armv6: Accelerate ff_imdct_half for general case (mdct_bits != 6)

Multimedia / Libav - Ben Avison [riscosopen.org] - 17 July 2014 17:34 UTC

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%

5c22e8e armv6: Accelerate ff_imdct_half for general case (mdct_bits != 6)
libavcodec/arm/mdct_vfp.S | 146 ++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 144 insertions(+), 2 deletions(-)

Upstream: git.libav.org

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