[FFmpeg-devel] suggestions for debugging ff_fft_calc_altivec?

[Timothy Gu]

On Aug 13, 2013 11:50 PM, "Pavel Koshevoy" <pkoshevoy at gmail.com> wrote:
>
> On 8/13/13 2:58 AM, Michael Niedermayer wrote:
>>
>> On Sat, Aug 10, 2013 at 10:27:27PM -0600, Pavel Koshevoy wrote:
>>>
>>> Hi,
>>>
>>> I've decided to take a closer look at OSX 10.5 PPC build with
>>> gas-preprocessor.pl.  The last time I tried it more than half a year
>>> ago the result was that some audio decoders didn't sound right (AAC,
>>> AC3 -- both sound like loud white noise).
>>>
>>> I tried again today and compared decoder output from builds with and
>>> without gas-preprocessor.  I've narrowed it down to output of
>>> fft_calc_c (called in ff_imdct_half_c) being different from output of
>>> ff_fft_calc_altivec (called in imdct_half_altivec).
>>>
>>> I am not a PPC assembly expert, nor am I familiar with FFT
>>> implementation details to be able to easily spot what the problem is
>>> with ff_fft_calc_altivec.  Can anyone offer some suggestions to
>>> proceed further?
>>
>> If you havent yet found the issue
>> you could test the working and non working with various test patterns
>> like all 0 input with one coefficient non zero
>> If this still doesnt hint to where the issue is you can comment parts
>> of the asm out to see when the difference disappears
>> the idea would be sort of like bisecting the lines of code to narrow
>> down where a difference is
>>

>> also make sure you use a gas preprocessor with martins fixes:
>> https://github.com/michaelni/gas-preprocessor
>> and when it helps, pester yuvi to merge them

yuvi did not pull in the changes in official libav repo
(github/libav/gas-preprocessor). I suppose your repo is the same as the
libav one.

>>
>
> It appears the problem was with gas-preprocessor -- the one recommended
at http://ffmpeg.org/platform.html doesn't work, the version you've
mentioned works.

Then we should change the link to yuvi's repo to
http://git.libav.org/?p=gas-preprocessor.git;a=summary or
https://github.com/libav/gas-preprocessor

[...]

Timothy