Le tiistaina 30. heinäkuuta 2024, 20.57.28 EEST flow gg a écrit : > From my understanding, moving from supporting only 128b to adding 256b > versions can simultaneously improve LMUL and solve some issues related to > insufficient vector registers (vvc, vp9).
To the contrary, if vectors are too short to process a macroblock in a single round, then there should be a loop with maximum LMUL, and the code should be the same for all vector length. That is just normal textbook RVV coding style. There should *not* be vector length specialisation since the code can be shared. > If we continue to support 512, 1024, ..., it almost exclusively improves > LMUL. I don't think so. Even more so than 256-bit hardware, 512-bit and 1024-bit hardware really _needs_ to short-circuit vector processing based on VL and not simply follow LMUL. > Therefore, 256b is the most worthwhile addition, and we can skip > adding 512b, 1024b, etc. > > Additionally, even though longer hardware will continually be developed, > the most used will probably still be 128b and 256b. I wouldn't be so sure. Realistically, lower-end SoCs decode video with DSPs. So video decoder vector optimisations are mainly for the server side, and that's exactly where larger vector sizes are most likely (e.g. AVX-512). > If someone complains that FFmpeg's RVV doesn't support 1024b well, it can > be said that it's not just RISC-V that lacks good support. > However, if the 256b performance is not good, then it seems like an issue > with RISC-V. :) > > I think maybe we can give some preference to the two smallest lengths? As I wrote, I am not necessarily against specialising for 256-bit as such. I am against: 1) specialising functions that do not really need to be specialised, 2) adding tons of boilerplate (notably in the C code) for it. -- 雷米‧德尼-库尔蒙 http://www.remlab.net/ _______________________________________________ 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".
