RISC-V inferior to ARMv8

By: Maynard Handley (name99.delete@this.name99.org), December 21, 2018 3:58 pm
Room: Moderated Discussions
Gabriele Svelto (gabriele.svelto.delete@this.gmail.com) on December 21, 2018 1:09 pm wrote:
> Michael S (already5chosen.delete@this.yahoo.com) on December 21, 2018 2:27 am wrote:
> > IBM, on the other hand, had troubles with achieving maximal throughput for "with
> > update" addressing modes, at least in case of integer loads on many POWER cores.
> > May be, it does not apply to POWER8 and/or 9, I didn't analyze them that deeply.
> > It's all about contention on GP register file update bus.
> Load- and store-with-update instructions have been cracked on all IBM POWER cores I know of including POWER9.
> So they provide a denser encoding than two separate instructions but offer no performance benefit over them.

This argument seems to veering off into wildly different directions.

The RISC-V argument always starts off as "yay instruction density", and if you accept that premise, well then, denser encoding ain't nothing. BUT

If you're willing to move beyond the density-uber-alles argument, then, as I keep pointing out, we are no longer living in the 1980s. The SPECIFIC arguments being made about ISA here are silly because any decent performance core engages in aggressive cracking and fusion to restructure the instruction stream into something that's a better fit for the underlying engine.
In my opinion, for performance latency engines, the questions that matter are not whether it takes two instructions rather than one to branch a mid-range distance or to read a struct variable. (Though I agree that the RISC-V choices are pathologically silly for a performance design, and I see nothing appealing, attractive, or inspired in its design.) Rather the questions that matter are those that exacerbate or not the pain-points.

The pain points CAN be the front-end (x86 variable length instructions; I don't know enough about compressed RISC-V to comment on that), and they are likely register rename. Rename can be made less painful with semantically richer instructions, which can be either encoded directly or created by fusion (of instructions appropriately ordered by the compiler). But the more you're using fusion to handle stupid low-level issues, the even wider your front-end has to be to have any bandwidth left over for next-gen fusion.

To my (perhaps cataract'ed eyes) it looks like RISC-V is not as awful a starting place for high single-threaded performance as x86, but it's not nearly as good as AArch64. Maybe slightly behind POWER?

(Of course there's a whole separate side of performance stemming from the cost and overhead of OS-type operations and memory ordering/coherency choices. Again I don't know enough about RISC-V to comment except that one's expectation, given the design choices everywhere else, is that these are likely optimized for easy understanding and implementation over what's likely to be best performing in technology current and over the next ten years.)
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TopicPosted ByDate
RISC-V Summit ProceedingsGabriele Svelto2018/12/19 09:36 AM
  RISC-V gut feelingsKonrad Schwarz2018/12/20 05:30 AM
    RISC-V inferior to ARMv8Heikki Kultala2018/12/20 08:36 AM
      RISC-V inferior to ARMv8Wilco2018/12/20 02:31 PM
        RISC-V inferior to ARMv8Travis Downs2018/12/20 03:18 PM
          RISC-V inferior to ARMv8Wilco2018/12/21 04:43 AM
            RISC-V inferior to ARMv8Ronald Maas2018/12/21 10:35 AM
          RISC-V inferior to ARMv8juanrga2018/12/21 11:28 AM
            RISC-V inferior to ARMv8Maynard Handley2018/12/21 03:39 PM
              RISC-V inferior to ARMv8anon2018/12/21 04:38 PM
                RISC-V inferior to ARMv8juanrga2018/12/23 05:39 AM
                  With similar logic nor do frequency (NT)Megol2018/12/23 10:45 AM
              RISC-V inferior to ARMv8juanrga2018/12/23 05:44 AM
                RISC-V inferior to ARMv8Wilco2018/12/23 07:21 AM
      RISC-V inferior to ARMv8Michael S2018/12/20 04:24 PM
        RISC-V inferior to ARMv8anon2018/12/20 05:22 PM
          RISC-V inferior to ARMv8Travis Downs2018/12/21 07:16 PM
            RISC-V inferior to ARMv8anon2018/12/22 04:53 AM
              Execution runtimes and SpectreFoo_2018/12/22 07:02 AM
        RISC-V inferior to ARMv8Adrian2018/12/20 09:51 PM
          RISC-V inferior to ARMv8Doug S2018/12/21 12:10 AM
            RISC-V inferior to ARMv8Adrian2018/12/21 12:38 AM
              RISC-V inferior to ARMv8Michael S2018/12/21 03:31 AM
                RISC-V inferior to ARMv8Adrian2018/12/21 04:23 AM
            RISC-V inferior to ARMv8random person2018/12/21 03:04 AM
              RISC-V inferior to ARMv8dmcq2018/12/21 05:27 AM
              RISC-V inferior to ARMv8juanrga2018/12/21 11:36 AM
              RISC-V inferior to ARMv8Doug S2018/12/21 01:02 PM
            RISC-V inferior to ARMv8juanrga2018/12/21 11:23 AM
          RISC-V inferior to ARMv8Adrian2018/12/21 12:21 AM
          RISC-V inferior to ARMv8anon2018/12/21 02:48 AM
            RISC-V inferior to ARMv8Adrian2018/12/21 04:44 AM
              RISC-V inferior to ARMv8anon2018/12/21 06:24 AM
            RISC-V inferior to ARMv8Adrian2018/12/21 05:09 AM
              RISC-V inferior to ARMv8Wilco2018/12/21 05:28 AM
          RISC-V inferior to ARMv8Michael S2018/12/21 03:27 AM
            RISC-V inferior to ARMv8Gabriele Svelto2018/12/21 02:09 PM
              RISC-V inferior to ARMv8Maynard Handley2018/12/21 03:58 PM
              RISC-V inferior to ARMv8Wilco2018/12/21 04:43 PM
                RISC-V inferior to ARMv8Travis Downs2018/12/21 06:45 PM
                  RISC-V inferior to ARMv8Wilco2018/12/22 05:37 AM
                    RISC-V inferior to ARMv8Travis Downs2018/12/22 07:54 AM
                      RISC-V inferior to ARMv8Wilco2018/12/22 11:32 AM
                Cracking is not freeGabriele Svelto2018/12/22 03:09 AM
                  Cracking is not freeWilco2018/12/22 05:32 AM
                    Cracking is not freeTravis Downs2018/12/22 08:07 AM
                      Cracking is not freeWilco2018/12/22 08:38 AM
                        Cracking is not freeTravis Downs2018/12/22 08:47 AM
                          Cracking is not freeWilco2018/12/22 11:24 AM
                            Cracking is not freeTravis Downs2018/12/25 04:41 PM
                              Cracking is not freeanon.12018/12/25 09:14 PM
                        multi-instruction decode and renamePaul A. Clayton2018/12/22 07:45 PM
                    Cracking is not freeGabriele Svelto2018/12/22 01:30 PM
                      Cracking is not freeWilco2018/12/23 07:48 AM
                      Cracking is not freeMichael S2018/12/23 09:09 AM
                        Cracking is not freeGabriele Svelto2018/12/26 03:53 PM
          RISC-V inferior to ARMv8rwessel2018/12/21 02:13 PM
          RISC-V inferior to ARMv8Seni2018/12/21 03:33 PM
            RISC-V inferior to ARMv8Wilco2018/12/21 04:33 PM
              RISC-V inferior to ARMv8Travis Downs2018/12/21 06:49 PM
                RISC-V inferior to ARMv8Wilco2018/12/22 05:58 AM
                  RISC-V inferior to ARMv8Travis Downs2018/12/22 08:03 AM
                    RISC-V inferior to ARMv8Wilco2018/12/22 08:22 AM
                      RISC-V inferior to ARMv8Travis Downs2018/12/22 08:40 AM
        RISC-V inferior to ARMv8dmcq2018/12/21 04:57 AM
      RISC-V inferior to ARMv8Konrad Schwarz2018/12/21 03:25 AM
      RISC-V inferior to ARMv8j2018/12/21 11:46 AM
        RISC-V inferior to ARMv8Travis Downs2018/12/21 07:08 PM
          RISC-V inferior to ARMv8dmcq2018/12/22 08:45 AM
            RISC-V inferior to ARMv8Travis Downs2018/12/22 08:50 AM
              RISC-V inferior to ARMv8Michael S2018/12/22 09:15 AM
                RISC-V inferior to ARMv8dmcq2018/12/22 11:41 AM
        RISC-V inferior to ARMv8AnonQ2018/12/22 09:13 AM
    RISC-V gut feelingsdmcq2018/12/20 08:41 AM
      RISC-V initial takeKonrad Schwarz2018/12/21 03:17 AM
        RISC-V initial takedmcq2018/12/21 04:23 AM
      RISC-V gut feelingsMontaray Jack2018/12/22 03:56 PM
        RISC-V gut feelingsdmcq2018/12/23 05:38 AM
  RISC-V Summit Proceedingsjuanrga2018/12/21 11:47 AM
    RISC-V Summit Proceedingsdmcq2018/12/22 07:21 AM
      RISC-V Summit ProceedingsMontaray Jack2018/12/22 03:03 PM
        RISC-V Summit Proceedingsdmcq2018/12/23 05:39 AM
  RISC-V Summit Proceedingsanon22018/12/21 11:57 AM
    RISC-V Summit ProceedingsMichael S2018/12/22 09:36 AM
      RISC-V Summit ProceedingsAnon2018/12/22 06:51 PM
      Not Stanford MIPS but commercial MIPSPaul A. Clayton2018/12/23 04:05 AM
        Not Stanford MIPS but commercial MIPSMichael S2018/12/23 04:49 AM
        Not Stanford MIPS but commercial MIPSdmcq2018/12/23 05:52 AM
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