[FFmpeg-devel] Input requested on floating point decomposition for AAC Main

[Måns Rullgård]

"Alex Converse" <alex.converse at gmail.com> writes:

> On Tue, Nov 11, 2008 at 5:21 AM, Jason Garrett-Glaser
> <darkshikari at gmail.com> wrote:
>> On Mon, Nov 10, 2008 at 10:39 PM, Alex Converse <alex.converse at gmail.com> wrote:
>>> To do 16-bit floating point rounding for AAC-Main, I need a function
>>> that will decompose a float into a normalized scheme and it's
>>> exponent. Conveniently there exists the x87 instruction FXTRACT for
>>> this very purpose.
>>
>> Are you sure this is a good idea?  FXTRACT takes 170 clock cycles on
>> Core 2, according to Agner, making it the single slowest floating
>> point operation in x86 history, perhaps competing with FBSTP.
>>
>
> Hmm, FXTRACT may be slow but it appears to not be as slow as frexpf.
> On my Core2 Duo I'm getting:
> 40092570 dezicycles in t_fxtract, 2 runs, 0 skips
> 142385760 dezicycles in t_frexpf, 2 runs, 0 skips
>
> for this test code:
> float t1() {
>     float j, f, g;
>     START_TIMER("t_fxtract");
>     for (j = 1.0f; j <= 1048576.0f; j++)
>         g = fxtract(j, &f);
>     STOP_TIMER("t_fxtract");
>     return g;
> }
>
> float t2() {
>     int i, j;
>     float h;
>     START_TIMER("t_frexpf");
>     for (j = 1.0f; j <= 1048576.0f; j++)
>         h = frexpf(j, &i);
>     STOP_TIMER("t_frexpf");
>     return h;
> }

What do frexpf() and fxtract() contain?

-- 
M?ns Rullg?rd
mans at mansr.com