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//===--- Threading.h - Abstractions for multithreading -----------*- C++-*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "support/Context.h"
#include "llvm/ADT/FunctionExtras.h"
#include "llvm/ADT/Twine.h"
#include <atomic>
#include <cassert>
#include <condition_variable>
#include <future>
#include <memory>
#include <mutex>
#include <thread>
#include <vector>
namespace clang {
namespace clangd {
/// A threadsafe flag that is initially clear.
class Notification {
// Sets the flag. No-op if already set.
void notify();
// Blocks until flag is set.
void wait() const;
bool Notified = false;
mutable std::condition_variable CV;
mutable std::mutex Mu;
/// Limits the number of threads that can acquire the lock at the same time.
class Semaphore {
Semaphore(std::size_t MaxLocks);
bool try_lock();
void lock();
void unlock();
std::mutex Mutex;
std::condition_variable SlotsChanged;
std::size_t FreeSlots;
/// A point in time we can wait for.
/// Can be zero (don't wait) or infinity (wait forever).
/// (Not time_point::max(), because many std::chrono implementations overflow).
class Deadline {
Deadline(std::chrono::steady_clock::time_point Time)
: Type(Finite), Time(Time) {}
static Deadline zero() { return Deadline(Zero); }
static Deadline infinity() { return Deadline(Infinite); }
std::chrono::steady_clock::time_point time() const {
assert(Type == Finite);
return Time;
bool expired() const {
return (Type == Zero) ||
(Type == Finite && Time < std::chrono::steady_clock::now());
bool operator==(const Deadline &Other) const {
return (Type == Other.Type) && (Type != Finite || Time == Other.Time);
enum Type { Zero, Infinite, Finite };
Deadline(enum Type Type) : Type(Type) {}
enum Type Type;
std::chrono::steady_clock::time_point Time;
/// Makes a deadline from a timeout in seconds. None means wait forever.
Deadline timeoutSeconds(llvm::Optional<double> Seconds);
/// Wait once on CV for the specified duration.
void wait(std::unique_lock<std::mutex> &Lock, std::condition_variable &CV,
Deadline D);
/// Waits on a condition variable until F() is true or D expires.
template <typename Func>
LLVM_NODISCARD bool wait(std::unique_lock<std::mutex> &Lock,
std::condition_variable &CV, Deadline D, Func F) {
while (!F()) {
if (D.expired())
return false;
wait(Lock, CV, D);
return true;
/// Runs tasks on separate (detached) threads and wait for all tasks to finish.
/// Objects that need to spawn threads can own an AsyncTaskRunner to ensure they
/// all complete on destruction.
class AsyncTaskRunner {
/// Destructor waits for all pending tasks to finish.
void wait() const { (void)wait(Deadline::infinity()); }
LLVM_NODISCARD bool wait(Deadline D) const;
// The name is used for tracing and debugging (e.g. to name a spawned thread).
void runAsync(const llvm::Twine &Name, llvm::unique_function<void()> Action);
mutable std::mutex Mutex;
mutable std::condition_variable TasksReachedZero;
std::size_t InFlightTasks = 0;
/// Runs \p Action asynchronously with a new std::thread. The context will be
/// propagated.
template <typename T>
std::future<T> runAsync(llvm::unique_function<T()> Action) {
return std::async(
[](llvm::unique_function<T()> &&Action, Context Ctx) {
WithContext WithCtx(std::move(Ctx));
return Action();
std::move(Action), Context::current().clone());
/// Memoize is a cache to store and reuse computation results based on a key.
/// Memoize<DenseMap<int, bool>> PrimeCache;
/// for (int I : RepetitiveNumbers)
/// if (PrimeCache.get(I, [&] { return expensiveIsPrime(I); }))
/// llvm::errs() << "Prime: " << I << "\n";
/// The computation will only be run once for each key.
/// This class is threadsafe. Concurrent calls for the same key may run the
/// computation multiple times, but each call will return the same result.
template <typename Container> class Memoize {
mutable Container Cache;
std::unique_ptr<std::mutex> Mu;
Memoize() : Mu(std::make_unique<std::mutex>()) {}
template <typename T, typename Func>
typename Container::mapped_type get(T &&Key, Func Compute) const {
std::lock_guard<std::mutex> Lock(*Mu);
auto It = Cache.find(Key);
if (It != Cache.end())
return It->second;
// Don't hold the mutex while computing.
auto V = Compute();
std::lock_guard<std::mutex> Lock(*Mu);
auto R = Cache.try_emplace(std::forward<T>(Key), V);
// Insert into cache may fail if we raced with another thread.
if (!R.second)
return R.first->second; // Canonical value, from other thread.
return V;
/// Used to guard an operation that should run at most every N seconds.
/// Usage:
/// mutable PeriodicThrottler ShouldLog(std::chrono::seconds(1));
/// void calledFrequently() {
/// if (ShouldLog())
/// log("this is not spammy");
/// }
/// This class is threadsafe. If multiple threads are involved, then the guarded
/// operation still needs to be threadsafe!
class PeriodicThrottler {
using Stopwatch = std::chrono::steady_clock;
using Rep = Stopwatch::duration::rep;
Rep Period;
std::atomic<Rep> Next;
/// If Period is zero, the throttler will return true every time.
PeriodicThrottler(Stopwatch::duration Period, Stopwatch::duration Delay = {})
: Period(Period.count()),
Next((Stopwatch::now() + Delay).time_since_epoch().count()) {}
/// Returns whether the operation should run at this time.
/// operator() is safe to call concurrently.
bool operator()();
} // namespace clangd
} // namespace clang