[FFmpeg-devel] [PATCH 1/4] armv6: Accelerate ff_imdct_half for general case (mdct_bits != 6)

[Michael Niedermayer]

On Fri, Jul 11, 2014 at 12:14:28AM +0100, Ben Avison wrote:
> The previous implementation targeted DTS Coherent Acoustics, which only
> requires mdct_bits == 6. This relatively small size lent itself to
> unrolling the loops a small number of times, and encoding offsets
> calculated at assembly time within the load/store instructions of each
> iteration.
> 
> In the more general case (codecs such as AAC and AC3) much larger arrays
> are used - mdct_bits == [8, 9, 11]. The old method does not scale for
> these cases, so more integer registers are used with non-unrolled versions
> of the loops (and with some stack spillage). The postrotation filter loop
> is still unrolled by a factor of 2 to permit the double-buffering of some
> VFP registers to facilitate overlap of neighbouring iterations.
> 
> 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 ff_imdct_half_c / ff_imdct_half_vfp, for the same
> example AAC stream:
> 
>                   Before          After
>                   Mean   StdDev   Mean   StdDev  Confidence  Change
> aac_decode_frame  2368.1 35.8     2117.2 35.3    100.0%      +11.8%
> ff_imdct_half_*   457.5  22.4     251.2  16.2    100.0%      +82.1%
> ---
>  libavcodec/arm/mdct_vfp.S |  146 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 144 insertions(+), 2 deletions(-)

applied

thanks

[...]
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