Cyclone Macro-op fusion

By: Maynard Handley (name99.delete@this.name99.org), July 6, 2015 11:55 am
Room: Moderated Discussions
Maynard Handley (name99.delete@this.name99.org) on July 6, 2015 12:07 pm wrote:
> Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on July 6, 2015 11:17 am wrote:
> > Maynard Handley (name99.delete@this.name99.org) on July 6, 2015 10:25 am wrote:
> > > Gabriele Svelto (gabriele.svelto.delete@this.gmail.com) on July 6, 2015 2:11 am wrote:
> > > > Maynard Handley (name99.delete@this.name99.org) on July 5, 2015 6:01 pm wrote:
> > > > > Interestingly I did not see any OTHER fusion possibilities in the code. In particular the possibility
> > > > > IBM selected (fusing instructions to create a large immediate) is not utilized, which might, of course
> > > > > reflect not enough time to add this to the design; but maybe also reflects something about ARM's constant
> > > > > generation and so a less frequent need for generating immediates through successive instructions.
> > > >
> > > > In POWER8's case instruction fusion applies to add immediate / add immediate
> > > > shifted + loads which covers a weakness in the ISA which might not apply to
> > > > ARM (lack of base + immediate addressing for certain types of load).
> > > >
> > > > IBM has also been doing more aggressive fusion such as turning conditional-jump-over-a-single-instruction
> > > > sequences into a single predicated µop (the instruction can be an add, and, or, xor plus some
> > > > immediate forms as well as a store). See the POWER8 user manual section 10.1.4.7.
> > > >
> > > > Beside being different ISAs, IBM doesn't have many constraints as far as power
> > > > goes so the tradeoffs they're making might not apply to Apple's design.
> > >
> > > The IBM branch over one instruction is neat, but, like you said
> > > for forming immediates, it reflects a hole in the iSA.
> > > I am guessing that Apple (and the whole ARM camp)'s answer to that is to use a conditional
> > > select. And of course Apple, in particular, have very little of a legacy code problem...
> > >
> > > That does raise the issue that perhaps the next lowest lying fruit for ARM op-fusion
> > > might be op+predicated move as a single unit? That and three input add seem like
> > > the most common patterns left after compare and branch have been handled.
> >
> > Various ARM cores fuse cmp+bcc, mov+movk, adrp+ldr, adrp+add already. There aren't that many cases
> > left that make sense - most fusions are already in the architecture (ldp, writeback, shift+alu etc),
> > and you also need to form instructions that are simple enough not to need splitting later.
>
> Do we know for a FACT that all the options you are suggesting
> are in use (as opposed to "the sensible ones to use")?
> That was the point of my initial post --- that we have some sort of confirmation about
> what Apple is doing in Cyclone. As far as I know, beyond that, it's all speculation. For
> example A72 is supposed to have enhanced fusion, but I've seen nothing beyond that detail
> as to what is being fused (and how it's enhanced beyond what gets fused in A57).

OK, going through the A57 optimization manual
http://infocenter.arm.com/help/topic/com.arm.doc.uan0015a/cortex_a57_software_optimisation_guide_external.pdf
I see no EXPLICIT reference to op fusion, but we are told that
mov+movk (and similar pairs) and
adrp+add
are "optimized" sequences, which I assume means they are fused. (adrp+ldr is not mentioned)

Seems that A57 is not using the compare+branch fusion, but maybe that's coming in A72?

I can't tell from the LLVM sources whether Cyclone implements these or not.
The sources seem to indicate (though they don't draw attention to the fact) that when one wants to translate the appropriate abstract op (eg load a 32bit imm, or perform a certain type of large offset load) on ANY ARMv8 core one generates these optimal sequences (which is kinda obvious, what else would you do), and I don't know enough about LLVM internals to know if there's a flag being set to glue these pairs together so that they are never subsequently split/rearranged.
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TopicPosted ByDate
Cyclone Macro-op fusionMaynard Handley2015/07/05 05:01 PM
  Cyclone Macro-op fusionGabriele Svelto2015/07/06 01:11 AM
    Cyclone Macro-op fusionMaynard Handley2015/07/06 09:25 AM
      Cyclone Macro-op fusionWilco2015/07/06 10:17 AM
        Cyclone Macro-op fusionMaynard Handley2015/07/06 11:07 AM
          Cyclone Macro-op fusionMaynard Handley2015/07/06 11:55 AM
            Cyclone Macro-op fusionGabriele Svelto2015/07/07 01:49 AM
      Power8 branch conversionSHK2015/07/06 11:41 AM
        Power8 branch conversionMaynard Handley2015/07/06 12:01 PM
          Power8 branch conversionMaynard Handley2015/07/06 12:09 PM
        Benefit of dynamic hammock predicationPaul A. Clayton2015/07/06 02:02 PM
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            Benefit of dynamic hammock predicationEduardoS2015/07/06 06:56 PM
            Benefit of dynamic hammock predicationPatrick Chase2015/07/06 09:30 PM
          Benefit of dynamic hammock predicationLinus Torvalds2015/07/06 02:59 PM
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              Benefit of dynamic hammock predicationLinus Torvalds2015/07/06 04:25 PM
                Benefit of dynamic hammock predicationanon2015/07/06 08:02 PM
            Benefit of dynamic hammock predicationPatrick Chase2015/07/06 09:31 PM
              Benefit of dynamic hammock predicationanon2015/07/06 10:02 PM
              Benefit of dynamic hammock predicationMichael S2015/07/07 12:07 AM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/07 08:23 AM
                Benefit of dynamic hammock predicationPatrick Chase2015/07/07 09:49 AM
                  Benefit of dynamic hammock predicationSHK2015/07/07 10:34 AM
                  Benefit of dynamic hammock predicationMaynard Handley2015/07/07 11:00 AM
                    Benefit of dynamic hammock predicationExophase2015/07/07 12:26 PM
                      Benefit of dynamic hammock predicationWilco2015/07/07 03:54 PM
                      Benefit of dynamic hammock predicationMaynard Handley2015/07/07 04:25 PM
                    Benefit of dynamic hammock predicationPatrick Chase2015/07/07 01:32 PM
                    Benefit of dynamic hammock predicationMichael S2015/07/08 12:57 AM
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                      Benefit of dynamic hammock predicationSylvain Collange2015/07/08 09:32 AM
                        Benefit of dynamic hammock predicationMaynard Handley2015/07/08 09:57 AM
                        Sadly, skewed associativity has not been broadly adoptedPaul A. Clayton2015/07/08 01:00 PM
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                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)David Kanter2015/07/09 06:20 AM
                            Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/09 09:04 AM
                              Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Tim McCaffrey2015/07/09 09:41 AM
                                Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 12:08 PM
                                  Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Sylvain Collange2015/07/10 01:03 AM
                              Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)David Kanter2015/07/09 03:15 PM
                                Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/09 04:21 PM
                            Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 11:44 AM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 08:43 AM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)someone2015/07/10 04:43 AM
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            Benefit of dynamic hammock predicationdmcq2015/07/07 01:38 PM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/08 07:23 AM
                Benefit of dynamic hammock predicationdmcq2015/07/08 09:11 AM
                  Benefit of dynamic hammock predicationLinus Torvalds2015/07/08 12:38 PM
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                      Example of memory ordering causing excitement in the real worlddmcq2015/07/09 03:37 AM
                        Example of memory ordering causing excitement in the real worldMark Roulo2015/07/09 07:24 AM
                          Example of memory ordering causing excitement in the real worlddmcq2015/07/09 09:09 AM
                          Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/09 09:12 AM
                            Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/09 11:14 AM
                              Example of memory ordering causing excitement in the real worlddmcq2015/07/09 03:24 PM
                                Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/10 12:07 PM
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                                        Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/14 12:21 PM
                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/14 01:54 PM
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                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/15 02:43 PM
                                                Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/16 11:41 AM
                                                  Example of memory ordering causing excitement in the real worldWilco2015/07/17 12:57 PM
                                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/17 03:29 PM
                                                      Example of memory ordering causing excitement in the real worlddmcq2015/07/18 03:07 AM
                                          Example of memory ordering causing excitement in the real worldJouni Osmala2015/07/16 02:52 AM
                                            Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/16 12:05 PM
                                              Example of memory ordering causing excitement in the real worldJouni Osmala2015/07/17 12:18 AM
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                                                    MPX on SkylakeMichael S2015/07/17 07:44 AM
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                                  Example of memory ordering causing excitement in the real worldanon2015/07/12 02:42 AM
                                    Example of memory ordering causing excitement in the real worlddmcq2015/07/12 09:27 AM
                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 10:24 AM
                                      Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 10:53 AM
                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 12:07 PM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/12 06:52 PM
                                            Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/13 10:05 AM
                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/13 11:20 AM
                                                Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/13 12:46 PM
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                                                    Example of memory ordering causing excitement in the real worldanon2015/07/13 07:51 PM
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                                                      David's Paul A. Clayton2015/07/14 08:37 AM
                                                      David's 2015-07-12 10:34 post confirmed some of that (NT)Paul A. Clayton2015/07/14 08:39 AM
                                                    Example of memory ordering causing excitement in the real worldEtienne2015/07/17 12:15 AM
                                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/13 01:14 PM
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                                                Example of memory ordering causing excitement in the real worldsylt2015/07/13 01:19 PM
                                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/13 02:24 PM
                                                    Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 03:48 PM
                                                      Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/13 06:18 PM
                                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 06:57 PM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/13 08:01 PM
                                                    What high level languages do you have in mind?Mark Roulo2015/07/13 04:00 PM
                                                      What high level languages do you have in mind?dmcq2015/07/14 05:54 AM
                                                  Example of memory ordering causing excitement in the real worldWilco2015/07/13 03:36 PM
                                                    Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 03:56 PM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/13 05:43 PM
                                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 07:11 PM
                                                          Example of memory ordering causing excitement in the real worldanon2015/07/13 08:07 PM
                                      Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/12 11:32 AM
                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 12:35 PM
                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 12:38 PM
                                          Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 05:47 PM
                                            Example of memory ordering causing excitement in the real worldNoSpammer2015/07/13 11:34 AM
                                              Example of memory ordering causing excitement in the real worldWilco2015/07/13 03:50 PM
                                                Example of memory ordering causing excitement in the real worldNoSpammer2015/07/14 01:02 PM
                                                  Example of memory ordering causing excitement in the real worldanon2015/07/14 08:37 PM
                                                    Example of memory ordering causing excitement in the real worldMichael S2015/07/15 12:19 AM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/15 01:26 AM
                                                        Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 06:05 AM
                                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/15 07:04 AM
                                                            Example of memory ordering causing excitement in the real worldMichael S2015/07/15 07:29 AM
                                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/15 09:09 AM
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                                                                    Can string ops use microarchitectural state?rwessel2015/07/16 08:34 PM
                                                                      Can string ops use microarchitectural state?Michael S2015/07/17 04:39 AM
                                                                        Can string ops use microarchitectural state?rwessel2015/07/17 08:02 AM
                                                                Consider interrupt followed by task preemptionMichael S2015/07/16 03:31 PM
                                                                  Consider interrupt followed by task preemptionPaul A. Clayton2015/07/16 05:23 PM
                                                            Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 11:50 AM
                                                              Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 01:27 PM
                                                                Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 02:14 PM
                                                                  Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 04:42 PM
                                                      Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 01:44 AM
                                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 01:17 PM
                                                      Example of memory ordering causing excitement in the real worldWilco2015/07/15 01:43 PM
                                                        Example of memory ordering causing excitement in the real worldanon2015/07/15 04:45 PM
                                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 05:35 PM
                                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/16 01:46 AM
                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/16 01:49 AM
                                        Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 06:15 AM
                                      Example of memory ordering causing excitement in the real worldanon2015/07/12 06:42 PM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/12 08:48 PM
                                        Example of memory ordering causing excitement in the real worldDavid Kanter2015/07/12 09:34 PM
                                          Example of memory ordering causing excitement in the real worldMichael S2015/07/13 12:51 AM
                                            Example of memory ordering causing excitement in the real worldDavid Kanter2015/07/13 07:31 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/13 05:05 AM
                                            Example of memory ordering causing excitement in the real worldGabriele Svelto2015/07/17 04:40 AM
                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/13 07:09 AM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/13 06:46 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/14 03:12 AM
                                        Example of memory ordering causing excitement in the real worlddmcq2015/07/15 02:51 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/15 03:38 AM
                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/15 05:21 AM
                                      Store queues in POWER/PowerPC processorsGabriele Svelto2015/07/16 12:55 AM
                                        Not sure about that conclusionDavid Kanter2015/07/16 06:34 AM
                                          The POWER4 had the same structureGabriele Svelto2015/07/17 04:20 AM
                                        Committing stores in program orderPaul A. Clayton2015/07/16 12:35 PM
                                          Committing stores in program orderMaynard Handley2015/07/17 11:52 AM
                                      Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/20 03:44 AM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/20 06:29 AM
                                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/20 11:08 PM
                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/21 12:27 AM
                                            Example of memory ordering causing excitement in the real worldMichael S2015/07/21 02:11 AM
                                              Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 04:24 AM
                                            Example of memory ordering causing excitement in the real worldanon2015/07/21 05:22 AM
                                              Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 06:36 AM
                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 07:25 AM
                                              Example of memory ordering causing excitement in the real worldanon2015/07/21 07:17 AM
                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 08:50 AM
                                                Example of memory ordering causing excitement in the real worldGabriele Svelto2015/07/21 11:46 AM
                                                Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 11:35 PM
                                                  Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/22 12:37 AM
                                                  Example of memory ordering causing excitement in the real worldanon2015/07/23 02:17 AM
                                                    Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 03:59 AM
                                                      Example of memory ordering causing excitement in the real worldMichael S2015/07/23 06:31 AM
                                                        Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 02:38 PM
                                                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 10:44 PM
                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/24 12:08 AM
                                    Example of memory ordering causing excitement in the real worlddmcq2015/07/13 06:27 AM
                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/16 04:11 AM
                      Example of memory ordering causing excitement in the real worldAnonymousse2015/07/16 04:07 AM
          Benefit of dynamic hammock predicationsomeotherdude2015/07/06 04:12 PM
            Benefit of dynamic hammock predicationMichael S2015/07/07 12:19 AM
          Benefit of dynamic hammock predicationPatrick Chase2015/07/06 09:20 PM
            SIMD predication is also helpful, of course (NT)Paul A. Clayton2015/07/07 06:45 AM
        Power8 branch conversionGabriele Svelto2015/07/07 01:50 AM
          isel works for storesPaul A. Clayton2015/07/07 06:43 AM
            isel works for storesPatrick Chase2015/07/07 10:08 AM
            isel DOES NOT work for storessomeotherdude2015/07/07 03:01 PM
              isel DOES NOT work for storesWilco2015/07/07 03:40 PM
              Yes it doesPatrick Chase2015/07/07 10:14 PM
                Yes it doesvvid2015/07/08 08:51 AM
            You don't need isel eitherGabriele Svelto2015/07/08 01:01 AM
              You don't need isel eitherPaul A. Clayton2015/07/08 03:46 AM
                You don't need isel eitherGabriele Svelto2015/07/08 04:22 AM
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