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The iPhone 6 Review

 by Joshua Ho, Brandon Chester, Chris Heinonen & Ryan Smith on September 30, 2014 8:01 AM EST
531 Comments | Add A Comment
531 Comments

A8’s GPU: Imagination Technologies’ PowerVR GX6450

Last but not least on our tour of the A8 SoC is Apple’s GPU of choice, Imagination’s PowerVR GX6450.

When Apple first announced the A8 SoC as part of their iPhone keynote, they told us to expect a nearly 50% increase in graphics performance. Based on that information and on the fact that that Apple was moving to a denser 20nm process, we initially believed that Apple would be upgrading from A7’s 4-core PowerVR design to a 6-core design, especially in light of the higher resolution displays present on the iPhone 6 and iPhone 6 Plus.

Instead our analysis with Chipworks found that only four GPU cores were present on A8, which ruled out the idea of a 6-core design but did narrow down the options considerably. Based on that information and more importantly Apple’s Metal Programming Guide, we have been able to narrow down our options to a single GPU, the PowerVR GX6450.

The GX6450 is the immediate successor to the G6430 first used in the A7 and is based on Imagination’s PowerVR Series6XT architecture. Imagination first announced PowerVR Series6XT to the public at CES 2014, and now just a short eight months later we are seeing the first Series6XT hardware reach retail.

We have already covered the PowerVR Series6/Series6XT architecture in some detail earlier this year so we won’t go through all of it again, but we would encourage anyone who is interested to take a look at our architectural analysis for additional information. Otherwise we will be spending the bulk of our time looking at how GX6450 differs from G6430 and why Apple would choose this specific GPU.

From a technical perspective Series6XT is a direct evolution over the previous Series6, and GX6450 is a direct evolution over G6430 as well. Given a 4-core configuration there are only a limited number of scenarios where GX6450 outright has more hardware than G6430 (e.g. additional ALUs), and instead Series6XT is focused on adding features and improving performance over Series6 through various tweaks and optimizations to the architecture. Series6 at this point is actually over two years old – it was first introduced to the public at CES 2012 – so a lot has happened in the mobile GPU landscape over the past couple of years.

The closest thing to a marquee feature on Series6XT is support for Adaptive Scalable Texture Compression (ASTC), a next-generation texture compression technology that is slowly making its way into GPUs from a number of manufacturers. Designed by the consortium responsible for OpenGL ES, Khronos, ASTC is designed to offer better texture compression (with finer grained quality options) than existing texture compression formats while also being a universal format supported by all GPUs. In Apple’s case they have always been using PowerVR GPUs – and hence all products support PVRTC and more recently PVRTC2 – however ASTC being exposed allows them to take advantage of the quality improvements while also making game development and porting from other platforms easier.

Less visible to users but certainly important to Apple, Series6XT also includes new power management capabilities to reduce power consumption under idle and light workloads. Through finer grained power gating technology that Imagination dubs “PowerGearing G6XT”, GX6450 can now have its shading clusters (USCs) powered down individually, allowing only as many of them as are necessary to be fired up. As Apple continues to min-max their designs, being able to idle at a lower power state can be used to improve battery life and/or increase how often and how long the A8’s GPU uses higher power states, improving overall efficiency.


Apple iPhone GPU Performance Estimate: Over The Years

And, perhaps most importantly overall, Series6XT comprises a series of under-the-hood optimizations to improve overall performance. When it comes to the internals of PowerVR architectures we only have limited details from Imagination on how they operate, so in some areas we know quite a bit about what Imagination has been up to and in other areas their architectures are still something akin to a black box. At any rate Imagination’s goal for Series6XT was to improve performance by up to 50% – this seems to be where Apple’s 50% performance improvement claim comes from – though as we’ll see the performance gains on real world applications are not going to be quite as potent.

What we do know about Series6XT is that Imagination has made some changes to the structure of the USCs themselves. Series6XT still uses a 16-wide SIMD design, but in each pipeline they have added another set of medium/half-precision (FP16) ALUs specifically to improve FP16 performance. Now instead of 2x3 (6) FP16 ALUs, Series6XT bumps that up to 4x2 (8) FP16 ALUs. This is the only outright increase in shader hardware when you compare Series6 to Series6XT, and on paper it improves FP16 performance by 33% at equivalent clock speeds.

The focus on FP16 is interesting, though for iOS it may be misplaced. These half-precision floating point operations are an excellent way to conserve bandwidth and power by not firing up more expensive FP32 ALUs, but the tradeoff is that the numbers they work with aren’t nearly as precise, hence their use has to be carefully planned. In practice what you will find is that while FP16 operations do see some use, they are by no means the predominant type of floating point GPU operation used, so the FP16 increase is a 33% increase only in the cases where performance is being constrained by the GPU’s FP16 performance.

FP32 performance meanwhile remains unchanged. Each USC pipeline contains two such ALUs, for up to four FP32 FLOPS per clock, or to use our typical metric, 128 MADs (Multiply-Adds) per clock.

The rest of Series6XT’s optimizations take place at the front and back ends, where geometry processing and pixel fill take place respectively. Imagination has not told us exactly what they have done here, but both these areas have been targeted to improve sustained polygon rates and pixel fillrate performance. These more generic optimizations stand to be more applicable to general performance, though by how much we cannot say.

One final optimization we want to point out for Series6XT is that Imagination has made some additional under-the-hood changes to improve GPU compute performance. We have not talked about GPU compute on iOS devices thus far, as until now Apple has not exposed any APIs suitable for it (e.g. OpenCL is not available on iOS). With iOS8 Apple is releasing their Metal API, which is robust enough to be used for both graphics and now compute. How developers put this capability to use remains to be seen, but GX6450 should perform even better than G6430.

Mobile SoC GPU Comparison
  PowerVR SGX 543MP2 PowerVR SGX 543MP3 PowerVR SGX 543MP4 PowerVR SGX 554MP4 PowerVR G6430 PowerVR GX6450
Used In iPad 2/iPhone 4S iPhone 5 iPad 3 iPad 4 iPad Air/iPhone 5s iPhone 6/iPhone 6Plus
SIMD Name USSE2 USSE2 USSE2 USSE2 USC USC
# of SIMDs 8 12 16 32 4 4
MADs per SIMD 4 4 4 4 32 32
Total MADs 32 48 64 128 128 128
GFLOPS @ 300MHz 19.2 GFLOPS 28.8 GFLOPS 38.4 GFLOPS 76.8 GFLOPS 76.8 GFLOPS 76.8 GFLOPS
Pixels/Clock N/A N/A N/A N/A 8 8
Texels/Clock N/A N/A N/A N/A 8 8

The one wildcard when talking about performance here is going to be clock speeds. Apple doesn’t expose these and they aren’t easy to test for (yet), though in the long term Metal offers some interesting possibilities for nailing that down, or at least getting a better idea of relative clock speeds.

In any case, we’ll take a look at our GPU benchmarks in depth in a bit, but overall GPU performance compared to A7 and its G6430 is consistently better, but the exact performance gain will depend on the test at hand. Some tests will come very close to reaching 50% while others will be just 15-20%. The dependent factor generally seems to be whether the test is ALU-bound or not; because the USC has not changed significantly from G6430 to GX6450 outside of those additional FP16 ALUs, tests that hit the FP32 ALUs in particular show less of an improvement. Otherwise more balanced tests (or at least tests more defined by pixel fillrate performance) can show greater gains. In general we should be looking at a 30-35% performance improvement.

Why Four Cores?

One thing that admittedly surprised us in the revelation that A8 was using a 4-core PowerVR design was that we figured a 6-core design would be a shoe-in for A8, especially since Apple was on the receiving end of the density improvements from TSMC’s 20nm process. But upon further reflection an additional two cores is likely more than Apple needed nor wanted.

The biggest factor here is that coming from G6430 in the A7, performance has seen a solid improvement despite sticking to only four GPU cores. Due to the combination of performance improvements from the Series6XT architecture and any clock speed increases from Apple, A8 gets quite a bit more GPU performance to play with. The increased resolution of the iPhone 6 screen in turn requires more performance if Apple wants to keep native resolution performance from significantly regressing, which GX6450 is capable of delivering on. Never mind the fact that G6430 also drove the iPad Air and its much larger 2048x1536 pixel display.

PowerVR Series6/6XT "Rogue"
GPU # of Clusters # of FP32 Ops per Cluster Total FP32 Ops Optimization
G6200 2 64 128 Area
G6230 2 64 128 Performance
GX6240 2 64 128 Area
GX6250 2 64 128 Performance
G6400 4 64 256 Area
G6430 4 64 256 Performance
GX6450 4 64 256 Performance
G6630 6 64 384 Performance
GX6650 6 64 384 Performance

These performance improvements in Series6XT have a cost as well, and that cost is suitably reflected in the estimated die sizes for each GPU. The G6430 was 22.1mm2 on the 28nm A7, while the GX6450 is 19.1mm2 on A8. Though GX6450 is smaller overall, it’s nowhere near the roughly 11.1mm2 a pure and perfect die shrink of G6430 would occupy. Limited area scaling aside, GX6450’s additional functionality and additional performance requires more transistors, and at the end of the day Apple doesn’t see a significantly smaller GPU because of this. In other words, the upgrade from G6430 to GX6450 has delivered much of the performance (and consumed much of the die space) we initially expected to be allocated to a 6-core GPU.

Overall the choice of GX6450 seems to be one of picking the GPU best for a phone, which is an area the G6430 proved effective with A7. As a step below Imagination’s 6-core PowerVR designs, GX6450 delivers a better balance between performance and power than a larger GPU would, which in turn is clearly a benefit to Apple. On the other hand this means A8 is not going to have the GPU performance to compete with the fastest SoCs specifically designed for tablets, though what this could mean for the obligatory iPad update remains to be seen.

A8’s CPU: What Comes After Cyclone? CPU Performance
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  • ninjaroll - Tuesday, September 30, 2014 - link

    You have to tell yourself that not everyone commenting is around your age range. I feel like a lot of the haters are about 12-18 years of age. If they're older...well.. I feel bad for them.

  • akdj - Saturday, October 4, 2014 - link

    I think you nailed it ninja
    The 'age & ambiguity' question. As well as age, the differences between generations and what 'we‘ had available (I'm 44) when growing up vs the 'gen Y' mid 20s-mid 30s (I think the higher end still appreciates both or the 'big three' for what they are ..not crapping on what 'they're not'). Then weve got the 'kids'. I'd even go so far as to say 12-21/22 year olds. As the 22 year old SAW the impact the Iphone had in 2007, the 'actual' Android fight 'start' in 2008 at 15 & 16. They were graduating and going into college or vocational training when the iPad broke and the Xoom filled (tablet computing). They've seen in their 'formative' years the evolution of HiDPI displays and developed personal opinions about their extremely 'personal' devices (I've got teenagers! Yikes, believe you me when I say their 'personal' devices:))
    Baby boomers, X & late Y didn't have cell phones growing up. Drug dealers and executives had pagers and computers were computers. They weren't 'connected' with the Internet (mainstream) and we paid a LOT of money for our Apple or Microsoft software and OS Updates. The incredible sea change Apple and Google have brought to the consumers and the masses regardless of income levels, location in the world and/or from developing countries ...they're penetration is significant. Obviously there are countries with their own restrictions, etc... But maybe they're the 'smart' ones for now...look at what the NSA/Patriot Act has done for the USA and her relationships even with our closest allies!
    We're still at the infancy of 'mobile' comms/computers and connectivity. These iPhones ARE computers. The G3, Z3, S5 or 5s/6/6+!!! All of them. I think as we age, we remember. It's easier not to take for granted the way technology has empowered our lives, folded the world in half, and the incredible benefits and convenience we enjoy OR despise with 'cellular' phones, phabs n tabs! At times they feel more like a leash than freedom. When you're working and paying a mortgage or two, car payments and student loans (from two decades ago or current kids going into post Ed), groceries and 'energy' (from gasoline to heating gas, cooling electricity or your battery in your fell phone of choice), groceries and your kids' entry fees, new 'cleats' and mitts, pads and summer camps....THEN you'll get it. I'd bet dollars to donuts (such a dumbass saying, very unhip I know;))
    As you age, technology will continue to evolve. Much of what we enjoy today is a direct and absolutely traceable line to developments during the 'Cold War'. Whether Russian or American, Chinese (anyone see their Olympics in Beijing? The opening and closing cereminies, etc? Kind of brings a new meaning to 'made in China' than it had when I was younger. IMHO they blew London completely outta the water ymmv as always)
    Point being there isn't 35+ folks on this board waging this ridiculous Holy War between OEMs or OS's. There ARE paid folks from both sides as again, social media in the last decade (another 'new') has become JUST as important as their thirty and sixty second TV spots, sponsorships or product placement in movies! It's HUGE. & IMHO a VERY important and crucial element in a free internet society to have sites like Anand's ...that he's passed along to Brian and Ryan and the rest of the crew. I've been here for years and have ALWAYS found what I've come for. Objective measurements and subjective reviews. We're all human. If we're reviewing a product its in our nature to 'add' our opinions now and then
    To me, as a user of OS X and Windows, UNIX, Android and iOS ...I feel like ANYone limiting themselves so blindly to what the 'enemy' is doing is ignorant, young and/or unemployed (if the latter, I feel for you if you're looking...but if you're lurking on forums like these zealots are they're NOT looking for employment. If you're out of work, you can spend 40-50 hours a week 'Looking' and in most developed nations...in other words ANYone that would criticize the other camp and not appreciate what they've already got)
    At the end of the day, it Samsung making Apple work harder. Cupertino making MtView work harder and ALL of them starting to reap the awards Microsoft seemed to 'leak' off over the past 10-15 years. We're no longer in an X86, workstation at your desk on the 'intranet' to collaborate with a fax machine to send the final product. If you don't remember those days, it's tough to take these complaints seriously as my 5s and from the time I've spent with the 6/6+, my Air and retina mini all have a 'place' in my life. And every ONE of them is faster with quicker connectivity and MORE software available than at ANY point in my life and I'm only, hopefully half way to the finish line. As you age, you'll understand what I'm saying
    That said ...If you're 44 & @ (mom's) home, in the basement, without a gig, and feeding your spiders backing these DBags arguing 'physical, objective, and factual' measurements of performance in the review....it's YOU that needs to reexamine your life and priorities.
    Love is family, kids, coaching and watching them grow, through good times and bad. It's the iPhone, the S5 or Note 3 you're carrying that's capturing those memories. Ten years from NOW, there won't be a 'lightning' connector. An iPhone 6+ or Note7/G7 or Z8! USB will be dead and history is indicative of the evolving future, only us 'old folk' will be using Facebook ...but giving it our BEST shot to 'learn' to new and HIP MySpace, Netscape, AOL or today's Twitter and Facebooks
    Remember kids, it's US, and my parents (your grandparents) that built this shit for you. Not YOU! You're reaping the benefits of the fruits of our labor. If you don't get out from behind the 600 dpi display you're so passionate about ...or get out of the house, learn how to ride a bike like Tony Hawk, snowboard like the 'Tomato' or innovate like Gates, Jobs and that snot nose kid from Stanford....young 'Zuke', you're futures are going to suck
    Don't be a slave to your tools. Let them work for you, choose what best fits your idea and vision and occupation and you'll find out soon there's a helluva lot more to life than MHz, GB of RAM, and PPI determining what you can and can't see. As your ears fail you so, too, will your eyes and damned if I can't tell the difference between the '6' &6+, the new HTC or my Note 3/5s, Air or mini! All different, ALL a helluva lot better than my green/orange monochrome displays I was 'working' on in the early 90s, how incredible '16 color displays' were and the transition from cathode ray tube 'monitors' to LCD and LED/AMOLED/Plasma displays showed us the difference between our VHS tapes, 480p DVD collection and the BluRay, 1080p displays. Now packaging those pixels into the palm of your hand is absolutely, and genuinely AMAZING. Nothing short of true miracles in engineering
    My dad graduated in 1972 with his bachelors in electrical engineering. Did it with a slide ruler and drafting kit he's still got today and the same kit myself and two of my three younger brothers took through engineering school with our TI calcs that did it all (early 90s), and my first 286 after my Apple IIe & IIc run. As a baby with the 8086 processor perhaps those of us born in the early to mid 70s and earlier are more 'appreciative' today than the younger generation. We're more patient, we've gained wisdom and most importantly we 'lived' without the Internet, with corded 'dial' phones (when I was a kid we had a party line...and only had to dial FOUR digits locally lol! Small town in northwest Montana). To me, I really HOPE there's youngsters as intrigued by ALL forms of operation systems and is the new 'Edison, Tesla, Carnegie or Jobs/Zuke/Gates' of a future era. Redesigning in his or her 13 year old mind an OS that's a 'learning OS'. Through the millions of lines of code to boot to the desk, half can be elimated as it learns YOUR usage and 'needs' ..that conforms to the individual and their needs ...regardless of how basic or how 'tough'.
    We run and have for over 20 years an audio and video production business. My wife and I are both experienced, high performance rated pilots and live in Alaska. It's paramount we fly with the business as we're living in an area nearly the physical size of the entire lower 48 with over 3 million lakes (sorry Minnesota, but we do only have just over 10,000 rivers;))---& more coastline that the ENTIRE CONUSA. With a pair of roads. No access without a plane or boat, or big balls and a four wheeler or snow 'machine' (it's Alaskan for snowmobile;)). We've been lucky enough to work with plenty of the largest cable and network broadcasters on documentaries and 'real TV' (not reality). Whether following the Troopers, fishing for crabbing on the ocean, flying into single resident 'zip codes' in the dead of winter with 2400 pounds of heating fuel in the plane with ya, it can be a kick in the ass and iOS has changed our operations in the last half decade for the better. Filing my FP, deciding how much fuel, traffic and weather conditions as well as updated Jep charts, plates and diversions ...it's becime my kneeboard, fifty LB flight bag, manuals and checklists, as well as maintenance and troubleshoot instructions ...ADSB and TCAS, 3D terrain mapping and tradfic following, it's a BIG change. While the Note 3 works GREAT for sketching rigging points with structural engineers, etc. The rMBP has been an absolute Home Run for us as has the new Mac Pro at the studios. We use several HP and Dell workstations as well, both systems are awesome and I think I'm one of the few enjoying Win 8.1 ...bought an HP 2in1 for about $750 and I've got a 13" core i5 slate with an SSD.
    Way TL/DR; youngsters don't be afraid to open your eyes and think for yourself. Try everything. Use what your need and stay away from the internet when you've made your chouce for a couple of weeks:). Something better is ALWAYS around the corner but each and every choice available today is better than yesterday's. Guaranteed

  • timbo24 - Tuesday, September 30, 2014 - link

    Great review, thanks for the hard work.

  • gevorg - Tuesday, September 30, 2014 - link

    Very nice to see audio tests, just another thing that makes Anandtech reviews unique.

  • paul4na - Wednesday, October 1, 2014 - link

    Unique? If you want proper phone reviews with detailed benchmarks then go to GSMArena.

  • doobydoo - Wednesday, October 1, 2014 - link

    LOL. You made a funny.

  • slatanek - Tuesday, September 30, 2014 - link

    still no sign of Windows 10 event...

  • Chaser - Tuesday, September 30, 2014 - link

    As objectively as I could I took up Costco on their 14 day return policy and tried the iPhone 6. I owned the first iPhone and had been Android flagship type ever since.

    Bottom line: compared to Android the iPhone does less. After 5+ years the interface is STILL the same square blobs that float on the screen. No shortcuts. No app widgets. Install a new app and it is placed in the next open spot with all the other square blobs. I liek how I can use shortcuts for my higher use app but hide others in the application folder with Android.

    No notification LED. My new G3 I can color code that notification to know just by sight what type of alert has popped up on my phone. Text, email, Facebook, more. With Apple you get a flash of the camera, if its upside down. What a joke.

    Despite Apple "allowing" Swiftkey's new keypad its a paltry joke compared to Android's version. Chrome can also be installed but make no mistake, any links through email or text will open the default browser Safari. iMessage is still the default text client with no alternatives that provide the same functionality.

    It's simple. While the Apple faithful will buy their new tech darling phones the boring, long on the tooth Apple interface does less. Android offers far more customization and openness. Back my gold iPhone 6 went to Costco and now I love my new G3. Sigh...maybe another 5 years.

  • Parhel - Tuesday, September 30, 2014 - link

    The customization and openness is exactly what turns me off of Android. It's not the only thing, but it's the main thing. I don't want that in a phone. I spend 10 hours a day coding and troubleshooting at work. For a phone, I want something that's already set up for me. Something that I barely need to even look at to use. I don't want to tinker with it.

  • Chaser - Tuesday, September 30, 2014 - link

    I tinker with nothing if I chose. However I'd rather have those choices than Apple's divine vision of how my phone should operate.
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