[FFmpeg-devel] [PATCH 2/3] lavu/internal: add avpriv_exp10

[Hendrik Leppkes]

On Fri, Dec 25, 2015 at 6:27 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Fri, Dec 25, 2015 at 9:26 AM, James Almer <jamrial at gmail.com> wrote:
>> On 12/25/2015 2:11 PM, Ganesh Ajjanagadde wrote:
>>> Fast, reasonably accurate 10^x. Alternative of detection of libm exp10 at configure
>>> time is not worth the trouble, since it is anyway not POSIX or ISO C,
>>> and currently only the GNU libm has it. Furthermore, GNU libm's variant
>>> is ~ 2x slower, and is ironically not correctly rounded (2 ulp off) to justify all
>>> that slowdown.
>>>
>>> Reviewed-by: Michael Niedermayer <michael at niedermayer.cc>
>>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>>> ---
>>>  libavutil/internal.h | 19 +++++++++++++++++++
>>>  1 file changed, 19 insertions(+)
>>>
>>> diff --git a/libavutil/internal.h b/libavutil/internal.h
>>> index f86b7fb..ae11601 100644
>>> --- a/libavutil/internal.h
>>> +++ b/libavutil/internal.h
>>> @@ -292,6 +292,25 @@ static av_always_inline av_const int64_t ff_rint64_clip(double a, int64_t amin,
>>>      return res;
>>>  }
>>>
>>> +/**
>>> + * Compute 10^x for floating point values. Note: this function is by no means
>>> + * "correctly rounded", and is meant as a fast, reasonably accurate approximation.
>>> + * For instance, maximum relative error for the double precision variant is
>>> + * ~ 1e-13 for very small and very large values.
>>> + * This is ~2x faster than GNU libm's approach, which is still off by 2ulp on
>>> + * some inputs.
>>> + * @param x exponent
>>> + * @return 10^x
>>> + */
>>> +static av_always_inline double avpriv_exp10(double x)
>>
>> It's an inline function in a header, and internal at that. Just call it ff_exp10.
>
> It is used in avcodec and avfilter. I thought this meant that it
> should be avpriv?
> Personally, I like ff_exp10 as it is shorter.
>

avpriv is a hack for private functions that need to be exported in
shared libraries, since we have a rule to only export av* functions.
inline functions in headers don't need to be exported in a library, so
no need for avpriv. ff* is fine there, and used for other similar
situations.

- Hendrik