory>
 
+#ifdef _MSC_VER
+// <future> depends on <eh.h> for __uncaught_exception.
+#include <eh.h>
+#endif
+
+#include <future>
+
 using namespace llvm;
 using namespace llvm::COFF;
 using llvm::sys::Process;
@@ -41,11 +50,13 @@ namespace coff {
 Configuration *Config;
 LinkerDriver *Driver;
 
-bool link(llvm::ArrayRef<const char *> Args) {
-  Configuration C;
-  LinkerDriver D;
-  Config = &C;
-  Driver = &D;
+BumpPtrAllocator BAlloc;
+StringSaver Saver{BAlloc};
+std::vector<SpecificAllocBase *> SpecificAllocBase::Instances;
+
+bool link(ArrayRef<const char *> Args) {
+  Config = make<Configuration>();
+  Driver = make<LinkerDriver>();
   Driver->link(Args);
   return true;
 }
@@ -58,26 +69,123 @@ static std::string getOutputPath(StringRef Path) {
   return (S.substr(0, S.rfind('.')) + E).str();
 }
 
-// Opens a file. Path has to be resolved already.
-// Newly created memory buffers are owned by this driver.
-MemoryBufferRef LinkerDriver::openFile(StringRef Path) {
-  std::unique_ptr<MemoryBuffer> MB =
-      check(MemoryBuffer::getFile(Path), "could not open " + Path);
-  MemoryBufferRef MBRef = MB->getMemBufferRef();
-  OwningMBs.push_back(std::move(MB)); // take ownership
+// ErrorOr is not default constructible, so it cannot be used as the type
+// parameter of a future.
+// FIXME: We could open the file in createFutureForFile and avoid needing to
+// return an error here, but for the moment that would cost us a file descriptor
+// (a limited resource on Windows) for the duration that the future is pending.
+typedef std::pair<std::unique_ptr<MemoryBuffer>, std::error_code> MBErrPair;
+
+// Create a std::future that opens and maps a file using the best strategy for
+// the host platform.
+static std::future<MBErrPair> createFutureForFile(std::string Path) {
+#if LLVM_ON_WIN32
+  // On Windows, file I/O is relatively slow so it is best to do this
+  // asynchronously.
+  auto Strategy = std::launch::async;
+#else
+  auto Strategy = std::launch::deferred;
+#endif
+  return std::async(Strategy, [=]() {
+    auto MBOrErr = MemoryBuffer::getFile(Path);
+    if (!MBOrErr)
+      return MBErrPair{nullptr, MBOrErr.getError()};
+    return MBErrPair{std::move(*MBOrErr), std::error_code()};
+  });
+}
+
+MemoryBufferRef LinkerDriver::takeBuffer(std::unique_ptr<MemoryBuffer> MB) {
+  MemoryBufferRef MBRef = *MB;
+  OwningMBs.push_back(std::move(MB));
+
+  if (Driver->Cpio)
+    Driver->Cpio->append(relativeToRoot(MBRef.getBufferIdentifier()),
+                         MBRef.getBuffer());
+
   return MBRef;
 }
 
-static std::unique_ptr<InputFile> createFile(MemoryBufferRef MB) {
+void LinkerDriver::addBuffer(std::unique_ptr<MemoryBuffer> MB) {
+  MemoryBufferRef MBRef = takeBuffer(std::move(MB));
+
   // File type is detected by contents, not by file extension.
-  file_magic Magic = identify_magic(MB.getBuffer());
+  file_magic Magic = identify_magic(MBRef.getBuffer());
+  if (Magic == file_magic::windows_resource) {
+    Resources.push_back(MBRef);
+    return;
+  }
+
+  FilePaths.push_back(MBRef.getBufferIdentifier());
   if (Magic == file_magic::archive)
-    return std::unique_ptr<InputFile>(new ArchiveFile(MB));
+    return Symtab.addFile(make<ArchiveFile>(MBRef));
   if (Magic == file_magic::bitcode)
-    return std::unique_ptr<InputFile>(new BitcodeFile(MB));
+    return Symtab.addFile(make<BitcodeFile>(MBRef));
+  if (Magic == file_magic::coff_cl_gl_object)
+    fatal(MBRef.getBufferIdentifier() + ": is not a native COFF file. "
+          "Recompile without /GL");
+  Symtab.addFile(make<ObjectFile>(MBRef));
+}
+
+void LinkerDriver::enqueuePath(StringRef Path) {
+  auto Future =
+      std::make_shared<std::future<MBErrPair>>(createFutureForFile(Path));
+  std::string PathStr = Path;
+  enqueueTask([=]() {
+    auto MBOrErr = Future->get();
+    if (MBOrErr.second)
+      fatal(MBOrErr.second, "could not open " + PathStr);
+    Driver->addBuffer(std::move(MBOrErr.first));
+  });
+
   if (Config->OutputFile == "")
-    Config->OutputFile = getOutputPath(MB.getBufferIdentifier());
-  return std::unique_ptr<InputFile>(new ObjectFile(MB));
+    Config->OutputFile = getOutputPath(Path);
+}
+
+void LinkerDriver::addArchiveBuffer(MemoryBufferRef MB, StringRef SymName,
+                                    StringRef ParentName) {
+  file_magic Magic = identify_magic(MB.getBuffer());
+  if (Magic == file_magic::coff_import_library) {
+    Symtab.addFile(make<ImportFile>(MB));
+    return;
+  }
+
+  InputFile *Obj;
+  if (Magic == file_magic::coff_object)
+    Obj = make<ObjectFile>(MB);
+  else if (Magic == file_magic::bitcode)
+    Obj = make<BitcodeFile>(MB);
+  else
+    fatal("unknown file type: " + MB.getBufferIdentifier());
+
+  Obj->ParentName = ParentName;
+  Symtab.addFile(Obj);
+  if (Config->Verbose)
+    outs() << "Loaded " << toString(Obj) << " for " << SymName << "\n";
+}
+
+void LinkerDriver::enqueueArchiveMember(const Archive::Child &C,
+                                        StringRef SymName,
+                                        StringRef ParentName) {
+  if (!C.getParent()->isThin()) {
+    MemoryBufferRef MB = check(
+        C.getMemoryBufferRef(),
+        "could not get the buffer for the member defining symbol " + SymName);
+    enqueueTask([=]() { Driver->addArchiveBuffer(MB, SymName, ParentName); });
+    return;
+  }
+
+  auto Future = std::make_shared<std::future<MBErrPair>>(createFutureForFile(
+      check(C.getFullName(),
+            "could not get the filename for the member defining symbol " +
+                SymName)));
+  enqueueTask([=]() {
+    auto MBOrErr = Future->get();
+    if (MBOrErr.second)
+      fatal(MBOrErr.second,
+            "could not get the buffer for the member defining " + SymName);
+    Driver->addArchiveBuffer(takeBuffer(std::move(MBOrErr.first)), SymName,
+                             ParentName);
+  });
 }
 
 static bool isDecorated(StringRef Sym) {
@@ -87,7 +195,7 @@ static bool isDecorated(StringRef Sym) {
 // Parses .drectve section contents and returns a list of files
 // specified by /defaultlib.
 void LinkerDriver::parseDirectives(StringRef S) {
-  llvm::opt::InputArgList Args = Parser.parse(S);
+  opt::InputArgList Args = Parser.parse(S);
 
   for (auto *Arg : Args) {
     switch (Arg->getOption().getID()) {
@@ -95,10 +203,8 @@ void LinkerDriver::parseDirectives(StringRef S) {
       parseAlternateName(Arg->getValue());
       break;
     case OPT_defaultlib:
-      if (Optional<StringRef> Path = findLib(Arg->getValue())) {
-        MemoryBufferRef MB = openFile(*Path);
-        Symtab.addFile(createFile(MB));
-      }
+      if (Optional<StringRef> Path = findLib(Arg->getValue()))
+        enqueuePath(*Path);
       break;
     case OPT_export: {
       Export E = parseExport(Arg->getValue());
@@ -135,19 +241,19 @@ void LinkerDriver::parseDirectives(StringRef S) {
 // Find file from search paths. You can omit ".obj", this function takes
 // care of that. Note that the returned path is not guaranteed to exist.
 StringRef LinkerDriver::doFindFile(StringRef Filename) {
-  bool hasPathSep = (Filename.find_first_of("/\\") != StringRef::npos);
-  if (hasPathSep)
+  bool HasPathSep = (Filename.find_first_of("/\\") != StringRef::npos);
+  if (HasPathSep)
     return Filename;
-  bool hasExt = (Filename.find('.') != StringRef::npos);
+  bool HasExt = (Filename.find('.') != StringRef::npos);
   for (StringRef Dir : SearchPaths) {
     SmallString<128> Path = Dir;
-    llvm::sys::path::append(Path, Filename);
-    if (llvm::sys::fs::exists(Path.str()))
-      return Alloc.save(Path.str());
-    if (!hasExt) {
+    sys::path::append(Path, Filename);
+    if (sys::fs::exists(Path.str()))
+      return Saver.save(Path.str());
+    if (!HasExt) {
       Path.append(".obj");
-      if (llvm::sys::fs::exists(Path.str()))
-        return Alloc.save(Path.str());
+      if (sys::fs::exists(Path.str()))
+        return Saver.save(Path.str());
     }
   }
   return Filename;
@@ -166,9 +272,9 @@ Optional<StringRef> LinkerDriver::findFile(StringRef Filename) {
 // Find library file from search path.
 StringRef LinkerDriver::doFindLib(StringRef Filename) {
   // Add ".lib" to Filename if that has no file extension.
-  bool hasExt = (Filename.find('.') != StringRef::npos);
-  if (!hasExt)
-    Filename = Alloc.save(Filename + ".lib");
+  bool HasExt = (Filename.find('.') != StringRef::npos);
+  if (!HasExt)
+    Filename = Saver.save(Filename + ".lib");
   return doFindFile(Filename);
 }
 
@@ -178,11 +284,12 @@ StringRef LinkerDriver::doFindLib(StringRef Filename) {
 Optional<StringRef> LinkerDriver::findLib(StringRef Filename) {
   if (Config->NoDefaultLibAll)
     return None;
+  if (!VisitedLibs.insert(Filename.lower()).second)
+    return None;
   StringRef Path = doFindLib(Filename);
   if (Config->NoDefaultLibs.count(Path))
     return None;
-  bool Seen = !VisitedFiles.insert(Path.lower()).second;
-  if (Seen)
+  if (!VisitedFiles.insert(Path.lower()).second)
     return None;
   return Path;
 }
@@ -192,7 +299,7 @@ void LinkerDriver::addLibSearchPaths() {
   Optional<std::string> EnvOpt = Process::GetEnv("LIB");
   if (!EnvOpt.hasValue())
     return;
-  StringRef Env = Alloc.save(*EnvOpt);
+  StringRef Env = Saver.save(*EnvOpt);
   while (!Env.empty()) {
     StringRef Path;
     std::tie(Path, Env) = Env.split(';');
@@ -200,17 +307,17 @@ void LinkerDriver::addLibSearchPaths() {
   }
 }
 
-Undefined *LinkerDriver::addUndefined(StringRef Name) {
-  Undefined *U = Symtab.addUndefined(Name);
-  Config->GCRoot.insert(U);
-  return U;
+SymbolBody *LinkerDriver::addUndefined(StringRef Name) {
+  SymbolBody *B = Symtab.addUndefined(Name);
+  Config->GCRoot.insert(B);
+  return B;
 }
 
 // Symbol names are mangled by appending "_" prefix on x86.
 StringRef LinkerDriver::mangle(StringRef Sym) {
   assert(Config->Machine != IMAGE_FILE_MACHINE_UNKNOWN);
   if (Config->Machine == I386)
-    return Alloc.save("_" + Sym);
+    return Saver.save("_" + Sym);
   return Sym;
 }
 
@@ -225,7 +332,7 @@ StringRef LinkerDriver::findDefaultEntry() {
   };
   for (auto E : Entries) {
     StringRef Entry = Symtab.findMangle(mangle(E[0]));
-    if (!Entry.empty() && !isa<Undefined>(Symtab.find(Entry)->Body))
+    if (!Entry.empty() && !isa<Undefined>(Symtab.find(Entry)->body()))
       return mangle(E[1]);
   }
   return "";
@@ -247,7 +354,83 @@ static uint64_t getDefaultImageBase() {
   return Config->DLL ? 0x10000000 : 0x400000;
 }
 
-void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
+static std::string createResponseFile(const opt::InputArgList &Args,
+                                      ArrayRef<StringRef> FilePaths,
+                                      ArrayRef<StringRef> SearchPaths) {
+  SmallString<0> Data;
+  raw_svector_ostream OS(Data);
+
+  for (auto *Arg : Args) {
+    switch (Arg->getOption().getID()) {
+    case OPT_linkrepro:
+    case OPT_INPUT:
+    case OPT_defaultlib:
+    case OPT_libpath:
+      break;
+    default:
+      OS << stringize(Arg) << "\n";
+    }
+  }
+
+  for (StringRef Path : SearchPaths) {
+    std::string RelPath = relativeToRoot(Path);
+    OS << "/libpath:" << quote(RelPath) << "\n";
+  }
+
+  for (StringRef Path : FilePaths)
+    OS << quote(relativeToRoot(Path)) << "\n";
+
+  return Data.str();
+}
+
+static unsigned getDefaultDebugType(const opt::InputArgList &Args) {
+  unsigned DebugTypes = static_cast<unsigned>(DebugType::CV);
+  if (Args.hasArg(OPT_driver))
+    DebugTypes |= static_cast<unsigned>(DebugType::PData);
+  if (Args.hasArg(OPT_profile))
+    DebugTypes |= static_cast<unsigned>(DebugType::Fixup);
+  return DebugTypes;
+}
+
+static unsigned parseDebugType(StringRef Arg) {
+  SmallVector<StringRef, 3> Types;
+  Arg.split(Types, ',', /*KeepEmpty=*/false);
+
+  unsigned DebugTypes = static_cast<unsigned>(DebugType::None);
+  for (StringRef Type : Types)
+    DebugTypes |= StringSwitch<unsigned>(Type.lower())
+                      .Case("cv", static_cast<unsigned>(DebugType::CV))
+                      .Case("pdata", static_cast<unsigned>(DebugType::PData))
+                      .Case("fixup", static_cast<unsigned>(DebugType::Fixup));
+  return DebugTypes;
+}
+
+static std::string getMapFile(const opt::InputArgList &Args) {
+  auto *Arg = Args.getLastArg(OPT_lldmap, OPT_lldmap_file);
+  if (!Arg)
+    return "";
+  if (Arg->getOption().getID() == OPT_lldmap_file)
+    return Arg->getValue();
+
+  assert(Arg->getOption().getID() == OPT_lldmap);
+  StringRef OutFile = Config->OutputFile;
+  return (OutFile.substr(0, OutFile.rfind('.')) + ".map").str();
+}
+
+void LinkerDriver::enqueueTask(std::function<void()> Task) {
+  TaskQueue.push_back(std::move(Task));
+}
+
+bool LinkerDriver::run() {
+  bool DidWork = !TaskQueue.empty();
+  while (!TaskQueue.empty()) {
+    TaskQueue.front()();
+    TaskQueue.pop_front();
+  }
+  return DidWork;
+}
+
+void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // If the first command line argument is "/lib", link.exe acts like lib.exe.
   // We call our own implementation of lib.exe that understands bitcode files.
   if (ArgsArr.size() > 1 && StringRef(ArgsArr[1]).equals_lower("/lib")) {
@@ -257,15 +440,15 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
   }
 
   // Needed for LTO.
-  llvm::InitializeAllTargetInfos();
-  llvm::InitializeAllTargets();
-  llvm::InitializeAllTargetMCs();
-  llvm::InitializeAllAsmParsers();
-  llvm::InitializeAllAsmPrinters();
-  llvm::InitializeAllDisassemblers();
+  InitializeAllTargetInfos();
+  InitializeAllTargets();
+  InitializeAllTargetMCs();
+  InitializeAllAsmParsers();
+  InitializeAllAsmPrinters();
+  InitializeAllDisassemblers();
 
   // Parse command line options.
-  llvm::opt::InputArgList Args = Parser.parseLINK(ArgsArr.slice(1));
+  opt::InputArgList Args = Parser.parseLINK(ArgsArr.slice(1));
 
   // Handle /help
   if (Args.hasArg(OPT_help)) {
@@ -273,6 +456,17 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
     return;
   }
 
+  if (auto *Arg = Args.getLastArg(OPT_linkrepro)) {
+    SmallString<64> Path = StringRef(Arg->getValue());
+    sys::path::append(Path, "repro");
+    ErrorOr<CpioFile *> F = CpioFile::create(Path);
+    if (F)
+      Cpio.reset(*F);
+    else
+      errs() << "/linkrepro: failed to open " << Path
+             << ".cpio: " << F.getError().message() << '\n';
+  }
+
   if (Args.filtered_begin(OPT_INPUT) == Args.filtered_end())
     fatal("no input files");
 
@@ -295,8 +489,17 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
     Config->Force = true;
 
   // Handle /debug
-  if (Args.hasArg(OPT_debug))
+  if (Args.hasArg(OPT_debug)) {
     Config->Debug = true;
+    Config->DebugTypes =
+        Args.hasArg(OPT_debugtype)
+            ? parseDebugType(Args.getLastArg(OPT_debugtype)->getValue())
+            : getDefaultDebugType(Args);
+  }
+
+  // Create a dummy PDB file to satisfy build sytem rules.
+  if (auto *Arg = Args.getLastArg(OPT_pdb))
+    Config->PDBPath = Arg->getValue();
 
   // Handle /noentry
   if (Args.hasArg(OPT_noentry)) {
@@ -447,72 +650,43 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
     Config->TerminalServerAware = false;
   if (Args.hasArg(OPT_nosymtab))
     Config->WriteSymtab = false;
+  Config->DumpPdb = Args.hasArg(OPT_dumppdb);
+  Config->DebugPdb = Args.hasArg(OPT_debugpdb);
 
   // Create a list of input files. Files can be given as arguments
   // for /defaultlib option.
-  std::vector<StringRef> Paths;
   std::vector<MemoryBufferRef> MBs;
   for (auto *Arg : Args.filtered(OPT_INPUT))
     if (Optional<StringRef> Path = findFile(Arg->getValue()))
-      Paths.push_back(*Path);
+      enqueuePath(*Path);
   for (auto *Arg : Args.filtered(OPT_defaultlib))
     if (Optional<StringRef> Path = findLib(Arg->getValue()))
-      Paths.push_back(*Path);
-  for (StringRef Path : Paths)
-    MBs.push_back(openFile(Path));
+      enqueuePath(*Path);
 
   // Windows specific -- Create a resource file containing a manifest file.
-  if (Config->Manifest == Configuration::Embed) {
-    std::unique_ptr<MemoryBuffer> MB = createManifestRes();
-    MBs.push_back(MB->getMemBufferRef());
-    OwningMBs.push_back(std::move(MB)); // take ownership
-  }
+  if (Config->Manifest == Configuration::Embed)
+    addBuffer(createManifestRes());
 
-  // Windows specific -- Input files can be Windows resource files (.res files).
-  // We invoke cvtres.exe to convert resource files to a regular COFF file
-  // then link the result file normally.
-  std::vector<MemoryBufferRef> Resources;
-  auto NotResource = [](MemoryBufferRef MB) {
-    return identify_magic(MB.getBuffer()) != file_magic::windows_resource;
-  };
-  auto It = std::stable_partition(MBs.begin(), MBs.end(), NotResource);
-  if (It != MBs.end()) {
-    Resources.insert(Resources.end(), It, MBs.end());
-    MBs.erase(It, MBs.end());
-  }
+  // Read all input files given via the command line.
+  run();
 
-  // Read all input files given via the command line. Note that step()
-  // doesn't read files that are specified by directive sections.
-  for (MemoryBufferRef MB : MBs)
-    Symtab.addFile(createFile(MB));
-  Symtab.step();
-
-  // Determine machine type and check if all object files are
-  // for the same CPU type. Note that this needs to be done before
-  // any call to mangle().
-  for (std::unique_ptr<InputFile> &File : Symtab.getFiles()) {
-    MachineTypes MT = File->getMachineType();
-    if (MT == IMAGE_FILE_MACHINE_UNKNOWN)
-      continue;
-    if (Config->Machine == IMAGE_FILE_MACHINE_UNKNOWN) {
-      Config->Machine = MT;
-      continue;
-    }
-    if (Config->Machine != MT)
-      fatal(File->getShortName() + ": machine type " + machineToStr(MT) +
-            " conflicts with " + machineToStr(Config->Machine));
-  }
+  // We should have inferred a machine type by now from the input files, but if
+  // not we assume x64.
   if (Config->Machine == IMAGE_FILE_MACHINE_UNKNOWN) {
-    llvm::errs() << "warning: /machine is not specified. x64 is assumed.\n";
+    errs() << "warning: /machine is not specified. x64 is assumed.\n";
     Config->Machine = AMD64;
   }
 
-  // Windows specific -- Convert Windows resource files to a COFF file.
-  if (!Resources.empty()) {
-    std::unique_ptr<MemoryBuffer> MB = convertResToCOFF(Resources);
-    Symtab.addFile(createFile(MB->getMemBufferRef()));
-    OwningMBs.push_back(std::move(MB)); // take ownership
-  }
+  // Windows specific -- Input files can be Windows resource files (.res files).
+  // We invoke cvtres.exe to convert resource files to a regular COFF file
+  // then link the result file normally.
+  if (!Resources.empty())
+    addBuffer(convertResToCOFF(Resources));
+
+  if (Cpio)
+    Cpio->append("response.txt",
+                 createResponseFile(Args, FilePaths,
+                                    ArrayRef<StringRef>(SearchPaths).slice(1)));
 
   // Handle /largeaddressaware
   if (Config->is64() || Args.hasArg(OPT_largeaddressaware))
@@ -537,7 +711,7 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
       fatal("entry point must be defined");
     Config->Entry = addUndefined(S);
     if (Config->Verbose)
-      llvm::outs() << "Entry name inferred: " << S << "\n";
+      outs() << "Entry name inferred: " << S << "\n";
   }
 
   // Handle /export
@@ -545,18 +719,19 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
     Export E = parseExport(Arg->getValue());
     if (Config->Machine == I386) {
       if (!isDecorated(E.Name))
-        E.Name = Alloc.save("_" + E.Name);
+        E.Name = Saver.save("_" + E.Name);
       if (!E.ExtName.empty() && !isDecorated(E.ExtName))
-        E.ExtName = Alloc.save("_" + E.ExtName);
+        E.ExtName = Saver.save("_" + E.ExtName);
     }
     Config->Exports.push_back(E);
   }
 
   // Handle /def
   if (auto *Arg = Args.getLastArg(OPT_deffile)) {
-    MemoryBufferRef MB = openFile(Arg->getValue());
     // parseModuleDefs mutates Config object.
-    parseModuleDefs(MB, &Alloc);
+    parseModuleDefs(
+        takeBuffer(check(MemoryBuffer::getFile(Arg->getValue()),
+                         Twine("could not open ") + Arg->getValue())));
   }
 
   // Handle /delayload
@@ -585,14 +760,10 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
   Symtab.addAbsolute(mangle("__guard_fids_count"), 0);
   Symtab.addAbsolute(mangle("__guard_flags"), 0x100);
 
-  // Read as much files as we can from directives sections.
-  Symtab.run();
-
-  // Resolve auxiliary symbols until we get a convergence.
-  // (Trying to resolve a symbol may trigger a Lazy symbol to load a new file.
-  // A new file may contain a directive section to add new command line options.
-  // That's why we have to repeat until converge.)
-  for (;;) {
+  // This code may add new undefined symbols to the link, which may enqueue more
+  // symbol resolution tasks, so we need to continue executing tasks until we
+  // converge.
+  do {
     // Windows specific -- if entry point is not found,
     // search for its mangled names.
     if (Config->Entry)
@@ -615,7 +786,7 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
       Symbol *Sym = Symtab.find(From);
       if (!Sym)
         continue;
-      if (auto *U = dyn_cast<Undefined>(Sym->Body))
+      if (auto *U = dyn_cast<Undefined>(Sym->body()))
         if (!U->WeakAlias)
           U->WeakAlias = Symtab.addUndefined(To);
     }
@@ -623,18 +794,15 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
     // Windows specific -- if __load_config_used can be resolved, resolve it.
     if (Symtab.findUnderscore("_load_config_used"))
       addUndefined(mangle("_load_config_used"));
-
-    if (Symtab.queueEmpty())
-      break;
-    Symtab.run();
-  }
+  } while (run());
 
   // Do LTO by compiling bitcode input files to a set of native COFF files then
   // link those files.
   Symtab.addCombinedLTOObjects();
+  run();
 
   // Make sure we have resolved all symbols.
-  Symtab.reportRemainingUndefines(/*Resolve=*/true);
+  Symtab.reportRemainingUndefines();
 
   // Windows specific -- if no /subsystem is given, we need to infer
   // that from entry point name.
@@ -662,10 +830,6 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
   if (Config->Manifest == Configuration::SideBySide)
     createSideBySideManifest();
 
-  // Create a dummy PDB file to satisfy build sytem rules.
-  if (auto *Arg = Args.getLastArg(OPT_pdb))
-    createPDB(Arg->getValue());
-
   // Identify unreferenced COMDAT sections.
   if (Config->DoGC)
     markLive(Symtab.getChunks());
@@ -679,13 +843,15 @@ void LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
 
   // Create a symbol map file containing symbol VAs and their names
   // to help debugging.
-  if (auto *Arg = Args.getLastArg(OPT_lldmap)) {
+  std::string MapFile = getMapFile(Args);
+  if (!MapFile.empty()) {
     std::error_code EC;
-    llvm::raw_fd_ostream Out(Arg->getValue(), EC, OpenFlags::F_Text);
+    raw_fd_ostream Out(MapFile, EC, OpenFlags::F_Text);
     if (EC)
-      fatal(EC, "could not create the symbol map");
+      fatal(EC, "could not create the symbol map " + MapFile);
     Symtab.printMap(Out);
   }
+
   // Call exit to avoid calling destructors.
   exit(0);
 }
diff --git a/COFF/Driver.h b/COFF/Driver.h
index 23969ee802fb..e8114640edec 100644
--- a/COFF/Driver.h
+++ b/COFF/Driver.h
@@ -13,12 +13,13 @@
 #include "Config.h"
 #include "SymbolTable.h"
 #include "lld/Core/LLVM.h"
+#include "lld/Core/Reproduce.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Object/Archive.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
-#include "llvm/Support/StringSaver.h"
 #include <memory>
 #include <set>
 #include <vector>
@@ -42,7 +43,6 @@ void doICF(const std::vector<Chunk *> &Chunks);
 
 class ArgParser {
 public:
-  ArgParser() : Alloc(AllocAux) {}
   // Parses command line options.
   llvm::opt::InputArgList parse(llvm::ArrayRef<const char *> Args);
 
@@ -56,25 +56,26 @@ private:
   std::vector<const char *> tokenize(StringRef S);
 
   std::vector<const char *> replaceResponseFiles(std::vector<const char *>);
-
-  llvm::BumpPtrAllocator AllocAux;
-  llvm::StringSaver Alloc;
 };
 
 class LinkerDriver {
 public:
-  LinkerDriver() : Alloc(AllocAux) {}
+  LinkerDriver() { coff::Symtab = &Symtab; }
   void link(llvm::ArrayRef<const char *> Args);
 
   // Used by the resolver to parse .drectve section contents.
   void parseDirectives(StringRef S);
 
+  // Used by ArchiveFile to enqueue members.
+  void enqueueArchiveMember(const Archive::Child &C, StringRef SymName,
+                            StringRef ParentName);
+
 private:
-  llvm::BumpPtrAllocator AllocAux;
-  llvm::StringSaver Alloc;
   ArgParser Parser;
   SymbolTable Symtab;
 
+  std::unique_ptr<CpioFile> Cpio; // for /linkrepro
+
   // Opens a file. Path has to be resolved already.
   MemoryBufferRef openFile(StringRef Path);
 
@@ -90,8 +91,9 @@ private:
   // Library search path. The first element is always "" (current directory).
   std::vector<StringRef> SearchPaths;
   std::set<std::string> VisitedFiles;
+  std::set<std::string> VisitedLibs;
 
-  Undefined *addUndefined(StringRef Sym);
+  SymbolBody *addUndefined(StringRef Sym);
   StringRef mangle(StringRef Sym);
 
   // Windows specific -- "main" is not the only main function in Windows.
@@ -104,12 +106,26 @@ private:
   StringRef findDefaultEntry();
   WindowsSubsystem inferSubsystem();
 
+  MemoryBufferRef takeBuffer(std::unique_ptr<MemoryBuffer> MB);
+  void addBuffer(std::unique_ptr<MemoryBuffer> MB);
+  void addArchiveBuffer(MemoryBufferRef MBRef, StringRef SymName,
+                        StringRef ParentName);
+
+  void enqueuePath(StringRef Path);
+
+  void enqueueTask(std::function<void()> Task);
+  bool run();
+
   // Driver is the owner of all opened files.
   // InputFiles have MemoryBufferRefs to them.
   std::vector<std::unique_ptr<MemoryBuffer>> OwningMBs;
+
+  std::list<std::function<void()>> TaskQueue;
+  std::vector<StringRef> FilePaths;
+  std::vector<MemoryBufferRef> Resources;
 };
 
-void parseModuleDefs(MemoryBufferRef MB, llvm::StringSaver *Alloc);
+void parseModuleDefs(MemoryBufferRef MB);
 void writeImportLibrary();
 
 // Functions below this line are defined in DriverUtils.cpp.
@@ -161,8 +177,6 @@ void checkFailIfMismatch(StringRef Arg);
 std::unique_ptr<MemoryBuffer>
 convertResToCOFF(const std::vector<MemoryBufferRef> &MBs);
 
-void createPDB(StringRef Path);
-
 // Create enum with OPT_xxx values for each option in Options.td
 enum {
   OPT_INVALID = 0,
diff --git a/COFF/DriverUtils.cpp b/COFF/DriverUtils.cpp
index 5d7dc2bc65af..14dd004f1c04 100644
--- a/COFF/DriverUtils.cpp
+++ b/COFF/DriverUtils.cpp
@@ -16,6 +16,7 @@
 #include "Config.h"
 #include "Driver.h"
 #include "Error.h"
+#include "Memory.h"
 #include "Symbols.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringSwitch.h"
@@ -43,29 +44,29 @@ namespace {
 class Executor {
 public:
   explicit Executor(StringRef S) : Saver(Alloc), Prog(Saver.save(S)) {}
-  void add(StringRef S)    { Args.push_back(Saver.save(S)); }
-  void add(std::string &S) { Args.push_back(Saver.save(S)); }
-  void add(Twine S)        { Args.push_back(Saver.save(S)); }
-  void add(const char *S)  { Args.push_back(Saver.save(S)); }
+  void add(StringRef S) { Args.push_back(Saver.save(S).data()); }
+  void add(std::string &S) { Args.push_back(Saver.save(S).data()); }
+  void add(Twine S) { Args.push_back(Saver.save(S).data()); }
+  void add(const char *S) { Args.push_back(Saver.save(S).data()); }
 
   void run() {
-    ErrorOr<std::string> ExeOrErr = llvm::sys::findProgramByName(Prog);
+    ErrorOr<std::string> ExeOrErr = sys::findProgramByName(Prog);
     if (auto EC = ExeOrErr.getError())
       fatal(EC, "unable to find " + Prog + " in PATH: ");
-    const char *Exe = Saver.save(*ExeOrErr);
+    const char *Exe = Saver.save(*ExeOrErr).data();
     Args.insert(Args.begin(), Exe);
     Args.push_back(nullptr);
-    if (llvm::sys::ExecuteAndWait(Args[0], Args.data()) != 0) {
+    if (sys::ExecuteAndWait(Args[0], Args.data()) != 0) {
       for (const char *S : Args)
         if (S)
-          llvm::errs() << S << " ";
+          errs() << S << " ";
       fatal("ExecuteAndWait failed");
     }
   }
 
 private:
-  llvm::BumpPtrAllocator Alloc;
-  llvm::StringSaver Saver;
+  BumpPtrAllocator Alloc;
+  StringSaver Saver;
   StringRef Prog;
   std::vector<const char *> Args;
 };
@@ -75,10 +76,8 @@ private:
 // Returns /machine's value.
 MachineTypes getMachineType(StringRef S) {
   MachineTypes MT = StringSwitch<MachineTypes>(S.lower())
-                        .Case("x64", AMD64)
-                        .Case("amd64", AMD64)
-                        .Case("x86", I386)
-                        .Case("i386", I386)
+                        .Cases("x64", "amd64", AMD64)
+                        .Cases("x86", "i386", I386)
                         .Case("arm", ARMNT)
                         .Default(IMAGE_FILE_MACHINE_UNKNOWN);
   if (MT != IMAGE_FILE_MACHINE_UNKNOWN)
@@ -168,8 +167,8 @@ void parseMerge(StringRef S) {
   if (!Inserted) {
     StringRef Existing = Pair.first->second;
     if (Existing != To)
-      llvm::errs() << "warning: " << S << ": already merged into "
-                   << Existing << "\n";
+      errs() << "warning: " << S << ": already merged into " << Existing
+             << "\n";
   }
 }
 
@@ -279,18 +278,54 @@ static void quoteAndPrint(raw_ostream &Out, StringRef S) {
   }
 }
 
+// An RAII temporary file class that automatically removes a temporary file.
+namespace {
+class TemporaryFile {
+public:
+  TemporaryFile(StringRef Prefix, StringRef Extn) {
+    SmallString<128> S;
+    if (auto EC = sys::fs::createTemporaryFile("lld-" + Prefix, Extn, S))
+      fatal(EC, "cannot create a temporary file");
+    Path = S.str();
+  }
+
+  TemporaryFile(TemporaryFile &&Obj) {
+    std::swap(Path, Obj.Path);
+  }
+
+  ~TemporaryFile() {
+    if (Path.empty())
+      return;
+    if (sys::fs::remove(Path))
+      fatal("failed to remove " + Path);
+  }
+
+  // Returns a memory buffer of this temporary file.
+  // Note that this function does not leave the file open,
+  // so it is safe to remove the file immediately after this function
+  // is called (you cannot remove an opened file on Windows.)
+  std::unique_ptr<MemoryBuffer> getMemoryBuffer() {
+    // IsVolatileSize=true forces MemoryBuffer to not use mmap().
+    return check(MemoryBuffer::getFile(Path, /*FileSize=*/-1,
+                                       /*RequiresNullTerminator=*/false,
+                                       /*IsVolatileSize=*/true),
+                 "could not open " + Path);
+  }
+
+  std::string Path;
+};
+}
+
 // Create the default manifest file as a temporary file.
-static std::string createDefaultXml() {
+TemporaryFile createDefaultXml() {
   // Create a temporary file.
-  SmallString<128> Path;
-  if (auto EC = sys::fs::createTemporaryFile("tmp", "manifest", Path))
-    fatal(EC, "cannot create a temporary file");
+  TemporaryFile File("defaultxml", "manifest");
 
   // Open the temporary file for writing.
   std::error_code EC;
-  llvm::raw_fd_ostream OS(Path, EC, sys::fs::F_Text);
+  raw_fd_ostream OS(File.Path, EC, sys::fs::F_Text);
   if (EC)
-    fatal(EC, "failed to open " + Path);
+    fatal(EC, "failed to open " + File.Path);
 
   // Emit the XML. Note that we do *not* verify that the XML attributes are
   // syntactically correct. This is intentional for link.exe compatibility.
@@ -316,56 +351,48 @@ static std::string createDefaultXml() {
   }
   OS << "</assembly>\n";
   OS.close();
-  return StringRef(Path);
+  return File;
 }
 
 static std::string readFile(StringRef Path) {
   std::unique_ptr<MemoryBuffer> MB =
       check(MemoryBuffer::getFile(Path), "could not open " + Path);
-  std::unique_ptr<MemoryBuffer> Buf(std::move(MB));
-  return Buf->getBuffer();
+  return MB->getBuffer();
 }
 
 static std::string createManifestXml() {
   // Create the default manifest file.
-  std::string Path1 = createDefaultXml();
+  TemporaryFile File1 = createDefaultXml();
   if (Config->ManifestInput.empty())
-    return readFile(Path1);
+    return readFile(File1.Path);
 
   // If manifest files are supplied by the user using /MANIFESTINPUT
   // option, we need to merge them with the default manifest.
-  SmallString<128> Path2;
-  if (auto EC = sys::fs::createTemporaryFile("tmp", "manifest", Path2))
-    fatal(EC, "cannot create a temporary file");
-  FileRemover Remover1(Path1);
-  FileRemover Remover2(Path2);
+  TemporaryFile File2("user", "manifest");
 
   Executor E("mt.exe");
   E.add("/manifest");
-  E.add(Path1);
+  E.add(File1.Path);
   for (StringRef Filename : Config->ManifestInput) {
     E.add("/manifest");
     E.add(Filename);
   }
   E.add("/nologo");
-  E.add("/out:" + StringRef(Path2));
+  E.add("/out:" + StringRef(File2.Path));
   E.run();
-  return readFile(Path2);
+  return readFile(File2.Path);
 }
 
 // Create a resource file containing a manifest XML.
 std::unique_ptr<MemoryBuffer> createManifestRes() {
   // Create a temporary file for the resource script file.
-  SmallString<128> RCPath;
-  if (auto EC = sys::fs::createTemporaryFile("tmp", "rc", RCPath))
-    fatal(EC, "cannot create a temporary file");
-  FileRemover RCRemover(RCPath);
+  TemporaryFile RCFile("manifest", "rc");
 
   // Open the temporary file for writing.
   std::error_code EC;
-  llvm::raw_fd_ostream Out(RCPath, EC, sys::fs::F_Text);
+  raw_fd_ostream Out(RCFile.Path, EC, sys::fs::F_Text);
   if (EC)
-    fatal(EC, "failed to open " + RCPath);
+    fatal(EC, "failed to open " + RCFile.Path);
 
   // Write resource script to the RC file.
   Out << "#define LANG_ENGLISH 9\n"
@@ -379,17 +406,15 @@ std::unique_ptr<MemoryBuffer> createManifestRes() {
   Out.close();
 
   // Create output resource file.
-  SmallString<128> ResPath;
-  if (auto EC = sys::fs::createTemporaryFile("tmp", "res", ResPath))
-    fatal(EC, "cannot create a temporary file");
+  TemporaryFile ResFile("output-resource", "res");
 
   Executor E("rc.exe");
   E.add("/fo");
-  E.add(ResPath.str());
+  E.add(ResFile.Path);
   E.add("/nologo");
-  E.add(RCPath.str());
+  E.add(RCFile.Path);
   E.run();
-  return check(MemoryBuffer::getFile(ResPath), "could not open " + ResPath);
+  return ResFile.getMemoryBuffer();
 }
 
 void createSideBySideManifest() {
@@ -397,7 +422,7 @@ void createSideBySideManifest() {
   if (Path == "")
     Path = Config->OutputFile + ".manifest";
   std::error_code EC;
-  llvm::raw_fd_ostream Out(Path, EC, llvm::sys::fs::F_Text);
+  raw_fd_ostream Out(Path, EC, sys::fs::F_Text);
   if (EC)
     fatal(EC, "failed to create manifest");
   Out << createManifestXml();
@@ -485,12 +510,14 @@ void fixupExports() {
   }
 
   for (Export &E : Config->Exports) {
+    SymbolBody *Sym = E.Sym;
     if (!E.ForwardTo.empty()) {
       E.SymbolName = E.Name;
-    } else if (Undefined *U = cast_or_null<Undefined>(E.Sym->WeakAlias)) {
-      E.SymbolName = U->getName();
     } else {
-      E.SymbolName = E.Sym->getName();
+      if (auto *U = dyn_cast<Undefined>(Sym))
+        if (U->WeakAlias)
+          Sym = U->WeakAlias;
+      E.SymbolName = Sym->getName();
     }
   }
 
@@ -515,7 +542,7 @@ void fixupExports() {
     Export *Existing = Pair.first->second;
     if (E == *Existing || E.Name != Existing->Name)
       continue;
-    llvm::errs() << "warning: duplicate /export option: " << E.Name << "\n";
+    errs() << "warning: duplicate /export option: " << E.Name << "\n";
   }
   Config->Exports = std::move(V);
 
@@ -555,20 +582,39 @@ void checkFailIfMismatch(StringRef Arg) {
 std::unique_ptr<MemoryBuffer>
 convertResToCOFF(const std::vector<MemoryBufferRef> &MBs) {
   // Create an output file path.
-  SmallString<128> Path;
-  if (auto EC = llvm::sys::fs::createTemporaryFile("resource", "obj", Path))
-    fatal(EC, "could not create temporary file");
+  TemporaryFile File("resource-file", "obj");
 
   // Execute cvtres.exe.
   Executor E("cvtres.exe");
   E.add("/machine:" + machineToStr(Config->Machine));
   E.add("/readonly");
   E.add("/nologo");
-  E.add("/out:" + Path);
-  for (MemoryBufferRef MB : MBs)
-    E.add(MB.getBufferIdentifier());
+  E.add("/out:" + Twine(File.Path));
+
+  // We must create new files because the memory buffers we have may have no
+  // underlying file still existing on the disk.
+  // It happens if it was created from a TemporaryFile, which usually delete
+  // the file just after creating the MemoryBuffer.
+  std::vector<TemporaryFile> ResFiles;
+  ResFiles.reserve(MBs.size());
+  for (MemoryBufferRef MB : MBs) {
+    // We store the temporary file in a vector to avoid deletion
+    // before running cvtres
+    ResFiles.emplace_back("resource-file", "res");
+    TemporaryFile& ResFile = ResFiles.back();
+    // Write the content of the resource in a temporary file
+    std::error_code EC;
+    raw_fd_ostream OS(ResFile.Path, EC, sys::fs::F_None);
+    if (EC)
+      fatal(EC, "failed to open " + ResFile.Path);
+    OS << MB.getBuffer();
+    OS.close();
+
+    E.add(ResFile.Path);
+  }
+
   E.run();
-  return check(MemoryBuffer::getFile(Path), "could not open " + Path);
+  return File.getMemoryBuffer();
 }
 
 // Create OptTable
@@ -595,7 +641,7 @@ public:
 };
 
 // Parses a given list of options.
-llvm::opt::InputArgList ArgParser::parse(ArrayRef<const char *> ArgsArr) {
+opt::InputArgList ArgParser::parse(ArrayRef<const char *> ArgsArr) {
   // First, replace respnose files (@<file>-style options).
   std::vector<const char *> Argv = replaceResponseFiles(ArgsArr);
 
@@ -603,28 +649,28 @@ llvm::opt::InputArgList ArgParser::parse(ArrayRef<const char *> ArgsArr) {
   COFFOptTable Table;
   unsigned MissingIndex;
   unsigned MissingCount;
-  llvm::opt::InputArgList Args =
-      Table.ParseArgs(Argv, MissingIndex, MissingCount);
+  opt::InputArgList Args = Table.ParseArgs(Argv, MissingIndex, MissingCount);
 
   // Print the real command line if response files are expanded.
   if (Args.hasArg(OPT_verbose) && ArgsArr.size() != Argv.size()) {
-    llvm::outs() << "Command line:";
+    outs() << "Command line:";
     for (const char *S : Argv)
-      llvm::outs() << " " << S;
-    llvm::outs() << "\n";
+      outs() << " " << S;
+    outs() << "\n";
   }
 
   if (MissingCount)
-    fatal("missing arg value for \"" + Twine(Args.getArgString(MissingIndex)) +
-          "\", expected " + Twine(MissingCount) +
-          (MissingCount == 1 ? " argument." : " arguments."));
+    fatal(Twine(Args.getArgString(MissingIndex)) + ": missing argument");
   for (auto *Arg : Args.filtered(OPT_UNKNOWN))
-    llvm::errs() << "ignoring unknown argument: " << Arg->getSpelling() << "\n";
+    errs() << "ignoring unknown argument: " << Arg->getSpelling() << "\n";
   return Args;
 }
 
-llvm::opt::InputArgList ArgParser::parseLINK(ArrayRef<const char *> Args) {
-  // Concatenate LINK env and given arguments and parse them.
+// link.exe has an interesting feature. If LINK environment exists,
+// its contents are handled as a command line string. So you can pass
+// extra arguments using the environment variable.
+opt::InputArgList ArgParser::parseLINK(ArrayRef<const char *> Args) {
+  // Concatenate LINK env and command line arguments, and then parse them.
   Optional<std::string> Env = Process::GetEnv("LINK");
   if (!Env)
     return parse(Args);
@@ -635,8 +681,7 @@ llvm::opt::InputArgList ArgParser::parseLINK(ArrayRef<const char *> Args) {
 
 std::vector<const char *> ArgParser::tokenize(StringRef S) {
   SmallVector<const char *, 16> Tokens;
-  StringSaver Saver(AllocAux);
-  llvm::cl::TokenizeWindowsCommandLine(S, Saver, Tokens);
+  cl::TokenizeWindowsCommandLine(S, Saver, Tokens);
   return std::vector<const char *>(Tokens.begin(), Tokens.end());
 }
 
@@ -645,14 +690,13 @@ std::vector<const char *> ArgParser::tokenize(StringRef S) {
 std::vector<const char *>
 ArgParser::replaceResponseFiles(std::vector<const char *> Argv) {
   SmallVector<const char *, 256> Tokens(Argv.data(), Argv.data() + Argv.size());
-  StringSaver Saver(AllocAux);
   ExpandResponseFiles(Saver, TokenizeWindowsCommandLine, Tokens);
   return std::vector<const char *>(Tokens.begin(), Tokens.end());
 }
 
 void printHelp(const char *Argv0) {
   COFFOptTable Table;
-  Table.PrintHelp(llvm::outs(), Argv0, "LLVM Linker", false);
+  Table.PrintHelp(outs(), Argv0, "LLVM Linker", false);
 }
 
 } // namespace coff
diff --git a/COFF/Error.cpp b/COFF/Error.cpp
index 602a8544ce2b..b2bd557413df 100644
--- a/COFF/Error.cpp
+++ b/COFF/Error.cpp
@@ -11,14 +11,31 @@
 
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/Process.h"
 #include "llvm/Support/raw_ostream.h"
 
+#if !defined(_MSC_VER) && !defined(__MINGW32__)
+#include <unistd.h>
+#endif
+
+using namespace llvm;
+
 namespace lld {
 namespace coff {
 
 void fatal(const Twine &Msg) {
-  llvm::errs() << Msg << "\n";
-  exit(1);
+  if (sys::Process::StandardErrHasColors()) {
+    errs().changeColor(raw_ostream::RED, /*bold=*/true);
+    errs() << "error: ";
+    errs().resetColor();
+  } else {
+    errs() << "error: ";
+  }
+  errs() << Msg << "\n";
+
+  outs().flush();
+  errs().flush();
+  _exit(1);
 }
 
 void fatal(std::error_code EC, const Twine &Msg) {
diff --git a/COFF/Error.h b/COFF/Error.h
index c9f64c662580..47549327db2b 100644
--- a/COFF/Error.h
+++ b/COFF/Error.h
@@ -32,6 +32,23 @@ template <class T> T check(Expected<T> E, const Twine &Prefix) {
   return std::move(*E);
 }
 
+template <class T> T check(ErrorOr<T> EO) {
+  if (!EO)
+    fatal(EO.getError().message());
+  return std::move(*EO);
+}
+
+template <class T> T check(Expected<T> E) {
+  if (!E) {
+    std::string Buf;
+    llvm::raw_string_ostream OS(Buf);
+    logAllUnhandledErrors(E.takeError(), OS, "");
+    OS.flush();
+    fatal(Buf);
+  }
+  return std::move(*E);
+}
+
 } // namespace coff
 } // namespace lld
 
diff --git a/COFF/ICF.cpp b/COFF/ICF.cpp
index a2c5a90334d0..196fbe2610ea 100644
--- a/COFF/ICF.cpp
+++ b/COFF/ICF.cpp
@@ -7,43 +7,19 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// Identical COMDAT Folding is a feature to merge COMDAT sections not by
-// name (which is regular COMDAT handling) but by contents. If two COMDAT
-// sections have the same data, relocations, attributes, etc., then the two
-// are considered identical and merged by the linker. This optimization
-// makes outputs smaller.
+// ICF is short for Identical Code Folding. That is a size optimization to
+// identify and merge two or more read-only sections (typically functions)
+// that happened to have the same contents. It usually reduces output size
+// by a few percent.
 //
-// ICF is theoretically a problem of reducing graphs by merging as many
-// identical subgraphs as possible, if we consider sections as vertices and
-// relocations as edges. This may be a bit more complicated problem than you
-// might think. The order of processing sections matters since merging two
-// sections can make other sections, whose relocations now point to the same
-// section, mergeable. Graphs may contain cycles, which is common in COFF.
-// We need a sophisticated algorithm to do this properly and efficiently.
+// On Windows, ICF is enabled by default.
 //
-// What we do in this file is this. We split sections into groups. Sections
-// in the same group are considered identical.
-//
-// First, all sections are grouped by their "constant" values. Constant
-// values are values that are never changed by ICF, such as section contents,
-// section name, number of relocations, type and offset of each relocation,
-// etc. Because we do not care about some relocation targets in this step,
-// two sections in the same group may not be identical, but at least two
-// sections in different groups can never be identical.
-//
-// Then, we try to split each group by relocation targets. Relocations are
-// considered identical if and only if the relocation targets are in the
-// same group. Splitting a group may make more groups to be splittable,
-// because two relocations that were previously considered identical might
-// now point to different groups. We repeat this step until the convergence
-// is obtained.
-//
-// This algorithm is so-called "optimistic" algorithm described in
-// http://research.google.com/pubs/pub36912.html.
+// See ELF/ICF.cpp for the details about the algortihm.
 //
 //===----------------------------------------------------------------------===//
 
 #include "Chunks.h"
+#include "Error.h"
 #include "Symbols.h"
 #include "lld/Core/Parallel.h"
 #include "llvm/ADT/Hashing.h"
@@ -58,29 +34,34 @@ using namespace llvm;
 namespace lld {
 namespace coff {
 
-typedef std::vector<SectionChunk *>::iterator ChunkIterator;
-typedef bool (*Comparator)(const SectionChunk *, const SectionChunk *);
-
 class ICF {
 public:
   void run(const std::vector<Chunk *> &V);
 
 private:
-  static uint64_t getHash(SectionChunk *C);
-  static bool equalsConstant(const SectionChunk *A, const SectionChunk *B);
-  static bool equalsVariable(const SectionChunk *A, const SectionChunk *B);
-  bool forEachGroup(std::vector<SectionChunk *> &Chunks, Comparator Eq);
-  bool segregate(ChunkIterator Begin, ChunkIterator End, Comparator Eq);
+  void segregate(size_t Begin, size_t End, bool Constant);
 
-  std::atomic<uint64_t> NextID = { 1 };
-};
+  bool equalsConstant(const SectionChunk *A, const SectionChunk *B);
+  bool equalsVariable(const SectionChunk *A, const SectionChunk *B);
 
-// Entry point to ICF.
-void doICF(const std::vector<Chunk *> &Chunks) {
-  ICF().run(Chunks);
-}
+  uint32_t getHash(SectionChunk *C);
+  bool isEligible(SectionChunk *C);
+
+  size_t findBoundary(size_t Begin, size_t End);
+
+  void forEachColorRange(size_t Begin, size_t End,
+                         std::function<void(size_t, size_t)> Fn);
+
+  void forEachColor(std::function<void(size_t, size_t)> Fn);
+
+  std::vector<SectionChunk *> Chunks;
+  int Cnt = 0;
+  std::atomic<uint32_t> NextId = {1};
+  std::atomic<bool> Repeat = {false};
+};
 
-uint64_t ICF::getHash(SectionChunk *C) {
+// Returns a hash value for S.
+uint32_t ICF::getHash(SectionChunk *C) {
   return hash_combine(C->getPermissions(),
                       hash_value(C->SectionName),
                       C->NumRelocs,
@@ -89,16 +70,44 @@ uint64_t ICF::getHash(SectionChunk *C) {
                       C->Checksum);
 }
 
-bool ICF::equalsConstant(const SectionChunk *A, const SectionChunk *B) {
-  if (A->AssocChildren.size() != B->AssocChildren.size() ||
-      A->NumRelocs != B->NumRelocs) {
-    return false;
+// Returns true if section S is subject of ICF.
+bool ICF::isEligible(SectionChunk *C) {
+  bool Global = C->Sym && C->Sym->isExternal();
+  bool Writable = C->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
+  return C->isCOMDAT() && C->isLive() && Global && !Writable;
+}
+
+// Split a range into smaller ranges by recoloring sections
+void ICF::segregate(size_t Begin, size_t End, bool Constant) {
+  while (Begin < End) {
+    // Divide [Begin, End) into two. Let Mid be the start index of the
+    // second group.
+    auto Bound = std::stable_partition(
+        Chunks.begin() + Begin + 1, Chunks.begin() + End, [&](SectionChunk *S) {
+          if (Constant)
+            return equalsConstant(Chunks[Begin], S);
+          return equalsVariable(Chunks[Begin], S);
+        });
+    size_t Mid = Bound - Chunks.begin();
+
+    // Split [Begin, End) into [Begin, Mid) and [Mid, End).
+    uint32_t Id = NextId++;
+    for (size_t I = Begin; I < Mid; ++I)
+      Chunks[I]->Color[(Cnt + 1) % 2] = Id;
+
+    // If we created a group, we need to iterate the main loop again.
+    if (Mid != End)
+      Repeat = true;
+
+    Begin = Mid;
   }
+}
 
-  // Compare associative sections.
-  for (size_t I = 0, E = A->AssocChildren.size(); I != E; ++I)
-    if (A->AssocChildren[I]->GroupID != B->AssocChildren[I]->GroupID)
-      return false;
+// Compare "non-moving" part of two sections, namely everything
+// except relocation targets.
+bool ICF::equalsConstant(const SectionChunk *A, const SectionChunk *B) {
+  if (A->NumRelocs != B->NumRelocs)
+    return false;
 
   // Compare relocations.
   auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
@@ -106,14 +115,14 @@ bool ICF::equalsConstant(const SectionChunk *A, const SectionChunk *B) {
         R1.VirtualAddress != R2.VirtualAddress) {
       return false;
     }
-    SymbolBody *B1 = A->File->getSymbolBody(R1.SymbolTableIndex)->repl();
-    SymbolBody *B2 = B->File->getSymbolBody(R2.SymbolTableIndex)->repl();
+    SymbolBody *B1 = A->File->getSymbolBody(R1.SymbolTableIndex);
+    SymbolBody *B2 = B->File->getSymbolBody(R2.SymbolTableIndex);
     if (B1 == B2)
       return true;
     if (auto *D1 = dyn_cast<DefinedRegular>(B1))
       if (auto *D2 = dyn_cast<DefinedRegular>(B2))
         return D1->getValue() == D2->getValue() &&
-               D1->getChunk()->GroupID == D2->getChunk()->GroupID;
+               D1->getChunk()->Color[Cnt % 2] == D2->getChunk()->Color[Cnt % 2];
     return false;
   };
   if (!std::equal(A->Relocs.begin(), A->Relocs.end(), B->Relocs.begin(), Eq))
@@ -128,54 +137,57 @@ bool ICF::equalsConstant(const SectionChunk *A, const SectionChunk *B) {
          A->getContents() == B->getContents();
 }
 
+// Compare "moving" part of two sections, namely relocation targets.
 bool ICF::equalsVariable(const SectionChunk *A, const SectionChunk *B) {
-  // Compare associative sections.
-  for (size_t I = 0, E = A->AssocChildren.size(); I != E; ++I)
-    if (A->AssocChildren[I]->GroupID != B->AssocChildren[I]->GroupID)
-      return false;
-
   // Compare relocations.
   auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
-    SymbolBody *B1 = A->File->getSymbolBody(R1.SymbolTableIndex)->repl();
-    SymbolBody *B2 = B->File->getSymbolBody(R2.SymbolTableIndex)->repl();
+    SymbolBody *B1 = A->File->getSymbolBody(R1.SymbolTableIndex);
+    SymbolBody *B2 = B->File->getSymbolBody(R2.SymbolTableIndex);
     if (B1 == B2)
       return true;
     if (auto *D1 = dyn_cast<DefinedRegular>(B1))
       if (auto *D2 = dyn_cast<DefinedRegular>(B2))
-        return D1->getChunk()->GroupID == D2->getChunk()->GroupID;
+        return D1->getChunk()->Color[Cnt % 2] == D2->getChunk()->Color[Cnt % 2];
     return false;
   };
   return std::equal(A->Relocs.begin(), A->Relocs.end(), B->Relocs.begin(), Eq);
 }
 
-bool ICF::segregate(ChunkIterator Begin, ChunkIterator End, Comparator Eq) {
-  bool R = false;
-  for (auto It = Begin;;) {
-    SectionChunk *Head = *It;
-    auto Bound = std::partition(It + 1, End, [&](SectionChunk *SC) {
-      return Eq(Head, SC);
-    });
-    if (Bound == End)
-      return R;
-    uint64_t ID = NextID++;
-    std::for_each(It, Bound, [&](SectionChunk *SC) { SC->GroupID = ID; });
-    It = Bound;
-    R = true;
+size_t ICF::findBoundary(size_t Begin, size_t End) {
+  for (size_t I = Begin + 1; I < End; ++I)
+    if (Chunks[Begin]->Color[Cnt % 2] != Chunks[I]->Color[Cnt % 2])
+      return I;
+  return End;
+}
+
+void ICF::forEachColorRange(size_t Begin, size_t End,
+                            std::function<void(size_t, size_t)> Fn) {
+  if (Begin > 0)
+    Begin = findBoundary(Begin - 1, End);
+
+  while (Begin < End) {
+    size_t Mid = findBoundary(Begin, Chunks.size());
+    Fn(Begin, Mid);
+    Begin = Mid;
   }
 }
 
-bool ICF::forEachGroup(std::vector<SectionChunk *> &Chunks, Comparator Eq) {
-  bool R = false;
-  for (auto It = Chunks.begin(), End = Chunks.end(); It != End;) {
-    SectionChunk *Head = *It;
-    auto Bound = std::find_if(It + 1, End, [&](SectionChunk *SC) {
-      return SC->GroupID != Head->GroupID;
-    });
-    if (segregate(It, Bound, Eq))
-      R = true;
-    It = Bound;
+// Call Fn on each color group.
+void ICF::forEachColor(std::function<void(size_t, size_t)> Fn) {
+  // If the number of sections are too small to use threading,
+  // call Fn sequentially.
+  if (Chunks.size() < 1024) {
+    forEachColorRange(0, Chunks.size(), Fn);
+    return;
   }
-  return R;
+
+  // Split sections into 256 shards and call Fn in parallel.
+  size_t NumShards = 256;
+  size_t Step = Chunks.size() / NumShards;
+  parallel_for(size_t(0), NumShards, [&](size_t I) {
+    forEachColorRange(I * Step, (I + 1) * Step, Fn);
+  });
+  forEachColorRange(Step * NumShards, Chunks.size(), Fn);
 }
 
 // Merge identical COMDAT sections.
@@ -183,62 +195,62 @@ bool ICF::forEachGroup(std::vector<SectionChunk *> &Chunks, Comparator Eq) {
 // contents and relocations are all the same.
 void ICF::run(const std::vector<Chunk *> &Vec) {
   // Collect only mergeable sections and group by hash value.
-  parallel_for_each(Vec.begin(), Vec.end(), [&](Chunk *C) {
-    if (auto *SC = dyn_cast<SectionChunk>(C)) {
-      bool Global = SC->Sym && SC->Sym->isExternal();
-      bool Writable = SC->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
-      if (SC->isCOMDAT() && SC->isLive() && Global && !Writable)
-        SC->GroupID = getHash(SC) | (uint64_t(1) << 63);
-    }
-  });
-  std::vector<SectionChunk *> Chunks;
   for (Chunk *C : Vec) {
-    if (auto *SC = dyn_cast<SectionChunk>(C)) {
-      if (SC->GroupID) {
-        Chunks.push_back(SC);
-      } else {
-        SC->GroupID = NextID++;
-      }
+    auto *SC = dyn_cast<SectionChunk>(C);
+    if (!SC)
+      continue;
+
+    if (isEligible(SC)) {
+      // Set MSB to 1 to avoid collisions with non-hash colors.
+      SC->Color[0] = getHash(SC) | (1 << 31);
+      Chunks.push_back(SC);
+    } else {
+      SC->Color[0] = NextId++;
     }
   }
 
+  if (Chunks.empty())
+    return;
+
   // From now on, sections in Chunks are ordered so that sections in
   // the same group are consecutive in the vector.
-  std::sort(Chunks.begin(), Chunks.end(),
-            [](SectionChunk *A, SectionChunk *B) {
-              return A->GroupID < B->GroupID;
-            });
-
-  // Split groups until we get a convergence.
-  int Cnt = 1;
-  forEachGroup(Chunks, equalsConstant);
-
-  for (;;) {
-    if (!forEachGroup(Chunks, equalsVariable))
-      break;
+  std::stable_sort(Chunks.begin(), Chunks.end(),
+                   [](SectionChunk *A, SectionChunk *B) {
+                     return A->Color[0] < B->Color[0];
+                   });
+
+  // Compare static contents and assign unique IDs for each static content.
+  forEachColor([&](size_t Begin, size_t End) { segregate(Begin, End, true); });
+  ++Cnt;
+
+  // Split groups by comparing relocations until convergence is obtained.
+  do {
+    Repeat = false;
+    forEachColor(
+        [&](size_t Begin, size_t End) { segregate(Begin, End, false); });
     ++Cnt;
-  }
+  } while (Repeat);
+
   if (Config->Verbose)
-    llvm::outs() << "\nICF needed " << Cnt << " iterations.\n";
-
-  // Merge sections in the same group.
-  for (auto It = Chunks.begin(), End = Chunks.end(); It != End;) {
-    SectionChunk *Head = *It++;
-    auto Bound = std::find_if(It, End, [&](SectionChunk *SC) {
-      return Head->GroupID != SC->GroupID;
-    });
-    if (It == Bound)
-      continue;
+    outs() << "\nICF needed " << Cnt << " iterations\n";
+
+  // Merge sections in the same colors.
+  forEachColor([&](size_t Begin, size_t End) {
+    if (End - Begin == 1)
+      return;
+
     if (Config->Verbose)
-      llvm::outs() << "Selected " << Head->getDebugName() << "\n";
-    while (It != Bound) {
-      SectionChunk *SC = *It++;
+      outs() << "Selected " << Chunks[Begin]->getDebugName() << "\n";
+    for (size_t I = Begin + 1; I < End; ++I) {
       if (Config->Verbose)
-        llvm::outs() << "  Removed " << SC->getDebugName() << "\n";
-      Head->replace(SC);
+        outs() << "  Removed " << Chunks[I]->getDebugName() << "\n";
+      Chunks[Begin]->replace(Chunks[I]);
     }
-  }
+  });
 }
 
+// Entry point to ICF.
+void doICF(const std::vector<Chunk *> &Chunks) { ICF().run(Chunks); }
+
 } // namespace coff
 } // namespace lld
diff --git a/COFF/InputFiles.cpp b/COFF/InputFiles.cpp
index ff26826371fa..0a97c2185f89 100644
--- a/COFF/InputFiles.cpp
+++ b/COFF/InputFiles.cpp
@@ -7,11 +7,15 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "InputFiles.h"
 #include "Chunks.h"
 #include "Config.h"
+#include "Driver.h"
 #include "Error.h"
-#include "InputFiles.h"
+#include "Memory.h"
+#include "SymbolTable.h"
 #include "Symbols.h"
+#include "llvm-c/lto.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/ADT/Twine.h"
@@ -26,88 +30,58 @@
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Target/TargetOptions.h"
-#include "llvm-c/lto.h"
 #include <cstring>
 #include <system_error>
 #include <utility>
 
+using namespace llvm;
 using namespace llvm::COFF;
 using namespace llvm::object;
 using namespace llvm::support::endian;
 
 using llvm::Triple;
 using llvm::support::ulittle32_t;
+using llvm::sys::fs::file_magic;
+using llvm::sys::fs::identify_magic;
 
 namespace lld {
 namespace coff {
 
-int InputFile::NextIndex = 0;
-llvm::LLVMContext BitcodeFile::Context;
-
-// Returns the last element of a path, which is supposed to be a filename.
-static StringRef getBasename(StringRef Path) {
-  size_t Pos = Path.find_last_of("\\/");
-  if (Pos == StringRef::npos)
-    return Path;
-  return Path.substr(Pos + 1);
-}
+LLVMContext BitcodeFile::Context;
 
-// Returns a string in the format of "foo.obj" or "foo.obj(bar.lib)".
-std::string InputFile::getShortName() {
-  if (ParentName == "")
-    return getName().lower();
-  std::string Res = (getBasename(ParentName) + "(" +
-                     getBasename(getName()) + ")").str();
-  return StringRef(Res).lower();
-}
+ArchiveFile::ArchiveFile(MemoryBufferRef M) : InputFile(ArchiveKind, M) {}
 
 void ArchiveFile::parse() {
   // Parse a MemoryBufferRef as an archive file.
-  File = check(Archive::create(MB), "failed to parse static library");
-
-  // Allocate a buffer for Lazy objects.
-  size_t NumSyms = File->getNumberOfSymbols();
-  LazySymbols.reserve(NumSyms);
+  File = check(Archive::create(MB), toString(this));
 
   // Read the symbol table to construct Lazy objects.
   for (const Archive::Symbol &Sym : File->symbols())
-    LazySymbols.emplace_back(this, Sym);
-
-  // Seen is a map from member files to boolean values. Initially
-  // all members are mapped to false, which indicates all these files
-  // are not read yet.
-  Error Err;
-  for (auto &Child : File->children(Err))
-    Seen[Child.getChildOffset()].clear();
-  if (Err)
-    fatal(Err, "failed to parse static library");
+    Symtab->addLazy(this, Sym);
 }
 
 // Returns a buffer pointing to a member file containing a given symbol.
-// This function is thread-safe.
-MemoryBufferRef ArchiveFile::getMember(const Archive::Symbol *Sym) {
+void ArchiveFile::addMember(const Archive::Symbol *Sym) {
   const Archive::Child &C =
       check(Sym->getMember(),
             "could not get the member for symbol " + Sym->getName());
 
   // Return an empty buffer if we have already returned the same buffer.
-  if (Seen[C.getChildOffset()].test_and_set())
-    return MemoryBufferRef();
-  return check(C.getMemoryBufferRef(),
-               "could not get the buffer for the member defining symbol " +
-                   Sym->getName());
+  if (!Seen.insert(C.getChildOffset()).second)
+    return;
+
+  Driver->enqueueArchiveMember(C, Sym->getName(), getName());
 }
 
 void ObjectFile::parse() {
   // Parse a memory buffer as a COFF file.
-  std::unique_ptr<Binary> Bin =
-      check(createBinary(MB), "failed to parse object file");
+  std::unique_ptr<Binary> Bin = check(createBinary(MB), toString(this));
 
   if (auto *Obj = dyn_cast<COFFObjectFile>(Bin.get())) {
     Bin.release();
     COFFObj.reset(Obj);
   } else {
-    fatal(getName() + " is not a COFF file");
+    fatal(toString(this) + " is not a COFF file");
   }
 
   // Read section and symbol tables.
@@ -137,13 +111,28 @@ void ObjectFile::initializeChunks() {
       Directives = std::string((const char *)Data.data(), Data.size());
       continue;
     }
-    // Skip non-DWARF debug info. MSVC linker converts the sections into
-    // a PDB file, but we don't support that.
-    if (Name == ".debug" || Name.startswith(".debug$"))
-      continue;
-    // We want to preserve DWARF debug sections only when /debug is on.
+
+    // Object files may have DWARF debug info or MS CodeView debug info
+    // (or both).
+    //
+    // DWARF sections don't need any special handling from the perspective
+    // of the linker; they are just a data section containing relocations.
+    // We can just link them to complete debug info.
+    //
+    // CodeView needs a linker support. We need to interpret and debug
+    // info, and then write it to a separate .pdb file.
+
+    // Ignore debug info unless /debug is given.
     if (!Config->Debug && Name.startswith(".debug"))
       continue;
+
+    // CodeView sections are stored to a different vector because they are
+    // not linked in the regular manner.
+    if (Name == ".debug" || Name.startswith(".debug$")) {
+      DebugChunks.push_back(new (Alloc) SectionChunk(this, Sec));
+      continue;
+    }
+
     if (Sec->Characteristics & llvm::COFF::IMAGE_SCN_LNK_REMOVE)
       continue;
     auto *C = new (Alloc) SectionChunk(this, Sec);
@@ -156,12 +145,14 @@ void ObjectFile::initializeSymbols() {
   uint32_t NumSymbols = COFFObj->getNumberOfSymbols();
   SymbolBodies.reserve(NumSymbols);
   SparseSymbolBodies.resize(NumSymbols);
-  llvm::SmallVector<std::pair<Undefined *, uint32_t>, 8> WeakAliases;
+  SmallVector<std::pair<SymbolBody *, uint32_t>, 8> WeakAliases;
   int32_t LastSectionNumber = 0;
   for (uint32_t I = 0; I < NumSymbols; ++I) {
     // Get a COFFSymbolRef object.
-    COFFSymbolRef Sym =
-        check(COFFObj->getSymbol(I), "broken object file: " + getName());
+    ErrorOr<COFFSymbolRef> SymOrErr = COFFObj->getSymbol(I);
+    if (!SymOrErr)
+      fatal(SymOrErr.getError(), "broken object file: " + toString(this));
+    COFFSymbolRef Sym = *SymOrErr;
 
     const void *AuxP = nullptr;
     if (Sym.getNumberOfAuxSymbols())
@@ -175,7 +166,7 @@ void ObjectFile::initializeSymbols() {
       Body = createUndefined(Sym);
       uint32_t TagIndex =
           static_cast<const coff_aux_weak_external *>(AuxP)->TagIndex;
-      WeakAliases.emplace_back((Undefined *)Body, TagIndex);
+      WeakAliases.emplace_back(Body, TagIndex);
     } else {
       Body = createDefined(Sym, AuxP, IsFirst);
     }
@@ -186,23 +177,30 @@ void ObjectFile::initializeSymbols() {
     I += Sym.getNumberOfAuxSymbols();
     LastSectionNumber = Sym.getSectionNumber();
   }
-  for (auto WeakAlias : WeakAliases)
-    WeakAlias.first->WeakAlias = SparseSymbolBodies[WeakAlias.second];
+  for (auto WeakAlias : WeakAliases) {
+    auto *U = dyn_cast<Undefined>(WeakAlias.first);
+    if (!U)
+      continue;
+    // Report an error if two undefined symbols have different weak aliases.
+    if (U->WeakAlias && U->WeakAlias != SparseSymbolBodies[WeakAlias.second])
+      Symtab->reportDuplicate(U->symbol(), this);
+    U->WeakAlias = SparseSymbolBodies[WeakAlias.second];
+  }
 }
 
-Undefined *ObjectFile::createUndefined(COFFSymbolRef Sym) {
+SymbolBody *ObjectFile::createUndefined(COFFSymbolRef Sym) {
   StringRef Name;
   COFFObj->getSymbolName(Sym, Name);
-  return new (Alloc) Undefined(Name);
+  return Symtab->addUndefined(Name, this, Sym.isWeakExternal())->body();
 }
 
-Defined *ObjectFile::createDefined(COFFSymbolRef Sym, const void *AuxP,
-                                   bool IsFirst) {
+SymbolBody *ObjectFile::createDefined(COFFSymbolRef Sym, const void *AuxP,
+                                      bool IsFirst) {
   StringRef Name;
   if (Sym.isCommon()) {
     auto *C = new (Alloc) CommonChunk(Sym);
     Chunks.push_back(C);
-    return new (Alloc) DefinedCommon(this, Sym, C);
+    return Symtab->addCommon(this, Sym, C)->body();
   }
   if (Sym.isAbsolute()) {
     COFFObj->getSymbolName(Sym, Name);
@@ -215,7 +213,10 @@ Defined *ObjectFile::createDefined(COFFSymbolRef Sym, const void *AuxP,
         SEHCompat = true;
       return nullptr;
     }
-    return new (Alloc) DefinedAbsolute(Name, Sym);
+    if (Sym.isExternal())
+      return Symtab->addAbsolute(Name, Sym)->body();
+    else
+      return new (Alloc) DefinedAbsolute(Name, Sym);
   }
   int32_t SectionNumber = Sym.getSectionNumber();
   if (SectionNumber == llvm::COFF::IMAGE_SYM_DEBUG)
@@ -223,12 +224,12 @@ Defined *ObjectFile::createDefined(COFFSymbolRef Sym, const void *AuxP,
 
   // Reserved sections numbers don't have contents.
   if (llvm::COFF::isReservedSectionNumber(SectionNumber))
-    fatal("broken object file: " + getName());
+    fatal("broken object file: " + toString(this));
 
   // This symbol references a section which is not present in the section
   // header.
   if ((uint32_t)SectionNumber >= SparseChunks.size())
-    fatal("broken object file: " + getName());
+    fatal("broken object file: " + toString(this));
 
   // Nothing else to do without a section chunk.
   auto *SC = cast_or_null<SectionChunk>(SparseChunks[SectionNumber]);
@@ -245,7 +246,11 @@ Defined *ObjectFile::createDefined(COFFSymbolRef Sym, const void *AuxP,
     SC->Checksum = Aux->CheckSum;
   }
 
-  auto *B = new (Alloc) DefinedRegular(this, Sym, SC);
+  DefinedRegular *B;
+  if (Sym.isExternal())
+    B = cast<DefinedRegular>(Symtab->addRegular(this, Sym, SC)->body());
+  else
+    B = new (Alloc) DefinedRegular(this, Sym, SC);
   if (SC->isCOMDAT() && Sym.getValue() == 0 && !AuxP)
     SC->setSymbol(B);
 
@@ -307,28 +312,29 @@ void ImportFile::parse() {
     ExtName = ExtName.substr(0, ExtName.find('@'));
     break;
   }
-  ImpSym = new (Alloc) DefinedImportData(DLLName, ImpName, ExtName, Hdr);
-  SymbolBodies.push_back(ImpSym);
+
+  this->Hdr = Hdr;
+  ExternalName = ExtName;
+
+  ImpSym = cast<DefinedImportData>(
+      Symtab->addImportData(ImpName, this)->body());
 
   // If type is function, we need to create a thunk which jump to an
   // address pointed by the __imp_ symbol. (This allows you to call
   // DLL functions just like regular non-DLL functions.)
   if (Hdr->getType() != llvm::COFF::IMPORT_CODE)
     return;
-  ThunkSym = new (Alloc) DefinedImportThunk(Name, ImpSym, Hdr->Machine);
-  SymbolBodies.push_back(ThunkSym);
+  ThunkSym = cast<DefinedImportThunk>(
+      Symtab->addImportThunk(Name, ImpSym, Hdr->Machine)->body());
 }
 
 void BitcodeFile::parse() {
-  // Usually parse() is thread-safe, but bitcode file is an exception.
-  std::lock_guard<std::mutex> Lock(Mu);
-
   Context.enableDebugTypeODRUniquing();
   ErrorOr<std::unique_ptr<LTOModule>> ModOrErr = LTOModule::createFromBuffer(
       Context, MB.getBufferStart(), MB.getBufferSize(), llvm::TargetOptions());
   M = check(std::move(ModOrErr), "could not create LTO module");
 
-  llvm::StringSaver Saver(Alloc);
+  StringSaver Saver(Alloc);
   for (unsigned I = 0, E = M->getSymbolCount(); I != E; ++I) {
     lto_symbol_attributes Attrs = M->getSymbolAttributes(I);
     if ((Attrs & LTO_SYMBOL_SCOPE_MASK) == LTO_SYMBOL_SCOPE_INTERNAL)
@@ -337,15 +343,15 @@ void BitcodeFile::parse() {
     StringRef SymName = Saver.save(M->getSymbolName(I));
     int SymbolDef = Attrs & LTO_SYMBOL_DEFINITION_MASK;
     if (SymbolDef == LTO_SYMBOL_DEFINITION_UNDEFINED) {
-      SymbolBodies.push_back(new (Alloc) Undefined(SymName));
+      SymbolBodies.push_back(Symtab->addUndefined(SymName, this, false)->body());
     } else {
       bool Replaceable =
           (SymbolDef == LTO_SYMBOL_DEFINITION_TENTATIVE || // common
            (Attrs & LTO_SYMBOL_COMDAT) ||                  // comdat
            (SymbolDef == LTO_SYMBOL_DEFINITION_WEAK &&     // weak external
             (Attrs & LTO_SYMBOL_ALIAS)));
-      SymbolBodies.push_back(new (Alloc) DefinedBitcode(this, SymName,
-                                                        Replaceable));
+      SymbolBodies.push_back(
+          Symtab->addBitcode(this, SymName, Replaceable)->body());
     }
   }
 
@@ -367,7 +373,26 @@ MachineTypes BitcodeFile::getMachineType() {
   }
 }
 
-std::mutex BitcodeFile::Mu;
+// Returns the last element of a path, which is supposed to be a filename.
+static StringRef getBasename(StringRef Path) {
+  size_t Pos = Path.find_last_of("\\/");
+  if (Pos == StringRef::npos)
+    return Path;
+  return Path.substr(Pos + 1);
+}
+
+// Returns a string in the format of "foo.obj" or "foo.obj(bar.lib)".
+std::string toString(InputFile *File) {
+  if (!File)
+    return "(internal)";
+  if (File->ParentName.empty())
+    return File->getName().lower();
+
+  std::string Res =
+      (getBasename(File->ParentName) + "(" + getBasename(File->getName()) + ")")
+          .str();
+  return StringRef(Res).lower();
+}
 
 } // namespace coff
 } // namespace lld
diff --git a/COFF/InputFiles.h b/COFF/InputFiles.h
index 0ec01b5075f9..498a1743e985 100644
--- a/COFF/InputFiles.h
+++ b/COFF/InputFiles.h
@@ -12,13 +12,13 @@
 
 #include "lld/Core/LLVM.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/LTO/legacy/LTOModule.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Support/StringSaver.h"
 #include <memory>
-#include <mutex>
 #include <set>
 #include <vector>
 
@@ -31,6 +31,7 @@ using llvm::COFF::MachineTypes;
 using llvm::object::Archive;
 using llvm::object::COFFObjectFile;
 using llvm::object::COFFSymbolRef;
+using llvm::object::coff_import_header;
 using llvm::object::coff_section;
 
 class Chunk;
@@ -38,6 +39,8 @@ class Defined;
 class DefinedImportData;
 class DefinedImportThunk;
 class Lazy;
+class SectionChunk;
+struct Symbol;
 class SymbolBody;
 class Undefined;
 
@@ -51,67 +54,44 @@ public:
   // Returns the filename.
   StringRef getName() { return MB.getBufferIdentifier(); }
 
-  // Returns symbols defined by this file.
-  virtual std::vector<SymbolBody *> &getSymbols() = 0;
-
   // Reads a file (the constructor doesn't do that).
   virtual void parse() = 0;
 
   // Returns the CPU type this file was compiled to.
   virtual MachineTypes getMachineType() { return IMAGE_FILE_MACHINE_UNKNOWN; }
 
-  // Returns a short, human-friendly filename. If this is a member of
-  // an archive file, a returned value includes parent's filename.
-  // Used for logging or debugging.
-  std::string getShortName();
-
-  // Sets a parent filename if this file is created from an archive.
-  void setParentName(StringRef N) { ParentName = N; }
+  // An archive file name if this file is created from an archive.
+  StringRef ParentName;
 
   // Returns .drectve section contents if exist.
   StringRef getDirectives() { return StringRef(Directives).trim(); }
 
-  // Each file has a unique index. The index number is used to
-  // resolve ties in symbol resolution.
-  int Index;
-  static int NextIndex;
-
 protected:
-  InputFile(Kind K, MemoryBufferRef M)
-      : Index(NextIndex++), MB(M), FileKind(K) {}
+  InputFile(Kind K, MemoryBufferRef M) : MB(M), FileKind(K) {}
 
   MemoryBufferRef MB;
   std::string Directives;
 
 private:
   const Kind FileKind;
-  StringRef ParentName;
 };
 
 // .lib or .a file.
 class ArchiveFile : public InputFile {
 public:
-  explicit ArchiveFile(MemoryBufferRef M) : InputFile(ArchiveKind, M) {}
+  explicit ArchiveFile(MemoryBufferRef M);
   static bool classof(const InputFile *F) { return F->kind() == ArchiveKind; }
   void parse() override;
 
-  // Returns a memory buffer for a given symbol. An empty memory buffer
-  // is returned if we have already returned the same memory buffer.
-  // (So that we don't instantiate same members more than once.)
-  MemoryBufferRef getMember(const Archive::Symbol *Sym);
-
-  llvm::MutableArrayRef<Lazy> getLazySymbols() { return LazySymbols; }
-
-  // All symbols returned by ArchiveFiles are of Lazy type.
-  std::vector<SymbolBody *> &getSymbols() override {
-    llvm_unreachable("internal fatal");
-  }
+  // Enqueues an archive member load for the given symbol. If we've already
+  // enqueued a load for the same archive member, this function does nothing,
+  // which ensures that we don't load the same member more than once.
+  void addMember(const Archive::Symbol *Sym);
 
 private:
   std::unique_ptr<Archive> File;
   std::string Filename;
-  std::vector<Lazy> LazySymbols;
-  std::map<uint64_t, std::atomic_flag> Seen;
+  llvm::DenseSet<uint64_t> Seen;
 };
 
 // .obj or .o file. This may be a member of an archive file.
@@ -122,7 +102,8 @@ public:
   void parse() override;
   MachineTypes getMachineType() override;
   std::vector<Chunk *> &getChunks() { return Chunks; }
-  std::vector<SymbolBody *> &getSymbols() override { return SymbolBodies; }
+  std::vector<SectionChunk *> &getDebugChunks() { return DebugChunks; }
+  std::vector<SymbolBody *> &getSymbols() { return SymbolBodies; }
 
   // Returns a SymbolBody object for the SymbolIndex'th symbol in the
   // underlying object file.
@@ -146,8 +127,8 @@ private:
   void initializeSymbols();
   void initializeSEH();
 
-  Defined *createDefined(COFFSymbolRef Sym, const void *Aux, bool IsFirst);
-  Undefined *createUndefined(COFFSymbolRef Sym);
+  SymbolBody *createDefined(COFFSymbolRef Sym, const void *Aux, bool IsFirst);
+  SymbolBody *createUndefined(COFFSymbolRef Sym);
 
   std::unique_ptr<COFFObjectFile> COFFObj;
   llvm::BumpPtrAllocator Alloc;
@@ -157,6 +138,9 @@ private:
   // chunks and non-section chunks for common symbols.
   std::vector<Chunk *> Chunks;
 
+  // CodeView debug info sections.
+  std::vector<SectionChunk *> DebugChunks;
+
   // This vector contains the same chunks as Chunks, but they are
   // indexed such that you can get a SectionChunk by section index.
   // Nonexistent section indices are filled with null pointers.
@@ -182,7 +166,6 @@ public:
   explicit ImportFile(MemoryBufferRef M)
       : InputFile(ImportKind, M), StringAlloc(StringAllocAux) {}
   static bool classof(const InputFile *F) { return F->kind() == ImportKind; }
-  std::vector<SymbolBody *> &getSymbols() override { return SymbolBodies; }
 
   DefinedImportData *ImpSym = nullptr;
   DefinedImportThunk *ThunkSym = nullptr;
@@ -191,10 +174,14 @@ public:
 private:
   void parse() override;
 
-  std::vector<SymbolBody *> SymbolBodies;
   llvm::BumpPtrAllocator Alloc;
   llvm::BumpPtrAllocator StringAllocAux;
   llvm::StringSaver StringAlloc;
+
+public:
+  StringRef ExternalName;
+  const coff_import_header *Hdr;
+  Chunk *Location = nullptr;
 };
 
 // Used for LTO.
@@ -202,7 +189,7 @@ class BitcodeFile : public InputFile {
 public:
   explicit BitcodeFile(MemoryBufferRef M) : InputFile(BitcodeKind, M) {}
   static bool classof(const InputFile *F) { return F->kind() == BitcodeKind; }
-  std::vector<SymbolBody *> &getSymbols() override { return SymbolBodies; }
+  std::vector<SymbolBody *> &getSymbols() { return SymbolBodies; }
   MachineTypes getMachineType() override;
   std::unique_ptr<LTOModule> takeModule() { return std::move(M); }
 
@@ -214,9 +201,10 @@ private:
   std::vector<SymbolBody *> SymbolBodies;
   llvm::BumpPtrAllocator Alloc;
   std::unique_ptr<LTOModule> M;
-  static std::mutex Mu;
 };
 
+std::string toString(InputFile *File);
+
 } // namespace coff
 } // namespace lld
 
diff --git a/COFF/Librarian.cpp b/COFF/Librarian.cpp
index 25fb4a87b3eb..4c597fad7345 100644
--- a/COFF/Librarian.cpp
+++ b/COFF/Librarian.cpp
@@ -54,7 +54,7 @@ static uint16_t getImgRelRelocation() {
   }
 }
 
-template <class T> void append(std::vector<uint8_t> &B, const T &Data) {
+template <class T> static void append(std::vector<uint8_t> &B, const T &Data) {
   size_t S = B.size();
   B.resize(S + sizeof(T));
   memcpy(&B[S], &Data, sizeof(T));
@@ -352,15 +352,16 @@ ObjectFactory::createNullImportDescriptor(std::vector<uint8_t> &Buffer) {
 NewArchiveMember ObjectFactory::createNullThunk(std::vector<uint8_t> &Buffer) {
   static const uint32_t NumberOfSections = 2;
   static const uint32_t NumberOfSymbols = 1;
+  uint32_t VASize = is32bit() ? 4 : 8;
 
   // COFF Header
   coff_file_header Header{
       u16(Config->Machine), u16(NumberOfSections), u32(0),
       u32(sizeof(Header) + (NumberOfSections * sizeof(coff_section)) +
           // .idata$5
-          sizeof(export_address_table_entry) +
+          VASize +
           // .idata$4
-          sizeof(export_address_table_entry)),
+          VASize),
       u32(NumberOfSymbols), u16(0),
       u16(is32bit() ? IMAGE_FILE_32BIT_MACHINE : 0),
   };
@@ -371,36 +372,40 @@ NewArchiveMember ObjectFactory::createNullThunk(std::vector<uint8_t> &Buffer) {
       {{'.', 'i', 'd', 'a', 't', 'a', '$', '5'},
        u32(0),
        u32(0),
-       u32(sizeof(export_address_table_entry)),
+       u32(VASize),
        u32(sizeof(coff_file_header) + NumberOfSections * sizeof(coff_section)),
        u32(0),
        u32(0),
        u16(0),
        u16(0),
-       u32(IMAGE_SCN_ALIGN_4BYTES | IMAGE_SCN_CNT_INITIALIZED_DATA |
-           IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE)},
+       u32((is32bit() ? IMAGE_SCN_ALIGN_4BYTES : IMAGE_SCN_ALIGN_8BYTES) |
+           IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ |
+           IMAGE_SCN_MEM_WRITE)},
       {{'.', 'i', 'd', 'a', 't', 'a', '$', '4'},
        u32(0),
        u32(0),
-       u32(sizeof(export_address_table_entry)),
+       u32(VASize),
        u32(sizeof(coff_file_header) + NumberOfSections * sizeof(coff_section) +
-           sizeof(export_address_table_entry)),
+           VASize),
        u32(0),
        u32(0),
        u16(0),
        u16(0),
-       u32(IMAGE_SCN_ALIGN_4BYTES | IMAGE_SCN_CNT_INITIALIZED_DATA |
-           IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE)},
+       u32((is32bit() ? IMAGE_SCN_ALIGN_4BYTES : IMAGE_SCN_ALIGN_8BYTES) |
+           IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ |
+           IMAGE_SCN_MEM_WRITE)},
   };
   append(Buffer, SectionTable);
 
-  // .idata$5
-  static const export_address_table_entry ILT{u32(0)};
-  append(Buffer, ILT);
+  // .idata$5, ILT
+  append(Buffer, u32(0));
+  if (!is32bit())
+    append(Buffer, u32(0));
 
-  // .idata$4
-  static const export_address_table_entry IAT{u32(0)};
-  append(Buffer, IAT);
+  // .idata$4, IAT
+  append(Buffer, u32(0));
+  if (!is32bit())
+    append(Buffer, u32(0));
 
   // Symbol Table
   coff_symbol16 SymbolTable[NumberOfSymbols] = {
@@ -458,7 +463,7 @@ void lld::coff::writeImportLibrary() {
   std::vector<NewArchiveMember> Members;
 
   std::string Path = getImplibPath();
-  std::string DLLName = llvm::sys::path::filename(Config->OutputFile);
+  std::string DLLName = sys::path::filename(Config->OutputFile);
   ObjectFactory OF(DLLName);
 
   std::vector<uint8_t> ImportDescriptor;
diff --git a/COFF/MarkLive.cpp b/COFF/MarkLive.cpp
index 0870986ad81a..0156d238b672 100644
--- a/COFF/MarkLive.cpp
+++ b/COFF/MarkLive.cpp
@@ -38,8 +38,8 @@ void markLive(const std::vector<Chunk *> &Chunks) {
   };
 
   // Add GC root chunks.
-  for (Undefined *U : Config->GCRoot)
-    if (auto *D = dyn_cast<DefinedRegular>(U->repl()))
+  for (SymbolBody *B : Config->GCRoot)
+    if (auto *D = dyn_cast<DefinedRegular>(B))
       Enqueue(D->getChunk());
 
   while (!Worklist.empty()) {
@@ -48,7 +48,7 @@ void markLive(const std::vector<Chunk *> &Chunks) {
 
     // Mark all symbols listed in the relocation table for this section.
     for (SymbolBody *S : SC->symbols())
-      if (auto *D = dyn_cast<DefinedRegular>(S->repl()))
+      if (auto *D = dyn_cast<DefinedRegular>(S))
         Enqueue(D->getChunk());
 
     // Mark associative sections if any.
diff --git a/COFF/Memory.h b/COFF/Memory.h
new file mode 100644
index 000000000000..526f11344a09
--- /dev/null
+++ b/COFF/Memory.h
@@ -0,0 +1,52 @@
+//===- Memory.h -------------------------------------------------*- C++ -*-===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// See ELF/Memory.h
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_COFF_MEMORY_H
+#define LLD_COFF_MEMORY_H
+
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/StringSaver.h"
+#include <vector>
+
+namespace lld {
+namespace coff {
+
+extern llvm::BumpPtrAllocator BAlloc;
+extern llvm::StringSaver Saver;
+
+struct SpecificAllocBase {
+  SpecificAllocBase() { Instances.push_back(this); }
+  virtual ~SpecificAllocBase() = default;
+  virtual void reset() = 0;
+  static std::vector<SpecificAllocBase *> Instances;
+};
+
+template <class T> struct SpecificAlloc : public SpecificAllocBase {
+  void reset() override { Alloc.DestroyAll(); }
+  llvm::SpecificBumpPtrAllocator<T> Alloc;
+};
+
+template <typename T, typename... U> T *make(U &&... Args) {
+  static SpecificAlloc<T> Alloc;
+  return new (Alloc.Alloc.Allocate()) T(std::forward<U>(Args)...);
+}
+
+inline void freeArena() {
+  for (SpecificAllocBase *Alloc : SpecificAllocBase::Instances)
+    Alloc->reset();
+  BAlloc.Reset();
+}
+}
+}
+
+#endif
diff --git a/COFF/ModuleDef.cpp b/COFF/ModuleDef.cpp
index 5e393f45d184..a273b6f535db 100644
--- a/COFF/ModuleDef.cpp
+++ b/COFF/ModuleDef.cpp
@@ -18,6 +18,7 @@
 
 #include "Config.h"
 #include "Error.h"
+#include "Memory.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/StringSaver.h"
@@ -113,7 +114,7 @@ private:
 
 class Parser {
 public:
-  explicit Parser(StringRef S, StringSaver *A) : Lex(S), Alloc(A) {}
+  explicit Parser(StringRef S) : Lex(S) {}
 
   void parse() {
     do {
@@ -197,9 +198,9 @@ private:
 
     if (Config->Machine == I386) {
       if (!isDecorated(E.Name))
-        E.Name = Alloc->save("_" + E.Name);
+        E.Name = Saver.save("_" + E.Name);
       if (!E.ExtName.empty() && !isDecorated(E.ExtName))
-        E.ExtName = Alloc->save("_" + E.ExtName);
+        E.ExtName = Saver.save("_" + E.ExtName);
     }
 
     for (;;) {
@@ -278,14 +279,11 @@ private:
   Lexer Lex;
   Token Tok;
   std::vector<Token> Stack;
-  StringSaver *Alloc;
 };
 
 } // anonymous namespace
 
-void parseModuleDefs(MemoryBufferRef MB, StringSaver *Alloc) {
-  Parser(MB.getBuffer(), Alloc).parse();
-}
+void parseModuleDefs(MemoryBufferRef MB) { Parser(MB.getBuffer()).parse(); }
 
 } // namespace coff
 } // namespace lld
diff --git a/COFF/Options.td b/COFF/Options.td
index e5c9c5b4635b..9dfbcc8e188c 100644
--- a/COFF/Options.td
+++ b/COFF/Options.td
@@ -27,6 +27,7 @@ def failifmismatch : P<"failifmismatch", "">;
 def heap    : P<"heap", "Size of the heap">;
 def implib  : P<"implib", "Import library name">;
 def libpath : P<"libpath", "Additional library search path">;
+def linkrepro : P<"linkrepro", "Dump linker invocation and input files for debugging">;
 def machine : P<"machine", "Specify target platform">;
 def merge   : P<"merge", "Combine sections">;
 def mllvm   : P<"mllvm", "Options to pass to LLVM">;
@@ -61,7 +62,9 @@ def deffile : Joined<["/", "-"], "def:">,
     HelpText<"Use module-definition file">;
 
 def debug : F<"debug">, HelpText<"Embed a symbol table in the image">;
+def debugtype : P<"debugtype", "Debug Info Options">;
 def dll : F<"dll">, HelpText<"Create a DLL">;
+def driver : P<"driver", "Generate a Windows NT Kernel Mode Driver">;
 def nodefaultlib_all : F<"nodefaultlib">;
 def noentry : F<"noentry">;
 def profile : F<"profile">;
@@ -91,7 +94,10 @@ def help_q : Flag<["/?", "-?"], "">, Alias<help>;
 def nosymtab : F<"nosymtab">;
 
 // Flags for debugging
-def lldmap : Joined<["/", "-"], "lldmap:">;
+def debugpdb : F<"debugpdb">;
+def dumppdb : Joined<["/", "-"], "dumppdb">;
+def lldmap : F<"lldmap">;
+def lldmap_file : Joined<["/", "-"], "lldmap:">;
 
 //==============================================================================
 // The flags below do nothing. They are defined only for link.exe compatibility.
diff --git a/COFF/PDB.cpp b/COFF/PDB.cpp
index 7606ccc680d3..56d5a3651143 100644
--- a/COFF/PDB.cpp
+++ b/COFF/PDB.cpp
@@ -7,55 +7,187 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "Driver.h"
+#include "PDB.h"
+#include "Chunks.h"
+#include "Config.h"
 #include "Error.h"
+#include "SymbolTable.h"
 #include "Symbols.h"
+#include "llvm/DebugInfo/CodeView/SymbolDumper.h"
+#include "llvm/DebugInfo/CodeView/TypeDumper.h"
+#include "llvm/DebugInfo/MSF/ByteStream.h"
+#include "llvm/DebugInfo/MSF/MSFBuilder.h"
+#include "llvm/DebugInfo/MSF/MSFCommon.h"
+#include "llvm/DebugInfo/PDB/Raw/DbiStream.h"
+#include "llvm/DebugInfo/PDB/Raw/DbiStreamBuilder.h"
+#include "llvm/DebugInfo/PDB/Raw/InfoStream.h"
+#include "llvm/DebugInfo/PDB/Raw/InfoStreamBuilder.h"
+#include "llvm/DebugInfo/PDB/Raw/PDBFile.h"
+#include "llvm/DebugInfo/PDB/Raw/PDBFileBuilder.h"
+#include "llvm/DebugInfo/PDB/Raw/TpiStream.h"
+#include "llvm/DebugInfo/PDB/Raw/TpiStreamBuilder.h"
+#include "llvm/Object/COFF.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/FileOutputBuffer.h"
+#include "llvm/Support/ScopedPrinter.h"
 #include <memory>
 
+using namespace lld;
+using namespace lld::coff;
 using namespace llvm;
+using namespace llvm::codeview;
 using namespace llvm::support;
 using namespace llvm::support::endian;
 
-const int PageSize = 4096;
-const uint8_t Magic[32] = "Microsoft C/C++ MSF 7.00\r\n\032DS\0\0";
-
-namespace {
-struct PDBHeader {
-  uint8_t Magic[32];
-  ulittle32_t PageSize;
-  ulittle32_t FpmPage;
-  ulittle32_t PageCount;
-  ulittle32_t RootSize;
-  ulittle32_t Reserved;
-  ulittle32_t RootPointer;
-};
-}
-
-void lld::coff::createPDB(StringRef Path) {
-  // Create a file.
-  size_t FileSize = PageSize * 3;
-  ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
-      FileOutputBuffer::create(Path, FileSize);
-  if (auto EC = BufferOrErr.getError())
-    fatal(EC, "failed to open " + Path);
-  std::unique_ptr<FileOutputBuffer> Buffer = std::move(*BufferOrErr);
-
-  // Write the file header.
-  uint8_t *Buf = Buffer->getBufferStart();
-  auto *Hdr = reinterpret_cast<PDBHeader *>(Buf);
-  memcpy(Hdr->Magic, Magic, sizeof(Magic));
-  Hdr->PageSize = PageSize;
-  // I don't know what FpmPage field means, but it must not be 0.
-  Hdr->FpmPage = 1;
-  Hdr->PageCount = FileSize / PageSize;
-  // Root directory is empty, containing only the length field.
-  Hdr->RootSize = 4;
-  // Root directory is on page 1.
-  Hdr->RootPointer = 1;
-
-  // Write the root directory. Root stream is on page 2.
-  write32le(Buf + PageSize, 2);
-  Buffer->commit();
+using llvm::object::coff_section;
+
+static ExitOnError ExitOnErr;
+
+// Returns a list of all SectionChunks.
+static std::vector<coff_section> getInputSections(SymbolTable *Symtab) {
+  std::vector<coff_section> V;
+  for (Chunk *C : Symtab->getChunks())
+    if (auto *SC = dyn_cast<SectionChunk>(C))
+      V.push_back(*SC->Header);
+  return V;
+}
+
+static SectionChunk *findByName(std::vector<SectionChunk *> &Sections,
+                                StringRef Name) {
+  for (SectionChunk *C : Sections)
+    if (C->getSectionName() == Name)
+      return C;
+  return nullptr;
+}
+
+static ArrayRef<uint8_t> getDebugT(ObjectFile *File) {
+  SectionChunk *Sec = findByName(File->getDebugChunks(), ".debug$T");
+  if (!Sec)
+    return {};
+
+  // First 4 bytes are section magic.
+  ArrayRef<uint8_t> Data = Sec->getContents();
+  if (Data.size() < 4)
+    fatal(".debug$T too short");
+  if (read32le(Data.data()) != COFF::DEBUG_SECTION_MAGIC)
+    fatal(".debug$T has an invalid magic");
+  return Data.slice(4);
+}
+
+static void dumpDebugT(ScopedPrinter &W, ObjectFile *File) {
+  ArrayRef<uint8_t> Data = getDebugT(File);
+  if (Data.empty())
+    return;
+
+  msf::ByteStream Stream(Data);
+  CVTypeDumper TypeDumper(&W, false);
+  if (auto EC = TypeDumper.dump(Data))
+    fatal(EC, "CVTypeDumper::dump failed");
+}
+
+static void dumpDebugS(ScopedPrinter &W, ObjectFile *File) {
+  SectionChunk *Sec = findByName(File->getDebugChunks(), ".debug$S");
+  if (!Sec)
+    return;
+
+  msf::ByteStream Stream(Sec->getContents());
+  CVSymbolArray Symbols;
+  msf::StreamReader Reader(Stream);
+  if (auto EC = Reader.readArray(Symbols, Reader.getLength()))
+    fatal(EC, "StreamReader.readArray<CVSymbolArray> failed");
+
+  CVTypeDumper TypeDumper(&W, false);
+  CVSymbolDumper SymbolDumper(W, TypeDumper, nullptr, false);
+  if (auto EC = SymbolDumper.dump(Symbols))
+    fatal(EC, "CVSymbolDumper::dump failed");
+}
+
+// Dump CodeView debug info. This is for debugging.
+static void dumpCodeView(SymbolTable *Symtab) {
+  ScopedPrinter W(outs());
+
+  for (ObjectFile *File : Symtab->ObjectFiles) {
+    dumpDebugT(W, File);
+    dumpDebugS(W, File);
+  }
+}
+
+static void addTypeInfo(SymbolTable *Symtab,
+                        pdb::TpiStreamBuilder &TpiBuilder) {
+  for (ObjectFile *File : Symtab->ObjectFiles) {
+    ArrayRef<uint8_t> Data = getDebugT(File);
+    if (Data.empty())
+      continue;
+
+    msf::ByteStream Stream(Data);
+    codeview::CVTypeArray Records;
+    msf::StreamReader Reader(Stream);
+    if (auto EC = Reader.readArray(Records, Reader.getLength()))
+      fatal(EC, "Reader.readArray failed");
+    for (const codeview::CVType &Rec : Records)
+      TpiBuilder.addTypeRecord(Rec);
+  }
+}
+
+// Creates a PDB file.
+void coff::createPDB(StringRef Path, SymbolTable *Symtab,
+                     ArrayRef<uint8_t> SectionTable) {
+  if (Config->DumpPdb)
+    dumpCodeView(Symtab);
+
+  BumpPtrAllocator Alloc;
+  pdb::PDBFileBuilder Builder(Alloc);
+  ExitOnErr(Builder.initialize(4096)); // 4096 is blocksize
+
+  // Create streams in MSF for predefined streams, namely
+  // PDB, TPI, DBI and IPI.
+  for (int I = 0; I < (int)pdb::kSpecialStreamCount; ++I)
+    ExitOnErr(Builder.getMsfBuilder().addStream(0));
+
+  // Add an Info stream.
+  auto &InfoBuilder = Builder.getInfoBuilder();
+  InfoBuilder.setAge(1);
+
+  // Should be a random number, 0 for now.
+  InfoBuilder.setGuid({});
+
+  // Should be the current time, but set 0 for reproducibilty.
+  InfoBuilder.setSignature(0);
+  InfoBuilder.setVersion(pdb::PdbRaw_ImplVer::PdbImplVC70);
+
+  // Add an empty DPI stream.
+  auto &DbiBuilder = Builder.getDbiBuilder();
+  DbiBuilder.setVersionHeader(pdb::PdbDbiV110);
+
+  // Add an empty TPI stream.
+  auto &TpiBuilder = Builder.getTpiBuilder();
+  TpiBuilder.setVersionHeader(pdb::PdbTpiV80);
+  if (Config->DebugPdb)
+    addTypeInfo(Symtab, TpiBuilder);
+
+  // Add an empty IPI stream.
+  auto &IpiBuilder = Builder.getIpiBuilder();
+  IpiBuilder.setVersionHeader(pdb::PdbTpiV80);
+
+  // Add Section Contributions.
+  std::vector<pdb::SectionContrib> Contribs =
+      pdb::DbiStreamBuilder::createSectionContribs(getInputSections(Symtab));
+  DbiBuilder.setSectionContribs(Contribs);
+
+  // Add Section Map stream.
+  ArrayRef<object::coff_section> Sections = {
+      (const object::coff_section *)SectionTable.data(),
+      SectionTable.size() / sizeof(object::coff_section)};
+  std::vector<pdb::SecMapEntry> SectionMap =
+      pdb::DbiStreamBuilder::createSectionMap(Sections);
+  DbiBuilder.setSectionMap(SectionMap);
+
+  ExitOnErr(DbiBuilder.addModuleInfo("", "* Linker *"));
+
+  // Add COFF section header stream.
+  ExitOnErr(
+      DbiBuilder.addDbgStream(pdb::DbgHeaderType::SectionHdr, SectionTable));
+
+  // Write to a file.
+  ExitOnErr(Builder.commit(Path));
 }
diff --git a/COFF/PDB.h b/COFF/PDB.h
new file mode 100644
index 000000000000..091e90fa1ef1
--- /dev/null
+++ b/COFF/PDB.h
@@ -0,0 +1,25 @@
+//===- PDB.h ----------------------------------------------------*- C++ -*-===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_COFF_PDB_H
+#define LLD_COFF_PDB_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace lld {
+namespace coff {
+class SymbolTable;
+
+void createPDB(llvm::StringRef Path, SymbolTable *Symtab,
+               llvm::ArrayRef<uint8_t> SectionTable);
+}
+}
+
+#endif
diff --git a/COFF/Strings.cpp b/COFF/Strings.cpp
new file mode 100644
index 000000000000..d0558413f673
--- /dev/null
+++ b/COFF/Strings.cpp
@@ -0,0 +1,30 @@
+//===- Strings.cpp -------------------------------------------------------===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Strings.h"
+
+#if defined(_MSC_VER)
+#include <Windows.h>
+#include <DbgHelp.h>
+#pragma comment(lib, "dbghelp.lib")
+#endif
+
+using namespace lld;
+using namespace lld::coff;
+using namespace llvm;
+
+Optional<std::string> coff::demangle(StringRef S) {
+#if defined(_MSC_VER)
+  char Buf[4096];
+  if (S.startswith("?"))
+    if (size_t Len = UnDecorateSymbolName(S.str().c_str(), Buf, sizeof(Buf), 0))
+      return std::string(Buf, Len);
+#endif
+  return None;
+}
diff --git a/COFF/Strings.h b/COFF/Strings.h
new file mode 100644
index 000000000000..1f85f3e2da5c
--- /dev/null
+++ b/COFF/Strings.h
@@ -0,0 +1,23 @@
+//===- Strings.h ------------------------------------------------*- C++ -*-===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_COFF_STRINGS_H
+#define LLD_COFF_STRINGS_H
+
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringRef.h"
+#include <string>
+
+namespace lld {
+namespace coff {
+llvm::Optional<std::string> demangle(llvm::StringRef S);
+}
+}
+
+#endif
diff --git a/COFF/SymbolTable.cpp b/COFF/SymbolTable.cpp
index df9da4c36650..9cc0b75c1510 100644
--- a/COFF/SymbolTable.cpp
+++ b/COFF/SymbolTable.cpp
@@ -7,12 +7,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "SymbolTable.h"
 #include "Config.h"
 #include "Driver.h"
 #include "Error.h"
-#include "SymbolTable.h"
+#include "Memory.h"
 #include "Symbols.h"
-#include "lld/Core/Parallel.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/LTO/legacy/LTOCodeGenerator.h"
 #include "llvm/Support/Debug.h"
@@ -24,222 +24,265 @@ using namespace llvm;
 namespace lld {
 namespace coff {
 
-void SymbolTable::addFile(std::unique_ptr<InputFile> FileP) {
-#if LLVM_ENABLE_THREADS
-  std::launch Policy = std::launch::async;
-#else
-  std::launch Policy = std::launch::deferred;
-#endif
+SymbolTable *Symtab;
 
-  InputFile *File = FileP.get();
-  Files.push_back(std::move(FileP));
-  if (auto *F = dyn_cast<ArchiveFile>(File)) {
-    ArchiveQueue.push_back(
-        std::async(Policy, [=]() { F->parse(); return F; }));
-    return;
+void SymbolTable::addFile(InputFile *File) {
+  if (Config->Verbose)
+    outs() << "Reading " << toString(File) << "\n";
+  File->parse();
+
+  MachineTypes MT = File->getMachineType();
+  if (Config->Machine == IMAGE_FILE_MACHINE_UNKNOWN) {
+    Config->Machine = MT;
+  } else if (MT != IMAGE_FILE_MACHINE_UNKNOWN && Config->Machine != MT) {
+    fatal(toString(File) + ": machine type " + machineToStr(MT) +
+          " conflicts with " + machineToStr(Config->Machine));
   }
-  ObjectQueue.push_back(
-      std::async(Policy, [=]() { File->parse(); return File; }));
+
   if (auto *F = dyn_cast<ObjectFile>(File)) {
     ObjectFiles.push_back(F);
   } else if (auto *F = dyn_cast<BitcodeFile>(File)) {
     BitcodeFiles.push_back(F);
-  } else {
-    ImportFiles.push_back(cast<ImportFile>(File));
+  } else if (auto *F = dyn_cast<ImportFile>(File)) {
+    ImportFiles.push_back(F);
   }
-}
 
-void SymbolTable::step() {
-  if (queueEmpty())
+  StringRef S = File->getDirectives();
+  if (S.empty())
     return;
-  readObjects();
-  readArchives();
-}
 
-void SymbolTable::run() {
-  while (!queueEmpty())
-    step();
-}
-
-void SymbolTable::readArchives() {
-  if (ArchiveQueue.empty())
-    return;
-
-  // Add lazy symbols to the symbol table. Lazy symbols that conflict
-  // with existing undefined symbols are accumulated in LazySyms.
-  std::vector<Symbol *> LazySyms;
-  for (std::future<ArchiveFile *> &Future : ArchiveQueue) {
-    ArchiveFile *File = Future.get();
-    if (Config->Verbose)
-      llvm::outs() << "Reading " << File->getShortName() << "\n";
-    for (Lazy &Sym : File->getLazySymbols())
-      addLazy(&Sym, &LazySyms);
-  }
-  ArchiveQueue.clear();
-
-  // Add archive member files to ObjectQueue that should resolve
-  // existing undefined symbols.
-  for (Symbol *Sym : LazySyms)
-    addMemberFile(cast<Lazy>(Sym->Body));
-}
-
-void SymbolTable::readObjects() {
-  if (ObjectQueue.empty())
-    return;
-
-  // Add defined and undefined symbols to the symbol table.
-  std::vector<StringRef> Directives;
-  for (size_t I = 0; I < ObjectQueue.size(); ++I) {
-    InputFile *File = ObjectQueue[I].get();
-    if (Config->Verbose)
-      llvm::outs() << "Reading " << File->getShortName() << "\n";
-    // Adding symbols may add more files to ObjectQueue
-    // (but not to ArchiveQueue).
-    for (SymbolBody *Sym : File->getSymbols())
-      if (Sym->isExternal())
-        addSymbol(Sym);
-    StringRef S = File->getDirectives();
-    if (!S.empty()) {
-      Directives.push_back(S);
-      if (Config->Verbose)
-        llvm::outs() << "Directives: " << File->getShortName()
-                     << ": " << S << "\n";
-    }
-  }
-  ObjectQueue.clear();
-
-  // Parse directive sections. This may add files to
-  // ArchiveQueue and ObjectQueue.
-  for (StringRef S : Directives)
-    Driver->parseDirectives(S);
-}
-
-bool SymbolTable::queueEmpty() {
-  return ArchiveQueue.empty() && ObjectQueue.empty();
+  if (Config->Verbose)
+    outs() << "Directives: " << toString(File) << ": " << S << "\n";
+  Driver->parseDirectives(S);
 }
 
-void SymbolTable::reportRemainingUndefines(bool Resolve) {
-  llvm::SmallPtrSet<SymbolBody *, 8> Undefs;
+void SymbolTable::reportRemainingUndefines() {
+  SmallPtrSet<SymbolBody *, 8> Undefs;
   for (auto &I : Symtab) {
     Symbol *Sym = I.second;
-    auto *Undef = dyn_cast<Undefined>(Sym->Body);
+    auto *Undef = dyn_cast<Undefined>(Sym->body());
     if (!Undef)
       continue;
+    if (!Sym->IsUsedInRegularObj)
+      continue;
     StringRef Name = Undef->getName();
     // A weak alias may have been resolved, so check for that.
     if (Defined *D = Undef->getWeakAlias()) {
-      if (Resolve)
-        Sym->Body = D;
+      // We resolve weak aliases by replacing the alias's SymbolBody with the
+      // target's SymbolBody. This causes all SymbolBody pointers referring to
+      // the old symbol to instead refer to the new symbol. However, we can't
+      // just blindly copy sizeof(Symbol::Body) bytes from D to Sym->Body
+      // because D may be an internal symbol, and internal symbols are stored as
+      // "unparented" SymbolBodies. For that reason we need to check which type
+      // of symbol we are dealing with and copy the correct number of bytes.
+      if (isa<DefinedRegular>(D))
+        memcpy(Sym->Body.buffer, D, sizeof(DefinedRegular));
+      else if (isa<DefinedAbsolute>(D))
+        memcpy(Sym->Body.buffer, D, sizeof(DefinedAbsolute));
+      else
+        // No other internal symbols are possible.
+        Sym->Body = D->symbol()->Body;
       continue;
     }
     // If we can resolve a symbol by removing __imp_ prefix, do that.
     // This odd rule is for compatibility with MSVC linker.
     if (Name.startswith("__imp_")) {
       Symbol *Imp = find(Name.substr(strlen("__imp_")));
-      if (Imp && isa<Defined>(Imp->Body)) {
-        if (!Resolve)
-          continue;
-        auto *D = cast<Defined>(Imp->Body);
-        auto *S = new (Alloc) DefinedLocalImport(Name, D);
-        LocalImportChunks.push_back(S->getChunk());
-        Sym->Body = S;
+      if (Imp && isa<Defined>(Imp->body())) {
+        auto *D = cast<Defined>(Imp->body());
+        replaceBody<DefinedLocalImport>(Sym, Name, D);
+        LocalImportChunks.push_back(
+            cast<DefinedLocalImport>(Sym->body())->getChunk());
         continue;
       }
     }
     // Remaining undefined symbols are not fatal if /force is specified.
     // They are replaced with dummy defined symbols.
-    if (Config->Force && Resolve)
-      Sym->Body = new (Alloc) DefinedAbsolute(Name, 0);
-    Undefs.insert(Sym->Body);
+    if (Config->Force)
+      replaceBody<DefinedAbsolute>(Sym, Name, 0);
+    Undefs.insert(Sym->body());
   }
   if (Undefs.empty())
     return;
-  for (Undefined *U : Config->GCRoot)
-    if (Undefs.count(U->repl()))
-      llvm::errs() << "<root>: undefined symbol: " << U->getName() << "\n";
-  for (std::unique_ptr<InputFile> &File : Files)
-    if (!isa<ArchiveFile>(File.get()))
-      for (SymbolBody *Sym : File->getSymbols())
-        if (Undefs.count(Sym->repl()))
-          llvm::errs() << File->getShortName() << ": undefined symbol: "
-                       << Sym->getName() << "\n";
+  for (SymbolBody *B : Config->GCRoot)
+    if (Undefs.count(B))
+      errs() << "<root>: undefined symbol: " << B->getName() << "\n";
+  for (ObjectFile *File : ObjectFiles)
+    for (SymbolBody *Sym : File->getSymbols())
+      if (Undefs.count(Sym))
+        errs() << toString(File) << ": undefined symbol: " << Sym->getName()
+               << "\n";
   if (!Config->Force)
     fatal("link failed");
 }
 
-void SymbolTable::addLazy(Lazy *New, std::vector<Symbol *> *Accum) {
-  Symbol *Sym = insert(New);
-  if (Sym->Body == New)
-    return;
-  SymbolBody *Existing = Sym->Body;
-  if (isa<Defined>(Existing))
-    return;
-  if (Lazy *L = dyn_cast<Lazy>(Existing))
-    if (L->getFileIndex() < New->getFileIndex())
-      return;
-  Sym->Body = New;
-  New->setBackref(Sym);
-  if (isa<Undefined>(Existing))
-    Accum->push_back(Sym);
+std::pair<Symbol *, bool> SymbolTable::insert(StringRef Name) {
+  Symbol *&Sym = Symtab[CachedHashStringRef(Name)];
+  if (Sym)
+    return {Sym, false};
+  Sym = make<Symbol>();
+  Sym->IsUsedInRegularObj = false;
+  Sym->PendingArchiveLoad = false;
+  return {Sym, true};
 }
 
-void SymbolTable::addSymbol(SymbolBody *New) {
-  // Find an existing symbol or create and insert a new one.
-  assert(isa<Defined>(New) || isa<Undefined>(New));
-  Symbol *Sym = insert(New);
-  if (Sym->Body == New)
-    return;
-  SymbolBody *Existing = Sym->Body;
-
-  // If we have an undefined symbol and a lazy symbol,
-  // let the lazy symbol to read a member file.
-  if (auto *L = dyn_cast<Lazy>(Existing)) {
-    // Undefined symbols with weak aliases need not to be resolved,
-    // since they would be replaced with weak aliases if they remain
-    // undefined.
-    if (auto *U = dyn_cast<Undefined>(New)) {
-      if (!U->WeakAlias) {
-        addMemberFile(L);
-        return;
-      }
+Symbol *SymbolTable::addUndefined(StringRef Name, InputFile *F,
+                                  bool IsWeakAlias) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(Name);
+  if (!F || !isa<BitcodeFile>(F))
+    S->IsUsedInRegularObj = true;
+  if (WasInserted || (isa<Lazy>(S->body()) && IsWeakAlias)) {
+    replaceBody<Undefined>(S, Name);
+    return S;
+  }
+  if (auto *L = dyn_cast<Lazy>(S->body())) {
+    if (!S->PendingArchiveLoad) {
+      S->PendingArchiveLoad = true;
+      L->File->addMember(&L->Sym);
     }
-    Sym->Body = New;
+  }
+  return S;
+}
+
+void SymbolTable::addLazy(ArchiveFile *F, const Archive::Symbol Sym) {
+  StringRef Name = Sym.getName();
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(Name);
+  if (WasInserted) {
+    replaceBody<Lazy>(S, F, Sym);
     return;
   }
+  auto *U = dyn_cast<Undefined>(S->body());
+  if (!U || U->WeakAlias || S->PendingArchiveLoad)
+    return;
+  S->PendingArchiveLoad = true;
+  F->addMember(&Sym);
+}
+
+void SymbolTable::reportDuplicate(Symbol *Existing, InputFile *NewFile) {
+  fatal("duplicate symbol: " + toString(*Existing->body()) + " in " +
+        toString(Existing->body()->getFile()) + " and in " +
+        (NewFile ? toString(NewFile) : "(internal)"));
+}
 
-  // compare() returns -1, 0, or 1 if the lhs symbol is less preferable,
-  // equivalent (conflicting), or more preferable, respectively.
-  int Comp = Existing->compare(New);
-  if (Comp == 0)
-    fatal("duplicate symbol: " + Existing->getDebugName() + " and " +
-          New->getDebugName());
-  if (Comp < 0)
-    Sym->Body = New;
+Symbol *SymbolTable::addAbsolute(StringRef N, COFFSymbolRef Sym) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(N);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body()))
+    replaceBody<DefinedAbsolute>(S, N, Sym);
+  else if (!isa<DefinedCOFF>(S->body()))
+    reportDuplicate(S, nullptr);
+  return S;
 }
 
-Symbol *SymbolTable::insert(SymbolBody *New) {
-  Symbol *&Sym = Symtab[New->getName()];
-  if (Sym) {
-    New->setBackref(Sym);
-    return Sym;
+Symbol *SymbolTable::addAbsolute(StringRef N, uint64_t VA) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(N);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body()))
+    replaceBody<DefinedAbsolute>(S, N, VA);
+  else if (!isa<DefinedCOFF>(S->body()))
+    reportDuplicate(S, nullptr);
+  return S;
+}
+
+Symbol *SymbolTable::addRelative(StringRef N, uint64_t VA) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(N);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body()))
+    replaceBody<DefinedRelative>(S, N, VA);
+  else if (!isa<DefinedCOFF>(S->body()))
+    reportDuplicate(S, nullptr);
+  return S;
+}
+
+Symbol *SymbolTable::addRegular(ObjectFile *F, COFFSymbolRef Sym,
+                                SectionChunk *C) {
+  StringRef Name;
+  F->getCOFFObj()->getSymbolName(Sym, Name);
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(Name);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body()))
+    replaceBody<DefinedRegular>(S, F, Sym, C);
+  else if (auto *R = dyn_cast<DefinedRegular>(S->body())) {
+    if (!C->isCOMDAT() || !R->isCOMDAT())
+      reportDuplicate(S, F);
+  } else if (auto *B = dyn_cast<DefinedBitcode>(S->body())) {
+    if (B->IsReplaceable)
+      replaceBody<DefinedRegular>(S, F, Sym, C);
+    else if (!C->isCOMDAT())
+      reportDuplicate(S, F);
+  } else
+    replaceBody<DefinedRegular>(S, F, Sym, C);
+  return S;
+}
+
+Symbol *SymbolTable::addBitcode(BitcodeFile *F, StringRef N, bool IsReplaceable) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(N);
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body())) {
+    replaceBody<DefinedBitcode>(S, F, N, IsReplaceable);
+    return S;
   }
-  Sym = new (Alloc) Symbol(New);
-  New->setBackref(Sym);
-  return Sym;
+  if (isa<DefinedCommon>(S->body()))
+    return S;
+  if (IsReplaceable)
+    if (isa<DefinedRegular>(S->body()) || isa<DefinedBitcode>(S->body()))
+      return S;
+  reportDuplicate(S, F);
+  return S;
 }
 
-// Reads an archive member file pointed by a given symbol.
-void SymbolTable::addMemberFile(Lazy *Body) {
-  std::unique_ptr<InputFile> File = Body->getMember();
+Symbol *SymbolTable::addCommon(ObjectFile *F, COFFSymbolRef Sym,
+                               CommonChunk *C) {
+  StringRef Name;
+  F->getCOFFObj()->getSymbolName(Sym, Name);
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(Name);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || !isa<DefinedCOFF>(S->body()))
+    replaceBody<DefinedCommon>(S, F, Sym, C);
+  else if (auto *DC = dyn_cast<DefinedCommon>(S->body()))
+    if (Sym.getValue() > DC->getSize())
+      replaceBody<DefinedCommon>(S, F, Sym, C);
+  return S;
+}
 
-  // getMember returns an empty buffer if the member was already
-  // read from the library.
-  if (!File)
-    return;
-  if (Config->Verbose)
-    llvm::outs() << "Loaded " << File->getShortName() << " for "
-                 << Body->getName() << "\n";
-  addFile(std::move(File));
+Symbol *SymbolTable::addImportData(StringRef N, ImportFile *F) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(N);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body()))
+    replaceBody<DefinedImportData>(S, N, F);
+  else if (!isa<DefinedCOFF>(S->body()))
+    reportDuplicate(S, nullptr);
+  return S;
+}
+
+Symbol *SymbolTable::addImportThunk(StringRef Name, DefinedImportData *ID,
+                                    uint16_t Machine) {
+  Symbol *S;
+  bool WasInserted;
+  std::tie(S, WasInserted) = insert(Name);
+  S->IsUsedInRegularObj = true;
+  if (WasInserted || isa<Undefined>(S->body()) || isa<Lazy>(S->body()))
+    replaceBody<DefinedImportThunk>(S, Name, ID, Machine);
+  else if (!isa<DefinedCOFF>(S->body()))
+    reportDuplicate(S, nullptr);
+  return S;
 }
 
 std::vector<Chunk *> SymbolTable::getChunks() {
@@ -252,7 +295,7 @@ std::vector<Chunk *> SymbolTable::getChunks() {
 }
 
 Symbol *SymbolTable::find(StringRef Name) {
-  auto It = Symtab.find(Name);
+  auto It = Symtab.find(CachedHashStringRef(Name));
   if (It == Symtab.end())
     return nullptr;
   return It->second;
@@ -266,7 +309,7 @@ Symbol *SymbolTable::findUnderscore(StringRef Name) {
 
 StringRef SymbolTable::findByPrefix(StringRef Prefix) {
   for (auto Pair : Symtab) {
-    StringRef Name = Pair.first;
+    StringRef Name = Pair.first.val();
     if (Name.startswith(Prefix))
       return Name;
   }
@@ -275,7 +318,7 @@ StringRef SymbolTable::findByPrefix(StringRef Prefix) {
 
 StringRef SymbolTable::findMangle(StringRef Name) {
   if (Symbol *Sym = find(Name))
-    if (!isa<Undefined>(Sym->Body))
+    if (!isa<Undefined>(Sym->body()))
       return Name;
   if (Config->Machine != I386)
     return findByPrefix(("?" + Name + "@@Y").str());
@@ -289,39 +332,22 @@ StringRef SymbolTable::findMangle(StringRef Name) {
   return findByPrefix(("?" + Name.substr(1) + "@@Y").str());
 }
 
-void SymbolTable::mangleMaybe(Undefined *U) {
-  if (U->WeakAlias)
-    return;
-  if (!isa<Undefined>(U->repl()))
+void SymbolTable::mangleMaybe(SymbolBody *B) {
+  auto *U = dyn_cast<Undefined>(B);
+  if (!U || U->WeakAlias)
     return;
   StringRef Alias = findMangle(U->getName());
   if (!Alias.empty())
     U->WeakAlias = addUndefined(Alias);
 }
 
-Undefined *SymbolTable::addUndefined(StringRef Name) {
-  auto *New = new (Alloc) Undefined(Name);
-  addSymbol(New);
-  if (auto *U = dyn_cast<Undefined>(New->repl()))
-    return U;
-  return New;
-}
-
-DefinedRelative *SymbolTable::addRelative(StringRef Name, uint64_t VA) {
-  auto *New = new (Alloc) DefinedRelative(Name, VA);
-  addSymbol(New);
-  return New;
-}
-
-DefinedAbsolute *SymbolTable::addAbsolute(StringRef Name, uint64_t VA) {
-  auto *New = new (Alloc) DefinedAbsolute(Name, VA);
-  addSymbol(New);
-  return New;
+SymbolBody *SymbolTable::addUndefined(StringRef Name) {
+  return addUndefined(Name, nullptr, false)->body();
 }
 
 void SymbolTable::printMap(llvm::raw_ostream &OS) {
   for (ObjectFile *File : ObjectFiles) {
-    OS << File->getShortName() << ":\n";
+    OS << toString(File) << ":\n";
     for (SymbolBody *Body : File->getSymbols())
       if (auto *R = dyn_cast<DefinedRegular>(Body))
         if (R->getChunk()->isLive())
@@ -330,84 +356,32 @@ void SymbolTable::printMap(llvm::raw_ostream &OS) {
   }
 }
 
-void SymbolTable::addCombinedLTOObject(ObjectFile *Obj) {
-  for (SymbolBody *Body : Obj->getSymbols()) {
-    if (!Body->isExternal())
-      continue;
-    // We should not see any new undefined symbols at this point, but we'll
-    // diagnose them later in reportRemainingUndefines().
-    StringRef Name = Body->getName();
-    Symbol *Sym = insert(Body);
-    SymbolBody *Existing = Sym->Body;
-
-    if (Existing == Body)
-      continue;
-
-    if (isa<DefinedBitcode>(Existing)) {
-      Sym->Body = Body;
-      continue;
-    }
-    if (auto *L = dyn_cast<Lazy>(Existing)) {
-      // We may see new references to runtime library symbols such as __chkstk
-      // here. These symbols must be wholly defined in non-bitcode files.
-      addMemberFile(L);
-      continue;
-    }
-
-    int Comp = Existing->compare(Body);
-    if (Comp == 0)
-      fatal("LTO: unexpected duplicate symbol: " + Name);
-    if (Comp < 0)
-      Sym->Body = Body;
-  }
-}
-
 void SymbolTable::addCombinedLTOObjects() {
   if (BitcodeFiles.empty())
     return;
 
-  // Diagnose any undefined symbols early, but do not resolve weak externals,
-  // as resolution breaks the invariant that each Symbol points to a unique
-  // SymbolBody, which we rely on to replace DefinedBitcode symbols correctly.
-  reportRemainingUndefines(/*Resolve=*/false);
-
   // Create an object file and add it to the symbol table by replacing any
   // DefinedBitcode symbols with the definitions in the object file.
   LTOCodeGenerator CG(BitcodeFile::Context);
   CG.setOptLevel(Config->LTOOptLevel);
-  std::vector<ObjectFile *> Objs = createLTOObjects(&CG);
-
-  for (ObjectFile *Obj : Objs)
-    addCombinedLTOObject(Obj);
-
-  size_t NumBitcodeFiles = BitcodeFiles.size();
-  run();
-  if (BitcodeFiles.size() != NumBitcodeFiles)
-    fatal("LTO: late loaded symbol created new bitcode reference");
+  for (ObjectFile *Obj : createLTOObjects(&CG))
+    Obj->parse();
 }
 
 // Combine and compile bitcode files and then return the result
 // as a vector of regular COFF object files.
 std::vector<ObjectFile *> SymbolTable::createLTOObjects(LTOCodeGenerator *CG) {
-  // All symbols referenced by non-bitcode objects must be preserved.
-  for (ObjectFile *File : ObjectFiles)
-    for (SymbolBody *Body : File->getSymbols())
-      if (auto *S = dyn_cast<DefinedBitcode>(Body->repl()))
-        CG->addMustPreserveSymbol(S->getName());
-
-  // Likewise for bitcode symbols which we initially resolved to non-bitcode.
+  // All symbols referenced by non-bitcode objects, including GC roots, must be
+  // preserved. We must also replace bitcode symbols with undefined symbols so
+  // that they may be replaced with real definitions without conflicting.
   for (BitcodeFile *File : BitcodeFiles)
-    for (SymbolBody *Body : File->getSymbols())
-      if (isa<DefinedBitcode>(Body) && !isa<DefinedBitcode>(Body->repl()))
+    for (SymbolBody *Body : File->getSymbols()) {
+      if (!isa<DefinedBitcode>(Body))
+        continue;
+      if (Body->symbol()->IsUsedInRegularObj)
         CG->addMustPreserveSymbol(Body->getName());
-
-  // Likewise for other symbols that must be preserved.
-  for (Undefined *U : Config->GCRoot) {
-    if (auto *S = dyn_cast<DefinedBitcode>(U->repl()))
-      CG->addMustPreserveSymbol(S->getName());
-    else if (auto *S = dyn_cast_or_null<DefinedBitcode>(U->getWeakAlias()))
-      CG->addMustPreserveSymbol(S->getName());
-  }
+      replaceBody<Undefined>(Body->symbol(), Body->getName());
+    }
 
   CG->setModule(BitcodeFiles[0]->takeModule());
   for (unsigned I = 1, E = BitcodeFiles.size(); I != E; ++I)
@@ -434,10 +408,8 @@ std::vector<ObjectFile *> SymbolTable::createLTOObjects(LTOCodeGenerator *CG) {
 
   std::vector<ObjectFile *> ObjFiles;
   for (SmallString<0> &Obj : Objs) {
-    auto *ObjFile = new ObjectFile(MemoryBufferRef(Obj, "<LTO object>"));
-    Files.emplace_back(ObjFile);
+    auto *ObjFile = make<ObjectFile>(MemoryBufferRef(Obj, "<LTO object>"));
     ObjectFiles.push_back(ObjFile);
-    ObjFile->parse();
     ObjFiles.push_back(ObjFile);
   }
 
diff --git a/COFF/SymbolTable.h b/COFF/SymbolTable.h
index 8bf4387cdfff..703821f2e124 100644
--- a/COFF/SymbolTable.h
+++ b/COFF/SymbolTable.h
@@ -11,18 +11,12 @@
 #define LLD_COFF_SYMBOL_TABLE_H
 
 #include "InputFiles.h"
+#include "llvm/ADT/CachedHashString.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/raw_ostream.h"
 
-#ifdef _MSC_VER
-// <future> depends on <eh.h> for __uncaught_exception.
-#include <eh.h>
-#endif
-
-#include <future>
-
 namespace llvm {
 struct LTOCodeGenerator;
 }
@@ -31,8 +25,12 @@ namespace lld {
 namespace coff {
 
 class Chunk;
+class CommonChunk;
 class Defined;
+class DefinedAbsolute;
+class DefinedRelative;
 class Lazy;
+class SectionChunk;
 class SymbolBody;
 struct Symbol;
 
@@ -45,18 +43,17 @@ struct Symbol;
 // conflicts. For example, obviously, a defined symbol is better than
 // an undefined symbol. Or, if there's a conflict between a lazy and a
 // undefined, it'll read an archive member to read a real definition
-// to replace the lazy symbol. The logic is implemented in resolve().
+// to replace the lazy symbol. The logic is implemented in the
+// add*() functions, which are called by input files as they are parsed.
+// There is one add* function per symbol type.
 class SymbolTable {
 public:
-  void addFile(std::unique_ptr<InputFile> File);
-  std::vector<std::unique_ptr<InputFile>> &getFiles() { return Files; }
-  void step();
-  void run();
-  bool queueEmpty();
+  void addFile(InputFile *File);
 
-  // Print an error message on undefined symbols. If Resolve is true, try to
-  // resolve any undefined symbols and update the symbol table accordingly.
-  void reportRemainingUndefines(bool Resolve);
+  // Try to resolve any undefined symbols and update the symbol table
+  // accordingly, then print an error message for any remaining undefined
+  // symbols.
+  void reportRemainingUndefines();
 
   // Returns a list of chunks of selected symbols.
   std::vector<Chunk *> getChunks();
@@ -69,7 +66,7 @@ public:
   // mangled symbol. This function tries to find a mangled name
   // for U from the symbol table, and if found, set the symbol as
   // a weak alias for U.
-  void mangleMaybe(Undefined *U);
+  void mangleMaybe(SymbolBody *B);
   StringRef findMangle(StringRef Name);
 
   // Print a layout map to OS.
@@ -88,37 +85,44 @@ public:
   std::vector<ObjectFile *> ObjectFiles;
 
   // Creates an Undefined symbol for a given name.
-  Undefined *addUndefined(StringRef Name);
-  DefinedRelative *addRelative(StringRef Name, uint64_t VA);
-  DefinedAbsolute *addAbsolute(StringRef Name, uint64_t VA);
+  SymbolBody *addUndefined(StringRef Name);
+
+  Symbol *addRelative(StringRef N, uint64_t VA);
+  Symbol *addAbsolute(StringRef N, uint64_t VA);
+
+  Symbol *addUndefined(StringRef Name, InputFile *F, bool IsWeakAlias);
+  void addLazy(ArchiveFile *F, const Archive::Symbol Sym);
+  Symbol *addAbsolute(StringRef N, COFFSymbolRef S);
+  Symbol *addRegular(ObjectFile *F, COFFSymbolRef S, SectionChunk *C);
+  Symbol *addBitcode(BitcodeFile *F, StringRef N, bool IsReplaceable);
+  Symbol *addCommon(ObjectFile *F, COFFSymbolRef S, CommonChunk *C);
+  Symbol *addImportData(StringRef N, ImportFile *F);
+  Symbol *addImportThunk(StringRef Name, DefinedImportData *S,
+                         uint16_t Machine);
+
+  void reportDuplicate(Symbol *Existing, InputFile *NewFile);
 
   // A list of chunks which to be added to .rdata.
   std::vector<Chunk *> LocalImportChunks;
 
 private:
-  void readArchives();
+  void readArchive();
   void readObjects();
 
-  void addSymbol(SymbolBody *New);
-  void addLazy(Lazy *New, std::vector<Symbol *> *Accum);
-  Symbol *insert(SymbolBody *New);
+  std::pair<Symbol *, bool> insert(StringRef Name);
   StringRef findByPrefix(StringRef Prefix);
 
-  void addMemberFile(Lazy *Body);
   void addCombinedLTOObject(ObjectFile *Obj);
   std::vector<ObjectFile *> createLTOObjects(llvm::LTOCodeGenerator *CG);
 
-  llvm::DenseMap<StringRef, Symbol *> Symtab;
-
-  std::vector<std::unique_ptr<InputFile>> Files;
-  std::vector<std::future<ArchiveFile *>> ArchiveQueue;
-  std::vector<std::future<InputFile *>> ObjectQueue;
+  llvm::DenseMap<llvm::CachedHashStringRef, Symbol *> Symtab;
 
   std::vector<BitcodeFile *> BitcodeFiles;
   std::vector<SmallString<0>> Objs;
-  llvm::BumpPtrAllocator Alloc;
 };
 
+extern SymbolTable *Symtab;
+
 } // namespace coff
 } // namespace lld
 
diff --git a/COFF/Symbols.cpp b/COFF/Symbols.cpp
index 6e2db6631ce7..6de85d581f49 100644
--- a/COFF/Symbols.cpp
+++ b/COFF/Symbols.cpp
@@ -7,16 +7,17 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "Symbols.h"
 #include "Error.h"
 #include "InputFiles.h"
-#include "Symbols.h"
+#include "Memory.h"
+#include "Strings.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 
+using namespace llvm;
 using namespace llvm::object;
-using llvm::sys::fs::identify_magic;
-using llvm::sys::fs::file_magic;
 
 namespace lld {
 namespace coff {
@@ -36,130 +37,14 @@ StringRef SymbolBody::getName() {
   return Name;
 }
 
-// Returns 1, 0 or -1 if this symbol should take precedence
-// over the Other, tie or lose, respectively.
-int SymbolBody::compare(SymbolBody *Other) {
-  Kind LK = kind(), RK = Other->kind();
-
-  // Normalize so that the smaller kind is on the left.
-  if (LK > RK)
-    return -Other->compare(this);
-
-  // First handle comparisons between two different kinds.
-  if (LK != RK) {
-    if (RK > LastDefinedKind) {
-      if (LK == LazyKind && cast<Undefined>(Other)->WeakAlias)
-        return -1;
-
-      // The LHS is either defined or lazy and so it wins.
-      assert((LK <= LastDefinedKind || LK == LazyKind) && "Bad kind!");
-      return 1;
-    }
-
-    // Bitcode has special complexities.
-    if (RK == DefinedBitcodeKind) {
-      auto *RHS = cast<DefinedBitcode>(Other);
-
-      switch (LK) {
-      case DefinedCommonKind:
-        return 1;
-
-      case DefinedRegularKind:
-        // As an approximation, regular symbols win over bitcode symbols,
-        // but we definitely have a conflict if the regular symbol is not
-        // replaceable and neither is the bitcode symbol. We do not
-        // replicate the rest of the symbol resolution logic here; symbol
-        // resolution will be done accurately after lowering bitcode symbols
-        // to regular symbols in addCombinedLTOObject().
-        if (cast<DefinedRegular>(this)->isCOMDAT() || RHS->IsReplaceable)
-          return 1;
-
-        // Fallthrough to the default of a tie otherwise.
-      default:
-        return 0;
-      }
-    }
-
-    // Either of the object file kind will trump a higher kind.
-    if (LK <= LastDefinedCOFFKind)
-      return 1;
-
-    // The remaining kind pairs are ties amongst defined symbols.
-    return 0;
-  }
-
-  // Now handle the case where the kinds are the same.
-  switch (LK) {
-  case DefinedRegularKind: {
-    auto *LHS = cast<DefinedRegular>(this);
-    auto *RHS = cast<DefinedRegular>(Other);
-    if (LHS->isCOMDAT() && RHS->isCOMDAT())
-      return LHS->getFileIndex() < RHS->getFileIndex() ? 1 : -1;
-    return 0;
-  }
-
-  case DefinedCommonKind: {
-    auto *LHS = cast<DefinedCommon>(this);
-    auto *RHS = cast<DefinedCommon>(Other);
-    if (LHS->getSize() == RHS->getSize())
-      return LHS->getFileIndex() < RHS->getFileIndex() ? 1 : -1;
-    return LHS->getSize() > RHS->getSize() ? 1 : -1;
-  }
-
-  case DefinedBitcodeKind: {
-    auto *LHS = cast<DefinedBitcode>(this);
-    auto *RHS = cast<DefinedBitcode>(Other);
-    // If both are non-replaceable, we have a tie.
-    if (!LHS->IsReplaceable && !RHS->IsReplaceable)
-      return 0;
-
-    // Non-replaceable symbols win, but even two replaceable symboles don't
-    // tie. If both symbols are replaceable, choice is arbitrary.
-    if (RHS->IsReplaceable && LHS->IsReplaceable)
-      return uintptr_t(LHS) < uintptr_t(RHS) ? 1 : -1;
-    return LHS->IsReplaceable ? -1 : 1;
-  }
-
-  case LazyKind: {
-    // Don't tie, pick the earliest.
-    auto *LHS = cast<Lazy>(this);
-    auto *RHS = cast<Lazy>(Other);
-    return LHS->getFileIndex() < RHS->getFileIndex() ? 1 : -1;
-  }
-
-  case UndefinedKind: {
-    auto *LHS = cast<Undefined>(this);
-    auto *RHS = cast<Undefined>(Other);
-    // Tie if both undefined symbols have different weak aliases.
-    if (LHS->WeakAlias && RHS->WeakAlias) {
-      if (LHS->WeakAlias->getName() != RHS->WeakAlias->getName())
-        return 0;
-      return uintptr_t(LHS) < uintptr_t(RHS) ? 1 : -1;
-    }
-    return LHS->WeakAlias ? 1 : -1;
-  }
-
-  case DefinedLocalImportKind:
-  case DefinedImportThunkKind:
-  case DefinedImportDataKind:
-  case DefinedAbsoluteKind:
-  case DefinedRelativeKind:
-    // These all simply tie.
-    return 0;
-  }
-  llvm_unreachable("unknown symbol kind");
-}
-
-std::string SymbolBody::getDebugName() {
-  std::string N = getName().str();
-  if (auto *D = dyn_cast<DefinedCOFF>(this)) {
-    N += " ";
-    N += D->File->getShortName();
-  } else if (auto *D = dyn_cast<DefinedBitcode>(this)) {
-    N += " ";
-    N += D->File->getShortName();
-  }
-  return N;
+InputFile *SymbolBody::getFile() {
+  if (auto *Sym = dyn_cast<DefinedCOFF>(this))
+    return Sym->File;
+  if (auto *Sym = dyn_cast<DefinedBitcode>(this))
+    return Sym->File;
+  if (auto *Sym = dyn_cast<Lazy>(this))
+    return Sym->File;
+  return nullptr;
 }
 
 COFFSymbolRef DefinedCOFF::getCOFFSymbol() {
@@ -174,44 +59,27 @@ DefinedImportThunk::DefinedImportThunk(StringRef Name, DefinedImportData *S,
                                        uint16_t Machine)
     : Defined(DefinedImportThunkKind, Name) {
   switch (Machine) {
-  case AMD64: Data.reset(new ImportThunkChunkX64(S)); return;
-  case I386:  Data.reset(new ImportThunkChunkX86(S)); return;
-  case ARMNT: Data.reset(new ImportThunkChunkARM(S)); return;
+  case AMD64: Data = make<ImportThunkChunkX64>(S); return;
+  case I386:  Data = make<ImportThunkChunkX86>(S); return;
+  case ARMNT: Data = make<ImportThunkChunkARM>(S); return;
   default:    llvm_unreachable("unknown machine type");
   }
 }
 
-std::unique_ptr<InputFile> Lazy::getMember() {
-  MemoryBufferRef MBRef = File->getMember(&Sym);
-
-  // getMember returns an empty buffer if the member was already
-  // read from the library.
-  if (MBRef.getBuffer().empty())
-    return std::unique_ptr<InputFile>(nullptr);
-
-  file_magic Magic = identify_magic(MBRef.getBuffer());
-  if (Magic == file_magic::coff_import_library)
-    return std::unique_ptr<InputFile>(new ImportFile(MBRef));
-
-  std::unique_ptr<InputFile> Obj;
-  if (Magic == file_magic::coff_object)
-    Obj.reset(new ObjectFile(MBRef));
-  else if (Magic == file_magic::bitcode)
-    Obj.reset(new BitcodeFile(MBRef));
-  else
-    fatal("unknown file type: " + File->getName());
-
-  Obj->setParentName(File->getName());
-  return Obj;
-}
-
 Defined *Undefined::getWeakAlias() {
   // A weak alias may be a weak alias to another symbol, so check recursively.
   for (SymbolBody *A = WeakAlias; A; A = cast<Undefined>(A)->WeakAlias)
-    if (auto *D = dyn_cast<Defined>(A->repl()))
+    if (auto *D = dyn_cast<Defined>(A))
       return D;
   return nullptr;
 }
 
+// Returns a symbol name for an error message.
+std::string toString(SymbolBody &B) {
+  if (Optional<std::string> S = demangle(B.getName()))
+    return ("\"" + *S + "\" (" + B.getName() + ")").str();
+  return B.getName();
+}
+
 } // namespace coff
 } // namespace lld
diff --git a/COFF/Symbols.h b/COFF/Symbols.h
index f96c1fb3cc1d..bc9ad4aa8aff 100644
--- a/COFF/Symbols.h
+++ b/COFF/Symbols.h
@@ -12,6 +12,7 @@
 
 #include "Chunks.h"
 #include "Config.h"
+#include "Memory.h"
 #include "lld/Core/LLVM.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Object/Archive.h"
@@ -32,15 +33,8 @@ class ArchiveFile;
 class BitcodeFile;
 class InputFile;
 class ObjectFile;
-class SymbolBody;
-
-// A real symbol object, SymbolBody, is usually accessed indirectly
-// through a Symbol. There's always one Symbol for each symbol name.
-// The resolver updates SymbolBody pointers as it resolves symbols.
-struct Symbol {
-  explicit Symbol(SymbolBody *P) : Body(P) {}
-  SymbolBody *Body;
-};
+struct Symbol;
+class SymbolTable;
 
 // The base class for real symbol classes.
 class SymbolBody {
@@ -75,28 +69,19 @@ public:
   // Returns the symbol name.
   StringRef getName();
 
-  // A SymbolBody has a backreference to a Symbol. Originally they are
-  // doubly-linked. A backreference will never change. But the pointer
-  // in the Symbol may be mutated by the resolver. If you have a
-  // pointer P to a SymbolBody and are not sure whether the resolver
-  // has chosen the object among other objects having the same name,
-  // you can access P->Backref->Body to get the resolver's result.
-  void setBackref(Symbol *P) { Backref = P; }
-  SymbolBody *repl() { return Backref ? Backref->Body : this; }
-
-  // Decides which symbol should "win" in the symbol table, this or
-  // the Other. Returns 1 if this wins, -1 if the Other wins, or 0 if
-  // they are duplicate (conflicting) symbols.
-  int compare(SymbolBody *Other);
+  // Returns the file from which this symbol was created.
+  InputFile *getFile();
 
-  // Returns a name of this symbol including source file name.
-  // Used only for debugging and logging.
-  std::string getDebugName();
+  Symbol *symbol();
+  const Symbol *symbol() const {
+    return const_cast<SymbolBody *>(this)->symbol();
+  }
 
 protected:
+  friend SymbolTable;
   explicit SymbolBody(Kind K, StringRef N = "")
       : SymbolKind(K), IsExternal(true), IsCOMDAT(false),
-        IsReplaceable(false), Name(N) {}
+        IsReplaceable(false), WrittenToSymtab(false), Name(N) {}
 
   const unsigned SymbolKind : 8;
   unsigned IsExternal : 1;
@@ -107,8 +92,12 @@ protected:
   // This bit is used by the \c DefinedBitcode subclass.
   unsigned IsReplaceable : 1;
 
+public:
+  // This bit is used by Writer::createSymbolAndStringTable().
+  unsigned WrittenToSymtab : 1;
+
+protected:
   StringRef Name;
-  Symbol *Backref = nullptr;
 };
 
 // The base class for any defined symbols, including absolute symbols,
@@ -149,12 +138,13 @@ public:
     return S->kind() <= LastDefinedCOFFKind;
   }
 
-  int getFileIndex() { return File->Index; }
+  ObjectFile *getFile() { return File; }
 
   COFFSymbolRef getCOFFSymbol();
 
-protected:
   ObjectFile *File;
+
+protected:
   const coff_symbol_generic *Sym;
 };
 
@@ -194,7 +184,7 @@ public:
   uint64_t getRVA() { return Data->getRVA(); }
 
 private:
-  friend SymbolBody;
+  friend SymbolTable;
   uint64_t getSize() { return Sym->Value; }
   CommonChunk *Data;
 };
@@ -253,14 +243,12 @@ public:
 
   static bool classof(const SymbolBody *S) { return S->kind() == LazyKind; }
 
-  // Returns an object file for this symbol, or a nullptr if the file
-  // was already returned.
-  std::unique_ptr<InputFile> getMember();
+  ArchiveFile *File;
 
-  int getFileIndex() { return File->Index; }
+private:
+  friend SymbolTable;
 
 private:
-  ArchiveFile *File;
   const Archive::Symbol Sym;
 };
 
@@ -293,26 +281,22 @@ public:
 // table in an output. The former has "__imp_" prefix.
 class DefinedImportData : public Defined {
 public:
-  DefinedImportData(StringRef D, StringRef N, StringRef E,
-                    const coff_import_header *H)
-      : Defined(DefinedImportDataKind, N), DLLName(D), ExternalName(E), Hdr(H) {
+  DefinedImportData(StringRef N, ImportFile *F)
+      : Defined(DefinedImportDataKind, N), File(F) {
   }
 
   static bool classof(const SymbolBody *S) {
     return S->kind() == DefinedImportDataKind;
   }
 
-  uint64_t getRVA() { return Location->getRVA(); }
-  StringRef getDLLName() { return DLLName; }
-  StringRef getExternalName() { return ExternalName; }
-  void setLocation(Chunk *AddressTable) { Location = AddressTable; }
-  uint16_t getOrdinal() { return Hdr->OrdinalHint; }
+  uint64_t getRVA() { return File->Location->getRVA(); }
+  StringRef getDLLName() { return File->DLLName; }
+  StringRef getExternalName() { return File->ExternalName; }
+  void setLocation(Chunk *AddressTable) { File->Location = AddressTable; }
+  uint16_t getOrdinal() { return File->Hdr->OrdinalHint; }
 
 private:
-  StringRef DLLName;
-  StringRef ExternalName;
-  const coff_import_header *Hdr;
-  Chunk *Location = nullptr;
+  ImportFile *File;
 };
 
 // This class represents a symbol for a jump table entry which jumps
@@ -329,10 +313,10 @@ public:
   }
 
   uint64_t getRVA() { return Data->getRVA(); }
-  Chunk *getChunk() { return Data.get(); }
+  Chunk *getChunk() { return Data; }
 
 private:
-  std::unique_ptr<Chunk> Data;
+  Chunk *Data;
 };
 
 // If you have a symbol "__imp_foo" in your object file, a symbol name
@@ -343,17 +327,17 @@ private:
 class DefinedLocalImport : public Defined {
 public:
   DefinedLocalImport(StringRef N, Defined *S)
-      : Defined(DefinedLocalImportKind, N), Data(S) {}
+      : Defined(DefinedLocalImportKind, N), Data(make<LocalImportChunk>(S)) {}
 
   static bool classof(const SymbolBody *S) {
     return S->kind() == DefinedLocalImportKind;
   }
 
-  uint64_t getRVA() { return Data.getRVA(); }
-  Chunk *getChunk() { return &Data; }
+  uint64_t getRVA() { return Data->getRVA(); }
+  Chunk *getChunk() { return Data; }
 
 private:
-  LocalImportChunk Data;
+  LocalImportChunk *Data;
 };
 
 class DefinedBitcode : public Defined {
@@ -361,6 +345,11 @@ class DefinedBitcode : public Defined {
 public:
   DefinedBitcode(BitcodeFile *F, StringRef N, bool IsReplaceable)
       : Defined(DefinedBitcodeKind, N), File(F) {
+    // IsReplaceable tracks whether the bitcode symbol may be replaced with some
+    // other (defined, common or bitcode) symbol. This is the case for common,
+    // comdat and weak external symbols. We try to replace bitcode symbols with
+    // "real" symbols (see SymbolTable::add{Regular,Bitcode}), and resolve the
+    // result against the real symbol from the combined LTO object.
     this->IsReplaceable = IsReplaceable;
   }
 
@@ -368,7 +357,6 @@ public:
     return S->kind() == DefinedBitcodeKind;
   }
 
-private:
   BitcodeFile *File;
 };
 
@@ -397,6 +385,52 @@ inline uint64_t Defined::getRVA() {
   llvm_unreachable("unknown symbol kind");
 }
 
+// A real symbol object, SymbolBody, is usually stored within a Symbol. There's
+// always one Symbol for each symbol name. The resolver updates the SymbolBody
+// stored in the Body field of this object as it resolves symbols. Symbol also
+// holds computed properties of symbol names.
+struct Symbol {
+  // True if this symbol was referenced by a regular (non-bitcode) object.
+  unsigned IsUsedInRegularObj : 1;
+
+  // True if we've seen both a lazy and an undefined symbol with this symbol
+  // name, which means that we have enqueued an archive member load and should
+  // not load any more archive members to resolve the same symbol.
+  unsigned PendingArchiveLoad : 1;
+
+  // This field is used to store the Symbol's SymbolBody. This instantiation of
+  // AlignedCharArrayUnion gives us a struct with a char array field that is
+  // large and aligned enough to store any derived class of SymbolBody.
+  llvm::AlignedCharArrayUnion<DefinedRegular, DefinedCommon, DefinedAbsolute,
+                              DefinedRelative, Lazy, Undefined,
+                              DefinedImportData, DefinedImportThunk,
+                              DefinedLocalImport, DefinedBitcode>
+      Body;
+
+  SymbolBody *body() {
+    return reinterpret_cast<SymbolBody *>(Body.buffer);
+  }
+  const SymbolBody *body() const { return const_cast<Symbol *>(this)->body(); }
+};
+
+template <typename T, typename... ArgT>
+void replaceBody(Symbol *S, ArgT &&... Arg) {
+  static_assert(sizeof(T) <= sizeof(S->Body), "Body too small");
+  static_assert(alignof(T) <= alignof(decltype(S->Body)),
+                "Body not aligned enough");
+  assert(static_cast<SymbolBody *>(static_cast<T *>(nullptr)) == nullptr &&
+         "Not a SymbolBody");
+  new (S->Body.buffer) T(std::forward<ArgT>(Arg)...);
+}
+
+inline Symbol *SymbolBody::symbol() {
+  assert(isExternal());
+  return reinterpret_cast<Symbol *>(reinterpret_cast<char *>(this) -
+                                    offsetof(Symbol, Body));
+}
+
+std::string toString(SymbolBody &B);
+
 } // namespace coff
 } // namespace lld
 
diff --git a/COFF/Writer.cpp b/COFF/Writer.cpp
index d8077df95701..3e69aebbb424 100644
--- a/COFF/Writer.cpp
+++ b/COFF/Writer.cpp
@@ -7,13 +7,15 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "Writer.h"
 #include "Config.h"
 #include "DLL.h"
 #include "Error.h"
 #include "InputFiles.h"
+#include "Memory.h"
+#include "PDB.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
-#include "Writer.h"
 #include "lld/Core/Parallel.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
@@ -21,6 +23,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/FileOutputBuffer.h"
+#include "llvm/Support/RandomNumberGenerator.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cstdio>
@@ -42,6 +45,61 @@ static const int DOSStubSize = 64;
 static const int NumberfOfDataDirectory = 16;
 
 namespace {
+
+class DebugDirectoryChunk : public Chunk {
+public:
+  DebugDirectoryChunk(const std::vector<std::unique_ptr<Chunk>> &R)
+      : Records(R) {}
+
+  size_t getSize() const override {
+    return Records.size() * sizeof(debug_directory);
+  }
+
+  void writeTo(uint8_t *B) const override {
+    auto *D = reinterpret_cast<debug_directory *>(B + OutputSectionOff);
+
+    for (const std::unique_ptr<Chunk> &Record : Records) {
+      D->Characteristics = 0;
+      D->TimeDateStamp = 0;
+      D->MajorVersion = 0;
+      D->MinorVersion = 0;
+      D->Type = COFF::IMAGE_DEBUG_TYPE_CODEVIEW;
+      D->SizeOfData = Record->getSize();
+      D->AddressOfRawData = Record->getRVA();
+      // TODO(compnerd) get the file offset
+      D->PointerToRawData = 0;
+
+      ++D;
+    }
+  }
+
+private:
+  const std::vector<std::unique_ptr<Chunk>> &Records;
+};
+
+class CVDebugRecordChunk : public Chunk {
+  size_t getSize() const override {
+    return sizeof(codeview::DebugInfo) + Config->PDBPath.size() + 1;
+  }
+
+  void writeTo(uint8_t *B) const override {
+    // Save off the DebugInfo entry to backfill the file signature (build id)
+    // in Writer::writeBuildId
+    DI = reinterpret_cast<codeview::DebugInfo *>(B + OutputSectionOff);
+
+    DI->Signature.CVSignature = OMF::Signature::PDB70;
+
+    // variable sized field (PDB Path)
+    auto *P = reinterpret_cast<char *>(B + OutputSectionOff + sizeof(*DI));
+    if (!Config->PDBPath.empty())
+      memcpy(P, Config->PDBPath.data(), Config->PDBPath.size());
+    P[Config->PDBPath.size()] = '\0';
+  }
+
+public:
+  mutable codeview::DebugInfo *DI = nullptr;
+};
+
 // The writer writes a SymbolTable result to a file.
 class Writer {
 public:
@@ -62,6 +120,7 @@ private:
   void setSectionPermissions();
   void writeSections();
   void sortExceptionTable();
+  void writeBuildId();
   void applyRelocations();
 
   llvm::Optional<coff_symbol16> createSymbol(Defined *D);
@@ -76,9 +135,7 @@ private:
   std::map<StringRef, std::vector<DefinedImportData *>> binImports();
 
   SymbolTable *Symtab;
-  std::unique_ptr<llvm::FileOutputBuffer> Buffer;
-  llvm::SpecificBumpPtrAllocator<OutputSection> CAlloc;
-  llvm::SpecificBumpPtrAllocator<BaserelChunk> BAlloc;
+  std::unique_ptr<FileOutputBuffer> Buffer;
   std::vector<OutputSection *> OutputSections;
   std::vector<char> Strtab;
   std::vector<llvm::object::coff_symbol16> OutputSymtab;
@@ -87,6 +144,11 @@ private:
   EdataContents Edata;
   std::unique_ptr<SEHTableChunk> SEHTable;
 
+  std::unique_ptr<Chunk> DebugDirectory;
+  std::vector<std::unique_ptr<Chunk>> DebugRecords;
+  CVDebugRecordChunk *BuildId = nullptr;
+  ArrayRef<uint8_t> SectionTable;
+
   uint64_t FileSize;
   uint32_t PointerToSymbolTable = 0;
   uint64_t SizeOfImage;
@@ -239,6 +301,11 @@ void Writer::run() {
   fixSafeSEHSymbols();
   writeSections();
   sortExceptionTable();
+  writeBuildId();
+
+  if (!Config->PDBPath.empty())
+    createPDB(Config->PDBPath, Symtab, SectionTable);
+
   if (auto EC = Buffer->commit())
     fatal(EC, "failed to write the output file");
 }
@@ -274,7 +341,7 @@ void Writer::createSections() {
     StringRef Name = getOutputSection(Pair.first);
     OutputSection *&Sec = Sections[Name];
     if (!Sec) {
-      Sec = new (CAlloc.Allocate()) OutputSection(Name);
+      Sec = make<OutputSection>(Name);
       OutputSections.push_back(Sec);
     }
     std::vector<Chunk *> &Chunks = Pair.second;
@@ -286,25 +353,46 @@ void Writer::createSections() {
 }
 
 void Writer::createMiscChunks() {
+  OutputSection *RData = createSection(".rdata");
+
   // Create thunks for locally-dllimported symbols.
   if (!Symtab->LocalImportChunks.empty()) {
-    OutputSection *Sec = createSection(".rdata");
     for (Chunk *C : Symtab->LocalImportChunks)
-      Sec->addChunk(C);
+      RData->addChunk(C);
+  }
+
+  // Create Debug Information Chunks
+  if (Config->Debug) {
+    DebugDirectory = llvm::make_unique<DebugDirectoryChunk>(DebugRecords);
+
+    // TODO(compnerd) create a coffgrp entry if DebugType::CV is not enabled
+    if (Config->DebugTypes & static_cast<unsigned>(coff::DebugType::CV)) {
+      auto Chunk = llvm::make_unique<CVDebugRecordChunk>();
+
+      BuildId = Chunk.get();
+      DebugRecords.push_back(std::move(Chunk));
+    }
+
+    RData->addChunk(DebugDirectory.get());
+    for (const std::unique_ptr<Chunk> &C : DebugRecords)
+      RData->addChunk(C.get());
   }
 
   // Create SEH table. x86-only.
   if (Config->Machine != I386)
     return;
+
   std::set<Defined *> Handlers;
+
   for (lld::coff::ObjectFile *File : Symtab->ObjectFiles) {
     if (!File->SEHCompat)
       return;
     for (SymbolBody *B : File->SEHandlers)
-      Handlers.insert(cast<Defined>(B->repl()));
+      Handlers.insert(cast<Defined>(B));
   }
+
   SEHTable.reset(new SEHTableChunk(Handlers));
-  createSection(".rdata")->addChunk(SEHTable.get());
+  RData->addChunk(SEHTable.get());
 }
 
 // Create .idata section for the DLL-imported symbol table.
@@ -340,7 +428,7 @@ void Writer::createImportTables() {
       Sec->addChunk(C);
   }
   if (!DelayIdata.empty()) {
-    Defined *Helper = cast<Defined>(Config->DelayLoadHelper->repl());
+    Defined *Helper = cast<Defined>(Config->DelayLoadHelper);
     DelayIdata.create(Helper);
     OutputSection *Sec = createSection(".didat");
     for (Chunk *C : DelayIdata.getChunks())
@@ -383,6 +471,10 @@ size_t Writer::addEntryToStringTable(StringRef Str) {
 }
 
 Optional<coff_symbol16> Writer::createSymbol(Defined *Def) {
+  // Relative symbols are unrepresentable in a COFF symbol table.
+  if (isa<DefinedRelative>(Def))
+    return None;
+
   if (auto *D = dyn_cast<DefinedRegular>(Def))
     if (!D->getChunk()->isLive())
       return None;
@@ -409,7 +501,6 @@ Optional<coff_symbol16> Writer::createSymbol(Defined *Def) {
 
   switch (Def->kind()) {
   case SymbolBody::DefinedAbsoluteKind:
-  case SymbolBody::DefinedRelativeKind:
     Sym.Value = Def->getRVA();
     Sym.SectionNumber = IMAGE_SYM_ABSOLUTE;
     break;
@@ -445,13 +536,11 @@ void Writer::createSymbolAndStringTable() {
   for (lld::coff::ObjectFile *File : Symtab->ObjectFiles)
     for (SymbolBody *B : File->getSymbols())
       if (auto *D = dyn_cast<Defined>(B))
-        if (Optional<coff_symbol16> Sym = createSymbol(D))
-          OutputSymtab.push_back(*Sym);
-
-  for (ImportFile *File : Symtab->ImportFiles)
-    for (SymbolBody *B : File->getSymbols())
-      if (Optional<coff_symbol16> Sym = createSymbol(cast<Defined>(B)))
-        OutputSymtab.push_back(*Sym);
+        if (!D->WrittenToSymtab) {
+          D->WrittenToSymtab = true;
+          if (Optional<coff_symbol16> Sym = createSymbol(D))
+            OutputSymtab.push_back(*Sym);
+        }
 
   OutputSection *LastSection = OutputSections.back();
   // We position the symbol table to be adjacent to the end of the last section.
@@ -542,7 +631,7 @@ template <typename PEHeaderTy> void Writer::writeHeader() {
   PE->SizeOfImage = SizeOfImage;
   PE->SizeOfHeaders = SizeOfHeaders;
   if (!Config->NoEntry) {
-    Defined *Entry = cast<Defined>(Config->Entry->repl());
+    Defined *Entry = cast<Defined>(Config->Entry);
     PE->AddressOfEntryPoint = Entry->getRVA();
     // Pointer to thumb code must have the LSB set, so adjust it.
     if (Config->Machine == ARMNT)
@@ -584,33 +673,32 @@ template <typename PEHeaderTy> void Writer::writeHeader() {
     Dir[IAT].RelativeVirtualAddress = Idata.getIATRVA();
     Dir[IAT].Size = Idata.getIATSize();
   }
-  if (!DelayIdata.empty()) {
-    Dir[DELAY_IMPORT_DESCRIPTOR].RelativeVirtualAddress =
-        DelayIdata.getDirRVA();
-    Dir[DELAY_IMPORT_DESCRIPTOR].Size = DelayIdata.getDirSize();
-  }
   if (OutputSection *Sec = findSection(".rsrc")) {
     Dir[RESOURCE_TABLE].RelativeVirtualAddress = Sec->getRVA();
     Dir[RESOURCE_TABLE].Size = Sec->getVirtualSize();
   }
-  if (OutputSection *Sec = findSection(".reloc")) {
-    Dir[BASE_RELOCATION_TABLE].RelativeVirtualAddress = Sec->getRVA();
-    Dir[BASE_RELOCATION_TABLE].Size = Sec->getVirtualSize();
-  }
   if (OutputSection *Sec = findSection(".pdata")) {
     Dir[EXCEPTION_TABLE].RelativeVirtualAddress = Sec->getRVA();
     Dir[EXCEPTION_TABLE].Size = Sec->getVirtualSize();
   }
+  if (OutputSection *Sec = findSection(".reloc")) {
+    Dir[BASE_RELOCATION_TABLE].RelativeVirtualAddress = Sec->getRVA();
+    Dir[BASE_RELOCATION_TABLE].Size = Sec->getVirtualSize();
+  }
   if (Symbol *Sym = Symtab->findUnderscore("_tls_used")) {
-    if (Defined *B = dyn_cast<Defined>(Sym->Body)) {
+    if (Defined *B = dyn_cast<Defined>(Sym->body())) {
       Dir[TLS_TABLE].RelativeVirtualAddress = B->getRVA();
       Dir[TLS_TABLE].Size = Config->is64()
                                 ? sizeof(object::coff_tls_directory64)
                                 : sizeof(object::coff_tls_directory32);
     }
   }
+  if (Config->Debug) {
+    Dir[DEBUG_DIRECTORY].RelativeVirtualAddress = DebugDirectory->getRVA();
+    Dir[DEBUG_DIRECTORY].Size = DebugDirectory->getSize();
+  }
   if (Symbol *Sym = Symtab->findUnderscore("_load_config_used")) {
-    if (auto *B = dyn_cast<DefinedRegular>(Sym->Body)) {
+    if (auto *B = dyn_cast<DefinedRegular>(Sym->body())) {
       SectionChunk *SC = B->getChunk();
       assert(B->getRVA() >= SC->getRVA());
       uint64_t OffsetInChunk = B->getRVA() - SC->getRVA();
@@ -626,12 +714,19 @@ template <typename PEHeaderTy> void Writer::writeHeader() {
       Dir[LOAD_CONFIG_TABLE].Size = LoadConfigSize;
     }
   }
+  if (!DelayIdata.empty()) {
+    Dir[DELAY_IMPORT_DESCRIPTOR].RelativeVirtualAddress =
+        DelayIdata.getDirRVA();
+    Dir[DELAY_IMPORT_DESCRIPTOR].Size = DelayIdata.getDirSize();
+  }
 
   // Write section table
   for (OutputSection *Sec : OutputSections) {
     Sec->writeHeaderTo(Buf);
     Buf += sizeof(coff_section);
   }
+  SectionTable = ArrayRef<uint8_t>(
+      Buf - OutputSections.size() * sizeof(coff_section), Buf);
 
   if (OutputSymtab.empty())
     return;
@@ -660,8 +755,10 @@ void Writer::openFile(StringRef Path) {
 void Writer::fixSafeSEHSymbols() {
   if (!SEHTable)
     return;
-  Config->SEHTable->setRVA(SEHTable->getRVA());
-  Config->SEHCount->setVA(SEHTable->getSize() / 4);
+  if (auto *T = dyn_cast<DefinedRelative>(Config->SEHTable->body()))
+    T->setRVA(SEHTable->getRVA());
+  if (auto *C = dyn_cast<DefinedAbsolute>(Config->SEHCount->body()))
+    C->setVA(SEHTable->getSize() / 4);
 }
 
 // Handles /section options to allow users to overwrite
@@ -715,6 +812,30 @@ void Writer::sortExceptionTable() {
   errs() << "warning: don't know how to handle .pdata.\n";
 }
 
+// Backfill the CVSignature in a PDB70 Debug Record.  This backfilling allows us
+// to get reproducible builds.
+void Writer::writeBuildId() {
+  // There is nothing to backfill if BuildId was not setup.
+  if (BuildId == nullptr)
+    return;
+
+  MD5 Hash;
+  MD5::MD5Result Res;
+
+  Hash.update(ArrayRef<uint8_t>{Buffer->getBufferStart(),
+                                Buffer->getBufferEnd()});
+  Hash.final(Res);
+
+  assert(BuildId->DI->Signature.CVSignature == OMF::Signature::PDB70 &&
+         "only PDB 7.0 is supported");
+  assert(sizeof(Res) == sizeof(BuildId->DI->PDB70.Signature) &&
+         "signature size mismatch");
+  memcpy(BuildId->DI->PDB70.Signature, Res,
+         sizeof(codeview::PDB70DebugInfo::Signature));
+  // TODO(compnerd) track the Age
+  BuildId->DI->PDB70.Age = 1;
+}
+
 OutputSection *Writer::findSection(StringRef Name) {
   for (OutputSection *Sec : OutputSections)
     if (Sec->getName() == Name)
@@ -744,16 +865,13 @@ OutputSection *Writer::createSection(StringRef Name) {
   uint32_t Perms = StringSwitch<uint32_t>(Name)
                        .Case(".bss", BSS | R | W)
                        .Case(".data", DATA | R | W)
-                       .Case(".didat", DATA | R)
-                       .Case(".edata", DATA | R)
-                       .Case(".idata", DATA | R)
-                       .Case(".rdata", DATA | R)
+                       .Cases(".didat", ".edata", ".idata", ".rdata", DATA | R)
                        .Case(".reloc", DATA | DISCARDABLE | R)
                        .Case(".text", CODE | R | X)
                        .Default(0);
   if (!Perms)
     llvm_unreachable("unknown section name");
-  auto Sec = new (CAlloc.Allocate()) OutputSection(Name);
+  auto Sec = make<OutputSection>(Name);
   Sec->addPermissions(Perms);
   OutputSections.push_back(Sec);
   return Sec;
@@ -784,13 +902,11 @@ void Writer::addBaserelBlocks(OutputSection *Dest, std::vector<Baserel> &V) {
     uint32_t P = V[J].RVA & Mask;
     if (P == Page)
       continue;
-    BaserelChunk *Buf = BAlloc.Allocate();
-    Dest->addChunk(new (Buf) BaserelChunk(Page, &V[I], &V[0] + J));
+    Dest->addChunk(make<BaserelChunk>(Page, &V[I], &V[0] + J));
     I = J;
     Page = P;
   }
   if (I == J)
     return;
-  BaserelChunk *Buf = BAlloc.Allocate();
-  Dest->addChunk(new (Buf) BaserelChunk(Page, &V[I], &V[0] + J));
+  Dest->addChunk(make<BaserelChunk>(Page, &V[I], &V[0] + J));
 }
diff --git a/COFF/Writer.h b/COFF/Writer.h
index 0473315ae50a..0d26090177d8 100644
--- a/COFF/Writer.h
+++ b/COFF/Writer.h
@@ -14,9 +14,7 @@
 
 namespace lld {
 namespace coff {
-
-class Chunk;
-class OutputSection;
+class SymbolTable;
 
 void writeResult(SymbolTable *T);