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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Peanutsrevenge - Friday, October 5, 2012 - link
What the hell are you guys bitching about?Of course the iPhone articles are going to be longer and more numerous than GS3 articles.
iPhone releases come with new iOS releases and have their own eco-system.
Android phone releases use a common OS across them and therefore much of what's in one article doesn't need repeating in another.
Anand liking Apple is not our problem, I can see why people like them (not so much Anand) and that's fine, personally I dislike them (hate was originally typed, but was edited due to being incorrect), but still respect them and respect people who purchase their products (and pay for their litigation).
An entire page of comments talking about how Anand isn't allowed to like or talk about Apple products because you guys don't like is ridiculous, they're a PC company and should exist on a PC website.
Grow up.
Kepe - Friday, October 5, 2012 - link
Sure, but I'm talking about dedicating entire, long articles to such things as the iPhone display or why it doesn't have a certain feature and so on. The SGS III has a very interesting display, too. Still it didn't get nearly as much attention. Of course Anand is allowed to talk about Apple products. What I want, though, is Anand(tech) to be as thorough in reviewing other products, too, or then stop making those huge articles only about Apple products. Because that is biased.In the Macbook Pro Retina article Anand talked about the cooling system and the fan blades for one page. When I read any other laptop review on Anandtech, cooling is briefly described in a sentence or two.
Dedicating so much attention to just one company's products makes it look like Anandtech is biased. And that is not good.
Magik_Breezy - Sunday, October 14, 2012 - link
Hopefully because of these comments they'll finally see what we want, not some Apple crap. Good engineering stupid managementSpunjji - Thursday, October 18, 2012 - link
Nailed it.vFunct - Saturday, October 6, 2012 - link
Android products would get more coverage if they bothered to do any engineering on them. Since they don't push the technology the way Apple does, they don't need a more in-depth review.StevoLincolnite - Saturday, October 6, 2012 - link
You're kidding right? Hardware wise Apple has always been behind the curve compared to the competition in every facet of it's product line-ups or very quickly beaten.lmcd - Saturday, October 6, 2012 - link
Umm, I would disagree there. Apple has always been ahead of the curve in GPUs and this is the FIRST TIME SINCE BEFORE THE A-SERIES that Apple has had a GPU without an overwhelming lead on the competition for more than half a year.*While GPU selection isn't always huge, it's one of the biggest points of differentiation in mobile chips, along with power use.
*excluding the A4 if you count from when it was first in a phone as opposed to in a tablet.
Magik_Breezy - Sunday, October 14, 2012 - link
The last time I played a game on my phone was about 8 months ago and I'm 15! To say that Apple pushes their hardware is naive as it gets.The Galaxy S III was the best purchase Ive made, even my mum doesn't like my iPhone 4.
vFunct - Sunday, October 7, 2012 - link
Yes, Apple products are always ahead of compromised Android products.Android devices are badly engineered, like incorporating LTE when the battery can't handle it, for example. Apple doesn't compromise on their design.
Kepe - Monday, October 8, 2012 - link
How much does Apple pay you for a comment praising them?