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diff --git a/clang-r344140b/include/clang/Basic/FileSystemStatCache.h b/clang-r344140b/include/clang/Basic/FileSystemStatCache.h
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+//===- FileSystemStatCache.h - Caching for 'stat' calls ---------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Defines the FileSystemStatCache interface.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_BASIC_FILESYSTEMSTATCACHE_H
+#define LLVM_CLANG_BASIC_FILESYSTEMSTATCACHE_H
+
+#include "clang/Basic/LLVM.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/FileSystem.h"
+#include <cstdint>
+#include <ctime>
+#include <memory>
+#include <string>
+#include <utility>
+
+namespace llvm {
+
+namespace vfs {
+
+class File;
+class FileSystem;
+
+} // namespace vfs
+} // namespace llvm
+
+namespace clang {
+
+// FIXME: should probably replace this with vfs::Status
+struct FileData {
+  std::string Name;
+  uint64_t Size = 0;
+  time_t ModTime = 0;
+  llvm::sys::fs::UniqueID UniqueID;
+  bool IsDirectory = false;
+  bool IsNamedPipe = false;
+  bool InPCH = false;
+
+  // FIXME: remove this when files support multiple names
+  bool IsVFSMapped = false;
+
+  FileData() = default;
+};
+
+/// Abstract interface for introducing a FileManager cache for 'stat'
+/// system calls, which is used by precompiled and pretokenized headers to
+/// improve performance.
+class FileSystemStatCache {
+  virtual void anchor();
+
+protected:
+  std::unique_ptr<FileSystemStatCache> NextStatCache;
+
+public:
+  virtual ~FileSystemStatCache() = default;
+
+  enum LookupResult {
+    /// We know the file exists and its cached stat data.
+    CacheExists,
+
+    /// We know that the file doesn't exist.
+    CacheMissing
+  };
+
+  /// Get the 'stat' information for the specified path, using the cache
+  /// to accelerate it if possible.
+  ///
+  /// \returns \c true if the path does not exist or \c false if it exists.
+  ///
+  /// If isFile is true, then this lookup should only return success for files
+  /// (not directories).  If it is false this lookup should only return
+  /// success for directories (not files).  On a successful file lookup, the
+  /// implementation can optionally fill in \p F with a valid \p File object and
+  /// the client guarantees that it will close it.
+  static bool get(StringRef Path, FileData &Data, bool isFile,
+                  std::unique_ptr<llvm::vfs::File> *F,
+                  FileSystemStatCache *Cache, llvm::vfs::FileSystem &FS);
+
+  /// Sets the next stat call cache in the chain of stat caches.
+  /// Takes ownership of the given stat cache.
+  void setNextStatCache(std::unique_ptr<FileSystemStatCache> Cache) {
+    NextStatCache = std::move(Cache);
+  }
+
+  /// Retrieve the next stat call cache in the chain.
+  FileSystemStatCache *getNextStatCache() { return NextStatCache.get(); }
+
+  /// Retrieve the next stat call cache in the chain, transferring
+  /// ownership of this cache (and, transitively, all of the remaining caches)
+  /// to the caller.
+  std::unique_ptr<FileSystemStatCache> takeNextStatCache() {
+    return std::move(NextStatCache);
+  }
+
+protected:
+  // FIXME: The pointer here is a non-owning/optional reference to the
+  // unique_ptr. Optional<unique_ptr<vfs::File>&> might be nicer, but
+  // Optional needs some work to support references so this isn't possible yet.
+  virtual LookupResult getStat(StringRef Path, FileData &Data, bool isFile,
+                               std::unique_ptr<llvm::vfs::File> *F,
+                               llvm::vfs::FileSystem &FS) = 0;
+
+  LookupResult statChained(StringRef Path, FileData &Data, bool isFile,
+                           std::unique_ptr<llvm::vfs::File> *F,
+                           llvm::vfs::FileSystem &FS) {
+    if (FileSystemStatCache *Next = getNextStatCache())
+      return Next->getStat(Path, Data, isFile, F, FS);
+
+    // If we hit the end of the list of stat caches to try, just compute and
+    // return it without a cache.
+    return get(Path, Data, isFile, F, nullptr, FS) ? CacheMissing : CacheExists;
+  }
+};
+
+/// A stat "cache" that can be used by FileManager to keep
+/// track of the results of stat() calls that occur throughout the
+/// execution of the front end.
+class MemorizeStatCalls : public FileSystemStatCache {
+public:
+  /// The set of stat() calls that have been seen.
+  llvm::StringMap<FileData, llvm::BumpPtrAllocator> StatCalls;
+
+  using iterator =
+      llvm::StringMap<FileData, llvm::BumpPtrAllocator>::const_iterator;
+
+  iterator begin() const { return StatCalls.begin(); }
+  iterator end() const { return StatCalls.end(); }
+
+  LookupResult getStat(StringRef Path, FileData &Data, bool isFile,
+                       std::unique_ptr<llvm::vfs::File> *F,
+                       llvm::vfs::FileSystem &FS) override;
+};
+
+} // namespace clang
+
+#endif // LLVM_CLANG_BASIC_FILESYSTEMSTATCACHE_H