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Sur d’autres sites (13082)

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

    11 juillet 2014, par Ben Avison
    armv6 : Accelerate ff_imdct_half for general case (mdct_bits != 6)

    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%
    

    Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
    • [DH] libavcodec/arm/mdct_vfp.S

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

    16 juillet 2014, par Ben Avison
    armv6 : Accelerate ff_fft_calc for general case (nbits != 4)

    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%
    

    Signed-off-by : Martin Storsjö <martin@martin.st>
    • [DBH] libavcodec/arm/fft_init_arm.c
    • [DBH] libavcodec/arm/fft_vfp.S

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

    11 juillet 2014, par Ben Avison
    armv6 : Accelerate ff_imdct_half for general case (mdct_bits != 6)

    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%
    

    Signed-off-by : Martin Storsjö <martin@martin.st>
    • [DBH] libavcodec/arm/mdct_vfp.S

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