The Haswell Front End

Conroe was a very wide machine. It brought us the first 4-wide front end of any x86 micro-architecture, meaning it could fetch and decode up to 4 instructions in parallel. We've seen improvements to the front end since Conroe, but the overall machine width hasn't changed - even with Haswell.

Haswell leaves the overall pipeline untouched. It's still the same 14 - 19 stage pipeline that we saw with Sandy Bridge depending on whether or not the instruction is found in the uop cache (which happens around 80% of the time). L1/L2 cache latencies are unchanged as well. Since Nehalem, Intel's Core micro-architectures have supported execution of two instruction threads per core to improve execution hardware utilization. Haswell also supports 2-way SMT/Hyper Threading.

The front end remains 4-wide, although Haswell features a better branch predictor and hardware prefetcher so we'll see better efficiency. Since the pipeline depth hasn't increased but overall branch prediction accuracy is up we'll see a positive impact on overall IPC (instructions executed per clock). Haswell is also more aggressive on the speculative memory access side.

The image below is a crude representation I put together of the Haswell front end compared to the two previous tocks. If you click the buttons below you'll toggle between Haswell, Sandy Bridge and Nehalem diagrams, with major changes highlighted.


In short, there aren't many major, high-level changes to see here. Instructions are fetched at the top, sent through a bunch of steps before getting to the decoders where they're converted from macro-ops (x86 instructions) to an internally understood format known to Intel as micro-ops (or µops). The instruction fetcher can grab 4 - 5 x86 instructions at a time, and the decoders can output up to 4 micro-ops per clock.

Sandy Bridge introduced the 1.5K µop cache that caches decoded micro-ops. When future instruction fetch requests are made, if the instructions are contained within the µop cache everything north of the cache is powered down and the instructions are serviced from the µop cache. The decode stages are very power hungry so being able to skip them is a boon to power efficiency. There are also performance benefits as well. A hit in the µop cache reduces the effective integer pipeline to 14 stages, the same length as it was in Conroe in 2006. Haswell retains all of these benefits. Even the µop cache size remains unchanged at 1.5K micro-ops (approximately 6KB in size).

Although it's noted above as a new/changed block, the updated instruction decode queue (aka allocation queue) was actually one of the changes made to improve single threaded performance in Ivy Bridge.

The instruction decode queue (where instructions go after they've been decoded) is no longer statically partitioned between the two threads that each core can service.

The big changes in Haswell are at the back end of the pipeline, in the execution engine.

CPU Architecture Improvements: Background Prioritizing ILP
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  • Kepe - Friday, October 05, 2012 - link

    Let's see. I think we can agree that the Samsung Galaxy S III was the most important Android phone launch of the summer, so it should get comparable treatment if Anandtech was completely neutral. Let's compare the articles about the SGS III vs. iPhone 5

    Doing a search on anandtech.com gives us 8 articles/news posts about the SGS III vs. 13 articles/news posts about the iPhone 5.

    SGS 3:

    Five news stories about product announcements
    Performance Preview article
    Preview article
    Review article

    iPhone 5:

    Why iPhone 5 isn't launched in 2011 article
    Analyzing rumours about iPhone 5 article
    New SoC in iPhone 5 article
    iPhone 5 Live Blog from the product launch seremony
    Three news stories about new features and product announcements
    iPhone 5 Hands On article
    Lack of simultaneous voice and LTE/EVDO article
    Analyzing Geekbench results article
    Sunspider Performance Analysis article
    Performance Preview article
    iPhone 5 Display Thoroughly Analyzed article

    + The upcoming iPhone 5 Review article
    + articles such as "iOS6 Maps Thoroughly Investigated"

    Look at the difference. It's quite clear which device gets more coverage. And it's the same thing for older iPhones. Articles such as "Camping out for the new iPhone 3GS".

    This is NOT equal treatment of all products. This is why my trust for Anandtech has started to slip. Yes, Anandtech still is the best place for reviews, but one really has to wonder if those reviews still are as neutral and objective as they used to be.
    Reply
  • vFunct - Saturday, October 06, 2012 - link

    It's an android device. Android devices do not matter. Everyone uses iPhones anyways, They are better. Apple makes better products, including laptops.

    No need to waste space on Android.
    Reply
  • Haugenshero - Saturday, October 06, 2012 - link

    Please take your pointless apple fanboy drivel to another site that doesn't care about actual hardware and software and just like shiny things. Reply
  • cjl - Saturday, October 06, 2012 - link

    Apple's (iOS) current sales are only 20% of the overall smartphone market share, while Android is over 60%, so if either one of the two is largely irrelevant, it's apple. Reply
  • HisDivineOrder - Sunday, October 07, 2012 - link

    Shhhh, icks-nay on the facts-nay.

    You might cause a fanboy's head to explode near one of those inconveniently placed explosive barrels we walk by in real life.

    Just imagine a chain reaction. Caused by an Apple fan's mind being blown. You might take out an entire city block.

    Do you want that kind of devastation on your karma? Think different. ;)
    Reply
  • vFunct - Sunday, October 07, 2012 - link

    The Android market is the cheap giveaways.

    No one willingly pays money for an Android phone.

    Not everyone can afford the premium quality of an Apple product. They will have to settle for an inferior Android devices instead until they can afford higher quality products.
    Reply
  • Kepe - Monday, October 08, 2012 - link

    Nice trolling there. Now go back under the bridge and stay there =) Reply
  • Old_Fogie_Late_Bloomer - Tuesday, October 09, 2012 - link

    "Not everyone can afford the premium quality of an Apple product. They will have to settle for an inferior Android devices instead until they can afford higher quality products."

    Ha ha ha! Well done, if you're screwing around.

    But seriously, if you actually believe that, seek psychiatric help. :-P
    Reply
  • Spunjji - Thursday, October 18, 2012 - link

    Hahahahahahahahahahahahahahahahahahahahaha Reply
  • solipsism - Tuesday, October 09, 2012 - link

    And how many of the 20% is on one phone? Let me know when you figure out how to cover every single Androd-based device that hits the market in a given year. Reply

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