No dynamic predication yet, I suspect

By: Patrick Chase (patrickjchase.delete@this.gmail.com), January 23, 2014 12:29 pm
Room: Moderated Discussions
Sorry about the re-reply, but:

Maynard Handley (name99.delete@this.name99.org) on January 23, 2014 8:51 am wrote:
> As far as I can tell - with current predictors, there's value in a ROB of at least five
> or six hundred instructions, perhaps a thousand. (That is, with misprediction accuracy as
> it is, you can go about that far and still have a reasonable chance of executing
> instructions that won't later be voided.)

Here's some data for SPEC2006 as of Merom/Woodcrest. See tables 3a and 3b in particular: http://www.spec.org/workshops/2007/austin/papers/Performance_Characterization_SPEC_CPU_Benchmarks.pdf

The executive summary is that Core2 had mispredict rates of about 1 per 100 instructions (not 1% of branches, 1% of *instructions*) on SPECint, and 0.7 per 100 instructions on SPECfp. It seens to me that you'd have a hard time keeping Core2's 100-entry ROB full with "good" instructions. Haswell would be similarly living on the edge even if mispredict rates have halved.

On the plus side many branches will be resolved long before retirement (unless they happen to depend upon the result of a load that missed L1...), so the situation isn't quite as dire as simple averages would suggest.
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        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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