godot/core/os/spin_lock.h

/**************************************************************************/
/*  spin_lock.h                                                           */
/**************************************************************************/
/*                         This file is part of:                          */
/*                             GODOT ENGINE                               */
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/* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur.                  */
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#ifndef SPIN_LOCK_H
#define SPIN_LOCK_H

#include "core/typedefs.h"

#ifdef _MSC_VER
#include <intrin.h>
#endif

#if defined(__APPLE__)

#include <os/lock.h>

class SpinLock {
	mutable os_unfair_lock _lock = OS_UNFAIR_LOCK_INIT;

public:
	_ALWAYS_INLINE_ void lock() const {
		os_unfair_lock_lock(&_lock);
	}

	_ALWAYS_INLINE_ void unlock() const {
		os_unfair_lock_unlock(&_lock);
	}
};

#else

#include "core/os/thread.h"

#include <atomic>

_ALWAYS_INLINE_ static void _cpu_pause() {
#if defined(_MSC_VER)
// ----- MSVC.
#if defined(_M_ARM) || defined(_M_ARM64) // ARM.
	__yield();
#elif defined(_M_IX86) || defined(_M_X64) // x86.
	_mm_pause();
#endif
#elif defined(__GNUC__) || defined(__clang__)
// ----- GCC/Clang.
#if defined(__i386__) || defined(__x86_64__) // x86.
	__builtin_ia32_pause();
#elif defined(__arm__) || defined(__aarch64__) // ARM.
	asm volatile("yield");
#elif defined(__powerpc__) || defined(__ppc__) || defined(__PPC__) // PowerPC.
	asm volatile("or 27,27,27");
#elif defined(__riscv) // RISC-V.
	asm volatile(".insn i 0x0F, 0, x0, x0, 0x010");
#endif
#endif
}

static_assert(std::atomic_bool::is_always_lock_free);

class alignas(Thread::CACHE_LINE_BYTES) SpinLock {
	mutable std::atomic<bool> locked = ATOMIC_VAR_INIT(false);

public:
	_ALWAYS_INLINE_ void lock() const {
		while (true) {
			bool expected = false;
			if (locked.compare_exchange_weak(expected, true, std::memory_order_acquire, std::memory_order_relaxed)) {
				break;
			}
			do {
				_cpu_pause();
			} while (locked.load(std::memory_order_relaxed));
		}
	}

	_ALWAYS_INLINE_ void unlock() const {
		locked.store(false, std::memory_order_release);
	}
};

#endif // __APPLE__

#endif // SPIN_LOCK_H