Benefit of dynamic hammock predication

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), July 8, 2015 1:38 pm
Room: Moderated Discussions
dmcq (dmcq.delete@this.fano.co.uk) on July 8, 2015 10:11 am wrote:
>
> If Linux copes with weak memory ordering and just using barriers then that is good. If a person wants to
> use shared data they should signal to the hardware that they are doing that - it should not be a general
> overhead.

You completely missed my argument.Apparently the whole "testing is meaningful" passed you by with no impact.

> The design of the hardware is becoming more automated and straightforward rules are what are needed.

.. I would say that _stricter_ rules are needed. We have a history of CPU's starting off with really badly designed and weak rules in general, and every single time that tends to be a big mistake. Undefined behavior in a CPU is bad. And weak memory ordering pretty much ends up being "undefined behavior".

Now, would I like memory ordering even stricter than what Intel gives me? Yeah, that would be lovely. I think s390 ends up actually doing that. But I'll take what I can get, and I'll point out the relative strengths and weaknesses. The x86 is a relatively strong model that helps make some orderign issues much easier to handle.

(In particular, causality is something that people don't think about, because we take it for granted. When your memory ordering breaks causality, I guarantee you that people can look at code that is "obviously correct" and have a really hard time debugging it)

> You're
> probably well aware of the problems that crop up because the compilers are getting more intelligent and
> follow the specs so they can produce better optimised code.

I wouldn't call that "intelligent". Not if it causes problems and depends on undefined behavior to do questionable optimizations. And yes, the C standards people are sadly giving compiler people more room to screw up (I particularly dislike the insane type-based aliasing).

For example, only a moron would decide that two accesses to the same variable cannot alias. But I have seen compilers that do exactly that, because they followed the standard and said that if you access it with two different types, the accesses cannot alias. Literally breaking obvious and traditional C code.

You apparently call things like that "intelligent". I call it stupid in the extreme. It's not a feature. It's idiocy. The fact that the C language definition allows it does not make the language definition "better". It makes it worse.

Same deal with CPU definitions.

> That is happening to processors. As they get
> more intelligent in following the specs we need proper markers on access to shared data.

No we don't. We need processors that are more robust, and don't do stupid things. We've got enough bugs to worry about that we don't need another source of really subtle ones.

And again, your definition of "intelligent" is very odd. Quite frankly, the Intel big0-core CPU's are clearly more intelligent than the ARM or Power cores.

If you really doubt that, go ask a Power person to explain their serialization logic. I can pretty much guarantee that that person will fail. Because the Power barriers arent' "intelligent". They are the exact opposite of that. They are confusing even to people who understand Power, much less anybody else. I guarantee you that even compiler people are afraid of them. They are the opposite of intelligent.

ARM is better, in at least having much clearer rules. Those rules are still much more fragile for users, and there are several very odd corner cases (the whole "data dependency" vs "control dependency" really is a pretty subtle rule that most people are not at all aware of - I can almost guarantee that most people on this forum had no idea).

Intel ordering rules are certainly not simple either (although they have improved a lot), so I'm by no means claiming they are perfect. But they are at least much less prone to nasty hidden races that expose bugs on new microarchitectures.

Linus
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
Cyclone Macro-op fusionMaynard Handley2015/07/05 06:01 PM
  Cyclone Macro-op fusionGabriele Svelto2015/07/06 02:11 AM
    Cyclone Macro-op fusionMaynard Handley2015/07/06 10:25 AM
      Cyclone Macro-op fusionWilco2015/07/06 11:17 AM
        Cyclone Macro-op fusionMaynard Handley2015/07/06 12:07 PM
          Cyclone Macro-op fusionMaynard Handley2015/07/06 12:55 PM
            Cyclone Macro-op fusionGabriele Svelto2015/07/07 02:49 AM
      Power8 branch conversionSHK2015/07/06 12:41 PM
        Power8 branch conversionMaynard Handley2015/07/06 01:01 PM
          Power8 branch conversionMaynard Handley2015/07/06 01:09 PM
        Benefit of dynamic hammock predicationPaul A. Clayton2015/07/06 03:02 PM
          Benefit of dynamic hammock predicationMaynard Handley2015/07/06 03:34 PM
            Benefit of dynamic hammock predicationEduardoS2015/07/06 07:56 PM
            Benefit of dynamic hammock predicationPatrick Chase2015/07/06 10:30 PM
          Benefit of dynamic hammock predicationLinus Torvalds2015/07/06 03:59 PM
            Benefit of dynamic hammock predicationWilco2015/07/06 04:35 PM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/06 05:25 PM
                Benefit of dynamic hammock predicationanon2015/07/06 09:02 PM
            Benefit of dynamic hammock predicationPatrick Chase2015/07/06 10:31 PM
              Benefit of dynamic hammock predicationanon2015/07/06 11:02 PM
              Benefit of dynamic hammock predicationMichael S2015/07/07 01:07 AM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/07 09:23 AM
                Benefit of dynamic hammock predicationPatrick Chase2015/07/07 10:49 AM
                  Benefit of dynamic hammock predicationSHK2015/07/07 11:34 AM
                  Benefit of dynamic hammock predicationMaynard Handley2015/07/07 12:00 PM
                    Benefit of dynamic hammock predicationExophase2015/07/07 01:26 PM
                      Benefit of dynamic hammock predicationWilco2015/07/07 04:54 PM
                      Benefit of dynamic hammock predicationMaynard Handley2015/07/07 05:25 PM
                    Benefit of dynamic hammock predicationPatrick Chase2015/07/07 02:32 PM
                    Benefit of dynamic hammock predicationMichael S2015/07/08 01:57 AM
                  Benefit of dynamic hammock predicationSylvain Collange2015/07/08 06:30 AM
                    Benefit of dynamic hammock predicationMaynard Handley2015/07/08 09:46 AM
                      Benefit of dynamic hammock predicationSylvain Collange2015/07/08 10:32 AM
                        Benefit of dynamic hammock predicationMaynard Handley2015/07/08 10:57 AM
                        Sadly, skewed associativity has not been broadly adoptedPaul A. Clayton2015/07/08 02:00 PM
                        Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/08 08:23 PM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)David Kanter2015/07/09 07:20 AM
                            Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/09 10:04 AM
                              Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Tim McCaffrey2015/07/09 10:41 AM
                                Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 01:08 PM
                                  Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Sylvain Collange2015/07/10 02:03 AM
                              Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)David Kanter2015/07/09 04:15 PM
                                Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/09 05:21 PM
                            Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 12:44 PM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 09:43 AM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)someone2015/07/10 05:43 AM
            Benefit of dynamic hammock predicationKonrad Schwarz2015/07/07 07:54 AM
            Benefit of dynamic hammock predicationdmcq2015/07/07 02:38 PM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/08 08:23 AM
                Benefit of dynamic hammock predicationdmcq2015/07/08 10:11 AM
                  Benefit of dynamic hammock predicationLinus Torvalds2015/07/08 01:38 PM
                    Example of memory ordering causing excitement in the real worldMark Roulo2015/07/08 04:54 PM
                      Example of memory ordering causing excitement in the real worlddmcq2015/07/09 04:37 AM
                        Example of memory ordering causing excitement in the real worldMark Roulo2015/07/09 08:24 AM
                          Example of memory ordering causing excitement in the real worlddmcq2015/07/09 10:09 AM
                          Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/09 10:12 AM
                            Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/09 12:14 PM
                              Example of memory ordering causing excitement in the real worlddmcq2015/07/09 04:24 PM
                                Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/10 01:07 PM
                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/10 03:49 PM
                                    Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/12 01:02 PM
                                      Example of memory ordering causing excitement in the real worlddmcq2015/07/13 05:03 AM
                                        Example of memory ordering causing excitement in the real worldMichael S2015/07/13 05:19 AM
                                        Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/14 01:21 PM
                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/14 02:54 PM
                                            Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/15 01:13 PM
                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/15 03:43 PM
                                                Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/16 12:41 PM
                                                  Example of memory ordering causing excitement in the real worldWilco2015/07/17 01:57 PM
                                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/17 04:29 PM
                                                      Example of memory ordering causing excitement in the real worlddmcq2015/07/18 04:07 AM
                                          Example of memory ordering causing excitement in the real worldJouni Osmala2015/07/16 03:52 AM
                                            Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/16 01:05 PM
                                              Example of memory ordering causing excitement in the real worldJouni Osmala2015/07/17 01:18 AM
                                                ISA support for bound checkingMichael S2015/07/17 04:51 AM
                                                  ISA support for bound checkingEtienne2015/07/17 06:39 AM
                                                  MPX on SkylakeMark Roulo2015/07/17 08:19 AM
                                                    MPX on SkylakeMichael S2015/07/17 08:44 AM
                                                      MPX on SkylakeMark Roulo2015/07/17 09:03 AM
                                  Example of memory ordering causing excitement in the real worldanon2015/07/12 03:42 AM
                                    Example of memory ordering causing excitement in the real worlddmcq2015/07/12 10:27 AM
                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 11:24 AM
                                      Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 11:53 AM
                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 01:07 PM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/12 07:52 PM
                                            Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/13 11:05 AM
                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/13 12:20 PM
                                                Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/13 01:46 PM
                                                  Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/13 02:10 PM
                                                    Example of memory ordering causing excitement in the real worldanon2015/07/13 08:51 PM
                                                    Simple cores justifying weak ordering?Paul A. Clayton2015/07/14 09:01 AM
                                                      David's Paul A. Clayton2015/07/14 09:37 AM
                                                      David's 2015-07-12 10:34 post confirmed some of that (NT)Paul A. Clayton2015/07/14 09:39 AM
                                                    Example of memory ordering causing excitement in the real worldEtienne2015/07/17 01:15 AM
                                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/13 02:14 PM
                                                  Reality is even worseTerry Gray2015/07/13 06:05 PM
                                                    Reality is even worseKonrad Schwarz2015/07/21 12:13 AM
                                                  A simple CAS/MCAS semantics2015/07/15 12:54 PM
                                                    A simple CAS/MCAS semanticsLinus Torvalds2015/07/15 01:55 PM
                                                    A simple CAS/MCAS semanticsEduardoS2015/07/15 02:31 PM
                                                Example of memory ordering causing excitement in the real worldsylt2015/07/13 02:19 PM
                                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/13 03:24 PM
                                                    Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 04:48 PM
                                                      Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/13 07:18 PM
                                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 07:57 PM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/13 09:01 PM
                                                    What high level languages do you have in mind?Mark Roulo2015/07/13 05:00 PM
                                                      What high level languages do you have in mind?dmcq2015/07/14 06:54 AM
                                                  Example of memory ordering causing excitement in the real worldWilco2015/07/13 04:36 PM
                                                    Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 04:56 PM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/13 06:43 PM
                                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 08:11 PM
                                                          Example of memory ordering causing excitement in the real worldanon2015/07/13 09:07 PM
                                      Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/12 12:32 PM
                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 01:35 PM
                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 01:38 PM
                                          Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 06:47 PM
                                            Example of memory ordering causing excitement in the real worldNoSpammer2015/07/13 12:34 PM
                                              Example of memory ordering causing excitement in the real worldWilco2015/07/13 04:50 PM
                                                Example of memory ordering causing excitement in the real worldNoSpammer2015/07/14 02:02 PM
                                                  Example of memory ordering causing excitement in the real worldanon2015/07/14 09:37 PM
                                                    Example of memory ordering causing excitement in the real worldMichael S2015/07/15 01:19 AM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/15 02:26 AM
                                                        Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 07:05 AM
                                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/15 08:04 AM
                                                            Example of memory ordering causing excitement in the real worldMichael S2015/07/15 08:29 AM
                                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/15 10:09 AM
                                                              Can string ops use microarchitectural state?Paul A. Clayton2015/07/16 09:33 AM
                                                                Can string ops use microarchitectural state?rwessel2015/07/16 01:00 PM
                                                                  Can string ops use microarchitectural state?Paul A. Clayton2015/07/16 06:16 PM
                                                                    Can string ops use microarchitectural state?rwessel2015/07/16 09:34 PM
                                                                      Can string ops use microarchitectural state?Michael S2015/07/17 05:39 AM
                                                                        Can string ops use microarchitectural state?rwessel2015/07/17 09:02 AM
                                                                Consider interrupt followed by task preemptionMichael S2015/07/16 04:31 PM
                                                                  Consider interrupt followed by task preemptionPaul A. Clayton2015/07/16 06:23 PM
                                                            Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 12:50 PM
                                                              Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 02:27 PM
                                                                Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 03:14 PM
                                                                  Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 05:42 PM
                                                      Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 02:44 AM
                                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 02:17 PM
                                                      Example of memory ordering causing excitement in the real worldWilco2015/07/15 02:43 PM
                                                        Example of memory ordering causing excitement in the real worldanon2015/07/15 05:45 PM
                                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 06:35 PM
                                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/16 02:46 AM
                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/16 02:49 AM
                                        Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 07:15 AM
                                      Example of memory ordering causing excitement in the real worldanon2015/07/12 07:42 PM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/12 09:48 PM
                                        Example of memory ordering causing excitement in the real worldDavid Kanter2015/07/12 10:34 PM
                                          Example of memory ordering causing excitement in the real worldMichael S2015/07/13 01:51 AM
                                            Example of memory ordering causing excitement in the real worldDavid Kanter2015/07/13 08:31 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/13 06:05 AM
                                            Example of memory ordering causing excitement in the real worldGabriele Svelto2015/07/17 05:40 AM
                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/13 08:09 AM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/13 07:46 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/14 04:12 AM
                                        Example of memory ordering causing excitement in the real worlddmcq2015/07/15 03:51 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/15 04:38 AM
                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/15 06:21 AM
                                      Store queues in POWER/PowerPC processorsGabriele Svelto2015/07/16 01:55 AM
                                        Not sure about that conclusionDavid Kanter2015/07/16 07:34 AM
                                          The POWER4 had the same structureGabriele Svelto2015/07/17 05:20 AM
                                        Committing stores in program orderPaul A. Clayton2015/07/16 01:35 PM
                                          Committing stores in program orderMaynard Handley2015/07/17 12:52 PM
                                      Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/20 04:44 AM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/20 07:29 AM
                                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 12:08 AM
                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/21 01:27 AM
                                            Example of memory ordering causing excitement in the real worldMichael S2015/07/21 03:11 AM
                                              Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 05:24 AM
                                            Example of memory ordering causing excitement in the real worldanon2015/07/21 06:22 AM
                                              Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 07:36 AM
                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 08:25 AM
                                              Example of memory ordering causing excitement in the real worldanon2015/07/21 08:17 AM
                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 09:50 AM
                                                Example of memory ordering causing excitement in the real worldGabriele Svelto2015/07/21 12:46 PM
                                                Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/22 12:35 AM
                                                  Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/22 01:37 AM
                                                  Example of memory ordering causing excitement in the real worldanon2015/07/23 03:17 AM
                                                    Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 04:59 AM
                                                      Example of memory ordering causing excitement in the real worldMichael S2015/07/23 07:31 AM
                                                        Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 03:38 PM
                                                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 11:44 PM
                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/24 01:08 AM
                                    Example of memory ordering causing excitement in the real worlddmcq2015/07/13 07:27 AM
                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/16 05:11 AM
                      Example of memory ordering causing excitement in the real worldAnonymousse2015/07/16 05:07 AM
          Benefit of dynamic hammock predicationsomeotherdude2015/07/06 05:12 PM
            Benefit of dynamic hammock predicationMichael S2015/07/07 01:19 AM
          Benefit of dynamic hammock predicationPatrick Chase2015/07/06 10:20 PM
            SIMD predication is also helpful, of course (NT)Paul A. Clayton2015/07/07 07:45 AM
        Power8 branch conversionGabriele Svelto2015/07/07 02:50 AM
          isel works for storesPaul A. Clayton2015/07/07 07:43 AM
            isel works for storesPatrick Chase2015/07/07 11:08 AM
            isel DOES NOT work for storessomeotherdude2015/07/07 04:01 PM
              isel DOES NOT work for storesWilco2015/07/07 04:40 PM
              Yes it doesPatrick Chase2015/07/07 11:14 PM
                Yes it doesvvid2015/07/08 09:51 AM
            You don't need isel eitherGabriele Svelto2015/07/08 02:01 AM
              You don't need isel eitherPaul A. Clayton2015/07/08 04:46 AM
                You don't need isel eitherGabriele Svelto2015/07/08 05:22 AM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell green?