Select provider of libav* libraries

[Andreas Cadhalpun]

Hi Sebastian,

On 01.06.2015 17:18, Sebastian Ramacher wrote:
> On 2015-05-31 13:21:07, Andreas Cadhalpun wrote:
>>> I'd rather keep the packaging of the package that is currently called
>>> "libav", because on many architectures, it compiles multiple "flavors"
>>> with hardware optimized flavors (on i386 for instance, there is a
>>> flavor without latest SEE, etc).
>>
>> I had considered producing optimized flavors, but I came to the conclusion
>> that it would complicate debian/rules without much benefit, because
>> the flavors are mostly unnecessary:
>>  * On i386 all the SSE etc. optimizations are guarded by runtime
>>    CPU detection, so that even when compiled with SSE the libraries work
>>    fine on CPUs without SSE.
>>    The only optimization were this doesn't apply are the i686 optimizations,
>>    which currently amount to a dozen assembler lines. So I don't think
>>    this justifies shipping two flavors.
> 
> Besides some assembler lines, the i686 optimized build also allows the compiler
> to use cmov instructions. Would ffmpeg benefit from using them?

I haven't benchmarked this, but I think ffmpeg gets the most important speedup
from the handwritten assembler optimizations. (That's also what I've been told
about the altivec optimizations on powerpc, i.e. that the altivec optimizations
of the compiler are less important than the handwritten ones.)

Best regards,
Andreas