Re: [FFmpeg-devel] PATCH - libmad MP3 decoding support

["Martin Storsjö" <martin@martin.st>]

On Mon, 2 May 2022, David Fletcher wrote:

> On 2/5/2022, "Nicolas George" <george@nsup.org> wrote:
>> Is there a trac ticket? If not, please fill one: we would not want to
>> keep that bug.
>>
>> Regards,
>>
>> --
>>  Nicolas George
>
> Hi Nicolas,
>
> I'll prepare a test case to demonstrate the issue and fill in a ticket.
> As far as I can tell this is an issue affecting older ARM hardware, it
> cannot be reproduced on x86 hardware. I now have some additional ARM
> hardware with a more capable CPU which should help pin down the problem.

That sounds like there's an issue with some of the ARM implementations of 
DSP functions, that only exists in implementations for less featureful ARM 
instruction sets.

(It could theoretically be a compiler bug also but I think it's much more 
probable that it's a bug in a seldom used codepath in ffmpeg.)

If you pass "-cpuflags 0" to ffmpeg, does the issue vanish? (If it does, 
it would suggest an issue in e.g. an armv5/armv6 specific implementation 
of some DSP functions.) Does if vanish if you configure with 
--arch=generic? (That would omit all ARM specific DSP routines and just 
use plain C code.)

On a more modern ARM cpu, if it works correctly normally, does the issue 
appear if you use "-cpuflags 0"? That would indicate an issue in one of 
the ARM specific routines that are enabled without cpuflags (e.g. that 
don't require anything above armv4).

// Martin

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