From 30de669b32b4b31c67c36171524cd539167fa916 Mon Sep 17 00:00:00 2001
From: Cameron Gutman <[EMAIL REDACTED]>
Date: Sun, 19 Apr 2026 14:06:44 -0500
Subject: [PATCH] atomic: Fix missing full memory barrier on GCC/Clang
__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.
---
CMakeLists.txt | 1 +
src/atomic/SDL_atomic.c | 28 +++++++++++++++++++++++++---
2 files changed, 26 insertions(+), 3 deletions(-)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 00c90ba60a77c..ff79c018a4e45 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2812,6 +2812,7 @@ elseif(PS2)
gskit
dmakit
ps2_drivers
+ atomic
)
elseif(OS2)
diff --git a/src/atomic/SDL_atomic.c b/src/atomic/SDL_atomic.c
index 5f4057bd34928..35c3ca42cd8be 100644
--- a/src/atomic/SDL_atomic.c
+++ b/src/atomic/SDL_atomic.c
@@ -35,12 +35,18 @@
#include <atomic.h>
#endif
-/* The __atomic_load_n() intrinsic showed up in different times for different compilers. */
-#if defined(__GNUC__) && (__GNUC__ >= 5)
+/* 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
-#elif _SDL_HAS_BUILTIN(__atomic_load_n) || (defined(__clang__) && defined(HAVE_GCC_ATOMICS))
+#define HAVE_ATOMIC_EXCHANGE_N 1
+#else
+#if _SDL_HAS_BUILTIN(__atomic_load_n)
#define HAVE_ATOMIC_LOAD_N 1
#endif
+#if _SDL_HAS_BUILTIN(__atomic_exchange_n)
+#define HAVE_ATOMIC_EXCHANGE_N 1
+#endif
+#endif
/* *INDENT-OFF* */ /* clang-format off */
#if defined(__WATCOMC__) && defined(__386__)
@@ -182,7 +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);
@@ -201,7 +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);