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diff --git a/clang-tools-extra/clangd/FS.h b/clang-tools-extra/clangd/FS.h
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+++ b/clang-tools-extra/clangd/FS.h
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+//===--- FS.h - File system related utils ------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_H
+
+#include "clang/Basic/VirtualFileSystem.h"
+#include "llvm/ADT/Optional.h"
+
+namespace clang {
+namespace clangd {
+
+/// Records status information for files open()ed or stat()ed during preamble
+/// build, so we can avoid stat()s on the underlying FS when reusing the
+/// preamble. For example, code completion can re-stat files when getting FileID
+/// for source locations stored in preamble (e.g. checking whether a location is
+/// in the main file).
+///
+/// The cache is keyed by absolute path of file name in cached status, as this
+/// is what preamble stores.
+///
+/// The cache is not thread-safe when updates happen, so the use pattern should
+/// be:
+///   - One FS writes to the cache from one thread (or several but strictly
+///   sequenced), e.g. when building preamble.
+///   - Sequence point (no writes after this point, no reads before).
+///   - Several FSs can read from the cache, e.g. code completions.
+///
+/// Note that the cache is only valid when reusing preamble.
+class PreambleFileStatusCache {
+public:
+  void update(const vfs::FileSystem &FS, vfs::Status S);
+  /// \p Path is a path stored in preamble.
+  llvm::Optional<vfs::Status> lookup(llvm::StringRef Path) const;
+
+  /// Returns a VFS that collects file status.
+  /// Only cache stats for files that exist because
+  ///   1) we only care about existing files when reusing preamble, unlike
+  ///   building preamble.
+  ///   2) we use the file name in the Status as the cache key.
+  ///
+  /// Note that the returned VFS should not outlive the cache.
+  IntrusiveRefCntPtr<vfs::FileSystem>
+  getProducingFS(IntrusiveRefCntPtr<vfs::FileSystem> FS);
+
+  /// Returns a VFS that uses the cache collected.
+  ///
+  /// Note that the returned VFS should not outlive the cache.
+  IntrusiveRefCntPtr<vfs::FileSystem>
+  getConsumingFS(IntrusiveRefCntPtr<vfs::FileSystem> FS) const;
+
+private:
+  llvm::StringMap<vfs::Status> StatCache;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_H