[Qemu-devel] [PATCH] atomic: using memory_order_relaxed for refcnt inc/dec ops

[Liu Ping Fan <qemulist@gmail.com>]

Refcnt's atomic inc/dec ops are frequent and its idiom need no seq_cst
order. So to get better performance, it worth to adopt _relaxed
other than _seq_cst memory model on them.

We resort to gcc builtins. If gcc supports C11 memory model, __atomic_*
buitlins is used, otherwise __sync_* builtins.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
---
 include/qemu/atomic.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 0aa8913..1f474b7 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -183,8 +183,15 @@
 #endif
 
 /* Provide shorter names for GCC atomic builtins.  */
+#ifndef _GLIBCXX_ATOMIC_BUILTINS
+/* close to C11 memory_order_seq_cst */
 #define atomic_fetch_inc(ptr)  __sync_fetch_and_add(ptr, 1)
 #define atomic_fetch_dec(ptr)  __sync_fetch_and_add(ptr, -1)
+#else
+/* C11 memory_order_relaxed */
+#define atomic_fetch_inc(ptr)  __atomic_fetch_add(ptr, 1, __ATOMIC_RELAXED)
+#define atomic_fetch_dec(ptr)  __atomic_fetch_add(ptr, -1, __ATOMIC_RELAXED)
+#endif
 #define atomic_fetch_add       __sync_fetch_and_add
 #define atomic_fetch_sub       __sync_fetch_and_sub
 #define atomic_fetch_and       __sync_fetch_and_and
@@ -192,8 +199,15 @@
 #define atomic_cmpxchg         __sync_val_compare_and_swap
 
 /* And even shorter names that return void.  */
+#ifndef _GLIBCXX_ATOMIC_BUILTINS
+/* close to C11 memory_order_seq_cst */
 #define atomic_inc(ptr)        ((void) __sync_fetch_and_add(ptr, 1))
 #define atomic_dec(ptr)        ((void) __sync_fetch_and_add(ptr, -1))
+#else
+/* C11 memory_order_relaxed */
+#define atomic_inc(ptr)        ((void) __atomic_fetch_add(ptr, 1, __ATOMIC_RELAXED))
+#define atomic_dec(ptr)        ((void) __atomic_fetch_add(ptr, -1, __ATOMIC_RELAXED))
+#endif
 #define atomic_add(ptr, n)     ((void) __sync_fetch_and_add(ptr, n))
 #define atomic_sub(ptr, n)     ((void) __sync_fetch_and_sub(ptr, n))
 #define atomic_and(ptr, n)     ((void) __sync_fetch_and_and(ptr, n))
-- 
1.8.1.4