[FFmpeg-devel] [PATCH 4/6] truehd: tune VLC decoding for ARM.

[Reimar Döffinger]

On 19.03.2014, at 22:39, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, Mar 19, 2014 at 05:26:19PM +0000, Ben Avison wrote:
>> Profiling on a Raspberry Pi revealed the best performance to correspond
>> with VLC_BITS = 5. Results for overall audio decode and the get_vlc2 function
>> in particular are as follows:
>> 
>>              Before          After
>>              Mean   StdDev   Mean   StdDev  Confidence  Change
>> 6:2 total     348.8  20.1     339.6  15.1    88.8%       +2.7%  (insignificant)
>> 6:2 function  38.1   8.1      26.4   4.1     100.0%      +44.5%
>> 8:2 total     339.1  15.4     324.5  15.5    99.4%       +4.5%
>> 8:2 function  33.8   7.0      27.3   5.6     99.7%       +23.6%
>> 6:6 total     604.6  20.8     572.8  20.6    100.0%      +5.6%
>> 6:6 function  95.8   8.4      68.9   8.2     100.0%      +39.1%
>> 8:8 total     766.4  17.6     741.5  21.2    100.0%      +3.4%
>> 8:8 function  106.0  11.4     86.1   9.9     100.0%      +23.1%
>> ---
>> libavcodec/mlpdec.c |   13 ++++++++++---
>> 1 files changed, 10 insertions(+), 3 deletions(-)
> 
> applied

This sounds to me like cache-tuning.
The Raspberry Pi CPU is really tiny, calling it "optimize for ARM" seems like a misleading title.
There is a good chance that with larger implementations the original values _might_ be faster, just like the smaller ones might be better with e.g. x86 Atoms.
Note: not tested and thus I do not suggest any changes, but pointing out that basing this on CPU architecture is likely far from optimal in many cases.