virtualx-engine/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"

#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.

// This is 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<std::atomic<m_type>>::value);

template <class 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;
	}

	// 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>(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;

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
		return count.decrement() == 0;
	}

	_ALWAYS_INLINE_ uint32_t unrefval() { // 0 if must be disposed of
		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