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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 509a36db16)
This commit is contained in:
Cameron Gutman
2026-04-11 13:44:35 -05:00
committed by Ryan C. Gordon
parent 350f873534
commit 97c154791b
2 changed files with 30 additions and 10 deletions
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1
CMakeLists.txt
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@@ -3235,6 +3235,7 @@ elseif(PS2)
gskit
dmakit
ps2_drivers
atomic
)
elseif(N3DS)
sdl_glob_sources("${SDL3_SOURCE_DIR}/src/main/n3ds/*.c")

39
src/atomic/SDL_atomic.c
View File

@@ -33,18 +33,16 @@
#include <atomic.h>
#endif
// The __atomic_load_n() intrinsic showed up in different times for different compilers.
#ifdef __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. */
#ifndef SDL_PLATFORM_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
@@ -182,7 +180,14 @@ int SDL_SetAtomicInt(SDL_AtomicInt *a, int v)
#ifdef HAVE_MSC_ATOMICS
SDL_COMPILE_TIME_ASSERT(atomic_set, sizeof(long) == sizeof(a->value));
return _InterlockedExchange((long *)&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(SDL_PLATFORM_SOLARIS)
SDL_COMPILE_TIME_ASSERT(atomic_set, sizeof(uint_t) == sizeof(a->value));
@@ -201,7 +206,14 @@ Uint32 SDL_SetAtomicU32(SDL_AtomicU32 *a, Uint32 v)
#ifdef HAVE_MSC_ATOMICS
SDL_COMPILE_TIME_ASSERT(atomic_set, sizeof(long) == sizeof(a->value));
return _InterlockedExchange((long *)&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(SDL_PLATFORM_SOLARIS)
SDL_COMPILE_TIME_ASSERT(atomic_set, sizeof(uint_t) == sizeof(a->value));
@@ -219,7 +231,14 @@ void *SDL_SetAtomicPointer(void **a, void *v)
{
#ifdef HAVE_MSC_ATOMICS
return _InterlockedExchangePointer(a, 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(SDL_PLATFORM_SOLARIS)
return atomic_swap_ptr(a, v);