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.
Facebook
Twitter
RSS
245 Comments
View All Comments
A5 - Friday, October 5, 2012 - link
8 years is a loooooong time in this space, and yes you (and most people here) are in the minority.Notebooks have been outselling desktops for several years, and in 2011 smartphone shipments were higher than all PC form-factors combined. It's pretty clear where the big bucks are going, and it isn't desktop PCs.
flamethrower - Friday, October 5, 2012 - link
In 8 years you'll have 50-inch OLED TVs on your walls. What's going to drive them? Possibly a computer integrated into them.Peanutsrevenge - Friday, October 5, 2012 - link
We'll just be using large screens, keyboards and mice wireless connected to our ultra portable devices.The desktop will likely still exist for people like us who frequent this site, however it's role will be far more specialised, possibly more as our personal cloud servers than our PCs.
yankeeDDL - Friday, October 5, 2012 - link
Wow. Thanks for the excellent article: I really enjoyed it.The thought of having a processor of the power level of Ivy bridge in my mobile phone blows my mind.
Honestly though, I really can't see how the volume of CPUs for desktop PCs and servers is going to drop so dramatically, that Intel will need the volume generated by mobile, to "survive".
Yes, of course more volume will help, but 8 years from now, even if the mobiles will have such kind of computational power, I would imagine that a Desktop would have 10~20x that performance, as it is today.
It's true that today's CPUs are typically more powerful than the average user ever needs, but raise the hand who wouldn't trade his CPU for one 10x faster (in the same power envelope) ...
That said, 10W still seems like a lot to fit in a mobile: who knows the power consumption of high-end mobile CPUs today? (quad-core Krait CPU, for example, or even Tegra3)
dagamer34 - Friday, October 5, 2012 - link
Intel's real problem is that the power needed for "good enough" computing in a typical desktop CPU came a couple of years ago Nd is rapidly approaching in mobile. With more and more tasks being offloaded to the cloud, battery life is becoming a stronger and stronger focus.What's sad is that because AMD isn't the major player it once was, Intel has allowed it's eye off the ball, revving Atom with only minor tweaks and having a laissez faire approach to GPU performance. It's only been recently when mobile has started to dominate in the minds of consumers and Intel's lack of any major design wins (the RAZR I doesn't count) which has forced Intel to push as hard as it is now.
sp3x0ps - Friday, October 5, 2012 - link
Where is the iPhone 5 review? I need details!! arghh.Demon-Xanth - Friday, October 5, 2012 - link
Atom was targeted to UMPCs, but quickly took over low power embedded systems who don't need much power but do run Windows.tipoo - Friday, October 5, 2012 - link
Poor Via.dgingeri - Friday, October 5, 2012 - link
"Within 8 years many expect all mainstream computing to move to smartphones, or whatever other ultra portable form factor computing device we're carrying around at that point."They said the same thing about laptops. Sure, laptops hold about 60-65% of the market these days, but the desktop is still very much around, and is the preferred platform for PC gamers and HTPC applications. They're far more flexible than any mobile form factor.
Smartphones also have the severe disadvantage of a very small screen. Even the largest are too small for most people to deal with. On top of that, actually surfing the net on those tiny screens is an exercise in frustration for many people. I try to tap on a link, only to get the link next to it, or above it, or below it, or possibly having my stupid phone just select the text instead of following the link.
Smartphones have their niche. There's no doubt there, but they are not going to be anyone's mainstream device unless they have needle thin fingers and 20/10 vision.
Anand Lal Shimpi - Friday, October 5, 2012 - link
I agree with the notebook/desktop comparison - these form factors won't go away. I should have said the majority of mainstream client computing goes to smartphones. And solving the display and input problems is easy: wireless display (WiDi/Miracast) and wireless keyboard/mouse (or a dock that does both over wires if you'd rather that).Take care,
Anand