Cracking is not free

By: Wilco (Wilco.dijkstra.delete@this.ntlworld.com), December 22, 2018 10:24 am
Room: Moderated Discussions
Travis Downs (travis.downs.delete@this.gmail.com) on December 22, 2018 7:47 am wrote:
> Wilco (Wilco.dijkstra.delete@this.ntlworld.com) on December 22, 2018 7:38 am wrote:
>
> > I don't see why you think it's any different: load-op and load-op-store have 2 destinations
> > too. It doesn't matter whether you crack before or after rename: they need a renamed
> > register for the result of the load and one for the final result.
>
> The result of the load doesn't need to participate in renaming in the same way because it is not architecturally
> visible and doesn't persist beyond the instruction. It just needs to get from the load to the single op
> that will ever consume it, which is a different problem and likely off the critical path.

You could special case it since no other instruction needs to read the renamed register.
But other than that it's like any other destination that needs to be renamed. As is the flags register.

> If this were correct though, then your original hypothesis of "saves rename bandwidth" holds in even fewer cases:
> the renamer still has to deal with the full complexity of the instruction whether it is cracked before or after
> since it has to do just as much work (rename just as many destination registers) in either case.

All instructions must be renamed either way. Early cracking means more micro ops to rename, so throughput is simply lower. Now you could widen rename, but that has a high cost. A cheaper approach is to add support for a 2nd destination register.

> Note that I agree with you that rename bandwidth is saved in the case of micro-fused/uncracked-until-after
> rename ops like load-op on x86 or call/store on ARM. It's only for 2-output
> ops like auto-increment this doesn't apply.

A modern Arm core can execute 2 loads with auto-increment every cycle using just 2 rename slots. The other slots are still free for other instructions, so yes it saves rename bandwidth.

Wilco
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TopicPosted ByDate
RISC-V Summit ProceedingsGabriele Svelto2018/12/19 08:36 AM
  RISC-V gut feelingsKonrad Schwarz2018/12/20 04:30 AM
    RISC-V inferior to ARMv8Heikki Kultala2018/12/20 07:36 AM
      RISC-V inferior to ARMv8Wilco2018/12/20 01:31 PM
        RISC-V inferior to ARMv8Travis Downs2018/12/20 02:18 PM
          RISC-V inferior to ARMv8Wilco2018/12/21 03:43 AM
            RISC-V inferior to ARMv8Ronald Maas2018/12/21 09:35 AM
          RISC-V inferior to ARMv8juanrga2018/12/21 10:28 AM
            RISC-V inferior to ARMv8Maynard Handley2018/12/21 02:39 PM
              RISC-V inferior to ARMv8anon2018/12/21 03:38 PM
                RISC-V inferior to ARMv8juanrga2018/12/23 04:39 AM
                  With similar logic nor do frequency (NT)Megol2018/12/23 09:45 AM
              RISC-V inferior to ARMv8juanrga2018/12/23 04:44 AM
                RISC-V inferior to ARMv8Wilco2018/12/23 06:21 AM
      RISC-V inferior to ARMv8Michael S2018/12/20 03:24 PM
        RISC-V inferior to ARMv8anon2018/12/20 04:22 PM
          RISC-V inferior to ARMv8Travis Downs2018/12/21 06:16 PM
            RISC-V inferior to ARMv8anon2018/12/22 03:53 AM
              Execution runtimes and SpectreFoo_2018/12/22 06:02 AM
        RISC-V inferior to ARMv8Adrian2018/12/20 08:51 PM
          RISC-V inferior to ARMv8Doug S2018/12/20 11:10 PM
            RISC-V inferior to ARMv8Adrian2018/12/20 11:38 PM
              RISC-V inferior to ARMv8Michael S2018/12/21 02:31 AM
                RISC-V inferior to ARMv8Adrian2018/12/21 03:23 AM
            RISC-V inferior to ARMv8random person2018/12/21 02:04 AM
              RISC-V inferior to ARMv8dmcq2018/12/21 04:27 AM
              RISC-V inferior to ARMv8juanrga2018/12/21 10:36 AM
              RISC-V inferior to ARMv8Doug S2018/12/21 12:02 PM
            RISC-V inferior to ARMv8juanrga2018/12/21 10:23 AM
          RISC-V inferior to ARMv8Adrian2018/12/20 11:21 PM
          RISC-V inferior to ARMv8anon2018/12/21 01:48 AM
            RISC-V inferior to ARMv8Adrian2018/12/21 03:44 AM
              RISC-V inferior to ARMv8anon2018/12/21 05:24 AM
            RISC-V inferior to ARMv8Adrian2018/12/21 04:09 AM
              RISC-V inferior to ARMv8Wilco2018/12/21 04:28 AM
          RISC-V inferior to ARMv8Michael S2018/12/21 02:27 AM
            RISC-V inferior to ARMv8Gabriele Svelto2018/12/21 01:09 PM
              RISC-V inferior to ARMv8Maynard Handley2018/12/21 02:58 PM
              RISC-V inferior to ARMv8Wilco2018/12/21 03:43 PM
                RISC-V inferior to ARMv8Travis Downs2018/12/21 05:45 PM
                  RISC-V inferior to ARMv8Wilco2018/12/22 04:37 AM
                    RISC-V inferior to ARMv8Travis Downs2018/12/22 06:54 AM
                      RISC-V inferior to ARMv8Wilco2018/12/22 10:32 AM
                Cracking is not freeGabriele Svelto2018/12/22 02:09 AM
                  Cracking is not freeWilco2018/12/22 04:32 AM
                    Cracking is not freeTravis Downs2018/12/22 07:07 AM
                      Cracking is not freeWilco2018/12/22 07:38 AM
                        Cracking is not freeTravis Downs2018/12/22 07:47 AM
                          Cracking is not freeWilco2018/12/22 10:24 AM
                            Cracking is not freeTravis Downs2018/12/25 03:41 PM
                              Cracking is not freeanon.12018/12/25 08:14 PM
                        multi-instruction decode and renamePaul A. Clayton2018/12/22 06:45 PM
                    Cracking is not freeGabriele Svelto2018/12/22 12:30 PM
                      Cracking is not freeWilco2018/12/23 06:48 AM
                      Cracking is not freeMichael S2018/12/23 08:09 AM
                        Cracking is not freeGabriele Svelto2018/12/26 02:53 PM
          RISC-V inferior to ARMv8rwessel2018/12/21 01:13 PM
          RISC-V inferior to ARMv8Seni2018/12/21 02:33 PM
            RISC-V inferior to ARMv8Wilco2018/12/21 03:33 PM
              RISC-V inferior to ARMv8Travis Downs2018/12/21 05:49 PM
                RISC-V inferior to ARMv8Wilco2018/12/22 04:58 AM
                  RISC-V inferior to ARMv8Travis Downs2018/12/22 07:03 AM
                    RISC-V inferior to ARMv8Wilco2018/12/22 07:22 AM
                      RISC-V inferior to ARMv8Travis Downs2018/12/22 07:40 AM
        RISC-V inferior to ARMv8dmcq2018/12/21 03:57 AM
      RISC-V inferior to ARMv8Konrad Schwarz2018/12/21 02:25 AM
      RISC-V inferior to ARMv8j2018/12/21 10:46 AM
        RISC-V inferior to ARMv8Travis Downs2018/12/21 06:08 PM
          RISC-V inferior to ARMv8dmcq2018/12/22 07:45 AM
            RISC-V inferior to ARMv8Travis Downs2018/12/22 07:50 AM
              RISC-V inferior to ARMv8Michael S2018/12/22 08:15 AM
                RISC-V inferior to ARMv8dmcq2018/12/22 10:41 AM
        RISC-V inferior to ARMv8AnonQ2018/12/22 08:13 AM
    RISC-V gut feelingsdmcq2018/12/20 07:41 AM
      RISC-V initial takeKonrad Schwarz2018/12/21 02:17 AM
        RISC-V initial takedmcq2018/12/21 03:23 AM
      RISC-V gut feelingsMontaray Jack2018/12/22 02:56 PM
        RISC-V gut feelingsdmcq2018/12/23 04:38 AM
  RISC-V Summit Proceedingsjuanrga2018/12/21 10:47 AM
    RISC-V Summit Proceedingsdmcq2018/12/22 06:21 AM
      RISC-V Summit ProceedingsMontaray Jack2018/12/22 02:03 PM
        RISC-V Summit Proceedingsdmcq2018/12/23 04:39 AM
  RISC-V Summit Proceedingsanon22018/12/21 10:57 AM
    RISC-V Summit ProceedingsMichael S2018/12/22 08:36 AM
      RISC-V Summit ProceedingsAnon2018/12/22 05:51 PM
      Not Stanford MIPS but commercial MIPSPaul A. Clayton2018/12/23 03:05 AM
        Not Stanford MIPS but commercial MIPSMichael S2018/12/23 03:49 AM
        Not Stanford MIPS but commercial MIPSdmcq2018/12/23 04:52 AM
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