Index: clang/include/clang/IndexSerialization/PathIndexer.h
===================================================================
--- /dev/null
+++ clang/include/clang/IndexSerialization/PathIndexer.h
@@ -0,0 +1,89 @@
+//===--- PathIndexer.h -- Index of paths ------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_INDEX_PATHINDEXER_H
+#define LLVM_CLANG_INDEX_PATHINDEXER_H
+
+#include "clang/Basic/FileManager.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringRef.h"
+
+#include <string>
+#include <vector>
+
+/// Represents a special root directory of a filesystem path.
+enum class RootDirKind {
+  Regular = 0,
+  CurrentWorkDir = 1,
+  SysRoot = 2,
+};
+
+/// Represents a single filesystem path backed by a buffer.
+struct BitPathComponent {
+  const size_t Offset;
+  const size_t Size;
+
+  BitPathComponent(size_t Offset, size_t Size) : Offset(Offset), Size(Size) {}
+};
+
+/// Represents a directory path backed by a buffer.
+struct DirBitPath {
+  /// Is this directory in any special root directory?
+  const RootDirKind PrefixKind;
+  /// Relative path to root represented by PrefixKind.
+  const BitPathComponent Dir;
+
+  DirBitPath(RootDirKind Kind, const BitPathComponent &Dir)
+      : PrefixKind(Kind), Dir(Dir) {}
+};
+
+/// Represents a file path backed by a buffer.
+struct FileBitPath : DirBitPath {
+  const BitPathComponent Filename;
+
+  FileBitPath(RootDirKind Kind, const BitPathComponent &Dir,
+              const BitPathComponent &Filename)
+      : DirBitPath(Kind, Dir), Filename(Filename) {}
+};
+
+/// Builds index of unique filesystem paths.
+class PathIndexer {
+  std::string WorkDir;
+  std::string SysrootPath;
+  llvm::SmallString<512> PathsBuf;
+  llvm::StringMap<DirBitPath, llvm::BumpPtrAllocator> Dirs;
+  std::vector<FileBitPath> FileBitPaths;
+  llvm::DenseMap<const clang::FileEntry *, size_t> FileToIndex;
+
+public:
+  /// \p CWD is current working directory.
+  PathIndexer(llvm::StringRef CWD, llvm::StringRef sysrootPath);
+
+  /// \returns buffer with stored paths.
+  llvm::StringRef getPathsBuffer() const { return PathsBuf.str(); }
+
+  /// \returns array of offsets and sizes of stored paths.
+  llvm::ArrayRef<FileBitPath> getBitPaths() const { return FileBitPaths; }
+
+  /// Stores path to \p FE if it hasn't been stored yet.
+  /// \returns index to FileToIndex or -1 if \p FE == nullptr.
+  int tryStoreFilePath(const clang::FileEntry *FE);
+
+  /// Stores \p Path if it is non-empty.
+  /// Warning: this method doesn't check for uniqueness.
+  /// \returns offset of \p Path value begin in buffer with stored paths.
+  size_t storePath(llvm::StringRef Path);
+
+private:
+  /// Stores \p dirStr path if it hasn't been stored yet.
+  DirBitPath tryStoreDirPath(llvm::StringRef dirStr);
+};
+
+#endif // LLVM_CLANG_INDEX_PATHINDEXER_H
Index: clang/lib/CMakeLists.txt
===================================================================
--- clang/lib/CMakeLists.txt
+++ clang/lib/CMakeLists.txt
@@ -20,6 +20,7 @@
 add_subdirectory(Tooling)
 add_subdirectory(DirectoryWatcher)
 add_subdirectory(Index)
+add_subdirectory(IndexSerialization)
 if(CLANG_ENABLE_STATIC_ANALYZER)
   add_subdirectory(StaticAnalyzer)
 endif()
Index: clang/lib/IndexSerialization/CMakeLists.txt
===================================================================
--- /dev/null
+++ clang/lib/IndexSerialization/CMakeLists.txt
@@ -0,0 +1,6 @@
+add_clang_library(clangIndexSerialization
+  PathIndexer.cpp
+
+  LINK_LIBS
+  clangBasic
+  )
Index: clang/lib/IndexSerialization/PathIndexer.cpp
===================================================================
--- /dev/null
+++ clang/lib/IndexSerialization/PathIndexer.cpp
@@ -0,0 +1,70 @@
+//===--- PathIndexer.cpp -- Index of paths ----------------------*- C++ -*-===//
+//
+// 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 "clang/IndexSerialization/PathIndexer.h"
+
+using namespace llvm;
+using namespace clang;
+
+PathIndexer::PathIndexer(StringRef workDir, StringRef sysrootPath)
+    : WorkDir(workDir),
+      SysrootPath(sysrootPath == "/" ? sysrootPath = StringRef()
+                                     : sysrootPath) {}
+
+int PathIndexer::tryStoreFilePath(const FileEntry *FE) {
+  if (!FE)
+    return -1;
+  auto Pair = FileToIndex.insert(std::make_pair(FE, FileBitPaths.size()));
+  bool IsNew = Pair.second;
+  size_t Index = Pair.first->getSecond();
+
+  if (IsNew) {
+    StringRef Filename = sys::path::filename(FE->getName());
+    DirBitPath Dir = tryStoreDirPath(sys::path::parent_path(FE->getName()));
+    FileBitPaths.emplace_back(
+        Dir.PrefixKind, Dir.Dir,
+        BitPathComponent(storePath(Filename), Filename.size()));
+  }
+  return Index;
+}
+
+size_t PathIndexer::storePath(StringRef Path) {
+  if (Path.empty())
+    return 0;
+  size_t offset = PathsBuf.size();
+  PathsBuf += Path;
+  return offset;
+}
+
+static bool isPathInDir(StringRef dir, StringRef path) {
+  if (dir.empty() || !path.startswith(dir))
+    return false;
+  StringRef rest = path.drop_front(dir.size());
+  return !rest.empty() && sys::path::is_separator(rest.front());
+}
+
+DirBitPath PathIndexer::tryStoreDirPath(StringRef dirStr) {
+  auto dirIt = Dirs.find(dirStr);
+  if (dirIt != Dirs.end())
+    return dirIt->second;
+
+  RootDirKind Kind = RootDirKind::Regular;
+  if (isPathInDir(SysrootPath, dirStr)) {
+    Kind = RootDirKind::SysRoot;
+    dirStr = dirStr.drop_front(SysrootPath.size());
+    while (!dirStr.empty() && dirStr[0] == '/')
+      dirStr = dirStr.drop_front();
+  } else if (isPathInDir(WorkDir, dirStr)) {
+    Kind = RootDirKind::CurrentWorkDir;
+    dirStr = dirStr.drop_front(WorkDir.size());
+    while (!dirStr.empty() && dirStr[0] == '/')
+      dirStr = dirStr.drop_front();
+  }
+
+  return DirBitPath(Kind, BitPathComponent(storePath(dirStr), dirStr.size()));
+}