Split the llvm::ThreadPool into an abstract base class and an implementation

This decouples the public API used to enqueue tasks and wait for completion
from the actual implementation, and opens up the possibility for clients to
set their own thread pool implementation for the pool.

https://discourse.llvm.org/t/construct-threadpool-from-vector-of-existing-threads/76883

Author: joker-eph

bolt/include/bolt/Core/ParallelUtilities.h

@@ -28,7 +28,6 @@ extern cl::opt<unsigned> TaskCount;
 } // namespace opts
 
 namespace llvm {
-class ThreadPool;
 
 namespace bolt {
 class BinaryContext;
@@ -51,7 +50,7 @@ enum SchedulingPolicy {
 };
 
 /// Return the managed thread pool and initialize it if not initiliazed.
-ThreadPool &getThreadPool();
+ThreadPoolInterface &getThreadPool();
 
 /// Perform the work on each BinaryFunction except those that are accepted
 /// by SkipPredicate, scheduling heuristic is based on SchedPolicy.

lldb/include/lldb/Core/Debugger.h

@@ -52,7 +52,7 @@
 
 namespace llvm {
 class raw_ostream;
-class ThreadPool;
+class ThreadPoolInterface;
 } // namespace llvm
 
 namespace lldb_private {
@@ -499,8 +499,8 @@ class Debugger : public std::enable_shared_from_this<Debugger>,
     return m_broadcaster_manager_sp;
   }
 
-  /// Shared thread poll. Use only with ThreadPoolTaskGroup.
-  static llvm::ThreadPool &GetThreadPool();
+  /// Shared thread pool. Use only with ThreadPoolTaskGroup.
+  static llvm::ThreadPoolInterface &GetThreadPool();
 
   /// Report warning events.
   ///

llvm/include/llvm/Debuginfod/Debuginfod.h

@@ -97,7 +97,7 @@ Expected<std::string> getCachedOrDownloadArtifact(
     StringRef UniqueKey, StringRef UrlPath, StringRef CacheDirectoryPath,
     ArrayRef<StringRef> DebuginfodUrls, std::chrono::milliseconds Timeout);
 
-class ThreadPool;
+class ThreadPoolInterface;
 
 struct DebuginfodLogEntry {
   std::string Message;
@@ -135,7 +135,7 @@ class DebuginfodCollection {
   // error.
   Expected<bool> updateIfStale();
   DebuginfodLog &Log;
-  ThreadPool &Pool;
+  ThreadPoolInterface &Pool;
   Timer UpdateTimer;
   sys::Mutex UpdateMutex;
 
@@ -145,7 +145,7 @@ class DebuginfodCollection {
 
 public:
   DebuginfodCollection(ArrayRef<StringRef> Paths, DebuginfodLog &Log,
-                       ThreadPool &Pool, double MinInterval);
+                       ThreadPoolInterface &Pool, double MinInterval);
   Error update();
   Error updateForever(std::chrono::milliseconds Interval);
   Expected<std::string> findDebugBinaryPath(object::BuildIDRef);

llvm/include/llvm/Support/BalancedPartitioning.h

@@ -50,7 +50,7 @@
 
 namespace llvm {
 
-class ThreadPool;
+class ThreadPoolInterface;
 /// A function with a set of utility nodes where it is beneficial to order two
 /// functions close together if they have similar utility nodes
 class BPFunctionNode {
@@ -115,7 +115,7 @@ class BalancedPartitioning {
   /// threads, so we need to track how many active threads that could spawn more
   /// threads.
   struct BPThreadPool {
-    ThreadPool &TheThreadPool;
+    ThreadPoolInterface &TheThreadPool;
     std::mutex mtx;
     std::condition_variable cv;
     /// The number of threads that could spawn more threads
@@ -128,7 +128,8 @@ class BalancedPartitioning {
     /// acceptable for other threads to add more tasks while blocking on this
     /// call.
     void wait();
-    BPThreadPool(ThreadPool &TheThreadPool) : TheThreadPool(TheThreadPool) {}
+    BPThreadPool(ThreadPoolInterface &TheThreadPool)
+        : TheThreadPool(TheThreadPool) {}
   };
 
   /// Run a recursive bisection of a given list of FunctionNodes

llvm/include/llvm/Support/ThreadPool.h

@@ -32,11 +32,8 @@ namespace llvm {
 
 class ThreadPoolTaskGroup;
 
-/// A ThreadPool for asynchronous parallel execution on a defined number of
-/// threads.
-///
-/// The pool keeps a vector of threads alive, waiting on a condition variable
-/// for some work to become available.
+/// This defines the abstract base interface for a ThreadPool allowing
+/// asynchronous parallel execution on a defined number of threads.
 ///
 /// It is possible to reuse one thread pool for different groups of tasks
 /// by grouping tasks using ThreadPoolTaskGroup. All tasks are processed using
@@ -49,16 +46,31 @@ class ThreadPoolTaskGroup;
 /// available threads are used up by tasks waiting for a task that has no thread
 /// left to run on (this includes waiting on the returned future). It should be
 /// generally safe to wait() for a group as long as groups do not form a cycle.
-class ThreadPool {
+class ThreadPoolInterface {
+  /// The actual method to enqueue a task to be defined by the concrete
+  /// implementation.
+  virtual void asyncEnqueue(std::function<void()> Task,
+                            ThreadPoolTaskGroup *Group) = 0;
+
 public:
-  /// Construct a pool using the hardware strategy \p S for mapping hardware
-  /// execution resources (threads, cores, CPUs)
-  /// Defaults to using the maximum execution resources in the system, but
-  /// accounting for the affinity mask.
-  ThreadPool(ThreadPoolStrategy S = hardware_concurrency());
+  /// Destroying the pool will drain the pending tasks and wait. The current
+  /// thread may participate in the execution of the pending tasks.
+  virtual ~ThreadPoolInterface();
 
-  /// Blocking destructor: the pool will wait for all the threads to complete.
-  ~ThreadPool();
+  /// Blocking wait for all the threads to complete and the queue to be empty.
+  /// It is an error to try to add new tasks while blocking on this call.
+  /// Calling wait() from a task would deadlock waiting for itself.
+  virtual void wait() = 0;
+
+  /// Blocking wait for only all the threads in the given group to complete.
+  /// It is possible to wait even inside a task, but waiting (directly or
+  /// indirectly) on itself will deadlock. If called from a task running on a
+  /// worker thread, the call may process pending tasks while waiting in order
+  /// not to waste the thread.
+  virtual void wait(ThreadPoolTaskGroup &Group) = 0;
+
+  /// Returns the maximum number of worker this pool can eventually grow to.
+  virtual unsigned getMaxConcurrency() const = 0;
 
   /// Asynchronous submission of a task to the pool. The returned future can be
   /// used to wait for the task to finish and is *non-blocking* on destruction.
@@ -92,30 +104,20 @@ class ThreadPool {
                      &Group);
   }
 
-  /// Blocking wait for all the threads to complete and the queue to be empty.
-  /// It is an error to try to add new tasks while blocking on this call.
-  /// Calling wait() from a task would deadlock waiting for itself.
-  void wait();
-
-  /// Blocking wait for only all the threads in the given group to complete.
-  /// It is possible to wait even inside a task, but waiting (directly or
-  /// indirectly) on itself will deadlock. If called from a task running on a
-  /// worker thread, the call may process pending tasks while waiting in order
-  /// not to waste the thread.
-  void wait(ThreadPoolTaskGroup &Group);
-
-  // Returns the maximum number of worker threads in the pool, not the current
-  // number of threads!
-  unsigned getMaxConcurrency() const { return MaxThreadCount; }
-
-  // TODO: misleading legacy name warning!
-  LLVM_DEPRECATED("Use getMaxConcurrency instead", "getMaxConcurrency")
-  unsigned getThreadCount() const { return MaxThreadCount; }
+private:
+  /// Asynchronous submission of a task to the pool. The returned future can be
+  /// used to wait for the task to finish and is *non-blocking* on destruction.
+  template <typename ResTy>
+  std::shared_future<ResTy> asyncImpl(std::function<ResTy()> Task,
+                                      ThreadPoolTaskGroup *Group) {
+    /// Wrap the Task in a std::function<void()> that sets the result of the
+    /// corresponding future.
+    auto R = createTaskAndFuture(Task);
 
-  /// Returns true if the current thread is a worker thread of this thread pool.
-  bool isWorkerThread() const;
+    asyncEnqueue(std::move(R.first), Group);
+    return R.second.share();
+  }
 
-private:
   /// Helpers to create a promise and a callable wrapper of \p Task that sets
   /// the result of the promise. Returns the callable and a future to access the
   /// result.
@@ -140,54 +142,74 @@ class ThreadPool {
             },
             std::move(F)};
   }
+};
+
+/// A ThreadPool implementation using std::threads.
+///
+/// The pool keeps a vector of threads alive, waiting on a condition variable
+/// for some work to become available.
+class StdThreadPool : public ThreadPoolInterface {
+public:
+  /// Construct a pool using the hardware strategy \p S for mapping hardware
+  /// execution resources (threads, cores, CPUs)
+  /// Defaults to using the maximum execution resources in the system, but
+  /// accounting for the affinity mask.
+  StdThreadPool(ThreadPoolStrategy S = hardware_concurrency());
+
+  /// Blocking destructor: the pool will wait for all the threads to complete.
+  ~StdThreadPool() override;
 
+  /// Blocking wait for all the threads to complete and the queue to be empty.
+  /// It is an error to try to add new tasks while blocking on this call.
+  /// Calling wait() from a task would deadlock waiting for itself.
+  void wait() override;
+
+  /// Blocking wait for only all the threads in the given group to complete.
+  /// It is possible to wait even inside a task, but waiting (directly or
+  /// indirectly) on itself will deadlock. If called from a task running on a
+  /// worker thread, the call may process pending tasks while waiting in order
+  /// not to waste the thread.
+  void wait(ThreadPoolTaskGroup &Group) override;
+
+  /// Returns the maximum number of worker threads in the pool, not the current
+  /// number of threads!
+  unsigned getMaxConcurrency() const override { return MaxThreadCount; }
+
+  // TODO: Remove, misleading legacy name warning!
+  LLVM_DEPRECATED("Use getMaxConcurrency instead", "getMaxConcurrency")
+  unsigned getThreadCount() const { return MaxThreadCount; }
+
+  /// Returns true if the current thread is a worker thread of this thread pool.
+  bool isWorkerThread() const;
+
+private:
   /// Returns true if all tasks in the given group have finished (nullptr means
   /// all tasks regardless of their group). QueueLock must be locked.
   bool workCompletedUnlocked(ThreadPoolTaskGroup *Group) const;
 
   /// Asynchronous submission of a task to the pool. The returned future can be
   /// used to wait for the task to finish and is *non-blocking* on destruction.
-  template <typename ResTy>
-  std::shared_future<ResTy> asyncImpl(std::function<ResTy()> Task,
-                                      ThreadPoolTaskGroup *Group) {
-
-#if LLVM_ENABLE_THREADS
-    /// Wrap the Task in a std::function<void()> that sets the result of the
-    /// corresponding future.
-    auto R = createTaskAndFuture(Task);
-
+  void asyncEnqueue(std::function<void()> Task,
+                    ThreadPoolTaskGroup *Group) override {
     int requestedThreads;
     {
       // Lock the queue and push the new task
       std::unique_lock<std::mutex> LockGuard(QueueLock);
 
       // Don't allow enqueueing after disabling the pool
       assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
-      Tasks.emplace_back(std::make_pair(std::move(R.first), Group));
+      Tasks.emplace_back(std::make_pair(std::move(Task), Group));
       requestedThreads = ActiveThreads + Tasks.size();
     }
     QueueCondition.notify_one();
     grow(requestedThreads);
-    return R.second.share();
-
-#else // LLVM_ENABLE_THREADS Disabled
-
-    // Get a Future with launch::deferred execution using std::async
-    auto Future = std::async(std::launch::deferred, std::move(Task)).share();
-    // Wrap the future so that both ThreadPool::wait() can operate and the
-    // returned future can be sync'ed on.
-    Tasks.emplace_back(std::make_pair([Future]() { Future.get(); }, Group));
-    return Future;
-#endif
   }
 
-#if LLVM_ENABLE_THREADS
-  // Grow to ensure that we have at least `requested` Threads, but do not go
-  // over MaxThreadCount.
+  /// Grow to ensure that we have at least `requested` Threads, but do not go
+  /// over MaxThreadCount.
   void grow(int requested);
 
   void processTasks(ThreadPoolTaskGroup *WaitingForGroup);
-#endif
 
   /// Threads in flight
   std::vector<llvm::thread> Threads;
@@ -209,25 +231,66 @@ class ThreadPool {
   /// Number of threads active for tasks in the given group (only non-zero).
   DenseMap<ThreadPoolTaskGroup *, unsigned> ActiveGroups;
 
-#if LLVM_ENABLE_THREADS // avoids warning for unused variable
   /// Signal for the destruction of the pool, asking thread to exit.
   bool EnableFlag = true;
-#endif
 
   const ThreadPoolStrategy Strategy;
 
   /// Maximum number of threads to potentially grow this pool to.
   const unsigned MaxThreadCount;
 };
 
+/// A non-threaded implementation.
+class SingleThreadExecutor : public ThreadPoolInterface {
+public:
+  /// Construct a non-threaded pool, ignoring using the hardware strategy.
+  SingleThreadExecutor(ThreadPoolStrategy ignored = {});
+
+  /// Blocking destructor: the pool will first execute the pending tasks.
+  ~SingleThreadExecutor() override;
+
+  /// Blocking wait for all the tasks to execute first
+  void wait() override;
+
+  /// Blocking wait for only all the tasks in the given group to complete.
+  void wait(ThreadPoolTaskGroup &Group) override;
+
+  /// Returns always 1: there is no concurrency.
+  unsigned getMaxConcurrency() const override { return 1; }
+
+  // TODO: Remove, misleading legacy name warning!
+  LLVM_DEPRECATED("Use getMaxConcurrency instead", "getMaxConcurrency")
+  unsigned getThreadCount() const { return 1; }
+
+  /// Returns true if the current thread is a worker thread of this thread pool.
+  bool isWorkerThread() const;
+
+private:
+  /// Asynchronous submission of a task to the pool. The returned future can be
+  /// used to wait for the task to finish and is *non-blocking* on destruction.
+  void asyncEnqueue(std::function<void()> Task,
+                    ThreadPoolTaskGroup *Group) override {
+    Tasks.emplace_back(std::make_pair(std::move(Task), Group));
+  }
+
+  /// Tasks waiting for execution in the pool.
+  std::deque<std::pair<std::function<void()>, ThreadPoolTaskGroup *>> Tasks;
+};
+
+#if LLVM_ENABLE_THREADS
+using ThreadPool = StdThreadPool;
+#else
+using ThreadPool = SingleThreadExecutor;
+#endif
+
 /// A group of tasks to be run on a thread pool. Thread pool tasks in different
 /// groups can run on the same threadpool but can be waited for separately.
 /// It is even possible for tasks of one group to submit and wait for tasks
 /// of another group, as long as this does not form a loop.
 class ThreadPoolTaskGroup {
 public:
   /// The ThreadPool argument is the thread pool to forward calls to.
-  ThreadPoolTaskGroup(ThreadPool &Pool) : Pool(Pool) {}
+  ThreadPoolTaskGroup(ThreadPoolInterface &Pool) : Pool(Pool) {}
 
   /// Blocking destructor: will wait for all the tasks in the group to complete
   /// by calling ThreadPool::wait().
@@ -244,7 +307,7 @@ class ThreadPoolTaskGroup {
   void wait() { Pool.wait(*this); }
 
 private:
-  ThreadPool &Pool;
+  ThreadPoolInterface &Pool;
 };
 
 } // namespace llvm

llvm/lib/Debuginfod/Debuginfod.cpp

@@ -348,7 +348,8 @@ DebuginfodLogEntry DebuginfodLog::pop() {
 }
 
 DebuginfodCollection::DebuginfodCollection(ArrayRef<StringRef> PathsRef,
-                                           DebuginfodLog &Log, ThreadPool &Pool,
+                                           DebuginfodLog &Log,
+                                           ThreadPoolInterface &Pool,
                                            double MinInterval)
     : Log(Log), Pool(Pool), MinInterval(MinInterval) {
   for (StringRef Path : PathsRef)