From d322e2fbe0369fe58163c5e336469c142b9632e5 Mon Sep 17 00:00:00 2001
From: "Emil J. Tywoniak" <emil@tywoniak.eu>
Date: Tue, 12 May 2026 11:43:32 +0200
Subject: [PATCH] threading: redirect locks to no-op when ENABLE_THREADS=0 or
 undefined YOSYS_ENABLE_THREADS

---
 kernel/threading.cc |  14 +++---
 kernel/threading.h  | 105 +++++++++++++++++++++++---------------------
 2 files changed, 60 insertions(+), 59 deletions(-)

diff --git a/kernel/threading.cc b/kernel/threading.cc
index eda6bb4cb..a49ee7d4e 100644
--- a/kernel/threading.cc
+++ b/kernel/threading.cc
@@ -70,7 +70,7 @@ ThreadPool::ThreadPool(int pool_size, std::function<void(int)> b)
         for (int i = 0; i < pool_size; i++)
                 threads.emplace_back([i, this]{ body(i); });
 #else
-        log_assert(pool_size == 0);
+        (void)pool_size;
 #endif
 }
 
@@ -96,11 +96,13 @@ IntRange item_range_for_worker(int num_items, int thread_num, int num_threads)
 }
 
 ParallelDispatchThreadPool::ParallelDispatchThreadPool(int pool_size)
-		: num_worker_threads_(std::max(1, pool_size) - 1)
-{
 #ifdef YOSYS_ENABLE_THREADS
-	main_to_workers_signal.resize(num_worker_threads_, 0);
+		: num_worker_threads_(std::max(1, pool_size) - 1)
+#else
+		: num_worker_threads_(0)
 #endif
+{
+	main_to_workers_signal.resize(num_worker_threads_, 0);
 	// Don't start the threads until we've constructed all our data members.
 	thread_pool = std::make_unique<ThreadPool>(num_worker_threads_, [this](int thread_num){
 		run_worker(thread_num);
@@ -109,14 +111,12 @@ ParallelDispatchThreadPool::ParallelDispatchThreadPool(int pool_size)
 
 ParallelDispatchThreadPool::~ParallelDispatchThreadPool()
 {
-#ifdef YOSYS_ENABLE_THREADS
 	if (num_worker_threads_ == 0)
 		return;
 	current_work = nullptr;
 	num_active_worker_threads_.store(num_worker_threads_, std::memory_order_relaxed);
 	signal_workers_start();
 	wait_for_workers_done();
-#endif
 }
 
 void ParallelDispatchThreadPool::run(std::function<void(const RunCtx &)> work, int max_threads)
@@ -127,13 +127,11 @@ void ParallelDispatchThreadPool::run(std::function<void(const RunCtx &)> work, i
 		work({{0}, 1});
 		return;
 	}
-#ifdef YOSYS_ENABLE_THREADS
 	num_active_worker_threads_.store(num_active_worker_threads, std::memory_order_relaxed);
 	current_work = &work;
 	signal_workers_start();
 	work({{0}, num_active_worker_threads + 1});
 	wait_for_workers_done();
-#endif
 }
 
 void ParallelDispatchThreadPool::run_worker(int thread_num)
diff --git a/kernel/threading.h b/kernel/threading.h
index 3a31b0633..98d3068c4 100644
--- a/kernel/threading.h
+++ b/kernel/threading.h
@@ -15,6 +15,33 @@
 
 YOSYS_NAMESPACE_BEGIN
 
+// Redirect to no-op to avoid dependence on <mutex>
+// and <condition_variable> in single-threaded builds
+#ifdef YOSYS_ENABLE_THREADS
+using Mutex = std::mutex;
+using CondVar = std::condition_variable;
+template <class M> using UniqueLock = std::unique_lock<M>;
+template <class M> using LockGuard = std::lock_guard<M>;
+#else
+struct Mutex {
+	void lock() {}
+	void unlock() {}
+	bool try_lock() { return true; }
+};
+struct CondVar {
+	template <class L> void wait(L &) {}
+	template <class L, class P> void wait(L &, P) {}
+	void notify_one() {}
+	void notify_all() {}
+};
+template <class M> struct UniqueLock {
+	UniqueLock(M &) {}
+};
+template <class M> struct LockGuard {
+	LockGuard(M &) {}
+};
+#endif
+
 // Concurrent queue implementation. Not fast, but simple.
 // Multi-producer, multi-consumer, optionally bounded.
 // When YOSYS_ENABLE_THREADS is not defined, this is just a non-thread-safe non-blocking deque.
@@ -27,26 +54,20 @@ public:
 	// Push an element into the queue. If it's at capacity, block until there is room.
 	void push_back(T t)
 	{
-#ifdef YOSYS_ENABLE_THREADS
-		std::unique_lock<std::mutex> lock(mutex);
+		UniqueLock<Mutex> lock(mutex);
 		not_full_condition.wait(lock, [this] { return static_cast<int>(contents.size()) < capacity; });
 		if (contents.empty())
 			not_empty_condition.notify_one();
-#endif
 		log_assert(!closed);
 		contents.push_back(std::move(t));
-#ifdef YOSYS_ENABLE_THREADS
 		if (static_cast<int>(contents.size()) < capacity)
 			not_full_condition.notify_one();
-#endif
 	}
 	// Signal that no more elements will be produced. `pop_front()` will return nullopt.
 	void close()
 	{
-#ifdef YOSYS_ENABLE_THREADS
-		std::unique_lock<std::mutex> lock(mutex);
+		UniqueLock<Mutex> lock(mutex);
 		not_empty_condition.notify_all();
-#endif
 		closed = true;
 	}
 	// Pop an element from the queue. Blocks until an element is available
@@ -62,39 +83,28 @@ public:
 		return pop_front_internal(false);
 	}
 private:
-#ifdef YOSYS_ENABLE_THREADS
 	std::optional<T> pop_front_internal(bool wait)
 	{
-		std::unique_lock<std::mutex> lock(mutex);
+		UniqueLock<Mutex> lock(mutex);
 		if (wait) {
 			not_empty_condition.wait(lock, [this] { return !contents.empty() || closed; });
 		}
-#else
-	std::optional<T> pop_front_internal(bool)
-	{
-#endif
 		if (contents.empty())
 			return std::nullopt;
-#ifdef YOSYS_ENABLE_THREADS
 		if (static_cast<int>(contents.size()) == capacity)
 			not_full_condition.notify_one();
-#endif
 		T result = std::move(contents.front());
 		contents.pop_front();
-#ifdef YOSYS_ENABLE_THREADS
 		if (!contents.empty())
 			not_empty_condition.notify_one();
-#endif
 		return std::move(result);
 	}
 
-#ifdef YOSYS_ENABLE_THREADS
-	std::mutex mutex;
+	Mutex mutex;
 	// Signals one waiter thread when the queue changes and is not full.
-	std::condition_variable not_full_condition;
+	CondVar not_full_condition;
 	// Signals one waiter thread when the queue changes and is not empty.
-	std::condition_variable not_empty_condition;
-#endif
+	CondVar not_empty_condition;
 	std::deque<T> contents;
 	int capacity;
 	bool closed = false;
@@ -245,15 +255,14 @@ private:
 	// is maintained.
 	std::atomic<int> num_active_worker_threads_ = 0;
 
-#ifdef YOSYS_ENABLE_THREADS
 	// Not especially efficient for large numbers of threads. Worker wakeup could scale
 	// better by conceptually organising workers into a tree and having workers wake
 	// up their children.
-	std::mutex main_to_workers_signal_mutex;
-	std::condition_variable main_to_workers_signal_cv;
+	Mutex main_to_workers_signal_mutex;
+	CondVar main_to_workers_signal_cv;
 	std::vector<uint8_t> main_to_workers_signal;
 	void signal_workers_start() {
-		std::unique_lock lock(main_to_workers_signal_mutex);
+		UniqueLock<Mutex> lock(main_to_workers_signal_mutex);
 		int num_active_worker_threads = num_active_worker_threads_.load(std::memory_order_relaxed);
 		std::fill(main_to_workers_signal.begin(), main_to_workers_signal.begin() + num_active_worker_threads, 1);
 		// When `num_active_worker_threads_` is small compared to `num_worker_threads_`, we have a "thundering herd"
@@ -261,14 +270,14 @@ private:
 		main_to_workers_signal_cv.notify_all();
 	}
 	void worker_wait_for_start(int thread_num) {
-		std::unique_lock lock(main_to_workers_signal_mutex);
+		UniqueLock<Mutex> lock(main_to_workers_signal_mutex);
 		main_to_workers_signal_cv.wait(lock, [this, thread_num] { return main_to_workers_signal[thread_num] > 0; });
 		main_to_workers_signal[thread_num] = 0;
 	}
 
 	std::atomic<int> done_workers = 0;
-	std::mutex workers_to_main_signal_mutex;
-	std::condition_variable workers_to_main_signal_cv;
+	Mutex workers_to_main_signal_mutex;
+	CondVar workers_to_main_signal_cv;
 	void signal_worker_done() {
 		// Must read `num_active_worker_threads_` before we increment `d`! Otherwise
 		// it is possible we would increment `d`, and then another worker signals the
@@ -277,19 +286,18 @@ private:
 		int num_active_worker_threads = num_active_worker_threads_.load(std::memory_order_relaxed);
 		int d = done_workers.fetch_add(1, std::memory_order_release);
 		if (d + 1 == num_active_worker_threads) {
-			std::unique_lock lock(workers_to_main_signal_mutex);
+			UniqueLock<Mutex> lock(workers_to_main_signal_mutex);
 			workers_to_main_signal_cv.notify_all();
 		}
 	}
 	void wait_for_workers_done() {
-		std::unique_lock lock(workers_to_main_signal_mutex);
+		UniqueLock<Mutex> lock(workers_to_main_signal_mutex);
 		workers_to_main_signal_cv.wait(lock, [this] {
 			int num_active_worker_threads = num_active_worker_threads_.load(std::memory_order_relaxed);
 			return done_workers.load(std::memory_order_acquire) == num_active_worker_threads;
 		});
 		done_workers.store(0, std::memory_order_relaxed);
 	}
-#endif
 	// Ensure `thread_pool` is destroyed before any other members,
 	// forcing all threads to be joined before destroying the
 	// members (e.g. workers_to_main_signal_mutex) they might be using.
@@ -301,15 +309,11 @@ class ConcurrentStack
 {
 public:
 	void push_back(T &&t) {
-#ifdef YOSYS_ENABLE_THREADS
-		std::lock_guard<std::mutex> lock(mutex);
-#endif
+		LockGuard<Mutex> lock(mutex);
 		contents.push_back(std::move(t));
 	}
 	std::optional<T> try_pop_back() {
-#ifdef YOSYS_ENABLE_THREADS
-		std::lock_guard<std::mutex> lock(mutex);
-#endif
+		LockGuard<Mutex> lock(mutex);
 		if (contents.empty())
 			return std::nullopt;
 		T result = std::move(contents.back());
@@ -317,9 +321,7 @@ public:
 		return result;
 	}
 private:
-#ifdef YOSYS_ENABLE_THREADS
-	std::mutex mutex;
-#endif
+	Mutex mutex;
 	std::vector<T> contents;
 };
 
@@ -596,12 +598,12 @@ public:
 			return;
 		bool was_empty;
 		{
-			std::unique_lock lock(thread_state.batches_lock);
+			UniqueLock<Mutex> lock(thread_state.batches_lock);
 			was_empty = thread_state.batches.empty();
 			thread_state.batches.push_back(std::move(thread_state.next_batch));
 		}
 		if (was_empty) {
-			std::unique_lock lock(waiters_lock);
+			UniqueLock<Mutex> lock(waiters_lock);
 			if (num_waiters > 0) {
 				waiters_cv.notify_one();
 			}
@@ -617,7 +619,7 @@ public:
 			return std::move(thread_state.next_batch);
 		// Empty our own work queue first.
 		{
-			std::unique_lock lock(thread_state.batches_lock);
+			UniqueLock<Mutex> lock(thread_state.batches_lock);
 			if (!thread_state.batches.empty()) {
 				std::vector<T> batch = std::move(thread_state.batches.back());
 				thread_state.batches.pop_back();
@@ -634,8 +636,9 @@ public:
 			// them will eventually enter this loop and there will be no further
 			// notifications on waiters_cv, so all will eventually increment
 			// num_waiters and wait, so num_waiters == num_threads()
-			// will become true.
-			std::unique_lock lock(waiters_lock);
+			// will become true. In single-threaded builds, num_threads() is 1,
+			// so we always terminate on the first iteration.
+			UniqueLock<Mutex> lock(waiters_lock);
 			++num_waiters;
 			if (num_waiters == num_threads()) {
 				waiters_cv.notify_all();
@@ -654,7 +657,7 @@ private:
 		for (int i = 1; i < num_threads(); i++) {
 			int other_thread_num = (thread.thread_num + i) % num_threads();
 			ThreadState &other_thread_state = thread_states[other_thread_num];
-			std::unique_lock lock(other_thread_state.batches_lock);
+			UniqueLock<Mutex> lock(other_thread_state.batches_lock);
 			if (!other_thread_state.batches.empty()) {
 				std::vector<T> batch = std::move(other_thread_state.batches.front());
 				other_thread_state.batches.pop_front();
@@ -670,15 +673,15 @@ private:
 		// Entirely thread-local.
 		std::vector<T> next_batch;
 
-		std::mutex batches_lock;
+		Mutex batches_lock;
 		// Only the associated thread ever adds to this, and only at the back.
 		// Other threads can remove elements from the front.
 		std::deque<std::vector<T>> batches;
 	};
 	std::vector<ThreadState> thread_states;
 
-	std::mutex waiters_lock;
-	std::condition_variable waiters_cv;
+	Mutex waiters_lock;
+	CondVar waiters_cv;
 	// Number of threads waiting for work. Their queues are empty.
 	int num_waiters = 0;
 };