12:30 "[ISA] do not matter very much"

By: Wilco (wilco.dijkstra.delete@this.ntlworld.com), February 24, 2021 4:37 am
Room: Moderated Discussions
Anon (no.delete@this.spam.com) on February 23, 2021 6:26 am wrote:
> Wilco (wilco.dijkstra.delete@this.ntlworld.com) on February 23, 2021 3:48 am wrote:
> > You forgot the sarcasm tag :-)
>
> Poor implementations don't prove an efficient implementation isn't possible.

Given even a bad software implementation (the SSE2 one) thrashes rep movsb on modern cores, it proves that it is not at all trivial to do a good hardware memcpy like you suggested. It's not like Intel/AMD haven't been trying for years.

> > bench-memcpy-random in GLIBC shows just how "efficient" rep movsb (__memcpy_erms) is on my 3700X:
>
> Your benchmark shows how hard it is to find the optimal software implementation of memcpy, there
> are 7 variations and a surprising fastest one (__memcpy_sse2_unaligned, what happens to AVX?), this
> show a somewhat lazy AMD that didn't even put their microcode to emit the best uop sequence.

There isn't a single optimal implementation of memcpy for all possible use-cases. Software allows you to select whichever one works best, and you can tweak it further, remove bottlenecks etc. However with hardware you are stuck with the one in your CPU. In order for hardware memcpy to work out, it has to be as fast as the best software implementation. So far nobody has proven this is feasible.

Wilco
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TopicPosted ByDate
New Lex Fridman interview with Jim KellerJohnG2021/02/19 11:25 PM
  12:30 "[ISA] do not matter very much" (NT)Moritz2021/02/20 09:45 AM
    AAARGH, what are we going to argue about then? (NT)j2021/02/20 02:43 PM
    Blasphemy! (NT):]2021/02/21 04:49 AM
    12:30 "[ISA] do not matter very much"anon22021/02/21 10:25 PM
      12:30 "[ISA] do not matter very much"Brett2021/02/21 11:59 PM
        12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/22 01:17 AM
      12:30 "[ISA] do not matter very much"Dummond D. Slow2021/02/22 08:57 AM
        12:30 "[ISA] do not matter very much"Anon2021/02/22 10:52 AM
          12:30 "[ISA] do not matter very much"juanrga2021/02/22 11:01 AM
          12:30 "[ISA] do not matter very much"Mark Roulo2021/02/22 11:54 AM
          ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/22 01:34 PM
            ARM being a good idea doesn't mean it would have worked for AMDAnon2021/02/22 03:25 PM
              ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/22 04:55 PM
                ARM being a good idea doesn't mean it would have worked for AMDDoug S2021/02/23 12:03 PM
                  ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/23 12:27 PM
                    ARM being a good idea doesn't mean it would have worked for AMDBrett2021/02/23 03:57 PM
                      3rd parties licensing ARM coresAnon2021/02/25 04:01 AM
                        3rd parties licensing ARM coresAnon2021/02/25 04:48 AM
                        3rd parties licensing ARM coresdmcq2021/02/25 06:01 AM
                          3rd parties licensing ARM coresDummond D. Slow2021/02/25 09:17 AM
                            3rd parties licensing ARM coresAnon2021/02/25 10:11 AM
                              3rd parties licensing ARM coresAnon2021/02/26 02:54 AM
                              3rd parties licensing ARM coresDummond D. Slow2021/02/26 10:01 AM
            ARM being a good idea doesn't mean it would have worked for AMDLinus Torvalds2021/02/22 05:06 PM
              ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/22 07:19 PM
              ARM being a good idea doesn't mean it would have worked for AMDanon22021/02/22 07:28 PM
              ARM being a good idea doesn't mean it would have worked for AMDdmcq2021/02/23 05:35 AM
                ARM being a good idea doesn't mean it would have worked for AMDJukka Larja2021/02/23 07:12 AM
                  ARM being a good idea doesn't mean it would have worked for AMDSimon Farnsworth2021/02/23 08:42 AM
                    ARM being a good idea doesn't mean it would have worked for AMDJukka Larja2021/02/24 06:03 AM
                ARM may have been a threat to Intelwumpus2021/02/23 08:30 AM
      12:30 "[ISA] do not matter very much"blaine2021/02/22 09:37 AM
        12:30 "[ISA] do not matter very much"anon22021/02/22 07:17 PM
          12:30 "[ISA] do not matter very much"Anon2021/02/23 03:05 AM
            12:30 "[ISA] do not matter very much"Wilco2021/02/23 03:48 AM
              12:30 "[ISA] do not matter very much"Bigos2021/02/23 03:55 AM
                12:30 "[ISA] do not matter very much"Wilco2021/02/23 04:15 AM
                  12:30 "[ISA] do not matter very much"Bigos2021/02/23 05:16 AM
                12:30 "[ISA] do not matter very much"Travis Downs2021/02/26 11:46 PM
              12:30 "[ISA] do not matter very much"Anon2021/02/23 06:26 AM
                12:30 "[ISA] do not matter very much"anon22021/02/23 04:35 PM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 07:57 AM
                12:30 "[ISA] do not matter very much"Wilco2021/02/24 04:37 AM
                  12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/24 06:24 AM
                    12:30 "[ISA] do not matter very much"Anon2021/02/24 08:11 AM
                    12:30 "[ISA] do not matter very much"rwessel2021/02/24 08:45 AM
                      12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/25 01:02 AM
                        12:30 "[ISA] do not matter very much"rwessel2021/02/25 04:51 AM
                        12:30 "[ISA] do not matter very much"Anon2021/02/25 04:53 AM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 08:07 AM
                    12:30 "[ISA] do not matter very much"Wilco2021/02/24 11:37 AM
                      runtime selection vs. heterogenous cores?Matt Sayler2021/02/24 06:10 PM
                        runtime selection vs. heterogenous cores?Wilco2021/02/26 05:22 AM
            12:30 "[ISA] do not matter very much"anon22021/02/23 04:20 AM
              12:30 "[ISA] do not matter very much"Anon2021/02/23 06:21 AM
                12:30 "[ISA] do not matter very much"none2021/02/23 07:37 AM
                  12:30 "[ISA] do not matter very much"rwessel2021/02/23 09:44 AM
                    12:30 "[ISA] do not matter very much"anon22021/02/23 04:30 PM
                      12:30 "[ISA] do not matter very much"Anon2021/02/24 08:25 AM
                        12:30 "[ISA] do not matter very much"anon.12021/02/25 06:13 AM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 08:44 AM
                12:30 "[ISA] do not matter very much"anon22021/02/23 03:51 PM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 08:31 AM
            12:30 "[ISA] do not matter very much"vvid2021/02/23 06:41 AM
              12:30 "[ISA] do not matter very much"Michael S2021/02/23 08:52 AM
                12:30 "[ISA] do not matter very much"rwessel2021/02/23 09:33 AM
                  12:30 "[ISA] do not matter very much"Linus Torvalds2021/02/23 11:44 AM
                    12:30 "[ISA] do not matter very much"rwessel2021/02/23 12:21 PM
                      12:30 "[ISA] do not matter very much"Linus Torvalds2021/02/23 12:30 PM
                        12:30 "[ISA] do not matter very much"Andrey2021/02/25 03:06 AM
                          12:30 "[ISA] do not matter very much"Anon2021/02/25 05:04 AM
                            12:30 "[ISA] do not matter very much"Andrey2021/02/25 05:54 AM
                              12:30 "[ISA] do not matter very much"Anon2021/02/25 06:33 AM
                          12:30 "[ISA] do not matter very much"Linus Torvalds2021/02/25 10:35 AM
                            12:30 "[ISA] do not matter very much"Andrey2021/02/25 01:34 PM
                              12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/26 01:18 AM
                              12:30 "[ISA] do not matter very much"dmcq2021/02/26 03:23 PM
                12:30 "[ISA] do not matter very much"Anon2021/02/24 08:45 AM
              12:30 "[ISA] do not matter very much"Gabriele Svelto2021/02/23 09:15 AM
          Context of ISA doesn't matterPaul A. Clayton2021/02/26 12:03 PM
  Is there a text version? (NT)Foo_2021/02/20 04:33 PM
    good question (NT)Michael S2021/02/21 04:31 AM
    Is there a text version?:]2021/02/21 10:34 AM
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