diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -140,6 +140,7 @@
   bool asNeeded = false;
   bool bsymbolic;
   bool bsymbolicFunctions;
+  bool cacheIRSymTabAheadOfTime;
   bool callGraphProfileSort;
   bool checkSections;
   bool compressDebugSections;
diff --git a/lld/ELF/Driver.h b/lld/ELF/Driver.h
--- a/lld/ELF/Driver.h
+++ b/lld/ELF/Driver.h
@@ -14,13 +14,25 @@
 #include "lld/Common/LLVM.h"
 #include "lld/Common/Reproduce.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/raw_ostream.h"
 
+namespace llvm {
+namespace lto {
+class IRSymtabFileCache;
+}
+} // namespace llvm
+
 namespace lld {
+extern llvm::lto::IRSymtabFileCache *irSymTabCache;
+
 namespace elf {
+void addFileToIrSymTabCache(StringRef path,
+                            llvm::StringMap<MemoryBufferRef> &LoadedFiles,
+                            std::vector<MemoryBufferRef> &ToCache);
 
 extern class LinkerDriver *driver;
 
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -47,6 +47,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compression.h"
@@ -69,6 +70,8 @@
 using namespace lld;
 using namespace lld::elf;
 
+llvm::lto::IRSymtabFileCache *lld::irSymTabCache;
+
 Configuration *elf::config;
 LinkerDriver *elf::driver;
 
@@ -193,9 +196,14 @@
 void LinkerDriver::addFile(StringRef path, bool withLOption) {
   using namespace sys::fs;
 
-  Optional<MemoryBufferRef> buffer = readFile(path);
+  Optional<MemoryBufferRef> buffer;
+  if (irSymTabCache)
+    buffer = irSymTabCache->getMemBufferForPath(path);
+  if (!buffer.hasValue())
+    buffer = readFile(path);
   if (!buffer.hasValue())
     return;
+
   MemoryBufferRef mbref = *buffer;
 
   if (config->formatBinary) {
@@ -495,6 +503,9 @@
   if (config->timeTraceEnabled)
     timeTraceProfilerInitialize(config->timeTraceGranularity, config->progName);
 
+  if (config->cacheIRSymTabAheadOfTime)
+    irSymTabCache = new lto::IRSymtabFileCache();
+
   {
     llvm::TimeTraceScope timeScope("ExecuteLinker");
 
@@ -890,6 +901,8 @@
       args.hasFlag(OPT_eh_frame_hdr, OPT_no_eh_frame_hdr, false);
   config->emitLLVM = args.hasArg(OPT_plugin_opt_emit_llvm, false);
   config->emitRelocs = args.hasArg(OPT_emit_relocs);
+  config->cacheIRSymTabAheadOfTime =
+      args.hasFlag(OPT_cache_ir_symtab, OPT_no_cache_ir_symtab, false);
   config->callGraphProfileSort = args.hasFlag(
       OPT_call_graph_profile_sort, OPT_no_call_graph_profile_sort, true);
   config->enableNewDtags =
@@ -1243,10 +1256,54 @@
   return false;
 }
 
+void lld::elf::addFileToIrSymTabCache(
+    StringRef path, llvm::StringMap<MemoryBufferRef> &LoadedFiles,
+    std::vector<MemoryBufferRef> &ToCache) {
+  using namespace sys::fs;
+
+  Optional<MemoryBufferRef> buffer = readFile(path);
+  if (!buffer.hasValue())
+    return;
+  MemoryBufferRef mbref = *buffer;
+  LoadedFiles.try_emplace(path, mbref);
+
+  switch (identify_magic(mbref.getBuffer())) {
+  case file_magic::archive: {
+    for (const std::pair<MemoryBufferRef, uint64_t> &p :
+         getArchiveMembers(mbref))
+      if (identify_magic(p.first.getBuffer()) == file_magic::bitcode)
+        ToCache.push_back(p.first);
+    break;
+  }
+
+  case file_magic::bitcode:
+    ToCache.push_back(mbref);
+    break;
+  default:
+    break;
+  }
+}
+
 void LinkerDriver::createFiles(opt::InputArgList &args) {
   // For --{push,pop}-state.
   std::vector<std::tuple<bool, bool, bool>> stack;
 
+  if (irSymTabCache) {
+    std::vector<std::string> Files;
+    for (auto *arg : args) {
+      switch (arg->getOption().getID()) {
+      case OPT_library:
+        if (Optional<std::string> path = searchLibrary(arg->getValue()))
+          Files.push_back(*path);
+        break;
+      case OPT_INPUT:
+        Files.push_back(arg->getValue());
+        break;
+      }
+    }
+    irSymTabCache->upgrade(Files, addFileToIrSymTabCache);
+  }
+
   // Iterate over argv to process input files and positional arguments.
   for (auto *arg : args) {
     switch (arg->getOption().getID()) {
@@ -1850,6 +1907,11 @@
       parseFile(files[i]);
   }
 
+  if (irSymTabCache) {
+    delete irSymTabCache;
+    irSymTabCache = nullptr;
+  }
+
   // Now that we have every file, we can decide if we will need a
   // dynamic symbol table.
   // We need one if we were asked to export dynamic symbols or if we are
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -486,6 +486,16 @@
   this->sections.resize(sections.size());
 }
 
+static Optional<std::pair<std::string, bool>> getPath(StringRef specifier) {
+  if (fs::exists(specifier))
+    return std::make_pair(specifier.str(), false);
+  else if (Optional<std::string> s = findFromSearchPaths(specifier))
+    return std::make_pair(*s, true);
+  else if (Optional<std::string> s = searchLibraryBaseName(specifier))
+    return std::make_pair(*s, true);
+  return None;
+}
+
 // An ELF object file may contain a `.deplibs` section. If it exists, the
 // section contains a list of library specifiers such as `m` for libm. This
 // function resolves a given name by finding the first matching library checking
@@ -495,12 +505,8 @@
 static void addDependentLibrary(StringRef specifier, const InputFile *f) {
   if (!config->dependentLibraries)
     return;
-  if (fs::exists(specifier))
-    driver->addFile(specifier, /*withLOption=*/false);
-  else if (Optional<std::string> s = findFromSearchPaths(specifier))
-    driver->addFile(*s, /*withLOption=*/true);
-  else if (Optional<std::string> s = searchLibraryBaseName(specifier))
-    driver->addFile(*s, /*withLOption=*/true);
+  if (Optional<std::pair<std::string, bool>> path = getPath(specifier))
+    driver->addFile(path->first, /*withLOption=*/path->second);
   else
     error(toString(f) +
           ": unable to find library from dependent library specifier: " +
@@ -1443,6 +1449,11 @@
   if (config->thinLTOIndexOnly)
     path = replaceThinLTOSuffix(mb.getBufferIdentifier());
 
+  if (irSymTabCache)
+    obj = CHECK(lto::InputFile::create(mb, *irSymTabCache), this);
+  else
+    obj = CHECK(lto::InputFile::create(mb), this);
+
   // ThinLTO assumes that all MemoryBufferRefs given to it have a unique
   // name. If two archives define two members with the same name, this
   // causes a collision which result in only one of the objects being taken
@@ -1454,9 +1465,7 @@
           ? saver.save(path)
           : saver.save(archiveName + "(" + path::filename(path) + " at " +
                        utostr(offsetInArchive) + ")");
-  MemoryBufferRef mbref(mb.getBuffer(), name);
-
-  obj = CHECK(lto::InputFile::create(mbref), this);
+  obj->setName(name);
 
   Triple t(obj->getTargetTriple());
   ekind = getBitcodeELFKind(t);
@@ -1515,6 +1524,14 @@
   for (const lto::InputFile::Symbol &objSym : obj->symbols())
     symbols.push_back(createBitcodeSymbol<ELFT>(keptComdats, objSym, *this));
 
+  if (config->dependentLibraries && irSymTabCache) {
+    std::vector<std::string> filesToUpgrade;
+    for (auto l : obj->getDependentLibraries())
+      if (Optional<std::pair<std::string, bool>> path = getPath(l))
+        filesToUpgrade.push_back(path->first);
+    irSymTabCache->upgrade(filesToUpgrade, addFileToIrSymTabCache);
+  }
+
   for (auto l : obj->getDependentLibraries())
     addDependentLibrary(l, this);
 }
diff --git a/lld/ELF/Options.td b/lld/ELF/Options.td
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -103,6 +103,10 @@
     "Only set DT_NEEDED for shared libraries if used",
     "Always set DT_NEEDED for shared libraries (default)">;
 
+defm cache_ir_symtab: BB<"cache-ir-symtab",
+    "Cache IR symbol table before parsing input files",
+    "Do not cache IR symbol table before parsing input files (default)">;
+
 defm call_graph_ordering_file:
   Eq<"call-graph-ordering-file", "Layout sections to optimize the given callgraph">;
 
diff --git a/lld/test/ELF/deplibs.s b/lld/test/ELF/deplibs.s
--- a/lld/test/ELF/deplibs.s
+++ b/lld/test/ELF/deplibs.s
@@ -31,6 +31,9 @@
 # RUN: ld.lld %t.o libcmdline.a -o /dev/null -L %t.dir --trace 2>&1 | \
 # RUN:   FileCheck %s -DOBJ=%t.o -DSO=%t.dir --check-prefix CMDLINE \
 # RUN:                --implicit-check-not=foo.a --implicit-check-not=libbar.so
+# RUN: ld.lld %t.o libcmdline.a -o /dev/null -L %t.dir --trace --cache-ir-symtab 2>&1 | \
+# RUN:   FileCheck %s -DOBJ=%t.o -DSO=%t.dir --check-prefix CMDLINE \
+# RUN:                --implicit-check-not=foo.a --implicit-check-not=libbar.so
 # CMDLINE:      [[OBJ]]
 # CMDLINE-NEXT: {{^libcmdline\.a}}
 
diff --git a/llvm/include/llvm/Bitcode/BitcodeReader.h b/llvm/include/llvm/Bitcode/BitcodeReader.h
--- a/llvm/include/llvm/Bitcode/BitcodeReader.h
+++ b/llvm/include/llvm/Bitcode/BitcodeReader.h
@@ -93,6 +93,7 @@
     StringRef getStrtab() const { return Strtab; }
 
     StringRef getModuleIdentifier() const { return ModuleIdentifier; }
+    void setModuleIdentifier(StringRef ModId) { ModuleIdentifier = ModId; }
 
     /// Read the bitcode module and prepare for lazy deserialization of function
     /// bodies. If ShouldLazyLoadMetadata is true, lazily load metadata as well.
diff --git a/llvm/include/llvm/LTO/LTO.h b/llvm/include/llvm/LTO/LTO.h
--- a/llvm/include/llvm/LTO/LTO.h
+++ b/llvm/include/llvm/LTO/LTO.h
@@ -17,10 +17,13 @@
 
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/IR/ModuleSummaryIndex.h"
 #include "llvm/LTO/Config.h"
+#include "llvm/Object/IRObjectFile.h"
 #include "llvm/Object/IRSymtab.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/thread.h"
 #include "llvm/Transforms/IPO/FunctionImport.h"
 
@@ -95,6 +98,25 @@
 struct SymbolResolution;
 class ThinBackendProc;
 
+// For parallelizing IR symtab upgrading.
+class IRSymtabFileCache {
+  ThreadPool BackendThreadPool;
+  using CacheType =
+      DenseMap<MemoryBufferRef, std::unique_ptr<object::IRSymtabFile>>;
+  CacheType Cache;
+  StringMap<MemoryBufferRef> LoadedFiles;
+
+  using AddFileFn = std::function<void(StringRef path,
+                                       StringMap<MemoryBufferRef> &LoadedFiles,
+                                       std::vector<MemoryBufferRef> &ToLoad)>;
+
+public:
+  IRSymtabFileCache() : BackendThreadPool(), Cache(), LoadedFiles() {}
+  void upgrade(const std::vector<std::string> &Files, AddFileFn AFF);
+  object::IRSymtabFile *get(const MemoryBufferRef &M);
+  Optional<MemoryBufferRef> getMemBufferForPath(StringRef Path);
+};
+
 /// An input file. This is a symbol table wrapper that only exposes the
 /// information that an LTO client should need in order to do symbol resolution.
 class InputFile {
@@ -105,6 +127,7 @@
   // FIXME: Remove LTO class friendship once we have bitcode symbol tables.
   friend LTO;
   InputFile() = default;
+  InputFile(object::IRSymtabFile &SF);
 
   std::vector<BitcodeModule> Mods;
   SmallVector<char, 0> Strtab;
@@ -122,6 +145,8 @@
 
   /// Create an InputFile.
   static Expected<std::unique_ptr<InputFile>> create(MemoryBufferRef Object);
+  static Expected<std::unique_ptr<InputFile>>
+  create(MemoryBufferRef Object, IRSymtabFileCache &IRSymTabCache);
 
   /// The purpose of this class is to only expose the symbol information that an
   /// LTO client should need in order to do symbol resolution.
@@ -161,6 +186,9 @@
   /// Returns the path to the InputFile.
   StringRef getName() const;
 
+  /// Set the path to the InputFile.
+  void setName(StringRef Name);
+
   /// Returns the input file's target triple.
   StringRef getTargetTriple() const { return TargetTriple; }
 
diff --git a/llvm/include/llvm/Object/IRObjectFile.h b/llvm/include/llvm/Object/IRObjectFile.h
--- a/llvm/include/llvm/Object/IRObjectFile.h
+++ b/llvm/include/llvm/Object/IRObjectFile.h
@@ -83,7 +83,7 @@
 
 /// Reads a bitcode file, creating its irsymtab if necessary.
 Expected<IRSymtabFile> readIRSymtab(MemoryBufferRef MBRef);
-
+bool needToUpgradeIRSymtab(MemoryBufferRef MBRef);
 }
 
 }
diff --git a/llvm/include/llvm/Object/IRSymtab.h b/llvm/include/llvm/Object/IRSymtab.h
--- a/llvm/include/llvm/Object/IRSymtab.h
+++ b/llvm/include/llvm/Object/IRSymtab.h
@@ -367,6 +367,7 @@
 
 /// Reads the contents of a bitcode file, creating its irsymtab if necessary.
 Expected<FileContents> readBitcode(const BitcodeFileContents &BFC);
+bool needUpgrade(const BitcodeFileContents &BFC);
 
 } // end namespace irsymtab
 } // end namespace llvm
diff --git a/llvm/include/llvm/Support/MemoryBuffer.h b/llvm/include/llvm/Support/MemoryBuffer.h
--- a/llvm/include/llvm/Support/MemoryBuffer.h
+++ b/llvm/include/llvm/Support/MemoryBuffer.h
@@ -15,6 +15,7 @@
 
 #include "llvm-c/Types.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/CBindingWrapping.h"
@@ -283,6 +284,28 @@
 // Create wrappers for C Binding types (see CBindingWrapping.h).
 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(MemoryBuffer, LLVMMemoryBufferRef)
 
+// Support MemoryBufferRef as DenseMap keys.
+template <> struct DenseMapInfo<MemoryBufferRef> {
+  static inline MemoryBufferRef getEmptyKey() {
+    static MemoryBufferRef EmptyKey(DenseMapInfo<StringRef>::getEmptyKey(),
+                                    DenseMapInfo<StringRef>::getEmptyKey());
+    return EmptyKey;
+  }
+  static inline MemoryBufferRef getTombstoneKey() {
+    static MemoryBufferRef TombstoneKey(
+        DenseMapInfo<StringRef>::getTombstoneKey(),
+        DenseMapInfo<StringRef>::getTombstoneKey());
+    return TombstoneKey;
+  }
+  static unsigned getHashValue(const MemoryBufferRef &mbref) {
+    return hash_combine(mbref.getBuffer().data(), mbref.getBuffer().size());
+  }
+  static bool isEqual(const MemoryBufferRef &LHS, const MemoryBufferRef &RHS) {
+    return LHS.getBuffer().data() == RHS.getBuffer().data() &&
+           LHS.getBuffer().size() == RHS.getBuffer().size();
+  }
+};
+
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_MEMORYBUFFER_H
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -32,13 +32,13 @@
 #include "llvm/Object/IRObjectFile.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/SHA1.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/TargetRegistry.h"
-#include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/TimeProfiler.h"
 #include "llvm/Support/VCSRevision.h"
@@ -420,39 +420,50 @@
 // Requires a destructor for std::vector<InputModule>.
 InputFile::~InputFile() = default;
 
-Expected<std::unique_ptr<InputFile>> InputFile::create(MemoryBufferRef Object) {
-  std::unique_ptr<InputFile> File(new InputFile);
-
-  Expected<IRSymtabFile> FOrErr = readIRSymtab(Object);
-  if (!FOrErr)
-    return FOrErr.takeError();
+InputFile::InputFile(IRSymtabFile &SF) {
+  TargetTriple = SF.TheReader.getTargetTriple();
+  SourceFileName = SF.TheReader.getSourceFileName();
+  COFFLinkerOpts = SF.TheReader.getCOFFLinkerOpts();
+  DependentLibraries = SF.TheReader.getDependentLibraries();
+  ComdatTable = SF.TheReader.getComdatTable();
 
-  File->TargetTriple = FOrErr->TheReader.getTargetTriple();
-  File->SourceFileName = FOrErr->TheReader.getSourceFileName();
-  File->COFFLinkerOpts = FOrErr->TheReader.getCOFFLinkerOpts();
-  File->DependentLibraries = FOrErr->TheReader.getDependentLibraries();
-  File->ComdatTable = FOrErr->TheReader.getComdatTable();
-
-  for (unsigned I = 0; I != FOrErr->Mods.size(); ++I) {
-    size_t Begin = File->Symbols.size();
+  for (unsigned I = 0; I != SF.Mods.size(); ++I) {
+    size_t Begin = Symbols.size();
     for (const irsymtab::Reader::SymbolRef &Sym :
-         FOrErr->TheReader.module_symbols(I))
+         SF.TheReader.module_symbols(I))
       // Skip symbols that are irrelevant to LTO. Note that this condition needs
       // to match the one in Skip() in LTO::addRegularLTO().
       if (Sym.isGlobal() && !Sym.isFormatSpecific())
-        File->Symbols.push_back(Sym);
-    File->ModuleSymIndices.push_back({Begin, File->Symbols.size()});
+        Symbols.push_back(Sym);
+    ModuleSymIndices.push_back({Begin, Symbols.size()});
   }
 
-  File->Mods = FOrErr->Mods;
-  File->Strtab = std::move(FOrErr->Strtab);
-  return std::move(File);
+  Mods = SF.Mods;
+  Strtab = std::move(SF.Strtab);
+}
+
+Expected<std::unique_ptr<InputFile>> InputFile::create(MemoryBufferRef Object) {
+  Expected<IRSymtabFile> FOrErr = readIRSymtab(Object);
+  if (!FOrErr)
+    return FOrErr.takeError();
+  return std::unique_ptr<InputFile>(new InputFile(*FOrErr));
+}
+
+Expected<std::unique_ptr<InputFile>>
+InputFile::create(MemoryBufferRef Object, IRSymtabFileCache &IRSymTabCache) {
+  if (IRSymtabFile *FOrErr = IRSymTabCache.get(Object))
+    return std::unique_ptr<InputFile>(new InputFile(*FOrErr));
+  return InputFile::create(Object);
 }
 
 StringRef InputFile::getName() const {
   return Mods[0].getModuleIdentifier();
 }
 
+void InputFile::setName(StringRef Name) {
+  return Mods[0].setModuleIdentifier(Name);
+}
+
 BitcodeModule &InputFile::getSingleBitcodeModule() {
   assert(Mods.size() == 1 && "Expect only one bitcode module");
   return Mods[0];
@@ -1409,6 +1420,77 @@
   return BackendProc->wait();
 }
 
+IRSymtabFile *IRSymtabFileCache::get(const MemoryBufferRef &M) {
+  auto Iter = Cache.find(M);
+  if (Iter != Cache.end())
+    return Iter->second.get();
+  return nullptr;
+}
+
+void IRSymtabFileCache::upgrade(const std::vector<std::string> &Files,
+                                AddFileFn AFF) {
+  std::vector<MemoryBufferRef> MBrefs;
+  for (auto &F : Files)
+    AFF(F, LoadedFiles, MBrefs);
+
+  // Cache IR SymTab only if it needs to be upgraded. Also try to balance the
+  // amount work each thread will do based on aggregate MemoryBufferRef size.
+  std::vector<MemoryBufferRef> ToCache;
+  unsigned TotalBufSize = 0;
+  for (auto &M : MBrefs)
+    if (Cache.count(M) == 0 && needToUpgradeIRSymtab(M)) {
+      ToCache.push_back(M);
+      TotalBufSize += M.getBufferSize();
+      Cache.try_emplace(M, nullptr);
+    }
+
+  if (ToCache.size() == 0)
+    return;
+
+  // If too few symtabs to cache, assign one to each thread.
+  const unsigned ThinLTOParallelismLevel = BackendThreadPool.getThreadCount();
+  const unsigned ThreadLocal = ToCache.size() >= ThinLTOParallelismLevel
+                                   ? TotalBufSize / ThinLTOParallelismLevel
+                                   : 0;
+
+  auto PreLoad = [&](std::vector<MemoryBufferRef>::iterator II,
+                     std::vector<MemoryBufferRef>::iterator IE) {
+    for (; II != IE; ++II) {
+      Expected<IRSymtabFile> FOrErr = readIRSymtab(*II);
+      if (FOrErr)
+        Cache.find(*II)->second.reset(new IRSymtabFile(std::move(*FOrErr)));
+      else
+        consumeError(FOrErr.takeError());
+    }
+  };
+
+  auto CI = ToCache.begin();
+  unsigned CurTotal = 0;
+  unsigned NumThread = 0;
+  for (auto I = ToCache.begin(), E = ToCache.end(); I != E; ++I) {
+    CurTotal += I->getBufferSize();
+    if (CurTotal < ThreadLocal)
+      continue;
+
+    CurTotal = 0;
+    BackendThreadPool.async(PreLoad, CI, I + 1);
+    CI = I + 1;
+    if (++NumThread == (ThinLTOParallelismLevel - 1)) {
+      BackendThreadPool.async(PreLoad, CI, E);
+      break;
+    }
+  }
+  BackendThreadPool.wait();
+}
+
+Optional<MemoryBufferRef>
+IRSymtabFileCache::getMemBufferForPath(StringRef Path) {
+  auto I = LoadedFiles.find(Path);
+  if (I != LoadedFiles.end())
+    return I->second;
+  return None;
+}
+
 Expected<std::unique_ptr<ToolOutputFile>> lto::setupLLVMOptimizationRemarks(
     LLVMContext &Context, StringRef RemarksFilename, StringRef RemarksPasses,
     StringRef RemarksFormat, bool RemarksWithHotness, int Count) {
diff --git a/llvm/lib/Object/IRObjectFile.cpp b/llvm/lib/Object/IRObjectFile.cpp
--- a/llvm/lib/Object/IRObjectFile.cpp
+++ b/llvm/lib/Object/IRObjectFile.cpp
@@ -133,6 +133,18 @@
       new IRObjectFile(*BCOrErr, std::move(Mods)));
 }
 
+bool object::needToUpgradeIRSymtab(MemoryBufferRef MBRef) {
+  Expected<MemoryBufferRef> BCOrErr =
+      IRObjectFile::findBitcodeInMemBuffer(MBRef);
+  if (!BCOrErr)
+    return false;
+
+  Expected<BitcodeFileContents> BFCOrErr = getBitcodeFileContents(*BCOrErr);
+  if (!BFCOrErr)
+    return false;
+  return irsymtab::needUpgrade(*BFCOrErr);
+}
+
 Expected<IRSymtabFile> object::readIRSymtab(MemoryBufferRef MBRef) {
   IRSymtabFile F;
   Expected<MemoryBufferRef> BCOrErr =
diff --git a/llvm/lib/Object/IRSymtab.cpp b/llvm/lib/Object/IRSymtab.cpp
--- a/llvm/lib/Object/IRSymtab.cpp
+++ b/llvm/lib/Object/IRSymtab.cpp
@@ -309,7 +309,10 @@
 
   assert(!IRMods.empty());
   Hdr.Version = storage::Header::kCurrentVersion;
-  setStr(Hdr.Producer, kExpectedProducerName);
+  if (getenv("LLVM_REEVAL_IR_SYMTAB_PRODUCER"))
+    setStr(Hdr.Producer, getExpectedProducerName());
+  else
+    setStr(Hdr.Producer, kExpectedProducerName);
   setStr(Hdr.TargetTriple, IRMods[0]->getTargetTriple());
   setStr(Hdr.SourceFileName, IRMods[0]->getSourceFileName());
   TT = Triple(IRMods[0]->getTargetTriple());
@@ -374,14 +377,13 @@
   return std::move(FC);
 }
 
-Expected<FileContents> irsymtab::readBitcode(const BitcodeFileContents &BFC) {
+bool irsymtab::needUpgrade(const BitcodeFileContents &BFC) {
   if (BFC.Mods.empty())
-    return make_error<StringError>("Bitcode file does not contain any modules",
-                                   inconvertibleErrorCode());
+    return false;
 
   if (BFC.StrtabForSymtab.empty() ||
       BFC.Symtab.size() < sizeof(storage::Header))
-    return upgrade(BFC.Mods);
+    return true;
 
   // We cannot use the regular reader to read the version and producer, because
   // it will expect the header to be in the current format. The only thing we
@@ -392,7 +394,7 @@
   StringRef Producer = Hdr->Producer.get(BFC.StrtabForSymtab);
   if (Version != storage::Header::kCurrentVersion ||
       Producer != kExpectedProducerName)
-    return upgrade(BFC.Mods);
+    return true;
 
   FileContents FC;
   FC.TheReader = {{BFC.Symtab.data(), BFC.Symtab.size()},
@@ -403,7 +405,20 @@
   // the bitcode file was created by binary concatenation, so we need to create
   // a new symbol table from scratch.
   if (FC.TheReader.getNumModules() != BFC.Mods.size())
-    return upgrade(std::move(BFC.Mods));
+    return true;
+
+  return false;
+}
+
+Expected<FileContents> irsymtab::readBitcode(const BitcodeFileContents &BFC) {
+  if (BFC.Mods.empty())
+    return make_error<StringError>("Bitcode file does not contain any modules",
+                                   inconvertibleErrorCode());
+  if (needUpgrade(BFC))
+    return upgrade(BFC.Mods);
 
+  FileContents FC;
+  FC.TheReader = {{BFC.Symtab.data(), BFC.Symtab.size()},
+                  {BFC.StrtabForSymtab.data(), BFC.StrtabForSymtab.size()}};
   return std::move(FC);
 }
diff --git a/llvm/unittests/CMakeLists.txt b/llvm/unittests/CMakeLists.txt
--- a/llvm/unittests/CMakeLists.txt
+++ b/llvm/unittests/CMakeLists.txt
@@ -41,6 +41,7 @@
 add_subdirectory(TableGen)
 add_subdirectory(Target)
 add_subdirectory(TextAPI)
+add_subdirectory(ThinLTO)
 add_subdirectory(Transforms)
 add_subdirectory(XRay)
 add_subdirectory(tools)
diff --git a/llvm/unittests/ThinLTO/CMakeLists.txt b/llvm/unittests/ThinLTO/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/llvm/unittests/ThinLTO/CMakeLists.txt
@@ -0,0 +1,11 @@
+set(LLVM_LINK_COMPONENTS
+  AsmParser
+  BinaryFormat
+  Core
+  LTO
+  Support
+  )
+
+add_llvm_unittest(ThinLTOTests
+  PreUpgradeIRSymbolTable.cpp
+  )
diff --git a/llvm/unittests/ThinLTO/PreUpgradeIRSymbolTable.cpp b/llvm/unittests/ThinLTO/PreUpgradeIRSymbolTable.cpp
new file mode 100644
--- /dev/null
+++ b/llvm/unittests/ThinLTO/PreUpgradeIRSymbolTable.cpp
@@ -0,0 +1,172 @@
+//===- llvm/unittest/ThinLTO/PreUpgradeIRSymbolTable.cpp ------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/ModuleSummaryAnalysis.h"
+#include "llvm/Analysis/ProfileSummaryInfo.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/BinaryFormat/Magic.h"
+#include "llvm/Bitcode/BitcodeWriter.h"
+#include "llvm/Config/config.h"
+#include "llvm/IR/Module.h"
+#include "llvm/LTO/LTO.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/ToolOutputFile.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+static ::testing::AssertionResult NoError(std::error_code EC) {
+  if (!EC)
+    return ::testing::AssertionSuccess();
+  return ::testing::AssertionFailure()
+         << "error " << EC.value() << ": " << EC.message();
+}
+
+std::vector<std::unique_ptr<MemoryBuffer>> AllBuffers;
+
+static void
+addFileToIrSymTabCache(StringRef path,
+                       llvm::StringMap<MemoryBufferRef> &LoadedFiles,
+                       std::vector<MemoryBufferRef> &ToCache) {
+  auto mbOrErr = MemoryBuffer::getFile(path, -1, false);
+  if (!mbOrErr)
+    return;
+  std::unique_ptr<MemoryBuffer> &mb = *mbOrErr;
+  MemoryBufferRef mbref = mb->getMemBufferRef();
+  AllBuffers.push_back(std::move(*mbOrErr));
+
+  LoadedFiles.try_emplace(path, mbref);
+  ToCache.push_back(mbref);
+}
+
+namespace {
+#ifdef _MSC_VER
+#define setenv(name, var, ignore) _putenv_s(name, var)
+#endif
+
+#if HAVE_SETENV || _MSC_VER
+struct PreUpgradeIRSymbolTableTest : public ::testing::Test {
+  LLVMContext Context;
+  SMDiagnostic Err;
+  std::unique_ptr<Module> ValidModule;
+  std::unique_ptr<Module> InvalidModule;
+
+  PreUpgradeIRSymbolTableTest()
+      : ValidModule(parseAssemblyString(
+            "target datalayout = \"e-m:e-i64:64-f80:128-n8:16:32:64-S128\"\n"
+            "define void @f() {\n"
+            "  ret void\n"
+            "}\n",
+            Err, Context)),
+        InvalidModule(parseAssemblyString("define void @g() {\n"
+                                          "  ret void\n"
+                                          "}\n",
+                                          Err, Context)) {}
+
+  enum class InputKind {
+    ValidAndUpgradable,
+    ValidAndNonUpgradable,
+    Invalid,
+  };
+
+  void createInputFiles(std::vector<std::string> &InputFiles,
+                        const std::vector<InputKind> Inputs) {
+    for (auto Kind : Inputs) {
+      SmallString<64> TmpName;
+      sys::fs::createUniquePath("thinlto-preload-ir-symtab-%%%.o", TmpName,
+                                /*MakeAbsolute=*/false);
+
+      std::error_code EC;
+      std::unique_ptr<llvm::ToolOutputFile> Out =
+          std::make_unique<llvm::ToolOutputFile>(TmpName, EC,
+                                                 llvm::sys::fs::OF_None);
+      ASSERT_TRUE(NoError(EC));
+      Module &M = Kind == InputKind::Invalid ? *InvalidModule : *ValidModule;
+      ProfileSummaryInfo PSI(M);
+      auto Index = buildModuleSummaryIndex(M, nullptr, &PSI);
+      if (Kind == InputKind::ValidAndUpgradable) {
+        setenv("LLVM_OVERRIDE_PRODUCER", "old-producer", 1);
+        setenv("LLVM_REEVAL_IR_SYMTAB_PRODUCER", "1", 1);
+      }
+      WriteBitcodeToFile(M, Out->os(), true, &Index);
+      if (Kind == InputKind::ValidAndUpgradable) {
+        unsetenv("LLVM_REEVAL_IR_SYMTAB_PRODUCER");
+        unsetenv("LLVM_OVERRIDE_PRODUCER");
+      }
+      Out->keep();
+      InputFiles.push_back(TmpName.c_str());
+    }
+  }
+
+  void testPreUpgradeForInputFiles(const std::vector<InputKind> Inputs) {
+    std::vector<std::string> InputFiles;
+    createInputFiles(InputFiles, Inputs);
+
+    // Upgrade the symbol tables of input files.
+    AllBuffers.clear();
+    std::unique_ptr<lto::IRSymtabFileCache> irSymTabCache =
+        std::make_unique<lto::IRSymtabFileCache>();
+    irSymTabCache->upgrade(InputFiles, addFileToIrSymTabCache);
+
+    // * If InputKind == ValidAndUpgradable, verify the input files are cached
+    //   and their symbol tables are upgraded.
+    // * If InputKind != ValidAndUpgradable, verify the input files are cached
+    //   and their symbol tables are not upgraded.
+    for (unsigned i = 0; i < Inputs.size(); ++i) {
+      // Verify the input is cached.
+      Optional<MemoryBufferRef> Object =
+          irSymTabCache->getMemBufferForPath(InputFiles[i]);
+      ASSERT_TRUE(Object.hasValue());
+
+      // Verify the input symbol table is upgraded.
+      object::IRSymtabFile *SymtabFile = irSymTabCache->get(*Object);
+      if (Inputs[i] == InputKind::ValidAndUpgradable)
+        ASSERT_TRUE(SymtabFile);
+      else
+        ASSERT_FALSE(SymtabFile);
+      (void)SymtabFile;
+    }
+
+    // Clean up.
+    for (auto &F : InputFiles) {
+      std::error_code EC = sys::fs::remove(F);
+      ASSERT_TRUE(NoError(EC));
+    }
+  }
+};
+
+TEST_F(PreUpgradeIRSymbolTableTest, AllUpgrade) {
+  const std::vector<InputKind> Inputs(10, InputKind::ValidAndUpgradable);
+  testPreUpgradeForInputFiles(Inputs);
+}
+
+TEST_F(PreUpgradeIRSymbolTableTest, AllNonUpgrade) {
+  const std::vector<InputKind> Inputs(10, InputKind::ValidAndNonUpgradable);
+  testPreUpgradeForInputFiles(Inputs);
+}
+
+TEST_F(PreUpgradeIRSymbolTableTest, AllInvalid) {
+  const std::vector<InputKind> Inputs(10, InputKind::Invalid);
+  testPreUpgradeForInputFiles(Inputs);
+}
+
+TEST_F(PreUpgradeIRSymbolTableTest, MixedInputs) {
+  const std::vector<InputKind> Inputs1(5, InputKind::ValidAndUpgradable);
+  const std::vector<InputKind> Inputs2(5, InputKind::ValidAndNonUpgradable);
+  const std::vector<InputKind> Inputs3(5, InputKind::Invalid);
+  std::vector<InputKind> MixedInputs;
+  MixedInputs.insert(MixedInputs.end(), Inputs1.begin(), Inputs1.end());
+  MixedInputs.insert(MixedInputs.end(), Inputs2.begin(), Inputs2.end());
+  MixedInputs.insert(MixedInputs.end(), Inputs3.begin(), Inputs3.end());
+  testPreUpgradeForInputFiles(MixedInputs);
+}
+
+#endif
+
+} // End anonymous namespace.