[FFmpeg-devel] [PATCH] avutil/libm: correct isnan, isinf compat hacks

[Michael Niedermayer]

On Sat, Nov 14, 2015 at 08:05:59PM -0500, Ganesh Ajjanagadde wrote:
> isnan and isinf are actually macros as per the standard. In particular,
> the existing implementation has incorrect signature. Furthermore, this
> results in undefined behavior for e.g double values outside float range
> as per the standard.
> 
> This patch corrects the undefined behavior for all usage within FFmpeg.
> There are some issues with long double, but they are theoretical at the
> moment. For instance, the usual culprit for lacking isinf and that uses
> this fallback is Microsoft, and on Microsoft, long double = double
> anyway. Furthermore, no client of isinf, isnan in the codebase actually
> uses long double arguments.
> 
> The above issue is harder because long double may be an IEEE 128 bit quad
> (very rare) or a 80 bit extended precision value (on GCC/Clang).
> 
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libavutil/libm.h | 50 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 48 insertions(+), 2 deletions(-)
> 
> diff --git a/libavutil/libm.h b/libavutil/libm.h
> index ab5df1b..04d9411 100644
> --- a/libavutil/libm.h
> +++ b/libavutil/libm.h
> @@ -91,23 +91,69 @@ static av_always_inline av_const double hypot(double x, double y)
>  #endif /* HAVE_HYPOT */
>  
>  #if !HAVE_ISINF
> -static av_always_inline av_const int isinf(float x)
> +#undef isinf
> +/* Note: these do not follow the BSD/Apple/GNU convention of returning -1 for
> +-Inf, +1 for Inf, 0 otherwise, but merely follow the POSIX/ISO mandated spec of
> +returning a non-zero value for +/-Inf, 0 otherwise. */
> +static av_always_inline av_const int avpriv_isinff(float x)
>  {
>      uint32_t v = av_float2int(x);
>      if ((v & 0x7f800000) != 0x7f800000)
>          return 0;
>      return !(v & 0x007fffff);
>  }
> +
> +static av_always_inline av_const int avpriv_isinf(double x)
> +{
> +    uint64_t v = av_double2int(x);
> +    if ((v & 0x7ff0000000000000) != 0x7ff0000000000000)
> +        return 0;
> +    return !(v & 0x000fffffffffffff);
> +}
> +

> +static av_always_inline av_const int avpriv_isinfl(long double x)
> +{
> +    // FIXME: just a stub, hard as long double width can vary between platforms
> +    // Also currently irrelevant
> +    return avpriv_isinf(x);
> +}

i think we should not add any long double code. It could break
build and is unused

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

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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