[FFmpeg-devel] [PATCH 1/3] avutil/get_pool: Remove redundant initial atomic operation

[Hendrik Leppkes]

On Sun, Mar 17, 2013 at 6:46 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> 602->442 dezicycles
>
> Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
> ---
>  libavutil/buffer.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/libavutil/buffer.c b/libavutil/buffer.c
> index d268a7f..3475e57 100644
> --- a/libavutil/buffer.c
> +++ b/libavutil/buffer.c
> @@ -239,14 +239,14 @@ void av_buffer_pool_uninit(AVBufferPool **ppool)
>  /* remove the whole buffer list from the pool and return it */
>  static BufferPoolEntry *get_pool(AVBufferPool *pool)
>  {
> -    BufferPoolEntry *cur = NULL, *last = NULL;
> +    BufferPoolEntry *cur = *(void * volatile *)&pool->pool, *last = NULL;
>
> -    do {
> +    while (cur != last) {
>          FFSWAP(BufferPoolEntry*, cur, last);
>          cur = avpriv_atomic_ptr_cas((void * volatile *)&pool->pool, last, NULL);
>          if (!cur)
>              return NULL;
> -    } while (cur != last);
> +    }
>
>      return cur;
>  }
> --
> 1.7.9.5
>


I don't think you can safely assume that the initial get is atomic
because there is no memory barrier, and considering this function is
called only a handful of times per frame, i would stay on the safe
side.