[FFmpeg-devel] [PATCH 2/2] libavutil: Make changes in softfloat needed for fixed point aac decoder.

[Nedeljko Babic]

> >
>> >> +static av_always_inline SoftFloat av_div_sf(SoftFloat a, SoftFloat b){
>> >
>> >missing documentation
>> 
>> I'll add it.
>> 
>> >is this exact ?
>> >if not what are the gurantees to the user
>> >
>> 
>> On our tests
>> For 80.3% of input values the output is exact
>> For 19.2% of input values the error is one LSB bit of mantissa
>> For 0.5% of input values the error is two LSB bits of mantissa
>
>I think av_div_sf() should be exact, approximate
>functions should have a clear and distinct name, be that
>av_div_sf_approx() or whatever
>

Ok, I'll rename the function.
Should I leave the original div function in the code?

>
>[...]
>> >
>> >> +    SoftFloat res;
>> >> +    SoftFloat iB, tmp;
>> >> +
>> >> +    if (b.mant != 0)
>> >> +    {
>> >> +        iB = av_recip_sf(b);
>> >> +        /* Newton iteration to double precision */
>> >> +        tmp = av_sub_sf(FLOAT_1, av_mul_sf(b, iB));
>> >> +        iB = av_add_sf(iB, av_mul_sf(iB, tmp));
>> >> +        tmp = av_sub_sf(FLOAT_1, av_mul_sf(b, iB));
>> >> +        iB = av_add_sf(iB, av_mul_sf(iB, tmp));
>> >> +        tmp = av_sub_sf(FLOAT_1, av_mul_sf(b, iB));
>> >> +        iB = av_add_sf(iB, av_mul_sf(iB, tmp));
>> >> +        res = av_mul_sf(a, iB);
>> >> +    }
>> >> +    else
>> >> +    {
>> >> +        /* handle division-by-zero */
>> >> +        res.mant = 1;
>> >> +        res.exp = 0x7FFFFFFF;
>> >> +    }
>> >> +
>> >> +    return res;
>> >> +#endif
>> >> +}
>> >> +
>> >> +//FIXME log, exp, pow
>> >>  
>> >
>> >>  static inline av_const SoftFloat av_int2sf(int v, int frac_bits){
>> >> -    return av_normalize_sf((SoftFloat){v, ONE_BITS-frac_bits});
>> >> +    return av_normalize_sf((SoftFloat){v, frac_bits});
>> >>  }
>> >
>> >missing documentation
>> 
>> I'll add it.
>> 
>> >also please make sure that the parameters make some logic sense
>> >and do not depend on the precission choosen by the implementation
>> >
>> >so a "1.0" shwould be generated from the same arguments no matter
>> >what the precision used in the implementation is
>> 
>> I am not sure I understand you on this.
>> 
>> Basic implementation of this function is the same as in original except
>> "ONE_BITS-frac_bits" is changed with "frac_bits".
>> This was done since algorithm is adjusted to be usable in implementation
>> of fixed point aac decoder.
>
>the numbers are of the form x * 2^y
>thus the interface to create 4.0 should be av_int2sf(1,2) not
>int2sf(1,123)
>The interface should be simple, logic and intuitive

As I said in the comment of this patch and before in comments to review,
I modified the code in the softfloat to be more usable in the implementation
of fixed point aac decoder.
Fixed point aac decoder was developed by using our float emulation and it was
more convenient to change ffmpeg softfloat than to make changes in aac since
softfloat is not used anywhere in ffmpeg currently.

It is clear to me that maybe for the number that is power of two it is more
convenient to use original way, but for other numbers it is basically the same.

On the other hand there is one subtraction less in the implementation of this
function if precision is used as argument.

And at the end, basically, I do not have a problem with changing this function
back to original if you want me to and comments above were just for the sake of
discussion :)

Thanks,
Nedeljko