[FFmpeg-devel] [PATCH 07/12] lpc: remove VLA in ff_lpc_compute_autocorr

[Justin Ruggles]

M?ns Rullg?rd wrote:

> Justin Ruggles <justin.ruggles at gmail.com> writes:
> 
>> M?ns Rullg?rd wrote:
>>
>>> Justin Ruggles <justin.ruggles at gmail.com> writes:
>>>
>>>> Mans Rullgard wrote:
>>>>
>>>>> ---
>>>>>  libavcodec/lpc.c |    4 +++-
>>>>>  1 files changed, 3 insertions(+), 1 deletions(-)
>>>>>
>>>> looks fine to me.  But I think the sse2 version of this function also
>>>> has the same VLA.
>>> Yes, it does, as I noticed later.  Requiring dsputil functions to
>>> allocate unknown amounts of memory us generally a bad idea.  Can we
>>> think of a better solution?  Make the caller pass a scratch buffer?
>> Well, I am currently working on revising the autocorrelation function,
>> mainly for the ALS encoder.  I'm considering splitting out the
>> windowing+int32-to-double part into a separate function or set of
>> functions.  So maybe we can change the autocorrelation function to take
>> double* input, with constraints that it be aligned and that at least
>> [-lag] be accessible (with negative array values either already zero or
>> containing actual sample values).  This is important for ALS since
>> previous sample values need to be included in certain situations.
> 
> That's all very well, but doesn't really address the issue of the
> dsputil function needing to allocate unbounded amounts of scratch
> memory.

If the input is changed to have those constraints, that temporary memory
wouldn't need to be allocated at all.

-Justin