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

@@ -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