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atomic: Fix missing full memory barrier on GCC/Clang

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__sync_lock_test_and_set() is designed for creating locks, not as
a general atomic exchange function. As a result, it only provides
an acquire memory barrier and isn't guaranteed to actually store
the provided value (though it does on architectures we care about).

__atomic_exchange_n() is supported on GCC/Clang for the last ~10
years, so let's use that instead if available. We will keep the
__sync_lock_test_and_set() fallback around for ancient platforms,
but add a full memory barrier to match the documented behavior.

(cherry picked from commit 30de669b32)
This commit is contained in:
Cameron Gutman
2026-04-19 14:06:44 -05:00
committed by Ryan C. Gordon
parent 1549cd38e6
commit 21ac66c8b3
2 changed files with 25 additions and 10 deletions
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1
CMakeLists.txt
View File

@@ -2802,6 +2802,7 @@ elseif(PS2)
gskit
dmakit
ps2_drivers
atomic
)
elseif(OS2)

34
src/atomic/SDL_atomic.c
View File

@@ -35,18 +35,16 @@
#include <atomic.h>
#endif
/* The __atomic_load_n() intrinsic showed up in different times for different compilers. */
#if defined(__clang__)
#if __has_builtin(__atomic_load_n) || defined(HAVE_GCC_ATOMICS)
/* !!! FIXME: this advertises as available in the NDK but uses an external symbol we don't have.
It might be in a later NDK or we might need an extra library? --ryan. */
#if !defined(__ANDROID__)
/* The __atomic intrinsics showed up in different times for different compilers. */
#if (defined(__GNUC__) && (__GNUC__ >= 5)) || (defined(__clang__) && defined(HAVE_GCC_ATOMICS))
#define HAVE_ATOMIC_LOAD_N 1
#define HAVE_ATOMIC_EXCHANGE_N 1
#else
#if _SDL_HAS_BUILTIN(__atomic_load_n)
#define HAVE_ATOMIC_LOAD_N 1
#endif
#endif
#elif defined(__GNUC__)
#if (__GNUC__ >= 5)
#define HAVE_ATOMIC_LOAD_N 1
#if _SDL_HAS_BUILTIN(__atomic_exchange_n)
#define HAVE_ATOMIC_EXCHANGE_N 1
#endif
#endif
@@ -190,7 +188,15 @@ int SDL_AtomicSet(SDL_atomic_t *a, int v)
return _InterlockedExchange((long *)&a->value, v);
#elif defined(HAVE_WATCOM_ATOMICS)
return _SDL_xchg_watcom(&a->value, v);
#elif defined(HAVE_ATOMIC_EXCHANGE_N)
return __atomic_exchange_n(&a->value, v, __ATOMIC_SEQ_CST);
#elif defined(HAVE_GCC_ATOMICS)
/* __sync_lock_test_and_set() is designed for locking rather than a
generic atomic exchange, so it only provides an acquire barrier
and may not store the exact value on all architectures. We prefer
__atomic_exchange_n() instead on all modern compilers.
*/
__sync_synchronize();
return __sync_lock_test_and_set(&a->value, v);
#elif defined(__SOLARIS__)
return (int)atomic_swap_uint((volatile uint_t *)&a->value, v);
@@ -209,7 +215,15 @@ void *SDL_AtomicSetPtr(void **a, void *v)
return _InterlockedExchangePointer(a, v);
#elif defined(HAVE_WATCOM_ATOMICS)
return (void *)_SDL_xchg_watcom((int *)a, (long)v);
#elif defined(HAVE_ATOMIC_EXCHANGE_N)
return __atomic_exchange_n(a, v, __ATOMIC_SEQ_CST);
#elif defined(HAVE_GCC_ATOMICS)
/* __sync_lock_test_and_set() is designed for locking rather than a
generic atomic exchange, so it only provides an acquire barrier
and may not store the exact value on all architectures. We prefer
__atomic_exchange_n() instead on all modern compilers.
*/
__sync_synchronize();
return __sync_lock_test_and_set(a, v);
#elif defined(__SOLARIS__)
return atomic_swap_ptr(a, v);