godot/core/templates/safe_refcount.h

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/*  safe_refcount.h                                                       */
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#ifndef SAFE_REFCOUNT_H
#define SAFE_REFCOUNT_H

#include "core/typedefs.h"

#ifdef DEV_ENABLED
#include "core/error/error_macros.h"
#endif

#include <atomic>
#include <type_traits>

// Design goals for these classes:
// - No automatic conversions or arithmetic operators,
//   to keep explicit the use of atomics everywhere.
// - Using acquire-release semantics, even to set the first value.
//   The first value may be set relaxedly in many cases, but adding the distinction
//   between relaxed and unrelaxed operation to the interface would make it needlessly
//   flexible. There's negligible waste in having release semantics for the initial
//   value and, as an important benefit, you can be sure the value is properly synchronized
//   even with threads that are already running.

// These are used in very specific areas of the engine where it's critical that these guarantees are held
#define SAFE_NUMERIC_TYPE_PUN_GUARANTEES(m_type)                    \
	static_assert(sizeof(SafeNumeric<m_type>) == sizeof(m_type));   \
	static_assert(alignof(SafeNumeric<m_type>) == alignof(m_type)); \
	static_assert(std::is_trivially_destructible_v<std::atomic<m_type>>);
#define SAFE_FLAG_TYPE_PUN_GUARANTEES                \
	static_assert(sizeof(SafeFlag) == sizeof(bool)); \
	static_assert(alignof(SafeFlag) == alignof(bool));

template <typename T>
class SafeNumeric {
	std::atomic<T> value;

	static_assert(std::atomic<T>::is_always_lock_free);

public:
	_ALWAYS_INLINE_ void set(T p_value) {
		value.store(p_value, std::memory_order_release);
	}

	_ALWAYS_INLINE_ T get() const {
		return value.load(std::memory_order_acquire);
	}

	_ALWAYS_INLINE_ T increment() {
		return value.fetch_add(1, std::memory_order_acq_rel) + 1;
	}

	// Returns the original value instead of the new one
	_ALWAYS_INLINE_ T postincrement() {
		return value.fetch_add(1, std::memory_order_acq_rel);
	}

	_ALWAYS_INLINE_ T decrement() {
		return value.fetch_sub(1, std::memory_order_acq_rel) - 1;
	}

	// Returns the original value instead of the new one
	_ALWAYS_INLINE_ T postdecrement() {
		return value.fetch_sub(1, std::memory_order_acq_rel);
	}

	_ALWAYS_INLINE_ T add(T p_value) {
		return value.fetch_add(p_value, std::memory_order_acq_rel) + p_value;
	}

	// Returns the original value instead of the new one
	_ALWAYS_INLINE_ T postadd(T p_value) {
		return value.fetch_add(p_value, std::memory_order_acq_rel);
	}

	_ALWAYS_INLINE_ T sub(T p_value) {
		return value.fetch_sub(p_value, std::memory_order_acq_rel) - p_value;
	}

	_ALWAYS_INLINE_ T bit_or(T p_value) {
		return value.fetch_or(p_value, std::memory_order_acq_rel);
	}
	_ALWAYS_INLINE_ T bit_and(T p_value) {
		return value.fetch_and(p_value, std::memory_order_acq_rel);
	}

	_ALWAYS_INLINE_ T bit_xor(T p_value) {
		return value.fetch_xor(p_value, std::memory_order_acq_rel);
	}

	// Returns the original value instead of the new one
	_ALWAYS_INLINE_ T postsub(T p_value) {
		return value.fetch_sub(p_value, std::memory_order_acq_rel);
	}

	_ALWAYS_INLINE_ T exchange_if_greater(T p_value) {
		while (true) {
			T tmp = value.load(std::memory_order_acquire);
			if (tmp >= p_value) {
				return tmp; // already greater, or equal
			}

			if (value.compare_exchange_weak(tmp, p_value, std::memory_order_acq_rel)) {
				return p_value;
			}
		}
	}

	_ALWAYS_INLINE_ T conditional_increment() {
		while (true) {
			T c = value.load(std::memory_order_acquire);
			if (c == 0) {
				return 0;
			}
			if (value.compare_exchange_weak(c, c + 1, std::memory_order_acq_rel)) {
				return c + 1;
			}
		}
	}

	_ALWAYS_INLINE_ explicit SafeNumeric(T p_value = static_cast<T>(0)) {
		set(p_value);
	}
};

class SafeFlag {
	std::atomic_bool flag;

	static_assert(std::atomic_bool::is_always_lock_free);

public:
	_ALWAYS_INLINE_ bool is_set() const {
		return flag.load(std::memory_order_acquire);
	}

	_ALWAYS_INLINE_ void set() {
		flag.store(true, std::memory_order_release);
	}

	_ALWAYS_INLINE_ void clear() {
		flag.store(false, std::memory_order_release);
	}

	_ALWAYS_INLINE_ void set_to(bool p_value) {
		flag.store(p_value, std::memory_order_release);
	}

	_ALWAYS_INLINE_ explicit SafeFlag(bool p_value = false) {
		set_to(p_value);
	}
};

class SafeRefCount {
	SafeNumeric<uint32_t> count;

#ifdef DEV_ENABLED
	_ALWAYS_INLINE_ void _check_unref_safety() {
		// This won't catch every misuse, but it's better than nothing.
		CRASH_COND_MSG(count.get() == 0,
				"Trying to unreference a SafeRefCount which is already zero is wrong and a symptom of it being misused.\n"
				"Upon a SafeRefCount reaching zero any object whose lifetime is tied to it, as well as the ref count itself, must be destroyed.\n"
				"Moreover, to guarantee that, no multiple threads should be racing to do the final unreferencing to zero.");
	}
#endif

public:
	_ALWAYS_INLINE_ bool ref() { // true on success
		return count.conditional_increment() != 0;
	}

	_ALWAYS_INLINE_ uint32_t refval() { // none-zero on success
		return count.conditional_increment();
	}

	_ALWAYS_INLINE_ bool unref() { // true if must be disposed of
#ifdef DEV_ENABLED
		_check_unref_safety();
#endif
		return count.decrement() == 0;
	}

	_ALWAYS_INLINE_ uint32_t unrefval() { // 0 if must be disposed of
#ifdef DEV_ENABLED
		_check_unref_safety();
#endif
		return count.decrement();
	}

	_ALWAYS_INLINE_ uint32_t get() const {
		return count.get();
	}

	_ALWAYS_INLINE_ void init(uint32_t p_value = 1) {
		count.set(p_value);
	}
};

#endif // SAFE_REFCOUNT_H