SVE alignment with non power-of-2 widths

By: Andrey (andrey.semashev.delete@this.gmail.com), September 25, 2021 3:46 am
Room: Moderated Discussions
-.- (blarg.delete@this.mailinator.com) on September 24, 2021 6:10 pm wrote:
> Andrey (andrey.semashev.delete@this.gmail.com) on September 24, 2021 8:29 am wrote:
> > Should SVE prohibit 136-bit
> > vectors? No, because your cache line and page sizes may be a multiple of 17. In such an implementation,
> > 136-bit vectors would be a reasonable choice. But in the power-of-2 world we live in, 136-bit vectors
> > are nonsensical (possibly, aside from some very special purpose hardware).
>
> Well, from what I can gather, it sounds like you're suggesting that non power-of-2 hardware may, in fact, eventuate.

Not in mass market, I don't think. Some specialized controllers, perhaps, though I have no idea what work loads would require a non-power-of-2 design, and SVE to boot.

> > By default - nothing. I mean, so long as you're targeting an unknown abstract implementation,
> > you may as well forget about alignment, instruction choices and scheduling and other micro-optimizations
> > and just write the unaligned loop that maintains correctness.
>
> Which is what I've been trying to portray the whole time really. The way
> SVE is designed encourages developers to not bother with aligning at all.

Again, you're talking about the spec, and I'm talking about the actual hardware. If all or the absolute majority of implementations don't care about alignment then that's great and developers don't need to optimize for it. But we don't have that many SVE implementations, and for other kind of instructions real hardware does care about alignment, so I'm going do assume SVE won't be an exception. Hardware designers will have to prove that alignment doesn't matter, they haven't done that yet.

> > Yes, heterogeneous cores are a pain, and it looks like x86 will follow suit in the near future.
>
> So far, it's Intel only, and I feel the core types on Alderlake aren't wildly different enough to be
> as important (to use your example of MOVDQU/PSHUFB). Of course, this could change in the future.

Rumor has it that AMD is also working on a hybrid CPU, possibly in Zen 5. I think, eventually hybrid designs will settle in x86 desktops and servers.

> On the other hand, ARM typically includes both in-order and OoO cores together.
> Whilst I've never really found a need to specifically target cores yet, my
> point was that it's generally more difficult to do in the ARM ecosystem.
>
> Considering the added difficulty of targeting specific processors on ARM, I get the feeling that support for
> such isn't a priority. I don't really think that's a bad idea - the vast majority of programs aren't going
> to optimise to such an extent - but it does feed into the idea of encouraging generic implementations.

ARM was not really present in the HPC domain, certainly not as long as x86 was. Traditionally, because of low CPU performance, heavy lifting tasks like video encoding was done with specialized hardware in the ARM world, while it was mostly in software in the x86 world. This is starting to change as ARM performance grows and it's starting to appear on desktops and servers, so there will be more incentive to optimize for it.

> > Side note about hybrid designs where different cores are radically different, like x86+ARM that AMD did.
> > That sort of combination is a somewhat different story.
> > The cores in such a hybrid are inherently incompatible
>
> I've never heard of such a configuration. The closest thing I've heard of is their K12 core being designed alongside
> Zen, which was planned to be socket compatible, but that doesn't mean you can run the two together.
> Am I missing something?

I thought I've seen somewhere news about a hybrid x86+ARM core from AMD, but I can't find that source now. Hmm, maybe I'm misremembering this, sorry.

AMD SkyBridge (https://www.extremetech.com/computing/181867-amds-project-skybridge-new-arm-and-x86-chips-that-are-pin-compatible) though was in the works. I imagine, in multi-socket systems you could use both x86 and ARM cores together. The project is dead now, so we probably will never know.
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Armv8.8-A and Armv9.3-Aanonymou52021/09/16 02:25 PM
  Armv8.8-A and Armv9.3-ADoug S2021/09/16 09:57 PM
    Armv8.8-A and Armv9.3-ABrett2021/09/16 10:32 PM
      Armv8.8-A and Armv9.3-Aanon2021/09/16 10:55 PM
        Armv8.8-A and Armv9.3-Anone2021/09/16 11:51 PM
      Armv8.8-A and Armv9.3-AJörn Engel2021/09/17 04:42 AM
        Armv8.8-A and Armv9.3-AMichael S2021/09/17 06:48 AM
          Armv8.8-A and Armv9.3-AJörn Engel2021/09/18 01:01 PM
            Armv8.8-A and Armv9.3-AMichael S2021/09/18 02:58 PM
              microbenchmark resultsMichael S2021/09/19 03:46 PM
                microbenchmark source codeMichael S2021/09/19 03:58 PM
                  microbenchmark source code-.-2021/09/20 03:49 PM
                    microbenchmark source codeMichael S2021/09/21 09:17 AM
                      microbenchmark source code-.-2021/09/21 03:33 PM
                        microbenchmark source codeMichael S2021/09/21 05:05 PM
                microbenchmark resultsAnon2021/09/19 04:32 PM
                  microbenchmark resultsJörn Engel2021/09/19 07:46 PM
                    microbenchmark resultsdmcq2021/09/20 01:19 AM
                      microbenchmark resultsMichael S2021/09/20 04:12 AM
                      microbenchmark results-.-2021/09/20 03:44 PM
                        microbenchmark resultsMichael S2021/09/21 09:23 AM
                          microbenchmark results-.-2021/09/21 03:35 PM
                            microbenchmark resultsAndrey2021/09/21 04:25 PM
                              I agree (NT)Michael S2021/09/21 05:07 PM
                              microbenchmark results-.-2021/09/22 04:56 PM
                                microbenchmark resultsMichael S2021/09/23 05:11 AM
                                  microbenchmark resultsdmcq2021/09/23 06:53 AM
                                  microbenchmark resultsAndrey2021/09/23 09:20 AM
                                microbenchmark resultsAndrey2021/09/23 09:11 AM
                                  microbenchmark results-.-2021/09/23 07:01 PM
                                    microbenchmark resultsSimon Farnsworth2021/09/24 01:47 AM
                                      microbenchmark results-.-2021/09/24 05:00 PM
                                    microbenchmark resultsAndrey2021/09/24 07:29 AM
                                      microbenchmark resultsdmcq2021/09/24 12:05 PM
                                        microbenchmark resultsDoug S2021/09/24 01:12 PM
                                          microbenchmark results---2021/09/24 06:06 PM
                                            microbenchmark resultsDoug S2021/09/24 10:46 PM
                                              microbenchmark results---2021/09/25 08:56 AM
                                                microbenchmark resultsJukka Larja2021/09/26 01:01 AM
                                                microbenchmark resultsDoug S2021/09/26 08:41 AM
                                                  microbenchmark resultsdmcq2021/09/26 12:37 PM
                                                    microbenchmark resultsDoug S2021/09/27 09:32 AM
                                                      microbenchmark resultsdmcq2021/09/28 06:56 AM
                                              microbenchmark resultsDummond D. Slow2021/09/25 11:49 AM
                                                microbenchmark resultsBrett2021/09/25 02:31 PM
                                              microbenchmark resultsdmcq2021/09/25 11:51 AM
                                                microbenchmark resultsDoug S2021/09/26 08:45 AM
                                            microbenchmark resultsRichard S2021/09/25 12:51 AM
                                              microbenchmark resultsDummond D. Slow2021/09/25 11:52 AM
                                                microbenchmark results---2021/09/25 02:04 PM
                                      SVE alignment with non power-of-2 widths-.-2021/09/24 05:10 PM
                                        SVE alignment with non power-of-2 widthsAndrey2021/09/25 03:46 AM
                                          SVE alignment with non power-of-2 widths-.-2021/09/25 04:35 PM
                                          SVE alignment with non power-of-2 widthsKevin G2021/09/27 08:46 AM
                                            SVE alignment with non power-of-2 widths-.-2021/09/27 08:06 PM
                                              SVE alignment with non power-of-2 widthsJukka Larja2021/09/28 05:37 AM
                                                SVE alignment with non power-of-2 widthsAndrey2021/09/28 11:12 AM
                                                  SVE alignment with non power-of-2 widthsdmcq2021/09/28 01:29 PM
                                                SVE alignment with non power-of-2 widths-.-2021/09/28 05:37 PM
                                                  SVE alignment with non power-of-2 widthsJukka Larja2021/09/29 05:50 AM
                    microbenchmark results---2021/09/20 06:11 AM
                    microbenchmark resultsJörn Engel2021/09/23 04:10 AM
                      microbenchmark resultsMichael S2021/09/23 04:55 AM
                        microbenchmark resultsJörn Engel2021/09/23 08:24 AM
                          microbenchmark resultsRoyi2021/09/26 03:25 PM
                      microbenchmark resultsdmcq2021/09/23 09:42 AM
                        microbenchmark results---2021/09/23 10:53 AM
                      microbenchmark resultsanon22021/09/23 01:40 PM
                microbenchmark results: Zen 3Adrian2021/09/22 12:57 AM
                  microbenchmark results: Zen 3Adrian2021/09/22 01:08 AM
                    microbenchmark results: Zen 3Michael S2021/09/22 04:48 AM
                      microbenchmark results: Zen 3Adrian2021/09/22 05:05 AM
        Armv8.8-A and Armv9.3-AKonrad Schwarz2021/09/28 04:45 AM
    Armv8.8-A and Armv9.3-ALinus Torvalds2021/09/17 07:59 AM
      Armv8.8-A and Armv9.3-ADoug S2021/09/17 10:35 AM
        Armv8.8-A and Armv9.3-Anksingh2021/09/17 11:23 AM
          Armv8.8-A and Armv9.3-ADoug S2021/09/17 01:35 PM
            Armv8.8-A and Armv9.3-AKonrad Schwarz2021/10/15 05:23 AM
              Armv8.8-A and Armv9.3-Arwessel2021/10/15 05:49 AM
        Armv8.8-A and Armv9.3-AAdrian2021/09/17 10:07 PM
          Armv8.8-A and Armv9.3-ADoug S2021/09/18 06:34 AM
            Armv8.8-A and Armv9.3-AAdrian2021/09/18 06:38 AM
      Armv8.8-A and Armv9.3-Ablaine2021/09/18 09:37 AM
      Armv8.8-A and Armv9.3-ABrett2021/09/19 12:06 PM
        Armv8.8-A and Armv9.3-Admcq2021/09/19 12:36 PM
        Armv8.8-A and Armv9.3-ADoug S2021/09/19 05:07 PM
          Armv8.8-A and Armv9.3-A - movesdmcq2021/09/28 08:54 AM
            Armv8.8-A and Armv9.3-A - movesDoug S2021/09/28 12:57 PM
              Armv8.8-A and Armv9.3-A - movesdmcq2021/09/28 01:21 PM
                Armv8.8-A and Armv9.3-A - movesNoSpammer2021/09/29 02:53 AM
                  Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 05:55 AM
                    Armv8.8-A and Armv9.3-A - movesdmcq2021/09/29 06:53 AM
                      Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 10:35 AM
                        Armv8.8-A and Armv9.3-A - movesdmcq2021/09/29 12:44 PM
                          Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 12:58 PM
                            Armv8.8-A and Armv9.3-A - movesdmcq2021/09/29 02:52 PM
                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 05:36 PM
                              Armv8.8-A and Armv9.3-A - movesAndrey2021/09/29 06:58 PM
                    Armv8.8-A and Armv9.3-A - movesDoug S2021/09/29 09:10 AM
                      Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 10:30 AM
                        Armv8.8-A and Armv9.3-A - movesDoug S2021/09/29 09:02 PM
                          Armv8.8-A and Armv9.3-A - movesrwessel2021/09/29 10:22 PM
                            Armv8.8-A and Armv9.3-A - movesMark Roulo2021/09/30 06:37 AM
                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 07:02 AM
                                Did they publish a full description? (NT)Michael S2021/09/30 07:12 AM
                                  Did they publish a full description?rwessel2021/09/30 08:18 AM
                                    Did they publish a full description?Michael S2021/09/30 09:24 AM
                                      Did they publish a full description?rwessel2021/09/30 09:42 AM
                                    Did they publish a full description?Adrian2021/09/30 11:22 PM
                                  Do we even okiw it's three instructions per move?Carson2021/09/30 09:28 PM
                                    Do we even okiw it's three instructions per move?Adrian2021/09/30 11:27 PM
                                    Do we even okiw it's three instructions per move?rwessel2021/10/01 03:19 AM
                            Armv8.8-A and Armv9.3-A - movesDoug S2021/09/30 08:48 AM
                              Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 09:39 AM
                                Armv8.8-A and Armv9.3-A - movesDoug S2021/09/30 01:56 PM
                                  Armv8.8-A and Armv9.3-A - movesrwessel2021/09/30 04:20 PM
                                    Armv8.8-A and Armv9.3-A - movesdmcq2021/10/01 03:38 AM
                                      Armv8.8-A and Armv9.3-A - movesMichael S2021/10/01 04:04 AM
                                        Armv8.8-A and Armv9.3-A - movesLinus Torvalds2021/10/01 10:01 AM
                                          memcpy - instruction cracking vs DMArpg2021/10/02 01:51 AM
                                            memcpy - instruction cracking vs DMAAdrian2021/10/02 02:45 AM
                                              memcpy - instruction cracking vs DMADoug S2021/10/02 08:47 AM
                                                memcpy - instruction cracking vs DMAAdrian2021/10/02 09:15 AM
                                                memcpy - instruction cracking vs DMArwessel2021/10/02 10:37 AM
                                                  memcpy - instruction cracking vs DMADoug S2021/10/02 05:49 PM
                                            memcpy - instruction cracking vs DMALinus Torvalds2021/10/02 09:43 AM
                                              memcpy - instruction cracking vs DMAdmcq2021/10/02 10:32 AM
                                              memcpy - instruction cracking vs DMABrett2021/10/02 10:45 AM
                                              memcpy - instruction cracking vs DMA---2021/10/02 02:03 PM
                                                memcpy - instruction cracking vs DMA---2021/10/02 02:12 PM
                                                  Moving copy to DRAM doesn't help for small copiesMark Roulo2021/10/02 02:59 PM
                                                    Moving copy to DRAM doesn't help for small copies---2021/10/02 06:32 PM
                                                      Moving copy to DRAM doesn't help for small copiesMichael S2021/10/03 12:40 AM
                                                        Moving copy to DRAM doesn't help for small copiesDoug S2021/10/03 09:09 AM
                                                          Moving copy to DRAM doesn't help for small copiesrwessel2021/10/03 09:51 AM
                                                          Moving copy to DRAM doesn't help for small copiesLinus Torvalds2021/10/03 10:09 AM
                                                            How about environments such as Java?Mark Roulo2021/10/03 11:41 AM
                                                              How about environments such as Java?rwessel2021/10/03 11:49 AM
                                                                How about environments such as Java?Mark Roulo2021/10/03 12:22 PM
                                                              How about environments such as Java?anon22021/10/03 06:58 PM
                                                                How about environments such as Java?Etienne Lorrain2021/10/04 04:08 AM
                                                                  Apart from "It depends" there is no short answer. (NT)Michael S2021/10/04 04:30 AM
                                                                  How about environments such as Java?Andrey2021/10/04 05:04 AM
                                                                  How about environments such as Java?anon22021/10/04 05:32 AM
                                                                How about environments such as Java?Mark Roulo2021/10/04 06:31 AM
                                                                How about environments such as Java?---2021/10/04 08:41 AM
                                                                  How about environments such as Java?Doug S2021/10/04 09:23 AM
                                                                    How about environments such as Java?Andrey2021/10/04 11:14 AM
                                                                      How about environments such as Java?Doug S2021/10/04 12:20 PM
                                                                  How about environments such as Java?anon22021/10/04 01:23 PM
                                                                  How about environments such as Java?rwessel2021/10/04 03:54 PM
                                                            Moving copy to DRAM doesn't help for small copiesJörn Engel2021/10/04 04:52 AM
                                                            Early software zeroing !=== early hardware zeroingPaul A. Clayton2021/10/05 10:19 AM
                                                              Early software zeroing !=== early hardware zeroingDoug S2021/10/05 11:21 AM
                                                memcpy - instruction cracking vs DMABrendan2021/10/02 03:53 PM
                                                  memcpy - instruction cracking vs DMALinus Torvalds2021/10/03 09:48 AM
                                                    memcpy - instruction cracking vs DMAdmcq2021/10/03 12:54 PM
                                              memcpy - instruction cracking vs DMAYuhong Bao2021/10/03 12:30 AM
                                                memcpy - instruction cracking vs DMADavid Hess2021/10/05 04:19 PM
                                                  memcpy - instruction cracking vs DMAAdrian2021/10/05 10:28 PM
                                                    memcpy - instruction cracking vs DMAEtienne Lorrain2021/10/06 01:24 AM
                                                    memcpy - instruction cracking vs DMArwessel2021/10/06 02:38 AM
                                                      memcpy - instruction cracking vs DMAAdrian2021/10/06 03:04 AM
                                                        memcpy - instruction cracking vs DMArwessel2021/10/06 04:59 AM
                                                    memcpy - instruction cracking vs DMA---2021/10/06 08:07 AM
                                                      memcpy - instruction cracking vs DMAAndrey2021/10/06 01:59 PM
                                                    memcpy - instruction cracking vs DMAgallier22021/10/06 10:06 PM
                                                      memcpy - instruction cracking vs DMAAdrian2021/10/06 10:59 PM
                                              memcpy - instruction cracking vs DMAMichael S2021/10/03 12:51 AM
                                                memcpy - instruction cracking vs DMArwessel2021/10/03 04:06 AM
                                                  memcpy - instruction cracking vs DMAMichael S2021/10/03 04:24 AM
                                                    memcpy - instruction cracking vs DMAMatt Sayler2021/10/03 07:02 AM
                                                    memcpy - instruction cracking vs DMADoug S2021/10/03 09:14 AM
                                      Armv8.8-A and Armv9.3-A - movesrwessel2021/10/01 04:10 AM
                                        Armv8.8-A and Armv9.3-A - movesEtienne Lorrain2021/10/01 06:55 AM
                                          Armv8.8-A and Armv9.3-A - movesrwessel2021/10/01 07:14 AM
                                            Armv8.8-A and Armv9.3-A - movesDoug S2021/10/01 10:17 AM
                                              Armv8.8-A and Armv9.3-A - movesrwessel2021/10/02 03:57 AM
  Armv8.8-A and Armv9.3-Anone2021/10/13 05:06 AM
    Armv8.8-A and Armv9.3-AAdrian2021/10/13 05:22 AM
      Armv8.8-A and Armv9.3-ADoug S2021/10/13 08:01 AM
        Armv8.8-A and Armv9.3-Admcq2021/10/13 09:17 AM
          Armv8.8-A and Armv9.3-Anone2021/10/13 09:26 PM
            Armv8.8-A and Armv9.3-Admcq2021/10/14 07:22 AM
    Armv8.8-A and Armv9.3-Arwessel2021/10/14 08:01 AM
      Armv8.8-A and Armv9.3-AAnon2021/10/14 10:08 AM
        Armv8.8-A and Armv9.3-AMichael S2021/10/14 12:25 PM
      Armv8.8-A and Armv9.3-ADoug S2021/10/14 10:18 AM
        Armv8.8-A and Armv9.3-Arwessel2021/10/14 06:07 PM
          Armv8.8-A and Armv9.3-ADoug S2021/10/14 09:23 PM
            Armv8.8-A and Armv9.3-Admcq2021/10/15 12:41 AM
              Armv8.8-A and Armv9.3-AGabriele Svelto2021/10/15 04:07 AM
            Armv8.8-A and Armv9.3-Arwessel2021/10/15 03:49 AM
              Armv8.8-A and Armv9.3-ADoug S2021/10/15 09:44 AM
                Armv8.8-A and Armv9.3-Ame2021/10/15 05:34 PM
                  Armv8.8-A and Armv9.3-ADoug S2021/10/16 08:47 AM
                    Armv8.8-A and Armv9.3-Ame2021/10/17 04:19 AM
                      Armv8.8-A and Armv9.3-ADoug S2021/10/17 09:17 AM
                        Armv8.8-A and Armv9.3-Ame2021/10/17 11:31 AM
                          Armv8.8-A and Armv9.3-ADoug S2021/10/17 12:33 PM
                            Armv8.8-A and Armv9.3-AzArchJon2021/10/18 09:35 AM
                              Armv8.8-A and Armv9.3-ADoug S2021/10/18 01:35 PM
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