Separate the OoO-ness from speculative-ness

Article: PhysX87: Software Deficiency
By: ? (, July 22, 2010 4:15 am
Room: Moderated Discussions
anon ( on 7/21/10 wrote:
>? ( on 7/21/10 wrote:
>>anon ( on 7/21/10 wrote:
>>>On the contrary, I think your assertion that pipelining requires speculative execution is wrong.

Notice the word "requires" in your sentence.

>>I am not saying that. I am saying this: Pipelining, the way it is implemented in
>>current CPUs (e.g: Core2), involves certain assumptions

No "requires" here. That is because I was trying to explicitly avoid it.

>I'm sorry, but you're just modifying your argument as you go along now. You can't
>tell me you were talking specifically about x86 and constraints of market realities
>in CPU design. Because we can all read what you posted to Linus.

No problem

>"NOTE that this theoretical OoO CPU I am mentioning here
>is *not* doing any *speculation* (except maybe those
>which also present in the in-order CPU). Once you have
>speculation, there by necessity exist some cases in which
>the CPU's prediction engine will make a wrong decision.
>Even mere pipelining entails a small dose of speculative
>execution !!!

Yes, it is slightly confusing. In the 1st paragraph I was talking about a "theoretical CPU", while in the 2nd I meant practical implementations of pipelining.

>I will repeat it one more time, because it seems to me
>that you are completely missing this: Even mere
>pipelining entails a small dose of speculation !!!"
>You are clearliy talking about theoretical designs, and you were saying that pipelining requires speculation.

So you want me to write a 50 pages long article in which I can it more clear? This is a discussion forum, the number of words I could have used is very limited.


(1) In the snipped text, I did not use the word "execution" anywhere. I use the word "speculative-ness".

>>>Caches have nothing to do with speculative execution that you were talking about.

(2) You jumped to the conclusion that I was talking about speculative execution. I was not. I was talking about speculative-ness in general.

>>What are you saying? That if a contemporary CPU (Core2 or whatever) decides to
>>allocate a cache-line for data at address 0x1230, it is not making any speculations
>>about future uses of that piece of data?

(3) I don't see any word "execution" there.

>>(Note: I am *not* against caches)
>Yes there is a speculative nature about caches, of course it is nothing to do with
>speculative execution

(4) Again, you used the word "execution".

>or anything in the CPU pipeline which is what this discussion
>was supposed to be about (out of order versus in order, pipelining, speculative execution etc). I'll quote you again
>"Even mere pipelining entails a small dose of speculative
>execution !!!"

In *actual* x86 CPUs pipelining is implemented in a way which entails speculative *execution*. As far as I can tell, Linus and Ian were talking about actual CPUs, so that sentence of mine was intended to make sense in that context.

I am not denying that pipelining as a theoretical concept does not necessarily imply speculative execution. But in practice, it mostly does.

>I'm not saying you don't know the difference, but I'm saying that the argument
>about execution engines is only muddied by you bringing up caches.

That's because you are trying to force the word "execution" into my texts ...
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                                          Separate the OoO-ness from speculative-nessDan Downs2010/07/22 08:14 AM
                                          Confusing and not very useful definitionDavid Kanter2010/07/22 12:41 PM
                                            Confusing and not very useful definition?2010/07/22 10:58 PM
                                              Confusing and not very useful definitionUngo2010/07/24 12:06 PM
                                                Confusing and not very useful definition?2010/07/25 10:23 PM
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                              Separate the OoO-ness from speculative-nessThiago Kurovski2010/07/21 04:13 PM
            You are just quoting SINGLE precision flops? OMG what planet do you live? Vincent Diepeveen2010/07/19 10:26 AM
              The prior poster was talking about SP (NT)David Kanter2010/07/19 11:34 AM
                All FFT's need double precisionVincent Diepeveen2010/07/19 02:02 PM
                  All FFT's need double precisionDavid Kanter2010/07/19 02:09 PM
                    All FFT's need double precisionVincent Diepeveen2010/07/19 04:06 PM
                  All FFT's need double precision - notMichael S2010/07/20 01:16 AM
                    All FFT's need double precision - notUngo2010/07/21 12:04 AM
                      All FFT's need double precision - notMichael S2010/07/21 02:35 PM
                      All FFT's need double precision - notEduardoS2010/07/21 02:52 PM
                        All FFT's need double precision - notAnon2010/07/21 05:23 PM
                          All FFT's need double precision - notRicardo B2010/07/26 07:46 AM
                        I'm on a boat!anon2010/07/22 11:42 AM
                        All FFT's need double precision - notVincent Diepeveen2010/07/24 11:39 PM
                          All FFT's need double precision - notslacker2010/07/25 03:27 AM
                            All FFT's need double precision - notRicardo B2010/07/26 07:40 AM
                          All FFT's need double precision - notEduardoS2010/07/25 08:37 AM
                            All FFT's need double precision - notMichael S2010/07/25 10:43 AM
                    All FFT's need double precision - notVincent Diepeveen2010/07/24 11:19 PM
      A bit off baseEduardoS2010/07/08 04:08 PM
        A bit off baseGroo2010/07/08 06:11 PM
          A bit off basejohn mann2010/07/08 06:58 PM
            All right...let's cool it...David Kanter2010/07/08 07:54 PM
    A bit off baseVincent Diepeveen2010/07/19 03:36 PM
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