virtualx-engine/core/safe_refcount.h

/*************************************************************************/
/*  safe_refcount.h                                                      */
/*************************************************************************/
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/*                           GODOT ENGINE                                */
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/* Copyright (c) 2007-2020 Juan Linietsky, Ariel Manzur.                 */
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#ifndef SAFE_REFCOUNT_H
#define SAFE_REFCOUNT_H

#include "core/os/mutex.h"
#include "core/typedefs.h"
#include "platform_config.h"

// Atomic functions, these are used for multithread safe reference counters!

#ifdef NO_THREADS

/* Bogus implementation unaware of multiprocessing */

template <class T>
static _ALWAYS_INLINE_ T atomic_conditional_increment(volatile T *pw) {
	if (*pw == 0) {
		return 0;
	}

	(*pw)++;

	return *pw;
}

template <class T>
static _ALWAYS_INLINE_ T atomic_decrement(volatile T *pw) {
	(*pw)--;

	return *pw;
}

template <class T>
static _ALWAYS_INLINE_ T atomic_increment(volatile T *pw) {
	(*pw)++;

	return *pw;
}

template <class T, class V>
static _ALWAYS_INLINE_ T atomic_sub(volatile T *pw, volatile V val) {
	(*pw) -= val;

	return *pw;
}

template <class T, class V>
static _ALWAYS_INLINE_ T atomic_add(volatile T *pw, volatile V val) {
	(*pw) += val;

	return *pw;
}

template <class T, class V>
static _ALWAYS_INLINE_ T atomic_exchange_if_greater(volatile T *pw, volatile V val) {
	if (val > *pw) {
		*pw = val;
	}

	return *pw;
}

#elif defined(__GNUC__)

/* Implementation for GCC & Clang */

// GCC guarantees atomic intrinsics for sizes of 1, 2, 4 and 8 bytes.
// Clang states it supports GCC atomic builtins.

template <class T>
static _ALWAYS_INLINE_ T atomic_conditional_increment(volatile T *pw) {
	while (true) {
		T tmp = static_cast<T const volatile &>(*pw);
		if (tmp == 0) {
			return 0; // if zero, can't add to it anymore
		}
		if (__sync_val_compare_and_swap(pw, tmp, tmp + 1) == tmp) {
			return tmp + 1;
		}
	}
}

template <class T>
static _ALWAYS_INLINE_ T atomic_decrement(volatile T *pw) {
	return __sync_sub_and_fetch(pw, 1);
}

template <class T>
static _ALWAYS_INLINE_ T atomic_increment(volatile T *pw) {
	return __sync_add_and_fetch(pw, 1);
}

template <class T, class V>
static _ALWAYS_INLINE_ T atomic_sub(volatile T *pw, volatile V val) {
	return __sync_sub_and_fetch(pw, val);
}

template <class T, class V>
static _ALWAYS_INLINE_ T atomic_add(volatile T *pw, volatile V val) {
	return __sync_add_and_fetch(pw, val);
}

template <class T, class V>
static _ALWAYS_INLINE_ T atomic_exchange_if_greater(volatile T *pw, volatile V val) {
	while (true) {
		T tmp = static_cast<T const volatile &>(*pw);
		if (tmp >= val) {
			return tmp; // already greater, or equal
		}
		if (__sync_val_compare_and_swap(pw, tmp, val) == tmp) {
			return val;
		}
	}
}

#elif defined(_MSC_VER)
// For MSVC use a separate compilation unit to prevent windows.h from polluting
// the global namespace.
uint32_t atomic_conditional_increment(volatile uint32_t *pw);
uint32_t atomic_decrement(volatile uint32_t *pw);
uint32_t atomic_increment(volatile uint32_t *pw);
uint32_t atomic_sub(volatile uint32_t *pw, volatile uint32_t val);
uint32_t atomic_add(volatile uint32_t *pw, volatile uint32_t val);
uint32_t atomic_exchange_if_greater(volatile uint32_t *pw, volatile uint32_t val);

uint64_t atomic_conditional_increment(volatile uint64_t *pw);
uint64_t atomic_decrement(volatile uint64_t *pw);
uint64_t atomic_increment(volatile uint64_t *pw);
uint64_t atomic_sub(volatile uint64_t *pw, volatile uint64_t val);
uint64_t atomic_add(volatile uint64_t *pw, volatile uint64_t val);
uint64_t atomic_exchange_if_greater(volatile uint64_t *pw, volatile uint64_t val);

#else
//no threads supported?
#error Must provide atomic functions for this platform or compiler!
#endif

struct SafeRefCount {
	uint32_t count;

public:
	// destroy() is called when weak_count_ drops to zero.

	_ALWAYS_INLINE_ bool ref() { // true on success

		return atomic_conditional_increment(&count) != 0;
	}

	_ALWAYS_INLINE_ uint32_t refval() { // none-zero on success

		return atomic_conditional_increment(&count);
	}

	_ALWAYS_INLINE_ bool unref() { // true if must be disposed of

		return atomic_decrement(&count) == 0;
	}

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

		return atomic_decrement(&count);
	}

	_ALWAYS_INLINE_ uint32_t get() const { // nothrow

		return count;
	}

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

#endif // SAFE_REFCOUNT_H