[FFmpeg-devel] suggestions for debugging ff_fft_calc_altivec?

Michael Niedermayer michaelni at gmx.at
Tue Aug 13 10:58:25 CEST 2013

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:
and when it helps, pester yuvi to merge them



Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have often repented speaking, but never of holding my tongue.
-- Xenocrates
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