[FFmpeg-devel] [PATCH 07/10] avcodec/vc1: Arm 64-bit NEON inverse transform fast paths

[Ben Avison]

On 30/03/2022 14:49, Martin Storsjö wrote:
> Looks generally reasonable. Is it possible to factorize out the 
> individual transforms (so that you'd e.g. invoke the same macro twice in 
> the 8x8 and 4x4 functions) without too much loss?

There is a close analogy here with the vertical/horizontal deblocking 
filters, because while there are similarities between the two matrix 
multiplications within a transform, one of them follows a series of 
loads and the other follows a matrix transposition.

If you look for example at ff_vc1_inv_trans_8x8_neon, you'll see I was 
able to do a fair amount of overlap between sections of the function - 
particularly between the transpose and the second matrix multiplication, 
but to a lesser extent between the loads and the first matrix 
multiplication and between the second multiplication and the stores. 
This sort of overlapping is tricky to maintain when using macros. Also, 
it means the the order of operations within each matrix multiply ended 
up quite different.

At first sight, you might think that the multiplies from the 8x8 
function (which you might also view as kind of 8-tap filter) would be 
re-usable for the size-8 multiplies in the 8x4 or 4x8 function. Yes, the 
instructions are similar, save for using .4h elements rather than .8h 
elements, but that has significant impacts on scheduling. For example, 
the Cortex-A72, which is my primary target, can only do NEON bit-shifts 
in one pipeline at once, irrespective of whether the vectors are 64-bit 
or 128-bit long, while other instructions don't have such restrictions.

So while in theory you could factor some of this code out more, I 
suspect any attempt to do so would have a detrimental effect on performance.

Ben