armv6: Accelerate ff_fft_calc for general case (nbits != 4)

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

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%

87552d5 armv6: Accelerate ff_fft_calc for general case (nbits != 4)
libavcodec/arm/fft_init_arm.c | 8 +-
libavcodec/arm/fft_vfp.S | 264 ++++++++++++++++++++++++++++++++++++++---
2 files changed, 255 insertions(+), 17 deletions(-)

Upstream: git.libav.org

  • Share