ISSCC Poulson Preview Online - OoO not unlikely?

Article: New Itanium Microarchitecture at ISSCC 2011
By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), November 17, 2010 12:12 pm
Room: Moderated Discussions
anon (anon@anon.com) on 11/17/10 wrote:
>
>Avoiding OoO probably made a lot more sense 10 years ago than it does now.

So I'm on record for (many times) saying that I'd love to
see an OoO Itanium.

But one reason I'd love to see it is because I think it
would be very interesting and challenging to do. Some of
the features of the Itanium architecture are nasty from
an OoO standpoint.

Predication is nice and simple for in-order cores. But it's
generally really nasty for OoO because it makes for big
uglyy data dependencies. Sure, you can fix that - using
prediction for predication is as valid as it is for any
normal conditional branch - but code that is compiled with
predication in mind tends to use it heavily for both the
nicely predictable cases and for unpredictable ones.

So while doing prediction for predicates is technically
possible, it's likely to not work that well. And without
prediction, you then easily end up with long dependent data
sequences. On an architecture that does not use
predication heavily, those long sequences get broken up
by branch prediction, and that helps avoid unnecessary
work and allows more parallel execution.

(The "conditional move" is a common predicated instruction
across many architectures. And on many OoO x86 cores, it
is quite expensive - latencies of several cycles etc. But
it's a win if it makes code smaller or the conditional is
not predictable)

Now, there are other areas in the architecture that I
suspect makes OoO more interesting - like the whole alias
model etc - but those seem less fundamental, and more an
issue of "ok, so it's different and people who have done
OoO cores before need to think about the issues".

Some other features - notably the register stack and the
rotation - are likely to be details that can be largely
handled by the early decoding cycles of the pipeline doing
another register renaming pass.

But then if you do certain models of instruction caching
that may not work well. For example, if you cache some
internal pre-decoded representation (like the "loop buffer"
or whatever it is called in many modern x86 chips), you
now have to do that register rotation thing later in the
pipeline.

I think predication is the big thing, though. The rest are
likely "details", but they are big enough details that it
wouldn't be easy to try to re-use what Intel has learnt
from doing OoO on x86.

Linus
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 03:40 AM
  ISSCC Poulson Preview Onlinesavantu2010/11/17 05:25 AM
  ISSCC Poulson Preview Onlinesomeone2010/11/17 08:51 AM
    ISSCC Poulson Preview OnlineLinus Torvalds2010/11/17 09:27 AM
      ISSCC Poulson Preview Onlinesomeone2010/11/17 10:31 AM
        ISSCC Poulson Preview OnlineRichard Cownie2010/11/17 10:55 AM
          ISSCC Poulson Preview OnlineLinus Torvalds2010/11/17 11:30 AM
          ISSCC Poulson Preview Onlinesomeone2010/11/17 11:42 AM
            ISSCC Poulson Preview Onlinea reader2010/11/17 02:07 PM
        ISSCC Poulson Preview OnlineGabriele Svelto2010/11/19 02:52 AM
          ISSCC Poulson Preview Onlinesavantu2010/11/19 05:37 AM
    ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 12:12 PM
      ISSCC Poulson Preview Onlinesomeone2010/11/17 12:40 PM
        ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 02:08 PM
          ISSCC Poulson Preview Onlineajensen2010/11/18 01:24 AM
  ISSCC Poulson Preview Onlinelubemark2010/11/17 09:52 AM
  ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 11:41 AM
    ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/17 12:12 PM
      ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 05:38 PM
        ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/17 08:03 PM
    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/17 12:14 PM
      ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/17 02:07 PM
        ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/17 02:59 PM
          ISSCC Poulson Preview Online - OoO not unlikely?sylt2010/11/17 03:26 PM
          ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 05:53 PM
            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/17 08:34 PM
              ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 09:22 PM
              ISSCC Poulson Preview Online - OoO not unlikely?Martin Høyer Kristiansen2010/11/18 12:35 AM
                ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/18 01:14 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?Martin Høyer Kristiansen2010/11/18 02:00 AM
          ISSCC Poulson Preview Online - OoO not unlikely?Groo2010/11/18 03:28 PM
            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/18 09:30 PM
              ISSCC Poulson Preview Online - OoO not unlikely?Doug Siebert2010/11/19 12:06 AM
                ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 07:52 AM
                  Negativity and ignorancePaul A. Clayton2010/11/19 06:11 PM
                  rhetoricshobold2010/11/20 05:01 AM
              ISSCC Poulson Preview Online - OoO not unlikely?anonymous2010/11/19 02:17 AM
                ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 08:22 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/19 09:26 AM
                    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 11:04 AM
                      ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/19 11:24 AM
                        ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 12:05 PM
                          ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/19 01:08 PM
                            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/20 11:02 AM
                              ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/20 12:59 PM
                              ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/20 01:01 PM
                                ISSCC Poulson Preview Online - OoO not unlikely?rwessel2010/11/20 01:27 PM
                                  ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/21 05:00 PM
                                    single-system single-location != RASanonymous2010/11/21 09:33 PM
                                      single-system single-location != RASdc2010/11/25 12:06 PM
                                        single-system single-location != RASDavid Kanter2010/11/25 01:58 PM
                    CS:SNAREPslacker2010/11/20 04:07 AM
                      CS:SNAREPnone2010/11/20 04:53 AM
                        CS:SNAREPS. Rao2010/11/20 11:20 AM
                      CS:SNAREPGabriele Svelto2010/11/22 07:11 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/19 11:08 AM
                    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 06:51 PM
                      ISSCC Poulson Preview Online - OoO not unlikely?rwessel2010/11/20 01:53 AM
                        ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/20 12:43 PM
                          ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/20 05:43 PM
                            ISSCC Poulson Preview Online - OoO not unlikely?Foo_2010/11/20 05:56 PM
              ISSCC Poulson Preview Online - OoO not unlikely?Groo2010/11/20 12:59 PM
                P = E/tslacker2010/11/20 11:31 PM
                  P = E/tAndi Kleen2010/11/21 03:02 AM
                ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/21 10:24 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/21 01:07 PM
                    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/21 04:50 PM
                      ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/21 05:38 PM
                        ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/21 06:27 PM
                          ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/22 08:37 AM
                            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/22 09:07 AM
                              ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/22 09:25 AM
                                ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/25 12:22 PM
                                  ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/26 10:44 AM
                              perfAM2010/11/24 03:28 AM
                                perfajensen2010/11/24 03:46 AM
                                perfsomeone2010/11/24 09:59 AM
                          OOOE and energy efficiencyDavid Kanter2010/11/23 11:28 AM
                            OOOE and energy efficiencysomeone2010/11/23 12:42 PM
                              OOOE and energy efficiencyDavid Kanter2010/11/23 06:00 PM
                                OOOE and energy efficiencyajensen2010/11/24 01:58 AM
                                  OOOE and energy efficiencyRichard Cownie2010/11/24 10:44 AM
                                    OOOE and energy efficiencyajensen2010/11/24 01:51 PM
                                      OOOE and energy efficiencyRichard Cownie2010/11/24 02:59 PM
                                        OOOE and energy efficiencyajensen2010/11/25 12:20 AM
                              OOOE and energy efficiencyChuck2010/11/23 10:04 PM
    ISSCC Poulson Preview Online - OoO not unlikely?ajensen2010/11/18 12:48 AM
  ISSCC Poulson Preview OnlineMatt Waldhauer2010/11/17 11:50 AM
    ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 12:21 PM
    ISSCC Poulson Preview Onlinesomeone2010/11/17 12:51 PM
      ISSCC Poulson Preview OnlineMatt Waldhauer2010/11/18 07:38 AM
    ISSCC Poulson Preview OnlineDan Downs2010/11/18 10:15 AM
      ISSCC Poulson Preview OnlineMatt Waldhauer2010/11/23 03:19 AM
  OoOE IPFajensen2010/11/22 02:12 AM
    OoOE IPFanon2010/11/22 03:24 AM
      OoOE IPFajensen2010/11/22 03:46 AM
  ISSCC Poulson Preview Online- Abstractlubemark2010/11/22 08:02 AM
    ISSCC Poulson Preview Online- AbstractIntelUser20002010/11/22 09:49 AM
      ISSCC Poulson Preview Online- AbstractDavid Kanter2010/11/22 11:08 AM
      Cache sharingPaul A. Clayton2010/11/22 01:42 PM
  ISSCC Poulson Preview OnlineTwelve2011/03/23 01:32 PM
    ISSCC Poulson Preview OnlineDaniel Bizó2011/03/24 06:23 AM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell green?