[FFmpeg-devel] [PATCH 3/4] avcodec/codec: Reorder elements to make AVCodec smaller

[Andreas Rheinhardt]

Nicolas George:
> Andreas Rheinhardt (12021-08-04):
>> Reordering max_lowres is an ABI break.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>> ---
>>  libavcodec/codec.h | 13 +++++++------
>>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> If it breaks API, then I would rather see it done properly rather than
> fortuitously: move capabilities and max_lowres after all the pointers.
> 
> For reference, the robust way of avoiding padding in structures is to
> order fields by decreasing size. Since the public part of AVCodec
> changes rarely, once it is done, it should mostly hold.
> 
It is about more than just padding; I'd like to see elements that belong
together to be together in the structure (like a pointer and a size
field, even if the size field is only four bytes); and commonly used
elements should be at the front. This makes it easier for API users to
get to the important points, it is better for cache locality and it also
means that one can sometimes use a smaller instruction in pointer +
offset addressing. E.g. on x86 offsets up to 128B need only one byte to
encode. Moving caps_internal (the most commonly used of the private
fields) to the front of the private part therefore resulted in savings
of 48B for me (tiny (actually negligible), I know, but this is already
enough to show that ordering everything by decreasing size is not
everything).

- Andreas