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diff --git a/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h b/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h
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+//===- DependencyScanningFilesystem.h - clang-scan-deps fs ===---*- 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_TOOLING_DEPENDENCY_SCANNING_FILESYSTEM_H
+#define LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_FILESYSTEM_H
+
+#include "clang/Basic/LLVM.h"
+#include "clang/Lex/PreprocessorExcludedConditionalDirectiveSkipMapping.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringSet.h"
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/VirtualFileSystem.h"
+#include <mutex>
+
+namespace clang {
+namespace tooling {
+namespace dependencies {
+
+/// An in-memory representation of a file system entity that is of interest to
+/// the dependency scanning filesystem.
+///
+/// It represents one of the following:
+/// - an opened source file with minimized contents and a stat value.
+/// - an opened source file with original contents and a stat value.
+/// - a directory entry with its stat value.
+/// - an error value to represent a file system error.
+/// - a placeholder with an invalid stat indicating a not yet initialized entry.
+class CachedFileSystemEntry {
+public:
+  /// Default constructor creates an entry with an invalid stat.
+  CachedFileSystemEntry() : MaybeStat(llvm::vfs::Status()) {}
+
+  CachedFileSystemEntry(std::error_code Error) : MaybeStat(std::move(Error)) {}
+
+  /// Create an entry that represents an opened source file with minimized or
+  /// original contents.
+  ///
+  /// The filesystem opens the file even for `stat` calls open to avoid the
+  /// issues with stat + open of minimized files that might lead to a
+  /// mismatching size of the file. If file is not minimized, the full file is
+  /// read and copied into memory to ensure that it's not memory mapped to avoid
+  /// running out of file descriptors.
+  static CachedFileSystemEntry createFileEntry(StringRef Filename,
+                                               llvm::vfs::FileSystem &FS,
+                                               bool Minimize = true);
+
+  /// Create an entry that represents a directory on the filesystem.
+  static CachedFileSystemEntry createDirectoryEntry(llvm::vfs::Status &&Stat);
+
+  /// \returns True if the entry is valid.
+  bool isValid() const { return !MaybeStat || MaybeStat->isStatusKnown(); }
+
+  /// \returns True if the current entry points to a directory.
+  bool isDirectory() const { return MaybeStat && MaybeStat->isDirectory(); }
+
+  /// \returns The error or the file's contents.
+  llvm::ErrorOr<StringRef> getContents() const {
+    if (!MaybeStat)
+      return MaybeStat.getError();
+    assert(!MaybeStat->isDirectory() && "not a file");
+    assert(isValid() && "not initialized");
+    return StringRef(Contents);
+  }
+
+  /// \returns The error or the status of the entry.
+  llvm::ErrorOr<llvm::vfs::Status> getStatus() const {
+    assert(isValid() && "not initialized");
+    return MaybeStat;
+  }
+
+  /// \returns the name of the file.
+  StringRef getName() const {
+    assert(isValid() && "not initialized");
+    return MaybeStat->getName();
+  }
+
+  /// Return the mapping between location -> distance that is used to speed up
+  /// the block skipping in the preprocessor.
+  const PreprocessorSkippedRangeMapping &getPPSkippedRangeMapping() const {
+    return PPSkippedRangeMapping;
+  }
+
+  CachedFileSystemEntry(CachedFileSystemEntry &&) = default;
+  CachedFileSystemEntry &operator=(CachedFileSystemEntry &&) = default;
+
+  CachedFileSystemEntry(const CachedFileSystemEntry &) = delete;
+  CachedFileSystemEntry &operator=(const CachedFileSystemEntry &) = delete;
+
+private:
+  llvm::ErrorOr<llvm::vfs::Status> MaybeStat;
+  // Store the contents in a small string to allow a
+  // move from the small string for the minimized contents.
+  // Note: small size of 1 allows us to store an empty string with an implicit
+  // null terminator without any allocations.
+  llvm::SmallString<1> Contents;
+  PreprocessorSkippedRangeMapping PPSkippedRangeMapping;
+};
+
+/// This class is a shared cache, that caches the 'stat' and 'open' calls to the
+/// underlying real file system.
+///
+/// It is sharded based on the hash of the key to reduce the lock contention for
+/// the worker threads.
+class DependencyScanningFilesystemSharedCache {
+public:
+  struct SharedFileSystemEntry {
+    std::mutex ValueLock;
+    CachedFileSystemEntry Value;
+  };
+
+  DependencyScanningFilesystemSharedCache();
+
+  /// Returns a cache entry for the corresponding key.
+  ///
+  /// A new cache entry is created if the key is not in the cache. This is a
+  /// thread safe call.
+  SharedFileSystemEntry &get(StringRef Key);
+
+private:
+  struct CacheShard {
+    std::mutex CacheLock;
+    llvm::StringMap<SharedFileSystemEntry, llvm::BumpPtrAllocator> Cache;
+  };
+  std::unique_ptr<CacheShard[]> CacheShards;
+  unsigned NumShards;
+};
+
+/// A virtual file system optimized for the dependency discovery.
+///
+/// It is primarily designed to work with source files whose contents was was
+/// preprocessed to remove any tokens that are unlikely to affect the dependency
+/// computation.
+///
+/// This is not a thread safe VFS. A single instance is meant to be used only in
+/// one thread. Multiple instances are allowed to service multiple threads
+/// running in parallel.
+class DependencyScanningWorkerFilesystem : public llvm::vfs::ProxyFileSystem {
+public:
+  DependencyScanningWorkerFilesystem(
+      DependencyScanningFilesystemSharedCache &SharedCache,
+      IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS,
+      ExcludedPreprocessorDirectiveSkipMapping *PPSkipMappings)
+      : ProxyFileSystem(std::move(FS)), SharedCache(SharedCache),
+        PPSkipMappings(PPSkipMappings) {}
+
+  llvm::ErrorOr<llvm::vfs::Status> status(const Twine &Path) override;
+  llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
+  openFileForRead(const Twine &Path) override;
+
+  /// The set of files that should not be minimized.
+  llvm::StringSet<> IgnoredFiles;
+
+private:
+  void setCachedEntry(StringRef Filename, const CachedFileSystemEntry *Entry) {
+    bool IsInserted = Cache.try_emplace(Filename, Entry).second;
+    (void)IsInserted;
+    assert(IsInserted && "local cache is updated more than once");
+  }
+
+  const CachedFileSystemEntry *getCachedEntry(StringRef Filename) {
+    auto It = Cache.find(Filename);
+    return It == Cache.end() ? nullptr : It->getValue();
+  }
+
+  llvm::ErrorOr<const CachedFileSystemEntry *>
+  getOrCreateFileSystemEntry(const StringRef Filename);
+
+  DependencyScanningFilesystemSharedCache &SharedCache;
+  /// The local cache is used by the worker thread to cache file system queries
+  /// locally instead of querying the global cache every time.
+  llvm::StringMap<const CachedFileSystemEntry *, llvm::BumpPtrAllocator> Cache;
+  /// The optional mapping structure which records information about the
+  /// excluded conditional directive skip mappings that are used by the
+  /// currently active preprocessor.
+  ExcludedPreprocessorDirectiveSkipMapping *PPSkipMappings;
+};
+
+} // end namespace dependencies
+} // end namespace tooling
+} // end namespace clang
+
+#endif // LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_FILESYSTEM_H