[FFmpeg-devel] [PATCHv2] configure+libm.h: add hypot emulation

[Ganesh Ajjanagadde]

On Sun, Nov 15, 2015 at 10:56 AM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> On Sun, Nov 15, 2015 at 10:03:37AM -0500, Ganesh Ajjanagadde wrote:
>> It is known that the naive sqrt(x*x + y*y) approach for computing the
>> hypotenuse suffers from overflow and accuracy issues, see e.g
>> http://www.johndcook.com/blog/2010/06/02/whats-so-hard-about-finding-a-hypotenuse/.
>> This adds hypot support to FFmpeg, a C99 function.
>>
>> On platforms without hypot, this patch does a reaonable workaround, that
>> although not as accurate as GNU libm, is readable and does not suffer
>> from the overflow issue. Improvements can be made separately.
>>
>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>> ---
>>  configure        |  2 ++
>>  libavutil/libm.h | 23 +++++++++++++++++++++++
>>  2 files changed, 25 insertions(+)
>>
>> diff --git a/configure b/configure
>> index d518b21..45df724 100755
>> --- a/configure
>> +++ b/configure
>> @@ -1774,6 +1774,7 @@ MATH_FUNCS="
>>      exp2
>>      exp2f
>>      expf
>> +    hypot
>>      isinf
>>      isnan
>>      ldexpf
>> @@ -5309,6 +5310,7 @@ disabled crystalhd || check_lib libcrystalhd/libcrystalhd_if.h DtsCrystalHDVersi
>>
>>  atan2f_args=2
>>  copysign_args=2
>> +hypot_args=2
>>  ldexpf_args=2
>>  powf_args=2
>>
>> diff --git a/libavutil/libm.h b/libavutil/libm.h
>> index 6c17b28..f7a2b41 100644
>> --- a/libavutil/libm.h
>> +++ b/libavutil/libm.h
>> @@ -102,6 +102,29 @@ static av_always_inline av_const int isnan(float x)
>>  }
>>  #endif /* HAVE_ISNAN */
>>
>> +#if !HAVE_HYPOT
>> +#undef hypot
>> +static inline av_const double hypot(double x, double y)
>> +{
>> +    double ret, temp;
>> +    x = fabs(x);
>> +    y = fabs(y);
>> +
>> +    if (isinf(x) || isinf(y))
>> +        return av_int2double(0x7ff0000000000000);
>
> if either is NaN the result should be NaN i think
> return x+y
> might achive this

No, quoting the man page/standard:
       If x or y is an infinity, positive infinity is returned.

       If x or y is a NaN, and the other argument is not an infinity,
a NaN is returned.

The inf case is handled first because of this.
Otherwise, NaN will anyway propagate and taint the result.

>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
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>
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