From 97c154791b20844026bd7469f56d13af7f0973d7 Mon Sep 17 00:00:00 2001
From: Cameron Gutman <[EMAIL REDACTED]>
Date: Sat, 11 Apr 2026 13:44:35 -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.
(cherry picked from commit 509a36db1648dfb69a644a26db1cf40264c16839)
---
CMakeLists.txt | 1 +
src/atomic/SDL_atomic.c | 39 +++++++++++++++++++++++++++++----------
2 files changed, 30 insertions(+), 10 deletions(-)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index bd51249011ef2..18258e513b7c4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3235,6 +3235,7 @@ elseif(PS2)
gskit
dmakit
ps2_drivers
+ atomic
)
elseif(N3DS)
sdl_glob_sources("${SDL3_SOURCE_DIR}/src/main/n3ds/*.c")
diff --git a/src/atomic/SDL_atomic.c b/src/atomic/SDL_atomic.c
index 40d57929aefda..ee036fee8d34b 100644
--- a/src/atomic/SDL_atomic.c
+++ b/src/atomic/SDL_atomic.c
@@ -33,19 +33,17 @@
#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
-#endif
-#endif
-#elif defined(__GNUC__)
-#if (__GNUC__ >= 5)
+#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
/*
@@ -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);