No dynamic predication yet, I suspect

By: dmcq (dmcq.delete@this.fano.co.uk), January 22, 2014 5:24 am
Room: Moderated Discussions
Exophase (exophase.delete@this.gmail.com) on January 22, 2014 12:29 am wrote:
> Paul A. Clayton (paaronclayton.delete@this.gmail.com) on January 21, 2014 10:04 pm wrote:
> > It seems they have not yet adopted dynamic predication for low-confidence hammock branches
> > (IBM seems to be providing this only for single instruction hammock branches, but a
> > more flexible mechanism might be useful), so it seems there is some room for improvement
> > (if one considers dynamic predication part of branch prediction).
>
> I'd consider it part of branch prediction in the sense that
> you're predicting not to actually use branch prediction.
>
> 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. 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. I would think there are some less obvious examples.

Getting the last couple of percent is very worthwhile as the processor may be looking a hundred or more instructions ahead to try and do worthwhile loads. One needs the chances of the load being executed being reasonably high and the chances go down for every branch along the path. If one can follow both sides of a conditional and know they come together again without changing the base register that is the sort of thing which would be well worth finding out if one can in a powerful processor.

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                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
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                            Fixed link to paperPaul A. Clayton2014/01/28 08:51 AM
                        No dynamic predication yet, I suspectPatrick Chase2014/01/23 12:29 PM
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      Is Cinebench a totally useless benchmark?Timothy McCaffrey2014/01/20 04:24 PM
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