[FFmpeg-devel] [PATCH] update doc/optimization.txt

[Ronald S. Bultje]

Hi,

On Wed, Sep 22, 2010 at 6:01 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> On Wed, Sep 22, 2010 at 5:50 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> id like to point out that xmm/mmx clobbers are only ok if either
>> 1. they are needed for correct functioning of actual binary code
>> 2. they cause no speedloss and add no instructions
>>
>> or to say it the other way around iam against making our code slower
>> for language lawyering correctness
>>
>> this applies to register saving in yasm as well
>> so yes i oppose saving xmm registers on win64 if its unneeded
>> you can do such saveing & restoring at an outer layer like the slice decode
>> function so the clobbering cannot leak but doing it in each optimized function
>> is silly if its unneeded. (of course its not silly if it is needed ...)
>
> Understood.
>
> XMM clobbering or the cglobal thing does nothing on Linux or other
> Unixy systems where it's not required. It only has some effect on
> Win64, where it is needed. On Win64 (at least), XMM registers are
> actually used in regular C code (e.g. in ffmpeg.c) when compiled with
> a regular compiler such as gcc, and they are callee-save, so not
> saving/restoring them has undefined effects. I haven't checked what
> they are used for (possibly float storage or so? The "undefined
> effect" always effected float code...), so in theory it may be
> possible to save/restore them similar to how we handle emms assuming
> that our decoders using fxp simd don't use any float code (basically
> same assumption as with emms). I'm not really sure about this, I'd
> like Loren's input before I change anything. More generally, since
> this has no effect on Unixy systems (neither negative nor positive)
> I'm not sure if anyone cares...
>
> Yasm never does anything for mmx registers. I don't think marking them
> as clobbered in inline asm does anything but I haven't checked this.
> The doc specifically excludes mentioning that mmx registers should be
> marked as clobbered; they are always caller-save.

With that said, applied.

(And yes, I'll keep to the "don't rewrite in yasm [i.e. cosmetics] and
fix stuff [i.e. functional] in the same patch", as I would do for C
code also.)

Ronald