No dynamic predication yet, I suspect

By: Patrick Chase (patickjchase.delete@this.gmail.com), January 22, 2014 11:36 pm
Room: Moderated Discussions
Exophase (exophase.delete@this.gmail.com) on January 22, 2014 12:29 am wrote:
> Personally I don't know the first thing about state of the art branch prediction. Techniques
> first used over 10 years ago still strike me as quite complex. But I have to wonder if the focus
> has not shifted away from trying to knock down the last few percent of unpredicted branches in
> some workloads and more towards trying to avoid doing a lot of work predicting the branches that
> could be predicted with simple heuristics.

One way to roughly assess whether this is the case is to look at ROB sizes (i.e. instruction window sizes). Branch prediction accuracy imposes an upper limit on the usable ROB size, because mispredicts cause ROB flushes (albeit partial flushes in SB/IB/Haswell). It would be pointless to design an ROB that's larger than the average number of instructions per mispredicted branch, and you'd probably want the latter to be quite a bit larger than the ROB size.

If you look at recent Intel core designs, the ROB has doubled in size between Core2 and Haswell, so the branch mispredict rate should ideally have been reduced by a factor of 2 (or perhaps a bit less: as noted above they no longer completely flush the ROB on mispredict, so that would allow a somewhat higher mispredict frequency). It seems likely that Intel would still focus heavily on accuracy and/or mitigation strategies like dynamic predication.

> Loop predictors are an obvious application, and I'm sure once you know a branch is a loop
> (with high confidence) you save a lot of power running it through the loop counter predictor
> instead of the general predictors full of big table lookups.

If you believe Agner Fog's results, Intel deployed a special-case loop predictor in P6 descendants from Pentium-M through Nehalem. Interestingly enough they appear to have discarded the loop predictor in Sandy/Ivy Bridge and Haswell.
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TopicPosted ByDate
Some cinebench scores and IPCTimothy McCaffrey2014/01/17 09:27 PM
  Many Thanks :) (NT)Alberto2014/01/18 02:12 AM
  Thanks! :-) (NT)Poindexter2014/01/19 04:46 AM
  Is Cinebench a totally useless benchmark?slacker2014/01/19 11:47 AM
    Is Cinebench a totally useless benchmark?Brett2014/01/19 12:48 PM
      Is Cinebench a totally useless benchmark?Alberto2014/01/19 02:08 PM
        Is Cinebench a totally useless benchmark?Exophase2014/01/19 07:48 PM
          Is Cinebench a totally useless benchmark?Alberto2014/01/20 02:15 AM
            Is Cinebench a totally useless benchmark?Exophase2014/01/20 02:45 AM
              Is Cinebench a totally useless benchmark?Alberto2014/01/21 04:42 AM
                Is Cinebench a totally useless benchmark?Exophase2014/01/21 08:10 AM
                  Monopolies holding back advancementsDoug S2014/01/21 06:04 PM
                    Monopolies holding back advancementsMaxwell2014/01/22 08:00 AM
                      Monopolies holding back advancementsDoug S2014/01/22 11:31 PM
                        Moore's Law provided Planned Obsolescencehobold2014/01/23 01:31 AM
                          Moore's Law provided Planned ObsolescenceDoug S2014/01/23 08:54 PM
                            Moore's Law provided Planned Obsolescencehobold2014/01/24 03:02 AM
                              Moore's Law provided Planned ObsolescenceDoug S2014/01/24 01:18 PM
        Is Cinebench a totally useless benchmark?Maynard Handley2014/01/19 10:26 PM
          Is Cinebench a totally useless benchmark?Exophase2014/01/19 11:01 PM
            Is Cinebench a totally useless benchmark?Maynard Handley2014/01/20 03:25 AM
              Is Cinebench a totally useless benchmark?Patrick Chase2014/01/21 10:13 AM
                Is Cinebench a totally useless benchmark?Patrick Chase2014/01/21 12:31 PM
          Is Cinebench a totally useless benchmark?Patrick Chase2014/01/21 09:19 AM
            Intel and branch predictionDavid Kanter2014/01/21 10:26 AM
              Intel and branch predictionMaynard Handley2014/01/21 08:52 PM
                Intel and branch predictionMaynard Handley2014/01/21 09:14 PM
                No dynamic predication yet, I suspectPaul A. Clayton2014/01/21 10:04 PM
                  No dynamic predication yet, I suspectExophase2014/01/22 12:29 AM
                    No dynamic predication yet, I suspectdmcq2014/01/22 05:24 AM
                    No dynamic predication yet, I suspectPatrick Chase2014/01/22 11:36 PM
                      No dynamic predication yet, I suspectMaynard Handley2014/01/23 08:51 AM
                        No dynamic predication yet, I suspectPatrick Chase2014/01/23 11:59 AM
                          No dynamic predication yet, I suspectPatrick Chase2014/01/23 12:01 PM
                          16 misses per core on Haswell?David Kanter2014/01/23 06:10 PM
                            16 misses per core on Haswell?Patrick Chase2014/01/23 08:12 PM
                          No dynamic predication yet, I suspectPatrick Chase2014/01/27 06:34 PM
                            Fixed link to paperPaul A. Clayton2014/01/28 08:51 AM
                        No dynamic predication yet, I suspectPatrick Chase2014/01/23 12:29 PM
                      SMT influence on ROB size?Paul A. Clayton2014/01/23 11:26 AM
                        SMT influence on ROB size?Patrick Chase2014/01/23 08:40 PM
    Is Cinebench a totally useless benchmark?Exophase2014/01/19 07:44 PM
    Is Cinebench a totally useless benchmark?anon2014/01/19 08:43 PM
      Is Cinebench a totally useless benchmark?Timothy McCaffrey2014/01/20 04:24 PM
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