[FFmpeg-devel] [PATCH 5/5] aarch64: me_cmp: Don't do uaddlv once per iteration

[Michael Niedermayer]

On Sat, Jul 16, 2022 at 03:30:23PM +0300, Martin Storsjö wrote:
> On Sat, 16 Jul 2022, Michael Niedermayer wrote:
> 
> > On Sat, Jul 16, 2022 at 12:25:37AM +0300, Martin Storsjö wrote:
> > > On Fri, 15 Jul 2022, Michael Niedermayer wrote:
> > > 
> > > > On Fri, Jul 15, 2022 at 10:56:03PM +0300, Martin Storsjö wrote:
> > > > > On Fri, 15 Jul 2022, Swinney, Jonathan wrote:
> > > > > 
> > > > > > If the max height is just 16, then this should be fine. I assumed that h
> > > > > > could have a much higher value (>1024), but if that is not the case,
> > > > > > then this is a useful optimization.
> > > > > 
> > > > > At least according to the me_cmp.h header, which says:
> > > > > 
> > > > > /* Motion estimation:
> > > > >  * h is limited to { width / 2, width, 2 * width },
> > > > >  * but never larger than 16 and never smaller than 2.
> > > > >  * Although currently h < 4 is not used as functions with
> > > > >  * width < 8 are neither used nor implemented. */
> > > > 
> > > > These rules where written with support for encoding of all
> > > > standard formats in mind at the time that was written.
> > > > today it may make sense to extend these rules to cover the
> > > > things which where created since then
> > > 
> > > Right, but if that suddenly changes, such a change also must expect that it
> > > might need updates to all assembly implementations that implement that
> > > interface currently. Right now, both the defacto case (any callers in the
> > > codebase) and the explicit documentation says that it can't be called with
> > > parameters outside of that range.
> > 
> > What i meant was that newly added functions should be more flexible than
> > these old rules. That is 2 sets of rules
> > 1. What a caller ATM can do and expect to work (thats whats written there)
> > 2. What an implementor of new functions should make sure is supported
> 
> With 2., do you mean if adding a new function into the same struct, or if
> implementing the existing pix_abs[0][..] functions?

i would say both


> 
> If you mean new implementations of the existing function interface, you say
> they "should be more flexible". How flexible must they be? Is it ok to
> assume h<=256 for the w=16 functions?

i think thats fine


> 
> Gradually increasing the requirements for existing function interfaces like
> you suggest is really problematic.

why ?
iam really just saying
"when you add new code, dont base it on old limitations"

And i suggest this because its much easier to do when a function is added than
when later one wants to use it 

When i originally wrote this list of restrictions it didnt list what our
code actually used but tried to look ahead to what we would need when we
write encoders for all standard formats at that time. We never wrote a
h264 encoder but this API was designed to support it.


> 
> If we want to require more of the functions, we should document it, and
> extend the checkasm test to test that new requirement - which also extends
> the requirement to the existing functions. If we don't have a checkasm test
> for the required behaviour, we can pretty much assume it's broken, even in
> new implementations.

yes

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No snowflake in an avalanche ever feels responsible. -- Voltaire
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 195 bytes
Desc: not available
URL: <https://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20220716/e9265fa8/attachment.sig>