[FFmpeg-devel] port mplayer eq filter to libavfilter

[William Yu]

2010/11/26 Ronald S. Bultje <rsbultje at gmail.com>:
> Hi,
>
> On Fri, Nov 26, 2010 at 9:38 AM, William Yu <genwillyu at gmail.com> wrote:
>> 2010/11/25 Ronald S. Bultje <rsbultje at gmail.com> wrote:
>>> On Thu, Nov 25, 2010 at 4:27 AM, William Yu <genwillyu at gmail.com> wrote:
>>>> + ? ? ? ?for (i = w&7; i; i--) {
>>>> + ? ? ? ? ? ?pel = ((*line* contrast)>>12) + brightness;
>>>> + ? ? ? ? ? ?if (pel&768) pel = (-pel)>>31;
>>>> + ? ? ? ? ? ?*line++ = pel;
>>>> + ? ? ? ?}
>>>
>>> Please don't mix and match C and ASM, this takes about 10-20 lines in
>>> asm, if you want you can even compile it using gcc and directly copy
>>> it (see above, use FASTDIV also). That will prevent it from using eax
>>> and then the function gets faster.
>>
>> Can you tell why don't mix and match C and ASM. I think compiler's optimizer
>> can does better than my manual code except those MMX instruction.
>
> See doc/optimization.txt
>
> Short version: there is no guarantee that values are maintained
> between asm blocks or between looped calls to the same asm block,
> unless the loop itself is within asm. This is correct:
>
> asm("setup .. loop .. inner .. end loop");
>
> This is not:
>
> asm("setup"); for (..) { asm("inner"); }
>
> which is why you do this:
>
> for (..) { asm("setup .. inner"); }
>
> but now you redo the setup for each row, unnecessarily.
>
> The same is true for the stuff at the end. Advantages of the
> do-it-all-in-asm is that you can make sure the compiler doesn't switch
> eax/stack (which it shouldn't). Also, you can then do the setup before
> the loop. Even though it's not the inner loop, as Michael said, it'd
> still make it faster. If Michael thinks I'm an a-hole here, then
> commit without and I'll fix it after commit.
>

Thanks for your comment, It let me known more about optimization.
I will read this doc carefully.

>>>[..]
>> Sorry, I am not enough experience on assembly. When i study these knowledge
>> enough, I will add those version. Or another people may more suitable
>> to does these job than me.
>
> Same, I'll add it after you commit, if I find time. It makes more
> sense to me to do it now (SSE2) since I already gave you the code, but
> I'm not maintainer so I'm easily overruled.
>
> Ronald
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