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

[Martin Storsjö]

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?

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?

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

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.

// Martin