From: Olivier Dion <odion@efficios.com>
Date: Mon, 2 Dec 2024 15:22:01 +0000 (-0500)
Subject: arm: Use atomic builtins for xchg if supported
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=ca62305c8956ae7c953c5f99be356a813c60265a;p=urcu.git

arm: Use atomic builtins for xchg if supported

If the toolchain supports the C11 memory model, then implement
`uatomic_xchg_mo' with `__atomic_exchange_n' instead of
`__sync_lock_test_and_set'.  This reduces the number of memory barriers
except for the default memory order FULL_FENCE.

Change-Id: I2261f93134071e37e152a23bb78b21332844429b
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/include/urcu/uatomic/arm.h b/include/urcu/uatomic/arm.h
index 6923371..d2ceb71 100644
--- a/include/urcu/uatomic/arm.h
+++ b/include/urcu/uatomic/arm.h
@@ -25,6 +25,25 @@ extern "C" {
 #endif
 
 /* xchg */
+
+/*
+ * If the toolchain supports the C11 memory model, then it is safe to implement
+ * `uatomic_xchg()' in term of __atomic builtins.  This has the effect of
+ * reducing the number of emitted memory barriers except for the
+ * CMM_SEQ_CST_FENCE memory order.
+ */
+#ifdef _CMM_TOOLCHAIN_SUPPORT_C11_MM
+#  define uatomic_xchg_mo(addr, v, mo)					\
+	__extension__							\
+	({								\
+		__typeof__((*addr)) _old =				\
+			__atomic_exchange_n(cmm_cast_volatile(addr), v,	\
+					cmm_to_c11(mo));		\
+		cmm_seq_cst_fence_after_atomic(mo);			\
+		_old;							\
+	})
+#else
+
 static inline void _cmm_compat_c11_smp_mb__before_xchg_mo(enum cmm_memorder mo)
 {
 	switch (mo) {
@@ -51,11 +70,12 @@ static inline void _cmm_compat_c11_smp_mb__before_xchg_mo(enum cmm_memorder mo)
  *
  * [1] https://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html
  */
-#define uatomic_xchg_mo(addr, v, mo)				\
+#  define uatomic_xchg_mo(addr, v, mo)				\
 	({							\
 		_cmm_compat_c11_smp_mb__before_xchg_mo(mo);	\
 		__sync_lock_test_and_set(addr, v);		\
 	})
+#endif	/* _CMM_TOOLCHAIN_SUPPORT_C11_MM */
 
 #ifdef __cplusplus
 }