[FFmpeg-devel] [PATCH 3/5] avcodec/fft_fixed: Hardcode cosine tables to save space

[Andreas Rheinhardt]

Lynne:
> Jan 7, 2021, 00:13 by andreas.rheinhardt at gmail.com:
> 
>> The tables that are used take 256B; the code to initialize them uses
>> 281B here (GCC 9.3, x64, -O3, but in av_cold functions). On top of that,
>> removing this code also allows to remove the array of AVOnce used to
>> guard the cosine tables against multiple initializations; this also
>> removes relocations.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>>  libavcodec/Makefile       |  2 +-
>>  libavcodec/fft.h          | 19 +++++++++---------
>>  libavcodec/fft_fixed.c    | 42 +++++++++++++++++++++++++++++++++++++++
>>  libavcodec/fft_template.c | 27 +++++++++----------------
>>  4 files changed, 62 insertions(+), 28 deletions(-)
>>
> 
> I'm not a big fan of this one. It saves minor amounts of space, introduces
> more hard/soft table init splits, and is for code due to be replaced by
> libavutil/tx anyway.

It actually removes a hard/soft table init split: Before this patch the
cos tables for the 16-bit fixed-point FFT were sometimes hardcoded and
sometimes not; now the latter option doesn't exist any more, reducing
complexity.
(I actually pondered removing the big #if at the beginning of
fft_template.c by moving the code for FFT_FIXED_32 to fft_fixed_32.c and
the code for FFT_FLOAT to fft_float.c.)

- Andreas