Core Tune-up

While the most significant sounding improvements were rolled into the SSE128 changes in Barcelona, they are merely the tip of the iceberg. The laundry list of improvements to Barcelona starts with the branch predictor.

In general, the accuracy of a CPU's branch predictor determines how wide and how deep of a design you can make. The average number of instructions before the predictor mispredicts governs how many instructions you can have in flight, which in turn controls how many execution units you can realistically keep fed on a regular basis. The K8's branch predictor was quite good and very well optimized for its architecture, but there were some advancements Intel introduced in the Pentium M and Pentium 4 that AMD could stand to benefit from.

Barcelona adds a 512-entry indirect predictor which, believe it or not, predicts indirect branches. An indirect branch is one where the target of the branch is a location pointed to by an address in memory, in other words, a branch with multiple targets. Instead of branching directly to a label indicated by the branch instruction, an indirect branch sends the CPU to a memory location that contains the location of the instruction that it should branch to.

Intel added an indirect predictor to its Pentium M processor based on the idea that the more you could limit the number of mispredicted branches, the more efficient your processor could be (thus lowering power consumption). The indirect predictor also made its way into Prescott in order to help minimize the performance deficit incurred by further pipelining the NetBurst architecture.

In Prescott, the simple addition of an indirect predictor resulted in over a 12% reduction in mispredicted branches in SPEC CPU2000. While details of how AMD and Intel differ in their predictor algorithms aren't public, we can expect similarly large improvements in areas where indirect branches are common. In the 253.perlbmk test of SPEC CPU2000 the reduction in mispredicted branches with Prescott was significant, reaching almost 55%. With Barcelona, fewer mispredicted branches means higher overall IPC and greater efficiency both from a power and performance standpoint. AMD doesn't have the incredibly deep pipeline to worry about that Intel did with Prescott, but the efficiency improvements should be significant.

The inclusion of an indirect predictor wasn't the only crystal ball improvement in Barcelona; the size of the return stack in the new core is double what it was in K8. In very deep call chains, for example code that calls many subroutines (e.g. recursive functions), the CPU will eventually run out of room to keep track of where it has been. Once it starts losing track of return addresses, it loses the ability to predict branches involved with those addresses. Barcelona helps alleviate the problem by doubling the size of the return stack. These sorts of improvements are generally implemented by profiling the behavior of software commonly used on a manufacturer's CPU, so we asked AMD what software or scenario drove this improvement of Barcelona. AMD wouldn't give us a concrete example of a situation other than to say that the return stack size improvements were made at the request of a "large software vendor".

The final improvement to the K8's branch prediction came through the usual channels - Barcelona now tracks more branches than its predecessor. There's no mystic science to branch prediction; a processor simply looks at branches it has taken and bases its predictions on historical data. The more historical data that is present, the more accurate a branch predictor becomes. When the K8 was designed it was built on a 130nm manufacturing process; with the first incarnation of Barcelona set to debut at 65nm AMD definitely has the die space to track more branch history data.

SSE128 Stacks and Loads of Optimizations
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  • JustKidding - Friday, March 2, 2007 - link

    So what you are saying is that it's not the size of your cache that matters as much as how well you use it. Reply
  • VooDooAddict - Thursday, March 1, 2007 - link

    quote:

    Smaller cache will mean fewer transistors which should mean better yields, lower power consumption and cheaper to produce.


    With Cache size differences usually having small impact on performance for Athlon64s, the slight trade off for better yields and margins seems the better choice for AMD here.
    Reply
  • Regs - Thursday, March 1, 2007 - link

    Where was this article 8 months ago? ;)


    I agree with Anands closing article that AMD now needs it's own "snowball effect" for the next couple of years. 4-5 years with a sitting target against a giant like Intel prooved to be costly in terms of competivness.

    We all saw it coming when Intel developed the first Pentium M. It looks like AMD got the message as well and started the Barcelona project. Maybe AMD learned their lesson.
    Reply
  • iwodo - Thursday, March 1, 2007 - link

    So bascially all intel 's C2D improvement are made into Barcelona. And apart from Virtualization improvement there are nothing new from AMD that Intel doesn't have?

    On performance note Barcelona doesn't seem to offer better clock scaling. I.e even if it is 30% faster then its current K8 it will only have slight advantage against C2D clock per clock. Not to mention it is up against Penryn. Although Penryn is nothing much then a few minor tweaks and more cache. It does allow intel to scale higher in clock speed.

    And given AMD slow roll out rate, and AMD limited production capacity Barcelona never seem like much of a threat.

    The article does not mention anything about FP improvement. Are AMD keeping them secret for now or is that all we are going to see?
    Reply
  • Spoelie - Thursday, March 1, 2007 - link

    The FP improvement is the SSE improvement, and according to the theory it's more powerful than what core2 duo is offering.

    There are improvements mentioned that are not in core2 (+ other way around, like instruction fusing), and improvements that are inspired on the same principle but implemented differently. The architectures themselves differ widely (see earlier article that compares K8 with Core2 - reservation station etc.) so different implementations of principally the same optimizations on a different architecture will have vastly different effects. Even after these improvements, the capabilities (how much can you decode, etc) of each read nothing alike. And if it were all the same, AMD has the platform advantage, so it would still end up faster by virtue of nothing else but that. Some guesstimates made by varying sites would put Barcelona ahead in FP code and at the same level or slightly behind in INT code. But those are just guesstimates.

    What I'm trying to say here is that barcelona is still very different from core2, and that we just don't know yet in which direction the pendulum will swing ;)
    Reply
  • Shintai - Thursday, March 1, 2007 - link

    No....precisely in theory is where Barcelona lacks. Core 2 Duo could in theory do 6 64bit or 3 128bit SSE instructions per cycle. Barcelona can do 4 64bit or 2 128bit. AMD provided this information aswell. Reply
  • Griswold - Thursday, March 1, 2007 - link

    Wishful thinking. Reply
  • Spoelie - Thursday, March 1, 2007 - link

    Hmmm, in the earlier article, there was explicit emphasis on the fact that 2 of the 3 units are symmetric in core2, but I'm not too sure what it means. It does imply however that those 3 units of core2 can only be used fully in certain combinations, and are not 3 independent units. On 128-bit performance, what was said is this: "so the Core architecture has essentially at least 2 times the processing power here [compared to K8]". Not 3 times, but "at least" 2 times, so again the 3 times will probably only be in certain situations.

    The next paragraph said this:
    "With 64-bit FP, Core can do 4 Double Precision FP calculations per cycle, while the *Athlon64* can do 3."
    So K8 was not at such a big disadvantage when it came to 64-bit SSE, if Barcelona doubles everything SSE, it should come ahead in this area.

    So to me it looks like for 128-bit, core2 will be faster in some situations, on par in others, and for 64-bit, Barcelona would be ahead.

    If this is wrong, I do not know where some of the articles I read over time came from, implying Barcelona would be better overall in SSE.
    Reply
  • Shintai - Friday, March 2, 2007 - link

    Core 2 got 3 individual SSE ports:
    http://www.realworldtech.com/page.cfm?ArticleID=RW...">http://www.realworldtech.com/page.cfm?ArticleID=RW...

    AMD says 4 double:
    http://www.anandtech.com/showdoc.aspx?i=2768&p...">http://www.anandtech.com/showdoc.aspx?i=2768&p...

    And 64 or 128bit doesnt matter. I dont know how you think that way.
    Barcelona got 2 SSE ports. They are able to do 2 128bit or 4 64bit. Most 128bit actually contains 2 64bit or 4 32bit.
    Core 2 got 3 SSE ports. They are able to do 3 128bit or 6 64bit.
    Reply
  • flyck - Friday, March 2, 2007 - link

    core duo has 3 SSE units but they are not symmetric, meaning that not every unit can execute all commands. Core duo can do at best 4DP flops/ cycle. the same as barcelona. Reply

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