[FFmpeg-devel] [PATCH] lavc/decode: do not use a context variable for function-local data

[James Almer]

On 5/22/2020 4:22 PM, Michael Niedermayer wrote:
> On Fri, May 22, 2020 at 02:32:07PM -0300, James Almer wrote:
>> On 5/22/2020 1:56 PM, Anton Khirnov wrote:
>>> ---
>>>  libavcodec/decode.c | 8 ++++----
>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/libavcodec/decode.c b/libavcodec/decode.c
>>> index f3327d74af..dbecdf3f46 100644
>>> --- a/libavcodec/decode.c
>>> +++ b/libavcodec/decode.c
>>> @@ -586,6 +586,7 @@ static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
>>>  int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt)
>>>  {
>>>      AVCodecInternal *avci = avctx->internal;
>>> +    AVPacket buffer_pkt = { NULL };
>>>      int ret;
>>>  
>>>      if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))
>>> @@ -597,16 +598,15 @@ int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacke
>>>      if (avpkt && !avpkt->size && avpkt->data)
>>>          return AVERROR(EINVAL);
>>>  
>>> -    av_packet_unref(avci->buffer_pkt);
>>>      if (avpkt && (avpkt->data || avpkt->side_data_elems)) {
>>> -        ret = av_packet_ref(avci->buffer_pkt, avpkt);
>>> +        ret = av_packet_ref(&buffer_pkt, avpkt);
>>>          if (ret < 0)
>>>              return ret;
>>>      }
>>>  
>>> -    ret = av_bsf_send_packet(avci->bsf, avci->buffer_pkt);
>>> +    ret = av_bsf_send_packet(avci->bsf, &buffer_pkt);
>>>      if (ret < 0) {
>>> -        av_packet_unref(avci->buffer_pkt);
>>> +        av_packet_unref(&buffer_pkt);
>>>          return ret;
>>>      }
>>
>> What's the gain here? You're not removing the context variable since
>> it's used in the encode framework as well, and you're introducing a
> 
>> stack AVPacket (that, while harmless, is not even properly initialized).
> 
> It would be nice if we could avoid all such code, so that we can extend
> AVPacket like other structs without Major bumping

Yes, some relatively recent commits went in the direction of removing as
much AVPacket on stack/heap usage as possible precisely to remove
sizeof(AVPacket) from the ABI, but at least within lavc it's not a problem.

> 
> thx
> 
> [...]
> 
> 
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