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

[Alex Converse]

On Tue, Nov 11, 2008 at 4:30 PM, M?ns Rullg?rd <mans at mansr.com> wrote:
> "Alex Converse" <alex.converse at gmail.com> writes:
>
>> On Tue, Nov 11, 2008 at 3:26 PM, M?ns Rullg?rd <mans at mansr.com> wrote:
>>> "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?
>>>
>>
>> frexpf() is the standard libm function, see man frexp.
>
> Yes, but what, exactly, does yours contain?
>

I think:
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/sysdeps/i386/fpu/s_frexpf.S?rev=1.4.4.1&content-type=text/x-cvsweb-markup&cvsroot=glibc

--Alex