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atomic: Implement loads on MSVC without RMW operations

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This fixes faults when loading from read-only memory and avoids
cache line bouncing across cores which reduces performance.
This commit is contained in:
Cameron Gutman
2026-05-11 19:52:54 -05:00
parent 4b69833bc5
commit f0d1101920
1 changed files with 29 additions and 5 deletions
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34
src/atomic/SDL_atomic.c
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@@ -263,9 +263,21 @@ int SDL_AtomicGet(SDL_atomic_t *a)
{
#ifdef HAVE_ATOMIC_LOAD_N
return __atomic_load_n(&a->value, __ATOMIC_SEQ_CST);
#elif defined(HAVE_MSC_ATOMICS)
SDL_COMPILE_TIME_ASSERT(atomic_get, sizeof(long) == sizeof(a->value));
return _InterlockedOr((long *)&a->value, 0);
#elif defined(HAVE_MSC_ATOMICS) && (defined(_M_ARM64) || defined(_M_ARM64EC))
SDL_COMPILE_TIME_ASSERT(atomic_get_int, sizeof(__int32) == sizeof(a->value));
return (int)__ldar32((unsigned __int32 *)&a->value);
#elif defined(HAVE_MSC_ATOMICS) && defined(_M_ARM)
SDL_COMPILE_TIME_ASSERT(atomic_get_int, sizeof(__int32) == sizeof(a->value));
__dmb(_ARM_BARRIER_ISH);
int value = __iso_volatile_load32((volatile __int32 *)&a->value);
__dmb(_ARM_BARRIER_ISH);
return value;
#elif defined(HAVE_MSC_ATOMICS) && (defined(_M_X64) || defined(_M_IX86))
SDL_COMPILE_TIME_ASSERT(atomic_get_int, sizeof(int) == sizeof(a->value));
SDL_CompilerBarrier();
int value = *(volatile int *)&a->value;
SDL_CompilerBarrier();
return value;
#elif defined(HAVE_WATCOM_ATOMICS)
return _SDL_xadd_watcom(&a->value, 0);
#elif defined(HAVE_GCC_ATOMICS)
@@ -287,8 +299,20 @@ void *SDL_AtomicGetPtr(void **a)
{
#ifdef HAVE_ATOMIC_LOAD_N
return __atomic_load_n(a, __ATOMIC_SEQ_CST);
#elif defined(HAVE_MSC_ATOMICS)
return _InterlockedCompareExchangePointer(a, NULL, NULL);
#elif defined(HAVE_MSC_ATOMICS) && (defined(_M_ARM64) || defined(_M_ARM64EC))
SDL_COMPILE_TIME_ASSERT(atomic_get_ptr, sizeof(__int64) == sizeof(*a));
return (void *)__ldar64((unsigned __int64 *)a);
#elif defined(HAVE_MSC_ATOMICS) && defined(_M_ARM)
SDL_COMPILE_TIME_ASSERT(atomic_get_ptr, sizeof(__int32) == sizeof(*a));
__dmb(_ARM_BARRIER_ISH);
void *value = (void*)__iso_volatile_load32((volatile __int32 *)a);
__dmb(_ARM_BARRIER_ISH);
return value;
#elif defined(HAVE_MSC_ATOMICS) && (defined(_M_X64) || defined(_M_IX86))
SDL_CompilerBarrier();
void *value = *(void * volatile *)a;
SDL_CompilerBarrier();
return value;
#elif defined(HAVE_GCC_ATOMICS)
return __sync_val_compare_and_swap(a, (void *)0, (void *)0);
#elif defined(__SOLARIS__)