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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  • JarredWalton - Thursday, March 1, 2007 - link

    Games have quite a lot of LOAD instructions, like most programs, as well as plenty of branches (esp. in the AI routines). Most likely the boost that Core 2 gets is due in a large part to the better instruction reordering and branch prediction, although the cache and prefetchers probably help as well. Given AMD was better than NetBurst due to memory latency, through in better OOE (Out of Order Execution) logic and keep the improved latency and they should do pretty well.

    Naturally, everything at this point is purely speculation, but in the next few months we should start to get a better idea of what's in store and how it will perform. One problem that still remains is that even if AMD can be competitive clock-for-clock, Intel looks primed to be able to go up to at least 3.6 GHz dual core and 3.46 GHz quad core if necessary. AMD has traditionally not reached clock speeds nearly as high as Intel, possibly due in part to having more metal layers (speculation again - process tech and other features naturally play a role), so if they release 2.9GHz Barcelona at $1000 you can pretty much guarantee Intel will launch 3.2 and/or 3.46 GHz Kentsfield (and/or FSB1333 3.33 GHz).

    On the bright side, at least things should stay interesting in the CPU world. :D
    Reply
  • yyrkoon - Thursday, March 1, 2007 - link

    Yes, interresting indeed, but from experience, AMD has always been too vocal in what they plan on doing, especially during the times they are in a 'rut'.

    What this usually means to me, is that AMD is trying to blow smoke up our backsides, we'll see though.

    Keep in mind, my main desktop system, and my backup server for that matter, both are AMD systems. The phrase "cost effective" applies here.
    Reply
  • kilkennycat - Thursday, March 1, 2007 - link

    Yesterday, Intel announced that they were converting a fourth fab to 45nm. A great deal of confidence in that process. And a few days ago they announced desktop shipments of Penryn-based CPUs pulled forward into 2007. Looks as if AMDs 'window of opportunity' is likely to be very small. IBM has not yet announced a successful implementation of a RAM on their 45nm process. Intel had their RAM design on 45nm up and running late 2005. Reply
  • archcommus - Thursday, March 1, 2007 - link

    True but the move to 45 nm might not make a huge difference in real world performance, just like the move to 65 nm didn't for AMD. Their next full blown architecture will still be a ways off. Reply
  • Roy2001 - Thursday, March 1, 2007 - link

    Dislike AMD's move to 65nm process, move to 45nm has shown that Penryn would eats less power and runs faster thanks to its high K material and metal gate. Reply
  • smitty3268 - Thursday, March 1, 2007 - link

    Every process shows that in theory before chips are actually being made on it. We'll see what actually happens when Penryn is released, not before. Reply
  • chucky2 - Thursday, March 1, 2007 - link

    Has AMD given any indication of how probable dropping an Agena or Kuma CPU into an existing AM2 motherboard will go?

    Especially AMD's own newly released 690G or the upcoming nVidia MCP68?

    Chuck
    Reply
  • mamisano - Thursday, March 1, 2007 - link

    It has been stated in the past that AM2+ based products will run in AM2 based boards. The limitation, if I understand it correctly, will be the lack of support of the new power features.

    Someone correct me if I am wrong :)
    Reply
  • chucky2 - Thursday, March 1, 2007 - link

    Then it should be no problem for AMD to confirm through AnandTech that this is the case.

    Surely if Barcelona is this close to shipping (only a few months away), AMD must know if Agena and/or Kuma will work in current AM2 motherboards, especially their own 690 series their just about to release.

    All I'm asking for is a definite either way, it shouldn't be that hard for AMD to do at this point.

    Chuck
    Reply
  • mino - Friday, March 2, 2007 - link

    AMD stated PUBLICLY to anyone who listened that AM2+ stuff will plug into AM2, just BIOS update needed.

    Why should they react to any consumer who ask on some forum the same question every second week ?

    Most important is they said it WILL(not "may") work with AM2-spec boards to big Tier 1 OEM's.
    They can not make it incompatible therefore. They would be out of bussines in no time.
    Reply

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