Barcelona optimization guide

By: EduardoS (, May 13, 2007 7:01 am
Room: Moderated Discussions
You didn't understand it correctly...

>It mentions for example latency of multiplication instruction is 5 cycles (!!).
>Even an instruction like PMULUDQ is 5 cycles whereas i don't see why this can't get done faster.
>Integer multiply 64 x 64 bits unsigned == 128 bits also seemingly has a latency
>of 5 cycles i read at appendix A page 219.
>Now i am not so good in counting and speaking for myself here, as i'm not very
>good in math, so i might have missed it from a previous generation K8, but i tend
>to remember that it was 4 cycles there, from an email i had.

The latency is 4 cycles for the lower 64 bits, 5 for the upper, comparing that upper to other processors:
K-8: 5 cycles
C2D: 7 cycles
P4E: 11 cycles
Multiply is a complex instruction, 5 cycles is ok.

>I was under the probably wrong misconception that it would be able to execute 4 instructions a cycle at integer area.
>When i read manual correct i deduce it's executing 3 instructions maximum a cycle.
>So Intel has bigger surplus potential of 33% for good programmers, did i read that
>correct, or am i wrong there and is core2 not having 4 integer units either?

Core 2 have 4 decoders (up to 5 instructions per clock with macro-fusion) but only 3 integer units and can retire only 4 uOPs per cycle, Barcelona isn't too far behind, if any.

>In the optimization manual it prefers to replace short loops of 4x4 already by
>hand written out code for those 16 cases.
>Why can't the processor perfectly predict such loops just like intel seemingly is doing?

You must have a history to predict a branch, the fetcher must go to somewhere else (throwing away some instructions fetched), and you must have an extra instruction, (a Jcc), so hand written still faster.

>It is 2007 now, no longer 1997.
>Seems their aim is to serve floating point.
>If a problem in its fundament is integer oriented and if you are not capable of
>rewriting an algorithm to integers, then you need to rethink your solution. Floating
>point is completely overrated IMHO.

IMHO floating point is a lazy way to handle non-integers numbers, but a lazy way is also faster, faster is also cheaper, and cost is one of most important factors in software development ;)

>SSEx is not paying enough attention to allow to vectorize integer codes. Putting
>all cards at floating point AMD obviously tries to conquer the highend market.

True, they could do a better work at integer SSEx.

>It is interesting therefore that AMD already includes popcount into its processor.

The easiest way to know if a integer is a power of 2.
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TopicPosted ByDate
Barcelona optimization guidemas2007/05/10 07:43 AM
  Barcelona optimization guideLinus Torvalds2007/05/10 10:00 AM
    Barcelona optimization guideRob Thorpe2007/05/10 10:23 AM
      Barcelona optimization guideLinus Torvalds2007/05/10 10:42 AM
        Barcelona optimization guideRob Thorpe2007/05/11 09:22 AM
          Barcelona optimization guideDavid Kanter2007/05/11 05:17 PM
            Barcelona optimization guideLinus Torvalds2007/05/11 05:30 PM
            Barcelona optimization guideanonymous2007/05/11 11:29 PM
              Barcelona optimization guideanonymous2007/05/12 07:47 AM
              Barcelona optimization guidehobold2007/05/14 05:30 AM
        Barcelona optimization guideAndreas Kaiser2007/05/12 09:32 AM
  Barcelona optimization guideVincent Diepeveen2007/05/13 05:20 AM
    Barcelona optimization guideEduardoS2007/05/13 07:01 AM
      Barcelona optimization guideVincent Diepeveen2007/05/13 09:18 AM
        Barcelona optimization guideMichael S2007/05/13 10:03 AM
        Barcelona optimization guideEduardoS2007/05/13 10:30 AM
        Barcelona optimization guideDresdenboy2007/05/14 08:18 AM
          Barcelona optimization guideVincent Diepeveen2007/05/16 02:36 AM
            Barcelona optimization guideEduardoS2007/05/16 06:57 AM
              Barcelona optimization guideVincent Diepeveen2007/05/16 09:51 AM
        Barcelona optimization guideDavid Kanter2007/05/16 04:13 AM
          Barcelona vs Core2 Vincent Diepeveen2007/05/16 06:35 AM
            Barcelona vs Core2 David Kanter2007/05/16 12:06 PM
            Barcelona vs Core2 EduardoS2007/05/16 12:41 PM
              Barcelona vs Core2 David Kanter2007/05/16 12:53 PM
                Barcelona vs Core2 EduardoS2007/05/16 01:37 PM
                  Barcelona vs Core2 David Kanter2007/05/16 02:43 PM
                    Barcelona vs Core2 EduardoS2007/05/16 04:32 PM
                    Barcelona vs Core2 Gabriele Svelto2007/05/17 06:38 AM
          Barcelona optimization guideanonymous2007/05/16 08:13 PM
            Barcelona optimization guideMichael S2007/05/17 05:26 AM
              Barcelona optimization guideanonymous2007/05/17 06:23 PM
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