SDL/src/atomic/SDL_spinlock.c

/*
  Simple DirectMedia Layer
  Copyright (C) 1997-2025 Sam Lantinga <slouken@libsdl.org>

  This software is provided 'as-is', without any express or implied
  warranty.  In no event will the authors be held liable for any damages
  arising from the use of this software.

  Permission is granted to anyone to use this software for any purpose,
  including commercial applications, and to alter it and redistribute it
  freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not
     claim that you wrote the original software. If you use this software
     in a product, an acknowledgment in the product documentation would be
     appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be
     misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.
*/
#include "SDL_internal.h"

#if defined(SDL_PLATFORM_WINDOWS)
#include "../core/windows/SDL_windows.h"
#endif

#if !defined(HAVE_GCC_ATOMICS) && defined(SDL_PLATFORM_SOLARIS)
#include <atomic.h>
#endif

#if !defined(HAVE_GCC_ATOMICS) && defined(SDL_PLATFORM_RISCOS)
#include <unixlib/local.h>
#endif

#if defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
#include <xmmintrin.h>
#endif

#ifdef PS2
#include <kernel.h>
#endif

#if !defined(HAVE_GCC_ATOMICS) && defined(SDL_PLATFORM_MACOS)
#include <libkern/OSAtomic.h>
#endif

// This function is where all the magic happens...
bool SDL_TryLockSpinlock(SDL_SpinLock *lock)
{
#if defined(HAVE_GCC_ATOMICS) || defined(HAVE_GCC_SYNC_LOCK_TEST_AND_SET)
    return __sync_lock_test_and_set(lock, 1) == 0;

#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
    return _InterlockedExchange_acq(lock, 1) == 0;

#elif defined(_MSC_VER)
    SDL_COMPILE_TIME_ASSERT(locksize, sizeof(*lock) == sizeof(long));
    return InterlockedExchange((long *)lock, 1) == 0;

#elif defined(__GNUC__) && defined(__arm__) &&               \
    (defined(__ARM_ARCH_3__) || defined(__ARM_ARCH_3M__) ||  \
     defined(__ARM_ARCH_4__) || defined(__ARM_ARCH_4T__) ||  \
     defined(__ARM_ARCH_5__) || defined(__ARM_ARCH_5TE__) || \
     defined(__ARM_ARCH_5TEJ__))
    int result;

#ifdef SDL_PLATFORM_RISCOS
    if (__cpucap_have_rex()) {
        __asm__ __volatile__(
            "ldrex %0, [%2]\nteq   %0, #0\nstrexeq %0, %1, [%2]"
            : "=&r"(result)
            : "r"(1), "r"(lock)
            : "cc", "memory");
        return result == 0;
    }
#endif

    __asm__ __volatile__(
        "swp %0, %1, [%2]\n"
        : "=&r,&r"(result)
        : "r,0"(1), "r,r"(lock)
        : "memory");
    return result == 0;

#elif defined(__GNUC__) && defined(__arm__)
    int result;
    __asm__ __volatile__(
        "ldrex %0, [%2]\nteq   %0, #0\nstrexeq %0, %1, [%2]"
        : "=&r"(result)
        : "r"(1), "r"(lock)
        : "cc", "memory");
    return result == 0;

#elif (defined(__GNUC__) || defined(__TINYC__)) && (defined(__i386__) || defined(__x86_64__))
    int result;
    __asm__ __volatile__(
        "lock ; xchgl %0, (%1)\n"
        : "=r"(result)
        : "r"(lock), "0"(1)
        : "cc", "memory");
    return result == 0;

#elif defined(SDL_PLATFORM_MACOS) || defined(SDL_PLATFORM_IOS) || defined(SDL_PLATFORM_TVOS)
    // Maybe used for PowerPC, but the Intel asm or gcc atomics are favored.
    return OSAtomicCompareAndSwap32Barrier(0, 1, lock);

#elif defined(SDL_PLATFORM_SOLARIS) && defined(_LP64)
    // Used for Solaris with non-gcc compilers.
    return ((int)atomic_cas_64((volatile uint64_t *)lock, 0, 1) == 0);

#elif defined(SDL_PLATFORM_SOLARIS) && !defined(_LP64)
    // Used for Solaris with non-gcc compilers.
    return ((int)atomic_cas_32((volatile uint32_t *)lock, 0, 1) == 0);
#elif defined(PS2)
    uint32_t oldintr;
    bool res = false;
    // disable interruption
    oldintr = DIntr();

    if (*lock == 0) {
        *lock = 1;
        res = true;
    }
    // enable interruption
    if (oldintr) {
        EIntr();
    }
    return res;
#else
    // Terrible terrible damage
    static SDL_Mutex *_spinlock_mutex;

    if (!_spinlock_mutex) {
        // Race condition on first lock...
        _spinlock_mutex = SDL_CreateMutex();
    }
    SDL_LockMutex(_spinlock_mutex);
    if (*lock == 0) {
        *lock = 1;
        SDL_UnlockMutex(_spinlock_mutex);
        return true;
    } else {
        SDL_UnlockMutex(_spinlock_mutex);
        return false;
    }
#endif
}

void SDL_LockSpinlock(SDL_SpinLock *lock)
{
    int iterations = 0;
    // FIXME: Should we have an eventual timeout?
    while (!SDL_TryLockSpinlock(lock)) {
        if (iterations < 32) {
            iterations++;
            SDL_CPUPauseInstruction();
        } else {
            // !!! FIXME: this doesn't definitely give up the current timeslice, it does different things on various platforms.
            SDL_Delay(0);
        }
    }
}

void SDL_UnlockSpinlock(SDL_SpinLock *lock)
{
#if defined(HAVE_GCC_ATOMICS) || defined(HAVE_GCC_SYNC_LOCK_TEST_AND_SET)
    __sync_lock_release(lock);

#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
    _InterlockedExchange_rel(lock, 0);

#elif defined(_MSC_VER)
    _ReadWriteBarrier();
    *lock = 0;

#elif defined(SDL_PLATFORM_SOLARIS)
    // Used for Solaris when not using gcc.
    *lock = 0;
    membar_producer();

#else
    *lock = 0;
#endif
}