[FFmpeg-devel] [PATCH 2/5] avcodec/fft_template: Remove unused fixed-point cosine tables

[Andreas Rheinhardt]

Michael Niedermayer:
> On Thu, Jan 07, 2021 at 12:13:05AM +0100, Andreas Rheinhardt wrote:
>> There are three types of FFTs: floating-point, 32-bit fixed-point and
>> 16-bit fixed-point. The latter has exactly one user: The fixed-point
>> AC-3-encoder; the cosine tables used by it use up to seven bits. The
>> tables corresponding to eight to seventeen bits are unused, as are the
>> FFT functions for these bits.
>>
>> Therefore this commit removes these tables and functions. This is
>> especially beneficial when using hardcoded tables as they take up moreFirst,
>> than 255 KiB. But even without it one saves said unused functions as
>> well as entries in corresponding tables (this also saves relocations).
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>> Thee changes to ARM assembly are honstely untested. I hope someone can
>> test them. Btw: It seems that the ARM assembly code wouldn't be able to
>> deal with an FFT with more than 16 bits (no function for this has been
>> defined), which only worked because no one ever used that many bits with
>> the fixed-point FFT.
>>
>>  libavcodec/arm/fft_fixed_neon.S | 18 ------------------
>>  libavcodec/cos_tablegen.c       |  4 ++--
>>  libavcodec/fft.h                |  4 +++-
>>  libavcodec/fft_fixed.c          |  1 +
>>  libavcodec/fft_template.c       | 31 +++++++++++++++++++++++--------
>>  tests/fate/fft.mak              |  8 ++++++--
>>  6 files changed, 35 insertions(+), 31 deletions(-)
> 
> make -j32 libavcodec/tests/fft-fixed && libavcodec/tests/fft-fixed
> Segmentation fault (core dumped)
> 
> (if you cant repro say so and ill rebuild with debug symbols ...)
> 
> thx
> [...]
> 
1. Lynne has an alternative patchset that makes the only user of
fft_fixed use fft_fixed_32 instead, so this is not important any more.
2. Are you testing the ARM assembly code (for which I ask for a test) or
not? If not, then this surprises me. Did you apply the changes to
fft.mak (some of the tests have been removed as they tested
functionality that was unused (apart from the tests) and has therefore
been removed).

- Andreas