[clang-scan-deps] Implementation of dependency scanner over minimized sources

[arphaman]

Summary

This patch implements the fast dependency scanning mode in clang-scan-deps: the preprocessing is done on files that are minimized using the dependency directives source minimizer.

A shared file system cache is used to ensure that the file system requests and source minimization is performed only once. The cache assumes that the underlying filesystem won't change during the course of the scan (or if it will, it will not affect the output), and it can't be evicted. This means that the service and workers can be used for a single run of a dependency scanner, and can't be reused across multiple, incremental runs. This is something that we'll most likely support in the future though. Note that the driver still utilizes the underlying real filesystem.

This patch is also still missing the fast skipped PP block skipping optimization that I mentioned at EuroLLVM talk.

[arphaman]

Fix a bug for empty minimized files where null terminator lookup by the lexer was an out of bounds read

[aganea]

I will take a look next week when I get back!

[aganea]

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

2

General comment for this file and the implementation: it seems there's nothing specific to the dependency scanning, except for replacing the content with minimized content? This cached FS could be very well used to compile a project with parallel workers. Could this be part of the VirtualFileSystem infrastructure? ( llvm/include/llvm/Support/VirtualFileSystem.h) If yes, can you possibly create a separate patch for this? @JDevlieghere @sammccall

102

The struct is self-explanatory, not sure the comment is needed here?

104

Would you mind renaming this to ValueLock so it is easier to search for? (and to remain consistent with CacheLock below)

130

Maybe worth mentioning this is NOT a global, thread-safe, class like DependencyScanningFilesystemSharedCache, but rather meant to be used as per-thread instances?

I am also wondering if there could be a better name to signify at first sight that this is a per-thread class. DependencyScanningLocalFilesystem? DependencyScanningWorkerFilesystem?

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

85

This line needs a comment. Is this value based on empirical results across different hardwares? (I can't recall your answer at the LLVM conf) I am wondering what would be the good strategy here? The more cores/HT in your PC, the more chances that you'll hit the same shard, thus locking. But the bigger we make this value, the more StringMaps we will have, and more cache misses possibly.
Should it be something like llvm::hardware_concurrency() / some_fudge? It'd be interesting to subsequently profile on high core count machines (or maybe you have done that).

211

Don't use auto when the type is not obvious.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

148

Can we not use caching all the time?

[arphaman]

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

2

I think it could be possible to separate out the cache, but we definitely need a subclass of a VFS to handle some Clang specific logic for how to determine how to deal with module files for instance. I wouldn't be opposed to factoring it out if people are interested. I think that can be done as a follow-up if there's an interest though.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

85

I rewrote it to use a heuristic we settled on after doing empirical testing on an 18 core / 36 thread machine ( max(2, thread_count / 4) ), and added a comment. This was the number 9 (36 / 4) I mentioned at the talk, so we only got to it because of a heuristic. I think now after some SDK/OS updates the effect from adding more shards is less pronounced, so it mostly flatlines past some number between 5-10. A better heuristic would probably be OS specific to take the cost of lock contention into account.

Note that the increased number of shards does not increase the number of cache misses, because the shard # is determined by the filename (we don't actually have global cache misses, as the cache is designed to have only one miss per file when it's first accessed)! It's just that an increased number of shards doesn't improve performance after hitting some specific limit, so we want to find a point where we can taper it off.

It would still be definitely interesting to profile it on other high core machines with different OSes to see if it's a reasonable heuristic for those scenarios too.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

148

We want to have a mode where it's as close to the regular clang -E invocation as possible for correctness CI and testing. We also haven't evaluated if the cost of keeping non-minimized sources in memory, so it might be too expensive for practical use? I can add a third option that caches but doesn't minimize though as a follow-up patch though

[aganea]

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

85

I'll give it a try on Windows 10, one project here has a large codebase and some wild machines to test on.

157

This makes only a short-lived objects, is that right? Just during the call to CachedFileSystemEntry::createFileEntry?

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

94

"might be" twice.

148

Yes that would be nice. As for the size taken in RAM, it would be only .H files, right? For Clang+LLVM+LLD I'm counting about 40 MB. But indeed with a large project, that would be about 1.5 GB of .H. Although I doubt all these files will be loaded at once in memory (I'll check)

As for the usage: Fastbuild works like distcc (plain mode, not pump) so we were also planning on extracting the fully preprocessed output, not only the dependencies. There's one use-case where we need to preprocess locally, then send the preprocessed output remotely for compilation. And another use-case where we only want to extract the dependency list, compute a digest, to retrieve the OBJ from a network cache.

[arphaman]

Fix comment typo

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

157

Yes, these VFS buffer files are only alive for a particular invocation.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

148

Right now it doesn't differentiate between .H and other files, but we could teach it do have a header only filter for the cache.

[aganea]

Thanks for the update Alex! Just a few more comments and we should be good to go:

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

117

Why not use a bump allocator here? (it would be per-thread) On Windows the heap is always thread-safe, which induce a lock in malloc for each new entry. You could also avoid the usage of unique_ptr by the same occasion:

llvm::StringMap<SharedFileSystemEntry, SpecificBumpPtrAllocator<SharedFileSystemEntry>> Cache;

(unless you're planning on removing entries from the cache later on?)

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

76

llvm::vfs::Status &&Stat to avoid a copy?

103

Shard.Cache.try_emplace(Key) will provide a default constructed value if it's not there.

117

CachedFileSystemEntry *Entry ?

198

CachedFileSystemEntry *Entry ?

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

148

No worries, I was simply wondering about the size in memory.

[arphaman]

Address review comments.

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

117

Good idea, I switched to a bump pointer allocator (I don't think I can use a specific one, as that would cause the values to be destroyed twice). I'm not planning on removing entries from the cache, no.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

76

The copy should already be avoided, as I move the argument when passing in, and when it's assigned to the result.

[arphaman]

Just for reference, this patch still doesn't reuse the FileManager across invocations in a thread. We expect to get even better performance once we reuse it, but I'm going have to improve its API first.

[aganea]

LGTM, thank you!

In D63907#1617417, @arphaman wrote:

Just for reference, this patch still doesn't reuse the FileManager across invocations in a thread. We expect to get even better performance once we reuse it, but I'm going have to improve its API first.

Can't wait! @SamChaps is already testing this patch. He found some minor edge-cases with the minimizer (most likely unrelated to this), I'll post a patch.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

76

If Stat is not a rvalue reference, wouldn't the std::move in the call site end up as a copy? See this. If you change int test(A a) to int test(A &&a) you can see the difference in the asm output. However if the function is inlined, the extra copy will probably be optimized out. Not a big deal - the OS calls like stat() will most likely dominate here.

[arphaman]

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

76

Isn't the difference in the output caused by the details of the calling convention (pass the structure on the stack by value)? The move constructor should still be called for the stat, it will not copy its members. We can optimize this though and pass by ref directly, I agree, so let me do that.

clang/include/clang/Basic/FileManager.h

public:
   getFile(StringRef Filename, bool OpenFile = false, bool CacheFailure = true);
 
   /// Returns the current file system options
   FileSystemOptions &getFileSystemOpts() { return FileSystemOpts; }
   const FileSystemOptions &getFileSystemOpts() const { return FileSystemOpts; }
 
   llvm::vfs::FileSystem &getVirtualFileSystem() const { return *FS; }
 
+  void setVirtualFileSystem(IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS) {
+    this->FS = std::move(FS);
+  }
+
   /// Retrieve a file entry for a "virtual" file that acts as
   /// if there were a file with the given name on disk.
   ///
   /// The file itself is not accessed.
   const FileEntry *getVirtualFile(StringRef Filename, off_t Size,
                                   time_t ModificationTime);
 
   /// Open the specified file as a MemoryBuffer, returning a new

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

+//===- 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 "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 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();
+  }
+
+  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;
+};
+
+/// 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<std::unique_ptr<SharedFileSystemEntry>> 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)
+      : ProxyFileSystem(std::move(FS)), SharedCache(SharedCache) {}
+
+  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();
+  }
+
+  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;
+};
+
+} // end namespace dependencies
+} // end namespace tooling
+} // end namespace clang
+
+#endif // LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_FILESYSTEM_H

clang/include/clang/Tooling/DependencyScanning/DependencyScanningService.h

+//===- DependencyScanningService.h - clang-scan-deps service ===-*- 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_SERVICE_H
+#define LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_SERVICE_H
+
+#include "clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h"
+
+namespace clang {
+namespace tooling {
+namespace dependencies {
+
+/// The mode in which the dependency scanner will operate to find the
+/// dependencies.
+enum class ScanningMode {
+  /// This mode is used to compute the dependencies by running the preprocessor
+  /// over
+  /// the unmodified source files.
+  CanonicalPreprocessing,
+
+  /// This mode is used to compute the dependencies by running the preprocessor
+  /// over
+  /// the source files that have been minimized to contents that might affect
+  /// the dependencies.
+  MinimizedSourcePreprocessing
+};
+
+/// The dependency scanning service contains the shared state that is used by
+/// the invidual dependency scanning workers.
+class DependencyScanningService {
+public:
+  DependencyScanningService(ScanningMode Mode);
+
+  ScanningMode getMode() const { return Mode; }
+
+  DependencyScanningFilesystemSharedCache &getSharedCache() {
+    return SharedCache;
+  }
+
+private:
+  const ScanningMode Mode;
+  /// The global file system cache.
+  DependencyScanningFilesystemSharedCache SharedCache;
+};
+
+} // end namespace dependencies
+} // end namespace tooling
+} // end namespace clang
+
+#endif // LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_SERVICE_H

clang/include/clang/Tooling/DependencyScanning/DependencyScanningWorker.h

 //===- DependencyScanningWorker.h - clang-scan-deps worker ===---*- 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_WORKER_H
 #define LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_WORKER_H
 
 #include "clang/Basic/DiagnosticOptions.h"
+#include "clang/Basic/FileManager.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Frontend/PCHContainerOperations.h"
 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include <string>
 
 namespace clang {
 namespace tooling {
 namespace dependencies {
 
+class DependencyScanningService;
+class DependencyScanningWorkerFilesystem;
+
 /// An individual dependency scanning worker that is able to run on its own
 /// thread.
 ///
 /// The worker computes the dependencies for the input files by preprocessing
 /// sources either using a fast mode where the source files are minimized, or
 /// using the regular processing run.
 class DependencyScanningWorker {
 public:
-  DependencyScanningWorker();
+  DependencyScanningWorker(DependencyScanningService &Service);
 
   /// Print out the dependency information into a string using the dependency
   /// file format that is specified in the options (-MD is the default) and
   /// return it.
   ///
   /// \returns A \c StringError with the diagnostic output if clang errors
   /// occurred, dependency file contents otherwise.
   llvm::Expected<std::string> getDependencyFile(const std::string &Input,
                                                 StringRef WorkingDirectory,
                                                 const CompilationDatabase &CDB);
 
 private:
   IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts;
   std::shared_ptr<PCHContainerOperations> PCHContainerOps;
 
+  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> RealFS;
   /// The file system that is used by each worker when scanning for
   /// dependencies. This filesystem persists accross multiple compiler
   /// invocations.
-  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> WorkerFS;
+  llvm::IntrusiveRefCntPtr<DependencyScanningWorkerFilesystem> DepFS;
 };
 
 } // end namespace dependencies
 } // end namespace tooling
 } // end namespace clang
 
 #endif // LLVM_CLANG_TOOLING_DEPENDENCY_SCANNING_WORKER_H

clang/lib/Tooling/DependencyScanning/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   Core
   Support
   )
 
 add_clang_library(clangDependencyScanning
+  DependencyScanningFilesystem.cpp
+  DependencyScanningService.cpp
   DependencyScanningWorker.cpp
 
   DEPENDS
   ClangDriverOptions
 
   LINK_LIBS
   clangAST
   clangBasic
   clangDriver
   clangFrontend
   clangFrontendTool
   clangLex
   clangParse
   clangSerialization
   clangTooling
 )

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

+//===- DependencyScanningFilesystem.cpp - clang-scan-deps fs --------------===//
+//
+// 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/Tooling/DependencyScanning/DependencyScanningFilesystem.h"
+#include "clang/Lex/DependencyDirectivesSourceMinimizer.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Threading.h"
+
+using namespace clang;
+using namespace tooling;
+using namespace dependencies;
+
+CachedFileSystemEntry CachedFileSystemEntry::createFileEntry(
+    StringRef Filename, llvm::vfs::FileSystem &FS, bool Minimize) {
+  // Load the file and its content from the file system.
+  llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> MaybeFile =
+      FS.openFileForRead(Filename);
+  if (!MaybeFile)
+    return MaybeFile.getError();
+  llvm::ErrorOr<llvm::vfs::Status> Stat = (*MaybeFile)->status();
+  if (!Stat)
+    return Stat.getError();
+
+  llvm::vfs::File &F = **MaybeFile;
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MaybeBuffer =
+      F.getBuffer(Stat->getName());
+  if (!MaybeBuffer)
+    return MaybeBuffer.getError();
+
+  llvm::SmallString<1024> MinimizedFileContents;
+  // Minimize the file down to directives that might affect the dependencies.
+  const auto &Buffer = *MaybeBuffer;
+  SmallVector<minimize_source_to_dependency_directives::Token, 64> Tokens;
+  if (!Minimize || minimizeSourceToDependencyDirectives(
+                       Buffer->getBuffer(), MinimizedFileContents, Tokens)) {
+    // Use the original file unless requested otherwise, or
+    // if the minimization failed.
+    // FIXME: Propage the diagnostic if desired by the client.
+    CachedFileSystemEntry Result;
+    Result.MaybeStat = std::move(*Stat);
+    Result.Contents.reserve(Buffer->getBufferSize() + 1);
+    Result.Contents.append(Buffer->getBufferStart(), Buffer->getBufferEnd());
+    // Implicitly null terminate the contents for Clang's lexer.
+    Result.Contents.push_back('\0');
+    Result.Contents.pop_back();
+    return Result;
+  }
+
+  CachedFileSystemEntry Result;
+  size_t Size = MinimizedFileContents.size();
+  Result.MaybeStat = llvm::vfs::Status(Stat->getName(), Stat->getUniqueID(),
+                                       Stat->getLastModificationTime(),
+                                       Stat->getUser(), Stat->getGroup(), Size,
+                                       Stat->getType(), Stat->getPermissions());
+  // The contents produced by the minimizer must be null terminated.
+  assert(MinimizedFileContents.data()[MinimizedFileContents.size()] == '\0' &&
+         "not null terminated contents");
+  // Even though there's an implicit null terminator in the minimized contents,
+  // we want to temporarily make it explicit. This will ensure that the
+  // std::move will preserve it even if it needs to do a copy if the
+  // SmallString still has the small capacity.
+  MinimizedFileContents.push_back('\0');
+  Result.Contents = std::move(MinimizedFileContents);
+  // Now make the null terminator implicit again, so that Clang's lexer can find
+  // it right where the buffer ends.
+  Result.Contents.pop_back();
+  return Result;
+}
+
+CachedFileSystemEntry
+CachedFileSystemEntry::createDirectoryEntry(llvm::vfs::Status Stat) {
+  assert(Stat.isDirectory() && "not a directory!");
+  auto Result = CachedFileSystemEntry();
+  Result.MaybeStat = std::move(Stat);
+  return Result;
+}
+
+DependencyScanningFilesystemSharedCache::
+    DependencyScanningFilesystemSharedCache() {
+  // This heuristic was chosen using a empirical testing on a
+  // reasonably high core machine (iMacPro 18 cores / 36 threads). The cache
+  // sharding gives a performance edge by reducing the lock contention.
+  // FIXME: A better heuristic might also consider the OS to account for
+  // the different cost of lock contention on different OSes.
+  NumShards = std::max(2u, llvm::hardware_concurrency() / 4);
+  CacheShards = llvm::make_unique<CacheShard[]>(NumShards);
+}
+
+/// 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.
+DependencyScanningFilesystemSharedCache::SharedFileSystemEntry &
+DependencyScanningFilesystemSharedCache::get(StringRef Key) {
+  CacheShard &Shard = CacheShards[llvm::hash_value(Key) % NumShards];
+  std::unique_lock<std::mutex> LockGuard(Shard.CacheLock);
+  auto It =
+      Shard.Cache.try_emplace(Key, std::unique_ptr<SharedFileSystemEntry>());
+  auto &Ptr = It.first->getValue();
+  // Create the actual cache entry if insert succeeded.
+  if (It.second)
+    Ptr = llvm::make_unique<SharedFileSystemEntry>();
+  return *Ptr;
+}
+
+llvm::ErrorOr<llvm::vfs::Status>
+DependencyScanningWorkerFilesystem::status(const Twine &Path) {
+  SmallString<256> OwnedFilename;
+  StringRef Filename = Path.toStringRef(OwnedFilename);
+
+  // Check the local cache first.
+  if (const auto *Entry = getCachedEntry(Filename))
+    return Entry->getStatus();
+
+  // FIXME: Handle PCM/PCH files.
+  // FIXME: Handle module map files.
+
+  bool KeepOriginalSource = IgnoredFiles.count(Filename);
+  DependencyScanningFilesystemSharedCache::SharedFileSystemEntry
+      &SharedCacheEntry = SharedCache.get(Filename);
+  const CachedFileSystemEntry *Result;
+  {
+    std::unique_lock<std::mutex> LockGuard(SharedCacheEntry.ValueLock);
+    CachedFileSystemEntry &CacheEntry = SharedCacheEntry.Value;
+
+    if (!CacheEntry.isValid()) {
+      llvm::vfs::FileSystem &FS = getUnderlyingFS();
+      auto MaybeStatus = FS.status(Filename);
+      if (!MaybeStatus)
+        CacheEntry = CachedFileSystemEntry(MaybeStatus.getError());
+      else if (MaybeStatus->isDirectory())
+        CacheEntry = CachedFileSystemEntry::createDirectoryEntry(
+            std::move(*MaybeStatus));
+      else
+        CacheEntry = CachedFileSystemEntry::createFileEntry(
+            Filename, FS, !KeepOriginalSource);
+    }
+
+    Result = &CacheEntry;
+  }
+
+  // Store the result in the local cache.
+  setCachedEntry(Filename, Result);
+  return Result->getStatus();
+}
+
+namespace {
+
+/// The VFS that is used by clang consumes the \c CachedFileSystemEntry using
+/// this subclass.
+class MinimizedVFSFile final : public llvm::vfs::File {
+public:
+  MinimizedVFSFile(std::unique_ptr<llvm::MemoryBuffer> Buffer,
+                   llvm::vfs::Status Stat)
+      : Buffer(std::move(Buffer)), Stat(std::move(Stat)) {}
+
+  llvm::ErrorOr<llvm::vfs::Status> status() override { return Stat; }
+
+  const llvm::MemoryBuffer *getBufferPtr() const { return Buffer.get(); }
+
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
+  getBuffer(const Twine &Name, int64_t FileSize, bool RequiresNullTerminator,
+            bool IsVolatile) override {
+    return std::move(Buffer);
+  }
+
+  std::error_code close() override { return {}; }
+
+private:
+  std::unique_ptr<llvm::MemoryBuffer> Buffer;
+  llvm::vfs::Status Stat;
+};
+
+llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
+createFile(const CachedFileSystemEntry *Entry) {
+  llvm::ErrorOr<StringRef> Contents = Entry->getContents();
+  if (!Contents)
+    return Contents.getError();
+  return llvm::make_unique<MinimizedVFSFile>(
+      llvm::MemoryBuffer::getMemBuffer(*Contents, Entry->getName(),
+                                       /*RequiresNullTerminator=*/false),
+      *Entry->getStatus());
+}
+
+} // end anonymous namespace
+
+llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
+DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
+  SmallString<256> OwnedFilename;
+  StringRef Filename = Path.toStringRef(OwnedFilename);
+
+  // Check the local cache first.
+  if (const auto *Entry = getCachedEntry(Filename))
+    return createFile(Entry);
+
+  // FIXME: Handle PCM/PCH files.
+  // FIXME: Handle module map files.
+
+  bool KeepOriginalSource = IgnoredFiles.count(Filename);
+  DependencyScanningFilesystemSharedCache::SharedFileSystemEntry
+      &SharedCacheEntry = SharedCache.get(Filename);
+  const CachedFileSystemEntry *Result;
+  {
+    std::unique_lock<std::mutex> LockGuard(SharedCacheEntry.ValueLock);
+    CachedFileSystemEntry &CacheEntry = SharedCacheEntry.Value;
+
+    if (!CacheEntry.isValid()) {
+      CacheEntry = CachedFileSystemEntry::createFileEntry(
+          Filename, getUnderlyingFS(), !KeepOriginalSource);
+    }
+
+    Result = &CacheEntry;
+  }
+
+  // Store the result in the local cache.
+  setCachedEntry(Filename, Result);
+  return createFile(Result);
+}

clang/lib/Tooling/DependencyScanning/DependencyScanningService.cpp

+//===- DependencyScanningService.cpp - clang-scan-deps service ------------===//
+//
+// 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/Tooling/DependencyScanning/DependencyScanningService.h"
+
+using namespace clang;
+using namespace tooling;
+using namespace dependencies;
+
+DependencyScanningService::DependencyScanningService(ScanningMode Mode)
+    : Mode(Mode) {}

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

 //===- DependencyScanningWorker.cpp - clang-scan-deps worker --------------===//
 //
 // 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/Tooling/DependencyScanning/DependencyScanningWorker.h"
 #include "clang/Frontend/CompilerInstance.h"
+#include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Frontend/Utils.h"
+#include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
 #include "clang/Tooling/Tooling.h"
 
 using namespace clang;
 using namespace tooling;
 using namespace dependencies;
 
 namespace {
 

 private:
   std::string CWD;
 };
 
 /// A clang tool that runs the preprocessor in a mode that's optimized for
 /// dependency scanning for the given compiler invocation.
 class DependencyScanningAction : public tooling::ToolAction {
 public:
-  DependencyScanningAction(StringRef WorkingDirectory,
-                           std::string &DependencyFileContents)
+  DependencyScanningAction(
+      StringRef WorkingDirectory, std::string &DependencyFileContents,
+      llvm::IntrusiveRefCntPtr<DependencyScanningWorkerFilesystem> DepFS)
       : WorkingDirectory(WorkingDirectory),
-        DependencyFileContents(DependencyFileContents) {}
+        DependencyFileContents(DependencyFileContents),
+        DepFS(std::move(DepFS)) {}
 
   bool runInvocation(std::shared_ptr<CompilerInvocation> Invocation,
                      FileManager *FileMgr,
                      std::shared_ptr<PCHContainerOperations> PCHContainerOps,
                      DiagnosticConsumer *DiagConsumer) override {
     // Create a compiler instance to handle the actual work.
     CompilerInstance Compiler(std::move(PCHContainerOps));
     Compiler.setInvocation(std::move(Invocation));
-    FileMgr->getFileSystemOpts().WorkingDir = WorkingDirectory;
-    Compiler.setFileManager(FileMgr);
 
     // Don't print 'X warnings and Y errors generated'.
     Compiler.getDiagnosticOpts().ShowCarets = false;
     // Create the compiler's actual diagnostics engine.
     Compiler.createDiagnostics(DiagConsumer, /*ShouldOwnClient=*/false);
     if (!Compiler.hasDiagnostics())
       return false;
 
+    // Use the dependency scanning optimized file system if we can.
+    if (DepFS) {
+      // FIXME: Purge the symlink entries from the stat cache in the FM.
+      const CompilerInvocation &CI = Compiler.getInvocation();
+      // Add any filenames that were explicity passed in the build settings and
+      // that might be opened, as we want to ensure we don't run source
+      // minimization on them.
+      DepFS->IgnoredFiles.clear();
+      for (const auto &Entry : CI.getHeaderSearchOpts().UserEntries)
+        DepFS->IgnoredFiles.insert(Entry.Path);
+      for (const auto &Entry : CI.getHeaderSearchOpts().VFSOverlayFiles)
+        DepFS->IgnoredFiles.insert(Entry);
+
+      // Support for virtual file system overlays on top of the caching
+      // filesystem.
+      FileMgr->setVirtualFileSystem(createVFSFromCompilerInvocation(
+          CI, Compiler.getDiagnostics(), DepFS));
+    }
+
+    FileMgr->getFileSystemOpts().WorkingDir = WorkingDirectory;
+    Compiler.setFileManager(FileMgr);
     Compiler.createSourceManager(*FileMgr);
 
     // Create the dependency collector that will collect the produced
     // dependencies.
     //
     // This also moves the existing dependency output options from the
     // invocation to the collector. The options in the invocation are reset,
     // which ensures that the compiler won't create new dependency collectors,
     // and thus won't write out the extra '.d' files to disk.
     auto Opts = llvm::make_unique<DependencyOutputOptions>(
         std::move(Compiler.getInvocation().getDependencyOutputOpts()));
     // We need at least one -MT equivalent for the generator to work.
     if (Opts->Targets.empty())
       Opts->Targets = {"clang-scan-deps dependency"};
     Compiler.addDependencyCollector(std::make_shared<DependencyPrinter>(
         std::move(Opts), DependencyFileContents));
 
     auto Action = llvm::make_unique<PreprocessOnlyAction>();
     const bool Result = Compiler.ExecuteAction(*Action);
+    if (!DepFS)
       FileMgr->clearStatCache();
     return Result;
   }
 
 private:
   StringRef WorkingDirectory;
   /// The dependency file will be written to this string.
   std::string &DependencyFileContents;
+  llvm::IntrusiveRefCntPtr<DependencyScanningWorkerFilesystem> DepFS;
 };
 
 } // end anonymous namespace
 
-DependencyScanningWorker::DependencyScanningWorker() {
+DependencyScanningWorker::DependencyScanningWorker(
+    DependencyScanningService &Service) {
   DiagOpts = new DiagnosticOptions();
   PCHContainerOps = std::make_shared<PCHContainerOperations>();
-  /// FIXME: Use the shared file system from the service for fast scanning
-  /// mode.
-  WorkerFS = new ProxyFileSystemWithoutChdir(llvm::vfs::getRealFileSystem());
+  RealFS = new ProxyFileSystemWithoutChdir(llvm::vfs::getRealFileSystem());
+  if (Service.getMode() == ScanningMode::MinimizedSourcePreprocessing)
+    DepFS = new DependencyScanningWorkerFilesystem(Service.getSharedCache(),
+                                                   RealFS);
 }
 
 llvm::Expected<std::string>
 DependencyScanningWorker::getDependencyFile(const std::string &Input,
                                             StringRef WorkingDirectory,
                                             const CompilationDatabase &CDB) {
   // Capture the emitted diagnostics and report them to the client
   // in the case of a failure.
   std::string DiagnosticOutput;
   llvm::raw_string_ostream DiagnosticsOS(DiagnosticOutput);
   TextDiagnosticPrinter DiagPrinter(DiagnosticsOS, DiagOpts.get());
 
-  WorkerFS->setCurrentWorkingDirectory(WorkingDirectory);
-  tooling::ClangTool Tool(CDB, Input, PCHContainerOps, WorkerFS);
+  RealFS->setCurrentWorkingDirectory(WorkingDirectory);
+  /// Create the tool that uses the underlying file system to ensure that any
+  /// file system requests that are made by the driver do not go through the
+  /// dependency scanning filesystem.
+  tooling::ClangTool Tool(CDB, Input, PCHContainerOps, RealFS);
   Tool.clearArgumentsAdjusters();
   Tool.setRestoreWorkingDir(false);
   Tool.setPrintErrorMessage(false);
   Tool.setDiagnosticConsumer(&DiagPrinter);
   std::string Output;
-  DependencyScanningAction Action(WorkingDirectory, Output);
+  DependencyScanningAction Action(WorkingDirectory, Output, DepFS);
   if (Tool.run(&Action)) {
     return llvm::make_error<llvm::StringError>(DiagnosticsOS.str(),
                                                llvm::inconvertibleErrorCode());
   }
   return Output;
 }

clang/test/ClangScanDeps/Inputs/frameworks/Framework.framework/Headers/Framework.h

+// This comment is stripped, so size is changed when file is opened
+#define FRAMEWORK 0

clang/test/ClangScanDeps/Inputs/frameworks/Framework.framework/PrivateHeaders/PrivateHeader.h

+// This comment is stripped when file is opened, so size will change
+#define PRIV 0

clang/test/ClangScanDeps/Inputs/header_stat_before_open_cdb.json

+[
+{
+  "directory": "DIR",
+  "command": "clang -E DIR/header_stat_before_open.m -iframework Inputs/frameworks",
+  "file": "DIR/header_stat_before_open.m"
+}
+]

clang/test/ClangScanDeps/Inputs/vfsoverlay.yaml

+{
+  'version': 0,
+  'roots': [
+    { 'name': 'DIR', 'type': 'directory',
+      'contents': [
+        { 'name': 'not_real.h', 'type': 'file',
+          'external-contents': 'DIR/Inputs/header.h'
+        }
+      ]
+    }
+  ]
+}

clang/test/ClangScanDeps/Inputs/vfsoverlay_cdb.json

+[
+{
+  "directory": "DIR",
+  "command": "clang -E DIR/vfsoverlay.cpp -IInputs -ivfsoverlay DIR/vfsoverlay.yaml",
+  "file": "DIR/vfsoverlay.cpp"
+}
+]

clang/test/ClangScanDeps/header_stat_before_open.m

+// RUN: rm -rf %t.dir
+// RUN: rm -rf %t.cdb
+// RUN: mkdir -p %t.dir
+// RUN: cp %s %t.dir/header_stat_before_open.m
+// RUN: mkdir %t.dir/Inputs
+// RUN: cp -R %S/Inputs/frameworks %t.dir/Inputs/frameworks
+// RUN: sed -e "s|DIR|%/t.dir|g" %S/Inputs/header_stat_before_open_cdb.json > %t.cdb
+//
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 1 | \
+// RUN:   FileCheck %s
+
+#include "Framework/Framework.h"
+#include "Framework/PrivateHeader.h"
+
+// CHECK: clang-scan-deps dependency
+// CHECK-NEXT: header_stat_before_open.m
+// CHECK-NEXT: Inputs/frameworks/Framework.framework/Headers/Framework.h
+// CHECK-NEXT: Inputs/frameworks/Framework.framework/PrivateHeaders/PrivateHeader.h

clang/test/ClangScanDeps/regular_cdb.cpp

 // RUN: rm -rf %t.dir
 // RUN: rm -rf %t.cdb
 // RUN: mkdir -p %t.dir
 // RUN: cp %s %t.dir/regular_cdb.cpp
 // RUN: cp %s %t.dir/regular_cdb2.cpp
 // RUN: mkdir %t.dir/Inputs
 // RUN: cp %S/Inputs/header.h %t.dir/Inputs/header.h
 // RUN: cp %S/Inputs/header2.h %t.dir/Inputs/header2.h
 // RUN: sed -e "s|DIR|%/t.dir|g" %S/Inputs/regular_cdb.json > %t.cdb
 //
-// RUN: clang-scan-deps -compilation-database %t.cdb -j 1 | \
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 1 -mode preprocess-minimized-sources | \
+// RUN:   FileCheck --check-prefixes=CHECK1,CHECK2,CHECK2NO %s
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 1 -mode preprocess | \
 // RUN:   FileCheck --check-prefixes=CHECK1,CHECK2,CHECK2NO %s
 //
 // Make sure we didn't produce any dependency files!
 // RUN: not cat %t.dir/regular_cdb.d
 // RUN: not cat %t.dir/regular_cdb2.d
 //
 // The output order is non-deterministic when using more than one thread,
 // so check the output using two runs. Note that the 'NOT' check is not used
 // as it might fail if the results for `regular_cdb.cpp` are reported before
 // `regular_cdb2.cpp`.
 //
-// RUN: clang-scan-deps -compilation-database %t.cdb -j 2 | \
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 2 -mode preprocess-minimized-sources | \
+// RUN:   FileCheck --check-prefix=CHECK1 %s
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 2 -mode preprocess | \
 // RUN:   FileCheck --check-prefix=CHECK1 %s
-// RUN: clang-scan-deps -compilation-database %t.cdb -j 2 | \
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 2 -mode preprocess-minimized-sources | \
+// RUN:   FileCheck --check-prefix=CHECK2 %s
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 2 -mode preprocess | \
 // RUN:   FileCheck --check-prefix=CHECK2 %s
 
 #include "header.h"
 
 // CHECK1: regular_cdb2.cpp
 // CHECK1-NEXT: Inputs{{/|\\}}header.h
 // CHECK1-NEXT: Inputs{{/|\\}}header2.h
 
 // CHECK2: regular_cdb.cpp
 // CHECK2-NEXT: Inputs{{/|\\}}header.h
 // CHECK2NO-NOT: header2

clang/test/ClangScanDeps/vfsoverlay.cpp

+// RUN: rm -rf %t.dir
+// RUN: rm -rf %t.cdb
+// RUN: mkdir -p %t.dir
+// RUN: cp %s %t.dir/vfsoverlay.cpp
+// RUN: sed -e "s|DIR|%/t.dir|g" %S/Inputs/vfsoverlay.yaml > %t.dir/vfsoverlay.yaml
+// RUN: mkdir %t.dir/Inputs
+// RUN: cp %S/Inputs/header.h %t.dir/Inputs/header.h
+// RUN: sed -e "s|DIR|%/t.dir|g" %S/Inputs/vfsoverlay_cdb.json > %t.cdb
+//
+// RUN: clang-scan-deps -compilation-database %t.cdb -j 1 | \
+// RUN:   FileCheck %s
+
+#include "not_real.h"
+
+// CHECK: clang-scan-deps dependency
+// CHECK-NEXT: vfsoverlay.cpp
+// CHECK-NEXT: Inputs{{/|\\}}header.h

clang/tools/clang-scan-deps/ClangScanDeps.cpp

 //===- ClangScanDeps.cpp - Implementation of clang-scan-deps --------------===//
 //
 // 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/Frontend/CompilerInstance.h"
 #include "clang/Tooling/CommonOptionsParser.h"
+#include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
 #include "clang/Tooling/DependencyScanning/DependencyScanningWorker.h"
 #include "clang/Tooling/JSONCompilationDatabase.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/Options.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/Threading.h"
 #include <mutex>

 /// The high-level implementation of the dependency discovery tool that runs on
 /// an individual worker thread.
 class DependencyScanningTool {
 public:
   /// Construct a dependency scanning tool.
   ///
   /// \param Compilations     The reference to the compilation database that's
   /// used by the clang tool.
-  DependencyScanningTool(const tooling::CompilationDatabase &Compilations,
+  DependencyScanningTool(DependencyScanningService &Service,
+                         const tooling::CompilationDatabase &Compilations,
                          SharedStream &OS, SharedStream &Errs)
-      : Compilations(Compilations), OS(OS), Errs(Errs) {}
+      : Worker(Service), Compilations(Compilations), OS(OS), Errs(Errs) {}
 
   /// Computes the dependencies for the given file and prints them out.
   ///
   /// \returns True on error.
   bool runOnFile(const std::string &Input, StringRef CWD) {
     auto MaybeFile = Worker.getDependencyFile(Input, CWD, Compilations);
     if (!MaybeFile) {
       llvm::handleAllErrors(
private:
   SharedStream &Errs;
 };
 
 llvm::cl::opt<bool> Help("h", llvm::cl::desc("Alias for -help"),
                          llvm::cl::Hidden);
 
 llvm::cl::OptionCategory DependencyScannerCategory("Tool options");
 
+static llvm::cl::opt<ScanningMode> ScanMode(
+    "mode",
+    llvm::cl::desc("The preprocessing mode used to compute the dependencies"),
+    llvm::cl::values(
+        clEnumValN(ScanningMode::MinimizedSourcePreprocessing,
+                   "preprocess-minimized-sources",
+                   "The set of dependencies is computed by preprocessing the "
+                   "source files that were minimized to only include the "
+                   "contents that might affect the dependencies"),
+        clEnumValN(ScanningMode::CanonicalPreprocessing, "preprocess",
+                   "The set of dependencies is computed by preprocessing the "
+                   "unmodified source files")),
+    llvm::cl::init(ScanningMode::MinimizedSourcePreprocessing));
+
 llvm::cl::opt<unsigned>
     NumThreads("j", llvm::cl::Optional,
                llvm::cl::desc("Number of worker threads to use (default: use "
                               "all concurrent threads)"),
                llvm::cl::init(0));
 
 llvm::cl::opt<std::string>
     CompilationDB("compilation-database",
AdjustingCompilations->appendArgumentsAdjuster(
         AdjustedArgs.push_back("-sys-header-deps");
         AdjustedArgs.push_back("-Wno-error");
         return AdjustedArgs;
       });
 
   SharedStream Errs(llvm::errs());
   // Print out the dependency results to STDOUT by default.
   SharedStream DependencyOS(llvm::outs());
+
+  DependencyScanningService Service(ScanMode);
   unsigned NumWorkers =
       NumThreads == 0 ? llvm::hardware_concurrency() : NumThreads;
   std::vector<std::unique_ptr<DependencyScanningTool>> WorkerTools;
   for (unsigned I = 0; I < NumWorkers; ++I)
     WorkerTools.push_back(llvm::make_unique<DependencyScanningTool>(
-        *AdjustingCompilations, DependencyOS, Errs));
+        Service, *AdjustingCompilations, DependencyOS, Errs));
 
   std::vector<std::thread> WorkerThreads;
   std::atomic<bool> HadErrors(false);
   std::mutex Lock;
   size_t Index = 0;
 
   llvm::outs() << "Running clang-scan-deps on " << Inputs.size()
                << " files using " << NumWorkers << " workers\n";