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

[Michael Niedermayer]

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

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

He who knows, does not speak. He who speaks, does not know. -- Lao Tsu
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