msvcrt: Fix bugs in math.c

[Kevin Chan]

Hi Piotr,

Please don't send unrelated changes in one patch, please split it into 2
> patches.
>

Done. Thanks :)


>
>    float CDECL MSVCRT__copysignf( float num, float sign )
>>   {
>> -    /* FIXME: Behaviour for Nan/Inf? */
>> -    if (sign < 0.0)
>> -        return num < 0.0 ? num : -num;
>> -    return num < 0.0 ? -num : num;
>> +    /* FIXME: Behaviour for signbit(NAN) is different in Linux and
>> +     *        Windows, where Windows gives a zero for -NAN
>> +     */
>> +    if (signbit(sign))
>> +        return signbit(num) ? num : -num;
>> +    return signbit(num) ? -num : num;
>>   }
>>
> Why don't you add an isnan() check? Is there any reason for returning
> "signbit(num) ? -num : num" instead of returning num?


Linux and Windows treats NAN differently and I am not quite sure which I
should follow here, so I just leave it for now...
I think the _copysignf() is supposed to return a (-num) when (num < 0 &&
sign >0), so I think this "signbit(num) ? -num : num" might be right.


> It would be nice to also change signbit definition on systems that doesn't
> support it, so it at least works for normal numbers.
>

The signbit() function is actually part of C99, not my implementation, so I
think it should be supported on most systems? Though I am not quite sure...


Thanks,
Kevin
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