[Qemu-devel] [PATCH v2] atomic: using memory_order_relaxed for refcnt inc/dec ops

[Liu Ping Fan]

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 <address@hidden>
---
v2:
  update atomics.txt
  fix the dependency MACRO
---
 docs/atomics.txt      |  4 ++++
 include/qemu/atomic.h | 14 ++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/docs/atomics.txt b/docs/atomics.txt
index 6f2997b..c8ad31d 100644
--- a/docs/atomics.txt
+++ b/docs/atomics.txt
@@ -45,6 +45,10 @@ consistency*, where "the result of any execution is the same 
as if the
 operations of all the processors were executed in some sequential order,
 and the operations of each individual processor appear in this sequence
 in the order specified by its program".
+Note that atomic_inc/_dec and atomic_fetch_inc/_dec tend to be exceptions.
+Once gcc provides atomic_* builtins, they adopt *relaxed* memory order.
+The reason is that these pairs are used by the refcnt's ops which is frequent
+and has more effect on performance.
 
 qemu/atomic.h provides the following set of atomic read-modify-write
 operations:
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 0aa8913..e8353d2 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -183,8 +183,15 @@
 #endif
 
 /* Provide shorter names for GCC atomic builtins.  */
+#ifdef __ATOMIC_RELAXED
+/* 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)
+#else
+/* 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)
+#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.  */
+#ifdef __ATOMIC_RELAXED
+/* 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))
+#else
+/* 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))
+#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