[FFmpeg-devel] [PATCHv2] avutil/libm: correct isnan, isinf compat hacks

Hendrik Leppkes h.leppkes at gmail.com
Tue Nov 24 09:17:18 CET 2015

On Sat, Nov 21, 2015 at 2:53 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Wed, Nov 18, 2015 at 3:04 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>> On Wed, Nov 18, 2015 at 2:58 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>>> On Tue, Nov 17, 2015 at 04:54:10PM -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.
>>>> Note that long double is not handled as it is not used in FFmpeg.
>>>> Furthermore, even if at some point long double gets used, it is likely
>>>> not needed to modify the macro in practice for usage in FFmpeg. See
>>>> below for analysis.
>>>> Getting long double to work strictly per the spec is significantly harder
>>>> since a long double may be an IEEE 128 bit quad (very rare), 80 bit
>>>> extended precision value (on GCC/Clang), or simply double (on recent Microsoft).
>>>> On the other hand, any potential future usage of long double is likely
>>>> for precision (when a platform offers extra precision) and not for range, since
>>>> the range anyway varies and is not as portable as IEEE 754 single/double
>>>> precision. In such cases, the implicit cast to a double is well defined
>>>> and isinf and isnan should work as intended.
>>>> Reviewed-by: Michael Niedermayer <michael at niedermayer.cc>
>>>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>>>> ---
>>>>  libavutil/libm.h | 34 ++++++++++++++++++++++++++++++++--
>>>>  1 file changed, 32 insertions(+), 2 deletions(-)
>>> probably ok
>>> maybe wait a day or 2 before pushing so people can test it on more
>>> obscure platforms
>>> thx
>> ok, will wait for 2 days for the hypot hack as well. Thanks.
> pushed, thanks

nan/inf behavior on VS2012 seems to have broken with this (probably
nan, but as the patch touches both..)
Reverting this particular patch resolves the failures.

(ebur128 was broken before)

Do any other systems use these fallbacks to verify they actually work?

I'm not at home until next week to do any further testing for the time being.

- Hendrik

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