[FFmpeg-devel] [PATCH] h264: remove clear_blocks call in threading init.

[Michael Niedermayer]

On Tue, Feb 12, 2013 at 09:18:34AM -0800, Ronald S. Bultje wrote:
> Hi,
> 
> On Tue, Feb 12, 2013 at 9:05 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Mon, Feb 11, 2013 at 06:08:45PM -0800, Ronald S. Bultje wrote:
> >> From: "Ronald S. Bultje" <rsbultje at gmail.com>
> >>
> >> Init code in that if statement goes down from 26716 cycles to 26047
> >> cycles, i.e. the removal of the clear_blocks and smaller memcpy()
> >> together save around 670 cycles.
> >> ---
> >>  libavcodec/h264.c | 7 +++----
> >>  1 file changed, 3 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/libavcodec/h264.c b/libavcodec/h264.c
> >> index 2d6c5e6..994e002 100644
> >> --- a/libavcodec/h264.c
> >> +++ b/libavcodec/h264.c
> >> @@ -1253,7 +1253,9 @@ static int decode_update_thread_context(AVCodecContext *dst,
> >>
> >>          // copy all fields after MpegEnc
> >>          memcpy(&h->s + 1, &h1->s + 1,
> >> -               sizeof(H264Context) - sizeof(MpegEncContext));
> >> +               offsetof(H264Context, intra_gb) - sizeof(MpegEncContext));
> >> +        memcpy(&h->cabac, &h1->cabac,
> >> +               sizeof(H264Context) - offsetof(H264Context, cabac));
> >>          memset(h->sps_buffers, 0, sizeof(h->sps_buffers));
> >>          memset(h->pps_buffers, 0, sizeof(h->pps_buffers));
> >>
> >
> > If either of these fields is moved in the structure or a new field
> > added between then threading can mysteriously break.
> > also the comment above would not match the code anymore
> > iam not sure this is a good idea but if it is done then the fields
> > should be documented in the struct so a developer touching them would
> > know they are special ...
> 
> Isn't that the case already? The non-init code in
> update_thread_context() for mpeg and h264 100% depends on ordering of
> the fields, any field ordering change will break the threading.

yes, the problem exists already with some fields


> 
> (I'm willing to document it, I'm just saying that I don't think I'm
> introducing a new problem that didn't already exist before, and that I
> don't really consider a problem anyway.)
> 
> Ronald
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-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I am the wisest man alive, for I know one thing, and that is that I know
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