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

[Måns Rullgård]

Justin Ruggles <justin.ruggles at gmail.com> writes:

> 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.

Even better.

-- 
M?ns Rullg?rd
mans at mansr.com