[FFmpeg-devel] [Updated PATCH 2/4] armv6: Accelerate ff_fft_calc for general case (nbits != 4)

[Michael Niedermayer]

On Fri, Jul 11, 2014 at 11:32:08AM +0100, Ben Avison wrote:
> The previous implementation targeted DTS Coherent Acoustics, which only
> requires nbits == 4 (fft16()). This case was (and still is) linked directly
> rather than being indirected through ff_fft_calc_vfp(), but now the full
> range from radix-4 up to radix-65536 is available. This benefits other codecs
> such as AAC and AC3.
> 
> The implementaion is based upon the C version, with each routine larger than
> radix-16 calling a hierarchy of smaller FFT functions, then performing a
> post-processing pass. This pass benefits a lot from loop unrolling to
> counter the long pipelines in the VFP. A relaxed calling standard also
> reduces the overhead of the call hierarchy, and avoiding the excessive
> inlining performed by GCC probably helps with I-cache utilisation too.
> 
> I benchmarked the result by measuring the number of gperftools samples that
> hit anywhere in the AAC decoder (starting from aac_decode_frame()) or
> specifically in the FFT routines (fft4() to fft512() and pass()) for the
> same sample AAC stream:
> 
>               Before          After
>               Mean   StdDev   Mean   StdDev  Confidence  Change
> Audio decode  2245.5 53.1     1599.6 43.8    100.0%      +40.4%
> FFT routines  940.6  22.0     348.1  20.8    100.0%      +170.2%
> ---
>  libavcodec/arm/fft_init_arm.c |    8 +-
>  libavcodec/arm/fft_vfp.S      |  284 +++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 275 insertions(+), 17 deletions(-)

merged a variant of this patch

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Complexity theory is the science of finding the exact solution to an
approximation. Benchmarking OTOH is finding an approximation of the exact
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