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author | Ben Avison <bavison@riscosopen.org> | 2014-07-11 00:12:31 +0100 |
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committer | Martin Storsjö <martin@martin.st> | 2014-07-18 01:34:08 +0300 |
commit | 5c22e8e4ad0852d61d5c4ba8d67d33fd72339497 (patch) | |
tree | 0203e0320cec765670ccd3388987963262c6272d /doc/nut.texi | |
parent | 2d60444331fca1910510038dd3817bea885c2367 (diff) | |
download | ffmpeg-5c22e8e4ad0852d61d5c4ba8d67d33fd72339497.tar.gz |
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>
Diffstat (limited to 'doc/nut.texi')
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