Intel's Haswell Architecture Analyzed: Building a New PC and a New Intel
by Anand Lal Shimpi on October 5, 2012 2:45 AM ESTThe 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.
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lmcd - Saturday, October 6, 2012 - link
Interestingly, this might be the first chance in forever AMD has at competing with Intel. If Haswell's sole goal is to hit lower power targets, and Piledriver hits its 15% and Steamroller its 15% over that, AMD is suddenly right up with Intel's i5 series with its GPU-less chips, and upper i3-range with their APUs, which is absolutely perfect positioning: most i5 purchases are for people planning to pair with discrete graphics, while most i3 series seem to go to the PC buyer looking for low price tags.The one downside is that the i7 series is Intel's money-maker: the clueless people who think they're getting maximum performance but are really just feeding the binning system and buying an unbalanced PC.
milkod2001 - Sunday, October 7, 2012 - link
u got it wrong bro, Intels money maker is not i7, it's i3 and i5(low end and a bit of mainstreem)as for Haswell, on paper it looks too good to be true as Ivy did last year and ended up everything but impressive.
Since Intel conroe core(2006) there actually were not any significant improvements worth mentioning.There's not much extra what todays CPUs can do and Pentium4 could not a decade ago.
I would love to see some innovations user could really benefit from(something like reattachable,thin, light, portable, firm solar panel hooked at the back of screen or even build in as last layer into screen itself) and not that crap Intel/AMD gives us year by year.
xeizo - Sunday, October 7, 2012 - link
Anand is very right, it's everything about power savings which in effect makes smaller and more portable form factors possible!As for mainstream perfomance, my Linux workstation still uses a Q9450 rev. C1 from 2008 clocked at 3.2GHz and a SSD of course. That box feels in every way as snappy as my Windows-box with Sandy Bridge at 4.8GHz. Which means, I really didn't need more performance than what C2Q already gave. Of course the SB-box benchmarks much faster, about twice as fast in most things, but the point is for myself I really don't need that perfromance except for some occasional game.
But I could use a smaller, cooler running device instead!
Teknobug - Tuesday, October 16, 2012 - link
LOL my Linux system still runs a Sempron and it's still fast.oomjcv - Sunday, October 7, 2012 - link
Very interesting article, enjoyed reading it.Something I would like to see is a decent comparison between Intel's and AMD's plans. Many might be able to outline the basics, but a thorough article on the subject should be rather enlightening... Comparing their design philosophies, architectures, possible pitfalls and successes etc, pretty much what's been done with this article only with both companies.
I know it might be time consuming but I imagine it could be quite a nice read.
zwillx - Monday, January 21, 2013 - link
agreed; it's difficult to find the common ground with so many different chip architectures. x86 is a big enough competition but now it's getting split wide open with ARM and BIG/litle etc etc so it's always helpful to have either more charts or real world examples lol.My take from this article though: Haswell still won't have the prowess to beat the GT650. I have GTX660 in my laptop w/ Optimus (TM). It works. Runs a game on HD4000 at 17 FPS. On the GTX660 I get 100+ fps, and am able to use higher anti-aliasing settings. So, clearly a 100% improvement over Ivy bridge is only putting the chip into "mediocre" category by the time its released.
alexandrio - Sunday, October 7, 2012 - link
"The bigger concern is whether or not the OEMs and ISVs will do their best to really take advantage of what Haswell offers. I know one will, but will the rest?"I am curious who is that one OME that will do their best to really take advantage of Haswell offers?
zwillx - Monday, January 21, 2013 - link
Apple. Or are you joking. I personally hate Apple and have since the original iMac but their engineering is top notch when it comes to getting ideal performance from silicon to user. So.. guessing that's the reference.Silma - Monday, October 8, 2012 - link
A fine read, technically very comprehensive, but still overly melodramatic.While it is true that it is crucial for Intel to step a foot in the byod market some things still hold true:
- In value and profit the PC processor market is much bigger than the byod processor market and will stay so for years because PCs, especially business PCs won't disappear anytime soon.
- Nobody can touch Intel in this market, it has been proved for decades. Not AMD at the height of its success, not mighty IBM, not Sun, nobody.
- Contrary to what you say Intel has a definitive production advantage and there are very few fabs able to compete. Note that Apple is incapable of producing processors, it is dependent on external manufacturers.
- What Apple does with its processor is interesting business wise for its iPods/Pads/Phones, but Apple doesn't have the research power Intel and others have in the chip space and I can't see how it will innovate better than Intel and other competitors.
- Intel is aware of its shortcomings, is pushing tremendously in the right direction. A competitor that doesn't rest on its laurels is a mighty threat, ARM beware.
- If Apple stops using Intel processors, it will of course wipe a few hundred millions of Intel's turnover but won't be anything remotely dangerous for Intel
- It remains to be seen that Apple users will accept yet another platform change.
- It remains to be seen that it would make sense business-wise for Apple
- I am quite sure many phone companies will be open about renewed chip competition and not letting a single platform become too powerful.
All in all it seems to me Intel is as dangerous as ever, executing very well in its core business and heading towards great things in the phone/pad space.
johnsmith9875 - Thursday, October 11, 2012 - link
Why couldn't they at least stick to LGA2011?