Zen 3

By: hobold (hobold.delete@this.vectorizer.org), October 11, 2020 11:32 am
Room: Moderated Discussions
anon (anon.delete@this.ymous.org) on October 11, 2020 2:58 am wrote:
[...]
> Value prediction may predict previously unknown values, much like prefetchers can predict
> unknown addresses (e.g., Stride prefetcher and its counterpart Stride value predictor).
> In practice those "unseen before" values may well be addresses (or values feeding address
> generation e.g., loop induction variable) that the prefetcher would have gotten too.
>
The words "unseen before" mean something other than "unknown". Just like "prediction" does not mean the same as accurately foretelling the actual future.

[...]
> On latency, I would contend that in isolation, latency is always greater than 0 because you have
> to at least fetch the instruction. However with superscalar and OoO the latency of some instructions
> does not appear on the critical path. Moreover, for instructions that *do* appear on the critical
> path, superscalar fetch + some optimizations (move elimination, zero/one-idiom elimination and
> now memory bypassing) can cause instructions to appear has having 0 latency.

The word "appear" means something other than "be".


I am splitting hairs here. I get that I may appear to be hell-bent on having the final word on this. But I hate it when engineers begin to think in concepts that are rooted more in marketing than in observable phenomena. Knowing the future beforehand is possible only under very specific circumstances. Negative latency is possible only with the help of a time machine (in the macroscopic world). Thinking in incorrect terminology misleads the thinkers to make blatant mistakes.

Take my favourite example from recent history: the retire stage of a microprocessor. Its function and purpose is to create the limited appearance of strictly sequential program execution, despite the reality of dynamic out of order execution.

If one's mental concept is that the retire stage creates anything more than a limited appearance, then one might be tempted to implement further functionality in that same pipeline stage. If this line of thought ever leads to the implementation of security features in the retire stage ("we can remove this from the critical path!"), then one ends up with a processor that creates the limited appearance of security. A very real meltdown.
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TopicPosted ByDate
Zen 3Blue2020/10/08 08:58 AM
  Zen 3Rayla2020/10/08 09:10 AM
  Zen 3Adrian2020/10/08 09:13 AM
    Does anyone know whether Zen 3 has AVX-512? (NT)Foo_2020/10/08 10:54 AM
      Does anyone know whether Zen 3 has AVX-512?Adrian2020/10/08 11:11 AM
  Zen 3 - Number of load/store units2020/10/08 09:21 AM
    Zen 3 - Number of load/store unitsRayla2020/10/08 09:28 AM
      Zen 3 - Number of load/store units2020/10/08 10:22 AM
        Zen 3 - Number of load/store unitsAdrian2020/10/08 10:53 AM
          Zen 3 - Number of load/store unitsTravis Downs2020/10/08 08:45 PM
          Zen 3 - CAD benchmarkPer Hesselgren2020/10/09 06:29 AM
            Zen 3 - CAD benchmarkAdrian2020/10/09 08:27 AM
        Zen 3 - Number of load/store unitsitsmydamnation2020/10/08 01:38 PM
          Zen 3 - Number of load/store unitsGroo2020/10/08 01:48 PM
            Zen 3 - Number of load/store unitsWilco2020/10/08 02:02 PM
              Zen 3 - Number of load/store unitsDummond D. Slow2020/10/08 03:39 PM
                Zen 3 - Number of load/store unitsDoug S2020/10/09 07:11 AM
                  Zen 3 - Number of load/store unitsDummond D. Slow2020/10/09 08:43 AM
                    Zen 3 - Number of load/store unitsDoug S2020/10/09 12:43 PM
                      N7 and N7P are not load/Store units - please fix the topic in your replies (NT)Heikki Kultala2020/10/10 06:37 AM
  Zen 3Jeff S.2020/10/08 11:16 AM
    Zen 3anon2020/10/08 12:57 PM
    Disappointing opening line in paperPaul A. Clayton2020/10/11 05:16 AM
      Thoughts on "Improving the Utilization of µop Caches..."Paul A. Clayton2020/10/14 11:11 AM
        Thoughts on "Improving the Utilization of µop Caches..."anon2020/10/15 10:56 AM
          Thoughts on "Improving the Utilization of µop Caches..."anon2020/10/15 10:57 AM
            Sorry about the messanon2020/10/15 10:58 AM
              Sorry about the messBrett2020/10/16 02:22 AM
          Caching dependence info in µop cachePaul A. Clayton2020/10/16 05:20 AM
            Caching dependence info in µop cacheanon2020/10/16 11:36 AM
              Caching dependence info in µop cachePaul A. Clayton2020/10/18 12:28 PM
  Zen 3juanrga2020/10/09 09:12 AM
  Zen 3Mr. Camel2020/10/09 05:30 PM
    Zen 3anon.12020/10/09 11:44 PM
      Cinebench is terrible benchmarkDavid Kanter2020/10/10 09:36 AM
        Cinebench is terrible benchmarkanon.12020/10/10 11:06 AM
        Cinebench is terrible benchmarkhobold2020/10/10 11:33 AM
          Some comments on benchmarksPaul A. Clayton2020/10/14 11:11 AM
            Some comments on benchmarksMark Roulo2020/10/14 02:21 PM
    Zen 3Adrian2020/10/10 12:59 AM
      Zen 3Adrian2020/10/10 01:18 AM
        Zen 3majord2020/10/15 03:02 AM
  Zen 3hobold2020/10/10 07:58 AM
    Zen 3Maynard Handley2020/10/10 09:36 AM
      Zen 3hobold2020/10/10 11:19 AM
        Zen 3anon2020/10/11 01:58 AM
          Zen 3hobold2020/10/11 11:32 AM
            Zen 3anon2020/10/11 12:07 PM
              Zen 3hobold2020/10/11 01:22 PM
    Zen 3anon2020/10/10 10:51 AM
    Zen 3Michael S2020/10/11 12:16 AM
      Zen 3hobold2020/10/11 01:13 AM
        Zen 3Michael S2020/10/11 01:18 AM
      Zen 3anon.12020/10/11 11:17 AM
  Zen 3David Hess2020/10/12 05:43 AM
    more power? (NT)anonymous22020/10/12 12:26 PM
      I think he's comparing 65W 3700X vs 105W 5800X (NT)John H2020/10/12 03:33 PM
        ?! Those are apples and oranges! (NT)anon2020/10/12 03:49 PM
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