[FFmpeg-devel] [PATCH] Add the relaxed and acquire/releae flavors of avpriv_atomic_int_get and avpriv_atomic_int_set.

Wan-Teh Chang wtc at google.com
Tue Mar 1 19:08:17 CET 2016


I removed the ff_thread_report_progress and ff_thread_await_progress
changes from this patch. This patch now only changes
libavutil/atomic*. I also added some comments to libavutil/atomic.h to
describe how the acquire and release operations are intended to be

While working on the comments in libavutil/atomic.h, I noticed that
the comment for avpriv_atomic_int_add_and_fetch contradicts the code.
The comment says:

 * @note This does NOT act as a memory barrier. This is primarily
 *       intended for reference counting.

But the primary implementation in libavutil/atomic_gcc.h uses the
sequentially consistent ordering rather than the relaxed ordering:

#define avpriv_atomic_int_add_and_fetch atomic_int_add_and_fetch_gcc
static inline int atomic_int_add_and_fetch_gcc(volatile int *ptr, int inc)
    return __atomic_add_fetch(ptr, inc, __ATOMIC_SEQ_CST);
    return __sync_add_and_fetch(ptr, inc);

How should we make the comment match the code? I suggest that the
default memory order of avpriv_atomic_int_add_and_fetch be the
sequentially consistent ordering, and add a
avpriv_atomic_int_add_and_fetch_relaxed function that does not issue a
memory barrier.

If you agree with my proposal, I can write a separate patch to do that.

Wan-Teh Chang

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