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 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
