godot/core/os/thread_work_pool.h

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/*  thread_work_pool.h                                                   */
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#ifndef THREAD_WORK_POOL_H
#define THREAD_WORK_POOL_H

#include "core/os/memory.h"
#include "core/os/semaphore.h"
#include "core/os/thread.h"

#include <atomic>

class ThreadWorkPool {
	std::atomic<uint32_t> index;

	struct BaseWork {
		std::atomic<uint32_t> *index = nullptr;
		uint32_t max_elements = 0;
		virtual void work() = 0;
		virtual ~BaseWork() = default;
	};

	template <class C, class M, class U>
	struct Work : public BaseWork {
		C *instance;
		M method;
		U userdata;
		virtual void work() override {
			while (true) {
				uint32_t work_index = index->fetch_add(1, std::memory_order_relaxed);
				if (work_index >= max_elements) {
					break;
				}
				(instance->*method)(work_index, userdata);
			}
		}
	};

	struct ThreadData {
		Thread thread;
		Semaphore start;
		Semaphore completed;
		std::atomic<bool> exit;
		BaseWork *work = nullptr;
	};

	ThreadData *threads = nullptr;
	uint32_t thread_count = 0;
	uint32_t threads_working = 0;
	BaseWork *current_work = nullptr;

	static void _thread_function(void *p_user);

public:
	template <class C, class M, class U>
	void begin_work(uint32_t p_elements, C *p_instance, M p_method, U p_userdata) {
		ERR_FAIL_COND(!threads); //never initialized
		ERR_FAIL_COND(current_work != nullptr);

		index.store(0, std::memory_order_release);

		Work<C, M, U> *w = memnew((Work<C, M, U>));
		w->instance = p_instance;
		w->userdata = p_userdata;
		w->method = p_method;
		w->index = &index;
		w->max_elements = p_elements;

		current_work = w;

		threads_working = MIN(p_elements, thread_count);

		for (uint32_t i = 0; i < threads_working; i++) {
			threads[i].work = w;
			threads[i].start.post();
		}
	}

	bool is_working() const {
		return current_work != nullptr;
	}

	bool is_done_dispatching() const {
		ERR_FAIL_COND_V(current_work == nullptr, true);
		return index.load(std::memory_order_acquire) >= current_work->max_elements;
	}

	uint32_t get_work_index() const {
		ERR_FAIL_COND_V(current_work == nullptr, 0);
		uint32_t idx = index.load(std::memory_order_acquire);
		return MIN(idx, current_work->max_elements);
	}

	void end_work() {
		ERR_FAIL_COND(current_work == nullptr);
		for (uint32_t i = 0; i < threads_working; i++) {
			threads[i].completed.wait();
			threads[i].work = nullptr;
		}

		threads_working = 0;
		memdelete(current_work);
		current_work = nullptr;
	}

	template <class C, class M, class U>
	void do_work(uint32_t p_elements, C *p_instance, M p_method, U p_userdata) {
		switch (p_elements) {
			case 0:
				// Nothing to do, so do nothing.
				break;
			case 1:
				// No value in pushing the work to another thread if it's a single job
				// and we're going to wait for it to finish. Just run it right here.
				(p_instance->*p_method)(0, p_userdata);
				break;
			default:
				// Multiple jobs to do; commence threaded business.
				begin_work(p_elements, p_instance, p_method, p_userdata);
				end_work();
		}
	}

	_FORCE_INLINE_ int get_thread_count() const { return thread_count; }
	void init(int p_thread_count = -1);
	void finish();
	~ThreadWorkPool();
};

#endif // THREAD_POOL_H