clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp - rust-lang/llvm-project - Git at Google

 //===- 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/SmallVectorMemoryBuffer.h"
 #include "llvm/Support/Threading.h"

 using namespace clang;
 using namespace tooling;
 using namespace dependencies;

 llvm::ErrorOr<DependencyScanningWorkerFilesystem::TentativeEntry>
 DependencyScanningWorkerFilesystem::readFile(StringRef Filename) {
   // Load the file and its content from the file system.
   auto MaybeFile = getUnderlyingFS().openFileForRead(Filename);
   if (!MaybeFile)
     return MaybeFile.getError();
   auto File = std::move(*MaybeFile);

   auto MaybeStat = File->status();
   if (!MaybeStat)
     return MaybeStat.getError();
   auto Stat = std::move(*MaybeStat);

   auto MaybeBuffer = File->getBuffer(Stat.getName());
   if (!MaybeBuffer)
     return MaybeBuffer.getError();
   auto Buffer = std::move(*MaybeBuffer);

   // If the file size changed between read and stat, pretend it didn't.
   if (Stat.getSize() != Buffer->getBufferSize())
     Stat = llvm::vfs::Status::copyWithNewSize(Stat, Buffer->getBufferSize());

   return TentativeEntry(Stat, std::move(Buffer));
 }

 EntryRef DependencyScanningWorkerFilesystem::minimizeIfNecessary(
     const CachedFileSystemEntry &Entry, StringRef Filename, bool Disable) {
   if (Entry.isError() || Entry.isDirectory() || Disable ||
       !shouldMinimize(Filename, Entry.getUniqueID()))
     return EntryRef(/*Minimized=*/false, Filename, Entry);

   CachedFileContents *Contents = Entry.getContents();
   assert(Contents && "contents not initialized");

   // Double-checked locking.
   if (Contents->MinimizedAccess.load())
     return EntryRef(/*Minimized=*/true, Filename, Entry);

   std::lock_guard<std::mutex> GuardLock(Contents->ValueLock);

   // Double-checked locking.
   if (Contents->MinimizedAccess.load())
     return EntryRef(/*Minimized=*/true, Filename, Entry);

   llvm::SmallString<1024> MinimizedFileContents;
   // Minimize the file down to directives that might affect the dependencies.
   SmallVector<minimize_source_to_dependency_directives::Token, 64> Tokens;
   if (minimizeSourceToDependencyDirectives(Contents->Original->getBuffer(),
                                            MinimizedFileContents, Tokens)) {
     // FIXME: Propagate the diagnostic if desired by the client.
     // Use the original file if the minimization failed.
     Contents->MinimizedStorage =
         llvm::MemoryBuffer::getMemBuffer(*Contents->Original);
     Contents->MinimizedAccess.store(Contents->MinimizedStorage.get());
     return EntryRef(/*Minimized=*/true, Filename, Entry);
   }

   // The contents produced by the minimizer must be null terminated.
   assert(MinimizedFileContents.data()[MinimizedFileContents.size()] == '\0' &&
          "not null terminated contents");

   // Compute the skipped PP ranges that speedup skipping over inactive
   // preprocessor blocks.
   llvm::SmallVector<minimize_source_to_dependency_directives::SkippedRange, 32>
       SkippedRanges;
   minimize_source_to_dependency_directives::computeSkippedRanges(Tokens,
                                                                  SkippedRanges);
   PreprocessorSkippedRangeMapping Mapping;
   for (const auto &Range : SkippedRanges) {
     if (Range.Length < 16) {
       // Ignore small ranges as non-profitable.
       // FIXME: This is a heuristic, its worth investigating the tradeoffs
       // when it should be applied.
       continue;
     }
     Mapping[Range.Offset] = Range.Length;
   }
   Contents->PPSkippedRangeMapping = std::move(Mapping);

   Contents->MinimizedStorage = std::make_unique<llvm::SmallVectorMemoryBuffer>(
       std::move(MinimizedFileContents));
   // This function performed double-checked locking using `MinimizedAccess`.
   // Assigning it must be the last thing this function does. If we were to
   // assign it before `PPSkippedRangeMapping`, other threads may skip the
   // critical section (`MinimizedAccess != nullptr`) and access the mappings
   // that are about to be initialized, leading to a data race.
   Contents->MinimizedAccess.store(Contents->MinimizedStorage.get());
   return EntryRef(/*Minimized=*/true, Filename, Entry);
 }

 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().compute_thread_count() / 4);
   CacheShards = std::make_unique<CacheShard[]>(NumShards);
 }

 DependencyScanningFilesystemSharedCache::CacheShard &
 DependencyScanningFilesystemSharedCache::getShardForFilename(
     StringRef Filename) const {
   return CacheShards[llvm::hash_value(Filename) % NumShards];
 }

 DependencyScanningFilesystemSharedCache::CacheShard &
 DependencyScanningFilesystemSharedCache::getShardForUID(
     llvm::sys::fs::UniqueID UID) const {
   auto Hash = llvm::hash_combine(UID.getDevice(), UID.getFile());
   return CacheShards[Hash % NumShards];
 }

 const CachedFileSystemEntry *
 DependencyScanningFilesystemSharedCache::CacheShard::findEntryByFilename(
     StringRef Filename) const {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto It = EntriesByFilename.find(Filename);
   return It == EntriesByFilename.end() ? nullptr : It->getValue();
 }

 const CachedFileSystemEntry *
 DependencyScanningFilesystemSharedCache::CacheShard::findEntryByUID(
     llvm::sys::fs::UniqueID UID) const {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto It = EntriesByUID.find(UID);
   return It == EntriesByUID.end() ? nullptr : It->getSecond();
 }

 const CachedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::CacheShard::
     getOrEmplaceEntryForFilename(StringRef Filename,
                                  llvm::ErrorOr<llvm::vfs::Status> Stat) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto Insertion = EntriesByFilename.insert({Filename, nullptr});
   if (Insertion.second)
     Insertion.first->second =
         new (EntryStorage.Allocate()) CachedFileSystemEntry(std::move(Stat));
   return *Insertion.first->second;
 }

 const CachedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::CacheShard::getOrEmplaceEntryForUID(
     llvm::sys::fs::UniqueID UID, llvm::vfs::Status Stat,
     std::unique_ptr<llvm::MemoryBuffer> Contents) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto Insertion = EntriesByUID.insert({UID, nullptr});
   if (Insertion.second) {
     CachedFileContents *StoredContents = nullptr;
     if (Contents)
       StoredContents = new (ContentsStorage.Allocate())
           CachedFileContents(std::move(Contents));
     Insertion.first->second = new (EntryStorage.Allocate())
         CachedFileSystemEntry(std::move(Stat), StoredContents);
   }
   return *Insertion.first->second;
 }

 const CachedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::CacheShard::
     getOrInsertEntryForFilename(StringRef Filename,
                                 const CachedFileSystemEntry &Entry) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   return *EntriesByFilename.insert({Filename, &Entry}).first->getValue();
 }

 /// Whitelist file extensions that should be minimized, treating no extension as
 /// a source file that should be minimized.
 ///
 /// This is kinda hacky, it would be better if we knew what kind of file Clang
 /// was expecting instead.
 static bool shouldMinimizeBasedOnExtension(StringRef Filename) {
   StringRef Ext = llvm::sys::path::extension(Filename);
   if (Ext.empty())
     return true; // C++ standard library
   return llvm::StringSwitch<bool>(Ext)
       .CasesLower(".c", ".cc", ".cpp", ".c++", ".cxx", true)
       .CasesLower(".h", ".hh", ".hpp", ".h++", ".hxx", true)
       .CasesLower(".m", ".mm", true)
       .CasesLower(".i", ".ii", ".mi", ".mmi", true)
       .CasesLower(".def", ".inc", true)
       .Default(false);
 }

 static bool shouldCacheStatFailures(StringRef Filename) {
   StringRef Ext = llvm::sys::path::extension(Filename);
   if (Ext.empty())
     return false; // This may be the module cache directory.
   // Only cache stat failures on source files.
   return shouldMinimizeBasedOnExtension(Filename);
 }

 void DependencyScanningWorkerFilesystem::disableMinimization(
     StringRef Filename) {
   // Since we're not done setting up `NotToBeMinimized` yet, we need to disable
   // minimization explicitly.
   if (llvm::ErrorOr<EntryRef> Result =
           getOrCreateFileSystemEntry(Filename, /*DisableMinimization=*/true))
     NotToBeMinimized.insert(Result->getStatus().getUniqueID());
 }

 bool DependencyScanningWorkerFilesystem::shouldMinimize(
     StringRef Filename, llvm::sys::fs::UniqueID UID) {
   return shouldMinimizeBasedOnExtension(Filename) &&
          !NotToBeMinimized.contains(UID);
 }

 const CachedFileSystemEntry &
 DependencyScanningWorkerFilesystem::getOrEmplaceSharedEntryForUID(
     TentativeEntry TEntry) {
   auto &Shard = SharedCache.getShardForUID(TEntry.Status.getUniqueID());
   return Shard.getOrEmplaceEntryForUID(TEntry.Status.getUniqueID(),
                                        std::move(TEntry.Status),
                                        std::move(TEntry.Contents));
 }

 const CachedFileSystemEntry *
 DependencyScanningWorkerFilesystem::findEntryByFilenameWithWriteThrough(
     StringRef Filename) {
   if (const auto *Entry = LocalCache.findEntryByFilename(Filename))
     return Entry;
   auto &Shard = SharedCache.getShardForFilename(Filename);
   if (const auto *Entry = Shard.findEntryByFilename(Filename))
     return &LocalCache.insertEntryForFilename(Filename, *Entry);
   return nullptr;
 }

 llvm::ErrorOr<const CachedFileSystemEntry &>
 DependencyScanningWorkerFilesystem::computeAndStoreResult(StringRef Filename) {
   llvm::ErrorOr<llvm::vfs::Status> Stat = getUnderlyingFS().status(Filename);
   if (!Stat) {
     if (!shouldCacheStatFailures(Filename))
       return Stat.getError();
     const auto &Entry =
         getOrEmplaceSharedEntryForFilename(Filename, Stat.getError());
     return insertLocalEntryForFilename(Filename, Entry);
   }

   if (const auto *Entry = findSharedEntryByUID(*Stat))
     return insertLocalEntryForFilename(Filename, *Entry);

   auto TEntry =
       Stat->isDirectory() ? TentativeEntry(*Stat) : readFile(Filename);

   const CachedFileSystemEntry *SharedEntry = [&]() {
     if (TEntry) {
       const auto &UIDEntry = getOrEmplaceSharedEntryForUID(std::move(*TEntry));
       return &getOrInsertSharedEntryForFilename(Filename, UIDEntry);
     }
     return &getOrEmplaceSharedEntryForFilename(Filename, TEntry.getError());
   }();

   return insertLocalEntryForFilename(Filename, *SharedEntry);
 }

 llvm::ErrorOr<EntryRef>
 DependencyScanningWorkerFilesystem::getOrCreateFileSystemEntry(
     StringRef Filename, bool DisableMinimization) {
   if (const auto *Entry = findEntryByFilenameWithWriteThrough(Filename))
     return minimizeIfNecessary(*Entry, Filename, DisableMinimization)
         .unwrapError();
   auto MaybeEntry = computeAndStoreResult(Filename);
   if (!MaybeEntry)
     return MaybeEntry.getError();
   return minimizeIfNecessary(*MaybeEntry, Filename, DisableMinimization)
       .unwrapError();
 }

 llvm::ErrorOr<llvm::vfs::Status>
 DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   SmallString<256> OwnedFilename;
   StringRef Filename = Path.toStringRef(OwnedFilename);

   llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
   if (!Result)
     return Result.getError();
   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)) {}

   static llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
   create(EntryRef Entry,
          ExcludedPreprocessorDirectiveSkipMapping *PPSkipMappings);

   llvm::ErrorOr<llvm::vfs::Status> status() override { return Stat; }

   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;
 };

 } // end anonymous namespace

 llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> MinimizedVFSFile::create(
     EntryRef Entry, ExcludedPreprocessorDirectiveSkipMapping *PPSkipMappings) {
   assert(!Entry.isError() && "error");

   if (Entry.isDirectory())
     return std::make_error_code(std::errc::is_a_directory);

   auto Result = std::make_unique<MinimizedVFSFile>(
       llvm::MemoryBuffer::getMemBuffer(Entry.getContents(),
                                        Entry.getStatus().getName(),
                                        /*RequiresNullTerminator=*/false),
       Entry.getStatus());

   const auto *EntrySkipMappings = Entry.getPPSkippedRangeMapping();
   if (EntrySkipMappings && !EntrySkipMappings->empty() && PPSkipMappings)
     (*PPSkipMappings)[Result->Buffer->getBufferStart()] = EntrySkipMappings;

   return llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>(
       std::unique_ptr<llvm::vfs::File>(std::move(Result)));
 }

 llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
 DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
   SmallString<256> OwnedFilename;
   StringRef Filename = Path.toStringRef(OwnedFilename);

   llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
   if (!Result)
     return Result.getError();
   return MinimizedVFSFile::create(Result.get(), PPSkipMappings);
 }
	//===- 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/SmallVectorMemoryBuffer.h"
	#include "llvm/Support/Threading.h"

	using namespace clang;
	using namespace tooling;
	using namespace dependencies;

	llvm::ErrorOr<DependencyScanningWorkerFilesystem::TentativeEntry>
	DependencyScanningWorkerFilesystem::readFile(StringRef Filename) {
	// Load the file and its content from the file system.
	auto MaybeFile = getUnderlyingFS().openFileForRead(Filename);
	if (!MaybeFile)
	return MaybeFile.getError();
	auto File = std::move(*MaybeFile);

	auto MaybeStat = File->status();
	if (!MaybeStat)
	return MaybeStat.getError();
	auto Stat = std::move(*MaybeStat);

	auto MaybeBuffer = File->getBuffer(Stat.getName());
	if (!MaybeBuffer)
	return MaybeBuffer.getError();
	auto Buffer = std::move(*MaybeBuffer);

	// If the file size changed between read and stat, pretend it didn't.
	if (Stat.getSize() != Buffer->getBufferSize())
	Stat = llvm::vfs::Status::copyWithNewSize(Stat, Buffer->getBufferSize());

	return TentativeEntry(Stat, std::move(Buffer));
	}

	EntryRef DependencyScanningWorkerFilesystem::minimizeIfNecessary(
	const CachedFileSystemEntry &Entry, StringRef Filename, bool Disable) {
	if (Entry.isError() \|\| Entry.isDirectory() \|\| Disable \|\|
	!shouldMinimize(Filename, Entry.getUniqueID()))
	return EntryRef(/Minimized=/false, Filename, Entry);

	CachedFileContents *Contents = Entry.getContents();
	assert(Contents && "contents not initialized");

	// Double-checked locking.
	if (Contents->MinimizedAccess.load())
	return EntryRef(/Minimized=/true, Filename, Entry);

	std::lock_guard<std::mutex> GuardLock(Contents->ValueLock);

	// Double-checked locking.
	if (Contents->MinimizedAccess.load())
	return EntryRef(/Minimized=/true, Filename, Entry);

	llvm::SmallString<1024> MinimizedFileContents;
	// Minimize the file down to directives that might affect the dependencies.
	SmallVector<minimize_source_to_dependency_directives::Token, 64> Tokens;
	if (minimizeSourceToDependencyDirectives(Contents->Original->getBuffer(),
	MinimizedFileContents, Tokens)) {
	// FIXME: Propagate the diagnostic if desired by the client.
	// Use the original file if the minimization failed.
	Contents->MinimizedStorage =
	llvm::MemoryBuffer::getMemBuffer(*Contents->Original);
	Contents->MinimizedAccess.store(Contents->MinimizedStorage.get());
	return EntryRef(/Minimized=/true, Filename, Entry);
	}

	// The contents produced by the minimizer must be null terminated.
	assert(MinimizedFileContents.data()[MinimizedFileContents.size()] == '\0' &&
	"not null terminated contents");

	// Compute the skipped PP ranges that speedup skipping over inactive
	// preprocessor blocks.
	llvm::SmallVector<minimize_source_to_dependency_directives::SkippedRange, 32>
	SkippedRanges;
	minimize_source_to_dependency_directives::computeSkippedRanges(Tokens,
	SkippedRanges);
	PreprocessorSkippedRangeMapping Mapping;
	for (const auto &Range : SkippedRanges) {
	if (Range.Length < 16) {
	// Ignore small ranges as non-profitable.
	// FIXME: This is a heuristic, its worth investigating the tradeoffs
	// when it should be applied.
	continue;
	}
	Mapping[Range.Offset] = Range.Length;
	}
	Contents->PPSkippedRangeMapping = std::move(Mapping);

	Contents->MinimizedStorage = std::make_unique<llvm::SmallVectorMemoryBuffer>(
	std::move(MinimizedFileContents));
	// This function performed double-checked locking using `MinimizedAccess`.
	// Assigning it must be the last thing this function does. If we were to
	// assign it before `PPSkippedRangeMapping`, other threads may skip the
	// critical section (`MinimizedAccess != nullptr`) and access the mappings
	// that are about to be initialized, leading to a data race.
	Contents->MinimizedAccess.store(Contents->MinimizedStorage.get());
	return EntryRef(/Minimized=/true, Filename, Entry);
	}

	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().compute_thread_count() / 4);
	CacheShards = std::make_unique<CacheShard[]>(NumShards);
	}

	DependencyScanningFilesystemSharedCache::CacheShard &
	DependencyScanningFilesystemSharedCache::getShardForFilename(
	StringRef Filename) const {
	return CacheShards[llvm::hash_value(Filename) % NumShards];
	}

	DependencyScanningFilesystemSharedCache::CacheShard &
	DependencyScanningFilesystemSharedCache::getShardForUID(
	llvm::sys::fs::UniqueID UID) const {
	auto Hash = llvm::hash_combine(UID.getDevice(), UID.getFile());
	return CacheShards[Hash % NumShards];
	}

	const CachedFileSystemEntry *
	DependencyScanningFilesystemSharedCache::CacheShard::findEntryByFilename(
	StringRef Filename) const {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto It = EntriesByFilename.find(Filename);
	return It == EntriesByFilename.end() ? nullptr : It->getValue();
	}

	const CachedFileSystemEntry *
	DependencyScanningFilesystemSharedCache::CacheShard::findEntryByUID(
	llvm::sys::fs::UniqueID UID) const {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto It = EntriesByUID.find(UID);
	return It == EntriesByUID.end() ? nullptr : It->getSecond();
	}

	const CachedFileSystemEntry &
	DependencyScanningFilesystemSharedCache::CacheShard::
	getOrEmplaceEntryForFilename(StringRef Filename,
	llvm::ErrorOr<llvm::vfs::Status> Stat) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto Insertion = EntriesByFilename.insert({Filename, nullptr});
	if (Insertion.second)
	Insertion.first->second =
	new (EntryStorage.Allocate()) CachedFileSystemEntry(std::move(Stat));
	return *Insertion.first->second;
	}

	const CachedFileSystemEntry &
	DependencyScanningFilesystemSharedCache::CacheShard::getOrEmplaceEntryForUID(
	llvm::sys::fs::UniqueID UID, llvm::vfs::Status Stat,
	std::unique_ptr<llvm::MemoryBuffer> Contents) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto Insertion = EntriesByUID.insert({UID, nullptr});
	if (Insertion.second) {
	CachedFileContents *StoredContents = nullptr;
	if (Contents)
	StoredContents = new (ContentsStorage.Allocate())
	CachedFileContents(std::move(Contents));
	Insertion.first->second = new (EntryStorage.Allocate())
	CachedFileSystemEntry(std::move(Stat), StoredContents);
	}
	return *Insertion.first->second;
	}

	const CachedFileSystemEntry &
	DependencyScanningFilesystemSharedCache::CacheShard::
	getOrInsertEntryForFilename(StringRef Filename,
	const CachedFileSystemEntry &Entry) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	return *EntriesByFilename.insert({Filename, &Entry}).first->getValue();
	}

	/// Whitelist file extensions that should be minimized, treating no extension as
	/// a source file that should be minimized.
	///
	/// This is kinda hacky, it would be better if we knew what kind of file Clang
	/// was expecting instead.
	static bool shouldMinimizeBasedOnExtension(StringRef Filename) {
	StringRef Ext = llvm::sys::path::extension(Filename);
	if (Ext.empty())
	return true; // C++ standard library
	return llvm::StringSwitch<bool>(Ext)
	.CasesLower(".c", ".cc", ".cpp", ".c++", ".cxx", true)
	.CasesLower(".h", ".hh", ".hpp", ".h++", ".hxx", true)
	.CasesLower(".m", ".mm", true)
	.CasesLower(".i", ".ii", ".mi", ".mmi", true)
	.CasesLower(".def", ".inc", true)
	.Default(false);
	}

	static bool shouldCacheStatFailures(StringRef Filename) {
	StringRef Ext = llvm::sys::path::extension(Filename);
	if (Ext.empty())
	return false; // This may be the module cache directory.
	// Only cache stat failures on source files.
	return shouldMinimizeBasedOnExtension(Filename);
	}

	void DependencyScanningWorkerFilesystem::disableMinimization(
	StringRef Filename) {
	// Since we're not done setting up `NotToBeMinimized` yet, we need to disable
	// minimization explicitly.
	if (llvm::ErrorOr<EntryRef> Result =
	getOrCreateFileSystemEntry(Filename, /DisableMinimization=/true))
	NotToBeMinimized.insert(Result->getStatus().getUniqueID());
	}

	bool DependencyScanningWorkerFilesystem::shouldMinimize(
	StringRef Filename, llvm::sys::fs::UniqueID UID) {
	return shouldMinimizeBasedOnExtension(Filename) &&
	!NotToBeMinimized.contains(UID);
	}

	const CachedFileSystemEntry &
	DependencyScanningWorkerFilesystem::getOrEmplaceSharedEntryForUID(
	TentativeEntry TEntry) {
	auto &Shard = SharedCache.getShardForUID(TEntry.Status.getUniqueID());
	return Shard.getOrEmplaceEntryForUID(TEntry.Status.getUniqueID(),
	std::move(TEntry.Status),
	std::move(TEntry.Contents));
	}

	const CachedFileSystemEntry *
	DependencyScanningWorkerFilesystem::findEntryByFilenameWithWriteThrough(
	StringRef Filename) {
	if (const auto *Entry = LocalCache.findEntryByFilename(Filename))
	return Entry;
	auto &Shard = SharedCache.getShardForFilename(Filename);
	if (const auto *Entry = Shard.findEntryByFilename(Filename))
	return &LocalCache.insertEntryForFilename(Filename, *Entry);
	return nullptr;
	}

	llvm::ErrorOr<const CachedFileSystemEntry &>
	DependencyScanningWorkerFilesystem::computeAndStoreResult(StringRef Filename) {
	llvm::ErrorOr<llvm::vfs::Status> Stat = getUnderlyingFS().status(Filename);
	if (!Stat) {
	if (!shouldCacheStatFailures(Filename))
	return Stat.getError();
	const auto &Entry =
	getOrEmplaceSharedEntryForFilename(Filename, Stat.getError());
	return insertLocalEntryForFilename(Filename, Entry);
	}

	if (const auto Entry = findSharedEntryByUID(Stat))
	return insertLocalEntryForFilename(Filename, *Entry);

	auto TEntry =
	Stat->isDirectory() ? TentativeEntry(*Stat) : readFile(Filename);

	const CachedFileSystemEntry *SharedEntry = [&]() {
	if (TEntry) {
	const auto &UIDEntry = getOrEmplaceSharedEntryForUID(std::move(*TEntry));
	return &getOrInsertSharedEntryForFilename(Filename, UIDEntry);
	}
	return &getOrEmplaceSharedEntryForFilename(Filename, TEntry.getError());
	}();

	return insertLocalEntryForFilename(Filename, *SharedEntry);
	}

	llvm::ErrorOr<EntryRef>
	DependencyScanningWorkerFilesystem::getOrCreateFileSystemEntry(
	StringRef Filename, bool DisableMinimization) {
	if (const auto *Entry = findEntryByFilenameWithWriteThrough(Filename))
	return minimizeIfNecessary(*Entry, Filename, DisableMinimization)
	.unwrapError();
	auto MaybeEntry = computeAndStoreResult(Filename);
	if (!MaybeEntry)
	return MaybeEntry.getError();
	return minimizeIfNecessary(*MaybeEntry, Filename, DisableMinimization)
	.unwrapError();
	}

	llvm::ErrorOr<llvm::vfs::Status>
	DependencyScanningWorkerFilesystem::status(const Twine &Path) {
	SmallString<256> OwnedFilename;
	StringRef Filename = Path.toStringRef(OwnedFilename);

	llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
	if (!Result)
	return Result.getError();
	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)) {}

	static llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
	create(EntryRef Entry,
	ExcludedPreprocessorDirectiveSkipMapping *PPSkipMappings);

	llvm::ErrorOr<llvm::vfs::Status> status() override { return Stat; }

	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;
	};

	} // end anonymous namespace

	llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> MinimizedVFSFile::create(
	EntryRef Entry, ExcludedPreprocessorDirectiveSkipMapping *PPSkipMappings) {
	assert(!Entry.isError() && "error");

	if (Entry.isDirectory())
	return std::make_error_code(std::errc::is_a_directory);

	auto Result = std::make_unique<MinimizedVFSFile>(
	llvm::MemoryBuffer::getMemBuffer(Entry.getContents(),
	Entry.getStatus().getName(),
	/RequiresNullTerminator=/false),
	Entry.getStatus());

	const auto *EntrySkipMappings = Entry.getPPSkippedRangeMapping();
	if (EntrySkipMappings && !EntrySkipMappings->empty() && PPSkipMappings)
	(*PPSkipMappings)[Result->Buffer->getBufferStart()] = EntrySkipMappings;

	return llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>(
	std::unique_ptr<llvm::vfs::File>(std::move(Result)));
	}

	llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
	DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
	SmallString<256> OwnedFilename;
	StringRef Filename = Path.toStringRef(OwnedFilename);

	llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
	if (!Result)
	return Result.getError();
	return MinimizedVFSFile::create(Result.get(), PPSkipMappings);
	}