Thoughts and questions on the Cortex A9

By: Gabriele Svelto (gabriele.svelto.delete@this.gmail.com), September 28, 2009 12:38 am
Room: Moderated Discussions
Wilco (Wilco.Dijkstra@ntlworld.com) on 9/26/09 wrote:
---------------------------
>You can write the result to the register file as soon as it has been computed.
>It's a big advantage over in-order as you don't have to have to pipeline (and forward)
>every result to the final writeback stage. This is what you could call OoO completion.
>If the instruction was not speculated then you could release resources used by that
>instruction early as you know that it will complete once it starts.

Thanks for the clarification, that's an interesting aspect which can boost resource utilization quite a bit in well behaved codes. For example in loops where the branch condition can be resolved ahead of the branch and thus doesn't require speculation I suppose that the renames will be released very soon making the OoOE-window more effective.

>Single cycle load-use sounds like a special forwarding case. OoO pipelines typically
>aren't skewed - it doesn't make sense to issue an instruction and do nothing for
>1 or 2 cycles when you could issue an independent intruction instead. Unlike in-order, you don't gain anything at all.

Yeah, that's what made me curious about it. See here at page 4:

http://www.jp.arm.com/event/pdf/forum2007/t1-2.pdf

"Low load-use penalty of a single clock cycle optimizes benefit from OoO dispatch."

I frankly doubt that their L1 has a single-cycle access so that must mean something else.

>Historically ldr-str latency has been zero on most ARMs - even mul-str is usually
>zero latency. The paper suggests ldr-str forwarding happens within the L1. That
>likely means zero latency and a power saving.

I see.

>The A8 uses 32-entry fully associative TLB's, but a TLB miss means 2 expensive
>L2 accesses. The A9 pipeline diagram mentions micro-TLB's, which means they are
>small (likely 8 to 16 entries) and on a miss use a larger main TLB (like ARM11).
>I hope the main TLB has at least 128 entries and ideally 256.

That makes sense, I suppose that te main TLB should be at least large enough to cover the L2.

>Doing it after decode would need a lot more area as decoded instructions can be
>significantly wider than 32 bits. Loops with branches are likely harder to deal
>with as well. Caching fetched data is easier and works well on Thumb-2 where an
>average instruction is just 21 bits. L1 fetch is one of the largest power consumers
>(27% on StrongARM), so avoiding it in most loops helps a lot.

I didn't knew that L1 accesses were 'very' expensive from a power POV, that explains it.
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TopicPosted ByDate
Thoughts and questions on the Cortex A9Gabriele Svelto2009/09/26 01:46 AM
  Thoughts and questions on the Cortex A9none2009/09/26 02:27 AM
    Thoughts and questions on the Cortex A9jeff2009/09/27 04:06 AM
      Thoughts and questions on the Cortex A9Michael S2009/09/27 04:29 AM
        Thoughts and questions on the Cortex A9none2009/09/27 05:01 AM
          Thoughts and questions on the Cortex A9Howard Chu2009/09/27 09:39 AM
      Thoughts and questions on the Cortex A9Wilco2009/09/27 06:03 AM
        Thoughts and questions on the Cortex A9jeff2009/09/27 07:00 AM
          Thoughts and questions on the Cortex A9a reader2009/09/27 07:17 AM
            Thoughts and questions on the Cortex A9David Kanter2009/09/27 07:37 AM
              Thoughts and questions on the Cortex A9a reader2009/09/27 07:46 AM
                Thoughts and questions on the Cortex A9Mat2009/10/01 12:04 PM
                  Thoughts and questions on the Cortex A9Wilco2009/10/01 05:09 PM
                    Thoughts and questions on the Cortex A9anon2009/10/01 07:19 PM
            Thoughts and questions on the Cortex A9RagingDragon2009/09/28 04:11 PM
          Thoughts and questions on the Cortex A9Linus Torvalds2009/09/27 08:05 AM
            OOO hw vs SW&in-order hwno thanks2009/09/27 03:47 PM
              OOO hw vs SW&in-order hwLinus Torvalds2009/09/28 05:22 AM
                OOO hw vs SW&in-order hw?2009/09/28 10:37 AM
                  OOO hw vs SW&in-order hwRagingDragon2009/09/28 04:22 PM
                  OOO hw vs SW&in-order hwMegol2009/09/29 03:35 AM
                OOO hw vs SW&in-order hwAnders Jensen2009/09/28 10:50 PM
                  OOO hw vs SW&in-order hwLinus Torvalds2009/09/29 06:44 AM
                    OOO hw vs SW&in-order hwMark Roulo2009/09/29 08:58 AM
                      OOO hw vs SW&in-order hwLinus Torvalds2009/09/29 09:30 AM
                        3- and 4-issue in-order CPUsMark Roulo2009/09/29 10:06 AM
                          3- and 4-issue in-order CPUsLinus Torvalds2009/09/29 10:29 AM
                          3- and 4-issue in-order CPUsGian-Carlo Pascutto2009/09/29 11:35 PM
                          3- and 4-issue in-order CPUsMichael S2009/09/30 01:01 AM
                    OOO hw vs SW&in-order hwmpx2009/09/30 03:14 AM
                    OOO hw vs SW&in-order hwPun Zu2009/10/02 01:44 AM
                      OOO hw vs SW&in-order hwnone2009/10/02 04:22 AM
                      OOO hw vs SW&in-order hwLinus Torvalds2009/10/02 06:11 AM
                        OOO hw vs SW&in-order hwa reader2009/10/02 08:30 AM
                          OOO hw vs SW&in-order hwLinus Torvalds2009/10/02 08:59 AM
                            MoorestownDavid Kanter2009/10/02 09:59 AM
                              What's the difference between Moorestown and Pine Trail cores?anon2009/10/03 07:37 PM
                              Moorestownnone2009/11/03 03:34 PM
                                MoorestownAnon2009/11/04 02:17 PM
                                  Moorestownnone2009/11/05 12:38 AM
                                    MoorestownDavid Kanter2009/11/05 03:45 PM
                                      MoorestownIntelUser20002009/11/06 03:17 AM
                                      MoorestownAnon2009/11/06 12:51 PM
                                        Moorestownnone2009/11/07 06:07 AM
                            OOO hw vs SW&in-order hwAnon2009/10/02 06:55 PM
                              Cluebat for graphicsDavid Kanter2009/10/02 08:19 PM
                                Cluebat for graphicsAnon2009/10/03 04:45 PM
                                  Cluebat for graphicsDavid Kanter2009/10/04 12:57 AM
                                    Cluebat for graphicsAnon2009/10/04 07:15 PM
                                      Cluebat for graphicsDavid Kanter2009/10/05 02:09 AM
                                        Cluebat for graphicsAnon2009/10/05 02:36 PM
                                          Cluebat for graphicsDavid Kanter2009/10/05 08:54 PM
                                            Cluebat for graphicsAnon2009/10/06 04:58 PM
                              OOO hw vs SW&in-order hwLinus Torvalds2009/10/03 05:58 AM
                            OOO hw vs SW&in-order hwslacker2009/10/02 08:11 PM
                            Linux graphics driversRagingDragon2009/10/03 07:27 PM
                              Linux graphics driversanon2009/10/04 06:15 AM
                                Linux graphics driversnone2009/10/04 09:12 AM
            Thoughts and questions on the Cortex A9jeff2009/09/27 05:31 PM
        Thoughts and questions on the Cortex A9someone2009/09/27 08:30 AM
          Thoughts and questions on the Cortex A9none2009/09/27 09:09 AM
            Thoughts and questions on the Cortex A9Wilco2009/09/27 10:35 AM
              Thoughts and questions on the Cortex A9someone2009/09/27 10:55 AM
                Thoughts and questions on the Cortex A9Wilco2009/09/28 01:08 AM
                  Thoughts and questions on the Cortex A9someone2009/09/28 04:58 AM
                    Thoughts and questions on the Cortex A9none2009/09/28 05:18 AM
                      Thoughts and questions on the Cortex A9someone2009/09/28 06:35 AM
                    Thoughts and questions on the Cortex A9Wilco2009/09/28 07:25 AM
                      Thoughts and questions on the Cortex A9Michael S2009/09/28 10:02 AM
                        Thoughts and questions on the Cortex A9Wilco2009/09/29 12:35 AM
                    Thoughts and questions on the Cortex A9Chuck2009/09/28 06:15 PM
              samplesAM2009/09/27 10:20 PM
                samplesWilco2009/09/28 12:51 AM
                  samplesAM2009/09/28 03:16 AM
              Shrinks and process techDavid Kanter2009/09/29 12:22 AM
            Thoughts and questions on the Cortex A9someone2009/09/27 10:42 AM
              Thoughts and questions on the Cortex A9none2009/09/27 11:52 AM
              Atom to stay in-oder or go OoO?AM2009/09/27 10:09 PM
                Atom to stay in-oder or go OoO?Ungo2009/09/28 04:34 AM
                  Atom to stay in-oder or go OoO?a reader2009/09/28 09:15 AM
                    Atom to stay in-oder or go OoO?anon2009/09/28 06:25 PM
                  Atom to stay in-oder or go OoO?AM2009/09/30 02:32 AM
                    Atom to stay in-oder or go OoO?baxeel2009/09/30 07:25 AM
                      Atom to stay in-oder or go OoO?AM2009/09/30 10:12 PM
                    Atom to stay in-oder or go OoO?Ungo2009/10/01 02:00 AM
                      Atom to stay in-oder or go OoO?AM2009/10/01 04:08 AM
                        Atom to stay in-oder or go OoO?anonymous2009/10/01 04:33 AM
                          Atom to stay in-oder or go OoO?AM2009/10/03 06:24 AM
                        Atom to stay in-oder or go OoO?Pun Zu2009/10/02 12:30 AM
                        Atom to stay in-oder or go OoO?Ungo2009/10/02 12:11 PM
                          Atom to stay in-oder or go OoO?AM2009/10/03 06:22 AM
                            Atom to stay in-oder or go OoO?Ungo2009/10/03 01:53 PM
                              Atom to stay in-oder or go OoO?AM2009/10/04 07:44 AM
                                Atom to stay in-oder or go OoO?David Kanter2009/10/04 10:02 PM
                                  Atom to stay in-oder or go OoO?AM2009/10/05 06:18 AM
                                    Atom to stay in-oder or go OoO?David Kanter2009/10/05 10:12 AM
                                      Atom to stay in-oder or go OoO?AM2009/10/06 03:51 AM
                                        Atom to stay in-oder or go OoO?anonymous2009/10/06 06:58 AM
                                        Do you have any proof?David Kanter2009/10/06 08:58 AM
                                          Do you?AM2009/10/06 10:30 PM
                                            Of course I do!anonymous2009/10/07 04:58 AM
                                              Thanks :-)AM2009/10/08 02:17 AM
                                                Thanks :-)anonymous2009/10/08 04:52 AM
                                                  Thanks :-)AM2009/10/09 02:13 AM
                                                    Thanks :-)anonymous2009/10/09 05:03 AM
                                                    Thanks :-)Foo_2009/10/09 05:47 AM
                                                      Thanks :-)AM2009/10/10 12:15 AM
                                            That's what I thought...David Kanter2009/10/07 08:00 AM
                                              That's what I thought...AM2009/10/08 02:26 AM
                                                That's what I thought...anonymous2009/10/08 05:02 AM
                                                  let's see...AM2009/10/09 02:09 AM
                                                    let's see...anonymous2009/10/09 04:43 AM
                                                      let's see...AM2009/10/09 04:52 AM
                                                        let's see...anonymous2009/10/09 05:15 AM
                                                          let's see...AM2009/10/10 12:18 AM
                Atom to stay in-oder or go OoO?someone2009/09/28 05:09 AM
          I call Trollhobold2009/09/28 03:51 AM
            I call Trollsomeone2009/09/28 05:15 AM
              OT: categories of motivation in a forumhobold2009/09/29 05:01 AM
          Thoughts and questions on the Cortex A9Michael S2009/09/28 09:43 AM
            Thoughts and questions on the Cortex A9a reader2009/09/28 03:12 PM
              Thoughts and questions on the Cortex A9someone else2009/09/28 11:25 PM
                Why Cortex A9?hobold2009/09/29 06:20 AM
                  Why Cortex A9?someone else2009/09/29 09:57 AM
                    Why Cortex A9?Richard Cownie2009/09/29 05:09 PM
                      Why Cortex A9?hobold2009/09/29 11:38 PM
                        Why Cortex A9?Richard Cownie2009/09/30 05:49 AM
                          Why Cortex A9?hobold2009/09/30 06:46 AM
                            Why Cortex A9?none2009/09/30 06:56 AM
                              Marvell Sheeva and plug computingRichard Cownie2009/09/30 08:03 AM
                              Why Cortex A9?Michael S2009/09/30 09:07 AM
                                Why Cortex A9?none2009/09/30 09:40 AM
                                Why Cortex A9?Gabriele Svelto2009/09/30 11:43 AM
                                  ARM architectural licenseDavid Kanter2009/09/30 04:57 PM
                                    ARM architectural licensea reader2009/10/01 06:25 AM
                                      ARM architectural licenseRichard Cownie2009/10/01 07:21 AM
                                Why Cortex A9?slacker2009/09/30 06:12 PM
                                  ARM architectural licenseDavid Kanter2009/09/30 06:16 PM
                                  Why Cortex A9?Michael S2009/10/01 06:45 AM
                                    Why Cortex A9?slacker2009/10/02 01:41 AM
                                      Why Cortex A9?Richard Cownie2009/10/02 09:28 AM
                                        Questions...David Kanter2009/10/02 09:56 AM
                                          Questions...Richard Cownie2009/10/02 10:29 AM
                                            Questions...Wilco2009/10/02 12:05 PM
                                          Questions...slacker2009/10/02 07:51 PM
                                        Why Cortex A9?slacker2009/10/02 07:44 PM
                            Why Cortex A9?David W. Hess2009/09/30 07:42 AM
    Thoughts and questions on the Cortex A9Gabriele Svelto2009/09/28 12:28 AM
  Thoughts and questions on the Cortex A9Wilco2009/09/26 06:38 AM
    Thoughts and questions on the Cortex A9Gabriele Svelto2009/09/28 12:38 AM
      Thoughts and questions on the Cortex A9Costanza2009/10/01 02:45 PM
    Thoughts and questions on the Cortex A9sylt2009/09/28 04:54 AM
      Thoughts and questions on the Cortex A9Wilco2009/09/29 12:15 AM
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