From 618a78995848158783640ffa3ef31612cfa0d106 Mon Sep 17 00:00:00 2001
From: Romain Vimont <rom1v@videolabs.io>
Date: Thu, 5 Jul 2018 10:30:48 +0200
Subject: [PATCH] core: add atomic refcounter helper
Implement an atomic refcounter with a weak but correct (1) memory order,
and expose a simple API.
(1) See for example "Using weakly ordered C++ atomics correctly" by Hans
Boehm at CppCon 2016 (the refcounting part also applies to C11):
<https://www.youtube.com/watch?v=M15UKpNlpeM&t=45m15s>
Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>
---
include/vlc_atomic.h | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
diff --git a/include/vlc_atomic.h b/include/vlc_atomic.h
index 868918fe8e62..e2d4fe5360a5 100644
--- a/include/vlc_atomic.h
+++ b/include/vlc_atomic.h
@@ -30,7 +30,9 @@
* Atomic operations do not require locking, but they are not very powerful.
*/
+# include <assert.h>
# include <stdatomic.h>
+# include <vlc_common.h>
typedef atomic_uint_least32_t vlc_atomic_float;
@@ -57,4 +59,30 @@ static inline void vlc_atomic_store_float(vlc_atomic_float *atom, float f)
atomic_store(atom, u.i);
}
+typedef struct vlc_atomic_rc_t {
+ atomic_uint refs;
+} vlc_atomic_rc_t;
+
+/** Init the RC to 1 */
+static inline void vlc_atomic_rc_init(vlc_atomic_rc_t *rc)
+{
+ atomic_init(&rc->refs, 1);
+}
+
+/** Increment the RC */
+static inline void vlc_atomic_rc_inc(vlc_atomic_rc_t *rc)
+{
+ unsigned prev = atomic_fetch_add_explicit(&rc->refs, 1, memory_order_relaxed);
+ vlc_assert(prev);
+ VLC_UNUSED(prev);
+}
+
+/** Decrement the RC and return true if it reaches 0 */
+static inline bool vlc_atomic_rc_dec(vlc_atomic_rc_t *rc)
+{
+ unsigned prev = atomic_fetch_sub_explicit(&rc->refs, 1, memory_order_acq_rel);
+ vlc_assert(prev);
+ return prev == 1;
+}
+
#endif
--
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