Parallel.cpp

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//===- llvm/Support/Parallel.cpp - Parallel algorithms --------------------===//
//
// 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 "llvm/Support/Parallel.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/ExponentialBackoff.h"
#include "llvm/Support/Jobserver.h"
#include "llvm/Support/ManagedStatic.h"
#include "llvm/Support/Threading.h"
 
#include <atomic>
#include <future>
#include <memory>
#include <mutex>
#include <thread>
#include <vector>
 
llvm::ThreadPoolStrategy llvm::parallel::strategy;
 
namespace llvm {
namespace parallel {
#if LLVM_ENABLE_THREADS
 
#ifdef _WIN32
static thread_local unsigned threadIndex = UINT_MAX;
 
unsigned getThreadIndex() { GET_THREAD_INDEX_IMPL; }
#else
thread_local unsigned threadIndex = UINT_MAX;
#endif
 
namespace detail {
 
namespace {
 
/// An abstract class that takes closures and runs them asynchronously.
class Executor {
public:
  virtual ~Executor() = default;
  virtual void add(std::function<void()> func) = 0;
  virtual size_t getThreadCount() const = 0;
 
  static Executor *getDefaultExecutor();
};
 
/// An implementation of an Executor that runs closures on a thread pool
///   in filo order.
class ThreadPoolExecutor : public Executor {
public:
  explicit ThreadPoolExecutor(ThreadPoolStrategy S) {
    if (S.UseJobserver)
      TheJobserver = JobserverClient::getInstance();
 
    ThreadCount = S.compute_thread_count();
    // Spawn all but one of the threads in another thread as spawning threads
    // can take a while.
    Threads.reserve(ThreadCount);
    Threads.resize(1);
    std::lock_guard<std::mutex> Lock(Mutex);
    // Use operator[] before creating the thread to avoid data race in .size()
    // in 'safe libc++' mode.
    auto &Thread0 = Threads[0];
    Thread0 = std::thread([this, S] {
      for (unsigned I = 1; I < ThreadCount; ++I) {
        Threads.emplace_back([this, S, I] { work(S, I); });
        if (Stop)
          break;
      }
      ThreadsCreated.set_value();
      work(S, 0);
    });
  }
 
  // To make sure the thread pool executor can only be created with a parallel
  // strategy.
  ThreadPoolExecutor() = delete;
 
  void stop() {
    {
      std::lock_guard<std::mutex> Lock(Mutex);
      if (Stop)
        return;
      Stop = true;
    }
    Cond.notify_all();
    ThreadsCreated.get_future().wait();
  }
 
  ~ThreadPoolExecutor() override {
    stop();
    std::thread::id CurrentThreadId = std::this_thread::get_id();
    for (std::thread &T : Threads)
      if (T.get_id() == CurrentThreadId)
        T.detach();
      else
        T.join();
  }
 
  struct Creator {
    static void *call() { return new ThreadPoolExecutor(strategy); }
  };
  struct Deleter {
    static void call(void *Ptr) { ((ThreadPoolExecutor *)Ptr)->stop(); }
  };
 
  void add(std::function<void()> F) override {
    {
      std::lock_guard<std::mutex> Lock(Mutex);
      WorkStack.push_back(std::move(F));
    }
    Cond.notify_one();
  }
 
  size_t getThreadCount() const override { return ThreadCount; }
 
private:
  void work(ThreadPoolStrategy S, unsigned ThreadID) {
    threadIndex = ThreadID;
    S.apply_thread_strategy(ThreadID);
    // Note on jobserver deadlock avoidance:
    // GNU Make grants each invoked process one implicit job slot. Our
    // JobserverClient models this by returning an implicit JobSlot on the
    // first successful tryAcquire() in a process. This guarantees forward
    // progress without requiring a dedicated "always-on" thread here.
 
    static thread_local std::unique_ptr<ExponentialBackoff> Backoff;
 
    while (true) {
      if (TheJobserver) {
        // Jobserver-mode scheduling:
        // - Acquire one job slot (with exponential backoff to avoid busy-wait).
        // - While holding the slot, drain and run tasks from the local queue.
        // - Release the slot when the queue is empty or when shutting down.
        // Rationale: Holding a slot amortizes acquire/release overhead over
        // multiple tasks and avoids requeue/yield churn, while still enforcing
        // the jobserver’s global concurrency limit. With K available slots,
        // up to K workers run tasks in parallel; within each worker tasks run
        // sequentially until the local queue is empty.
        ExponentialBackoff Backoff(std::chrono::hours(24));
        JobSlot Slot;
        do {
          if (Stop)
            return;
          Slot = TheJobserver->tryAcquire();
          if (Slot.isValid())
            break;
        } while (Backoff.waitForNextAttempt());
 
        auto SlotReleaser = llvm::make_scope_exit(
            [&] { TheJobserver->release(std::move(Slot)); });
 
        while (true) {
          std::function<void()> Task;
          {
            std::unique_lock<std::mutex> Lock(Mutex);
            Cond.wait(Lock, [&] { return Stop || !WorkStack.empty(); });
            if (Stop && WorkStack.empty())
              return;
            if (WorkStack.empty())
              break;
            Task = std::move(WorkStack.back());
            WorkStack.pop_back();
          }
          Task();
        }
      } else {
        std::unique_lock<std::mutex> Lock(Mutex);
        Cond.wait(Lock, [&] { return Stop || !WorkStack.empty(); });
        if (Stop)
          break;
        auto Task = std::move(WorkStack.back());
        WorkStack.pop_back();
        Lock.unlock();
        Task();
      }
    }
  }
 
  std::atomic<bool> Stop{false};
  std::vector<std::function<void()>> WorkStack;
  std::mutex Mutex;
  std::condition_variable Cond;
  std::promise<void> ThreadsCreated;
  std::vector<std::thread> Threads;
  unsigned ThreadCount;
 
  JobserverClient *TheJobserver = nullptr;
};
 
// A global raw pointer to the executor. Lifetime is managed by the
// objects created within createExecutor().
static Executor *TheExec = nullptr;
static std::once_flag Flag;
 
// This function will be called exactly once to create the executor.
// It contains the necessary platform-specific logic. Since functions
// called by std::call_once cannot return value, we have to set the
// executor as a global variable.
void createExecutor() {
#ifdef _WIN32
  // The ManagedStatic enables the ThreadPoolExecutor to be stopped via
  // llvm_shutdown() which allows a "clean" fast exit, e.g. via _exit(). This
  // stops the thread pool and waits for any worker thread creation to complete
  // but does not wait for the threads to finish. The wait for worker thread
  // creation to complete is important as it prevents intermittent crashes on
  // Windows due to a race condition between thread creation and process exit.
  //
  // The ThreadPoolExecutor will only be destroyed when the static unique_ptr to
  // it is destroyed, i.e. in a normal full exit. The ThreadPoolExecutor
  // destructor ensures it has been stopped and waits for worker threads to
  // finish. The wait is important as it prevents intermittent crashes on
  // Windows when the process is doing a full exit.
  //
  // The Windows crashes appear to only occur with the MSVC static runtimes and
  // are more frequent with the debug static runtime.
  //
  // This also prevents intermittent deadlocks on exit with the MinGW runtime.
 
  static ManagedStatic<ThreadPoolExecutor, ThreadPoolExecutor::Creator,
                       ThreadPoolExecutor::Deleter>
      ManagedExec;
  static std::unique_ptr<ThreadPoolExecutor> Exec(&(*ManagedExec));
  TheExec = Exec.get();
#else
  // ManagedStatic is not desired on other platforms. When `Exec` is destroyed
  // by llvm_shutdown(), worker threads will clean up and invoke TLS
  // destructors. This can lead to race conditions if other threads attempt to
  // access TLS objects that have already been destroyed.
  static ThreadPoolExecutor Exec(strategy);
  TheExec = &Exec;
#endif
}
 
Executor *Executor::getDefaultExecutor() {
  // Use std::call_once to lazily and safely initialize the executor.
  std::call_once(Flag, createExecutor);
  return TheExec;
}
} // namespace
} // namespace detail
 
size_t getThreadCount() {
  return detail::Executor::getDefaultExecutor()->getThreadCount();
}
#endif
 
// Latch::sync() called by the dtor may cause one thread to block. If is a dead
// lock if all threads in the default executor are blocked. To prevent the dead
// lock, only allow the root TaskGroup to run tasks parallelly. In the scenario
// of nested parallel_for_each(), only the outermost one runs parallelly.
TaskGroup::TaskGroup()
#if LLVM_ENABLE_THREADS
    : Parallel((parallel::strategy.ThreadsRequested != 1) &&
               (threadIndex == UINT_MAX)) {}
#else
    : Parallel(false) {}
#endif
TaskGroup::~TaskGroup() {
  // We must ensure that all the workloads have finished before decrementing the
  // instances count.
  L.sync();
}
 
void TaskGroup::spawn(std::function<void()> F) {
#if LLVM_ENABLE_THREADS
  if (Parallel) {
    L.inc();
    detail::Executor::getDefaultExecutor()->add([&, F = std::move(F)] {
      F();
      L.dec();
    });
    return;
  }
#endif
  F();
}
 
} // namespace parallel
} // namespace llvm
 
void llvm::parallelFor(size_t Begin, size_t End,
                       llvm::function_ref<void(size_t)> Fn) {
#if LLVM_ENABLE_THREADS
  if (parallel::strategy.ThreadsRequested != 1) {
    auto NumItems = End - Begin;
    // Limit the number of tasks to MaxTasksPerGroup to limit job scheduling
    // overhead on large inputs.
    auto TaskSize = NumItems / parallel::detail::MaxTasksPerGroup;
    if (TaskSize == 0)
      TaskSize = 1;
 
    parallel::TaskGroup TG;
    for (; Begin + TaskSize < End; Begin += TaskSize) {
      TG.spawn([=, &Fn] {
        for (size_t I = Begin, E = Begin + TaskSize; I != E; ++I)
          Fn(I);
      });
    }
    if (Begin != End) {
      TG.spawn([=, &Fn] {
        for (size_t I = Begin; I != End; ++I)
          Fn(I);
      });
    }
    return;
  }
#endif
 
  for (; Begin != End; ++Begin)
    Fn(Begin);
}