The iPhone 6 Reviewby Joshua Ho, Brandon Chester, Chris Heinonen & Ryan Smith on September 30, 2014 8:01 AM EST
- Posted in
- iPhone 6
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|
|# of SIMDs||8||12||16||32||4||4|
|MADs per SIMD||4||4||4||4||32||32|
|GFLOPS @ 300MHz||19.2 GFLOPS||28.8 GFLOPS||38.4 GFLOPS||76.8 GFLOPS||76.8 GFLOPS||76.8 GFLOPS|
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|
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.
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Caliko - Tuesday, October 6, 2015 - linkA good phone runs its own OS.
You and that moron don't understand HD so your obsession is invalid.
echtogammut - Tuesday, September 30, 2014 - linkHe isn't wrong about anything he said, but I just can't imagine getting worked up enough to say it. As a current Apple user/developer, I have every Apple product. I like their stuff, but I can't say I am obsessed about it. I also have a lot of the competing products, so I am constantly toying with all of them. When my current iPhone breaks, I will probably replace it with a Sony Xperia Z3 compact, because it looks like a perfect phone for me. I am not interested in a bigger phone and I would like something that is waterproof, because I run or ride a rain, sleet or shine. I personally think Apple is falling behind in the world of business and multi-use devices. I am seeing a lot of customers whom I developed business apps for iOS coming back and wanting to move to Microsoft or Android platforms because the devices are more powerful and offer more robust features.
GerryS - Wednesday, October 1, 2014 - linkI agree. He makes some good points, though most of them are seriously overstated. For most users (yes, nearly everyone in my experience) a "pretty dang good" display is about as good as we can be bothered to look for. I actually like the display on my iPhone 4s. I've never had a complaint, except that it is small, so I got an iPad.
See, I don't actually like it that Apple went for big phones. I carry my phone in my pants pocket. The 6 might be about the limit of the size phone I want. If I could get an updated 4s with the processor and other basic features of the 6, that's what I'd buy.
So, for most consumers, we want something that works. I had 4 Android phones before Verizon got the iPhone. I liked them pretty well, but they kept breaking. I went to Apple for dependability, and have not been disappointed. I've only had a couple of problems (even with my jailbreaks), and they've always been easy to fix.
I do like it when Apple leads the way, but it would be silly to expect them to have all of the advances. So many people compare the iPhones to Android phones, listing all of the things that came out first or "better" on Android phones. They seem to forget that there are dozens of companies making Android phones. The best of them have only one or tow innovations at a time - about the same as Apple manages. Apple continues to be one of the leaders, as long as you compare them to one company at a time.
If you want the universe of advances coming from Android manufacturers, then go buy an Android. We really won't hold it against you. For me, I like my iPhone and iPad. My wife has Android (at my recommendation) because we could get some features that were important to her.
Actius - Wednesday, October 1, 2014 - linkLol, he is wrong about his engine analogy! Seriously, none of that makes any sense. Haha, and what's a "V8 compressor"? My goodness...people shouldn't talk about things they really don't know. Just reading that was cringe worthy.
sigmatau - Thursday, October 2, 2014 - linkI'm going to guess you are not so dense as to be picking at the spelling but instead don't know what is a v8 Kompressor. Not only one of the best engines that Mercedes made, but also award winning by 3rd parties.
techconc - Thursday, October 2, 2014 - linkActually, he is either wrong or extremely misguided in almost everything he's said. Feel free to pick your argument of choice. He starts off by saying how the iPhone 6 launch was disastrous. As compared to what? What other vendor will sell 10 million devices on a launch weekend? Even for Apple, it broke their own previous records. I'm genuinely grateful for people that prove they're an idiot right up front. It lets me know I can either skip the rest of the post or read on for purely amusement purposes.
That's just the conceptual part of the post. His technical observations were equally misguided. Especially with regard to screen quality, etc. Clearly he didn't bother reading the Anandtech review he's commenting on.
shm224 - Thursday, October 2, 2014 - link@techconc: Sure, what other products you know starts to bend, or totally crap out after the first buggy update, or even have features withdrawn due to more bugs all within the first week of release?
akdj - Friday, October 3, 2014 - linkReally? You're still convinced the iPhone (6 OR 6+) actually and easily 'bends' in the first week? Buggy update? The one available for about two hours that few downloaded and within ten hours of pulling it a fixed update was released? Features withdrawn? I'm intrigued ..as an owner and realist like techconc, the person you responded to... I've GOT to know!
I'm patiently awaiting my pair of 6+s for my wife and I. We just returned from the mall and the Apple Store specifically playing for almost two hours with them. I built a two minute movie and. Rendered in 1080p in about 35/40 seconds, air dropped to myself. Un. Believable phone. It's. Amazing
We ordered launch day through our business representative. Lol. Silly me. Ship date estimate is pretty specific, actually a bit ambiguous with the latest update. 11/2-11/28/14:-)
Oh well. Plenty of time to allow developers to continue updating their apps.
Between my Air, rMBP 15" and the iPhone 5s /6+, my business of 27 years has been revolutionized. Literally, over the past half decade, as a pilot and sound/video producer...weight savings alone are enough to double our profits. And ½ our setup snd break down times. Even my 'flight bag' of nearly fifty pounds isn't necessary. With three retina minis in the cockpit for redundancy, the 'paper' is still there, but unnecessary any more for updates to plates and Jep charts, winds aloft and weather/traffic ...even diversion airports, filing my flight plan and telling me how thirsty she is! Fuel calculated, with a GPS dongle a tenth the price and 100 fold quicker to lock n track than avionics just a half decade ago provide incredible accuracy. ADSB and TCAS (3D terrain, weather and other traffic/with their specific info; altitude, speed, heading, and Xspnder --- TCAS, a warning system that 'tells you what to do' in conjunction with other planes fitted with the system including all commercial traffic and many GA pilots with ILS certifications ...Alaska can get nasty quick and having to 'duck down' these two systems alone are incredible and 'reasonably priced' advancements!).
I'm not sure there's a place in my life iOS has t changed. As a father, business owner and operator, little league and wrestling coach, and pilot...each iteration has improved signficantly enough in 'most situations' to justify yearly updates for me. iPhones hold their value. Until the 4s, AT&T was generous and 'allowed' a 12 month subsidized update. With the business I also provide 17 full time employee iPhones so we've been lucky enough to 'recoup' some of the money spent each update
I just sold an employee's iPhone 5, 32GB AT&T with a cracked screen, scratched to hell and working perfectly. With excellent battery health and perfect camera lens (only scratch-less area of the phone!) for $235 to a local repair guy. He saw me in the mall with the Gazelle box (more than a hundred bucks less) on my way to the USPS. Asked what I was sending. Told him. Showed him a 'picture' or seven before I unboxed it, but he was adamant ...he was able to shine it up with his pieces n parts for $45 or less! Told me it would cost a customer about $210 to do the work but parts were 20% the cost ...labor and time is the price. From there he was confident he could sell it for $325-350 at his kiosk within 24 hours.
My son has had his fifth generation iPod touch for two years. He's nine. No scratches. No scuffs. Clean a booger or twelve off once a week but other than that, it's completely 'perfect'. These iPhones are built damn near the same. Sleek, thin and well balanced ...no bugs, incredibly quick and unless you're a dumbass and put your $800 pocket computer in your rear pocket of your jeans and SIT on it, you're an idiot ...and you've got to be SIGNIFICANTLY overweight AND hit the precise angle in order to 'mash' 100 pounds of pressure to a 'single point'/torque.
Can you bend em? Yep. It's been proven and EVERY piece of proof we've seen visually demonstrates the incredible amount of force necessary and in such a way not indicative of daily use or situations a consumer would find themselves in 99.9% of the time.
Don't. Be. Silly. I respond not only to you but to all those talking like you. Until you've used one. Felt one. Actually spent time with it, it's difficult to understand.
These are absolute and unequivably the BEST two phones on the maket at this time. With the best and most abundant apps/software optimized to its specs. Support to back it up. And resale value when Ya get bored and ready for a new one. You'll recover your 'down payment' plus fifty percent in many cases ...if you take care of it. Seemingly, they're even more valuable than a same year flagship Samsung Note 2, as I wasn't able to get more than $125 for that joke. Note 3 got it right. But it took twice the cores, clocked at twice the speed with three times the memory ( ⅔ to ¾ of which is in use even without an app running! I've got one though and don't take me wrong, I love it) to get 'close' to the GUI fluency of the iPhone & iOS
That's. SAD. That's. Buggy. I'd much rather have a phone that bends with a hundred pounds of torque in a certain and specific area than I would gambling I'll get an update, deal with carriers and OEM bloat and shitty aesthetics and design. Lack of support or resale, lack of apps and software ...& the apps and software in parity are incredibly more enjoyable and stable on iOS. I like my Note for browsing and the SPen. I think an active digitizer would put the six plus over the top but as it is, it's perfect.
Indeed, I'll continue using (& owning) my N3. But I'm not the least bit compelled with the '4' and its 2540 display. While I'm sure there's noticeable and 'obvious' benefits to having 550/600+ PPI ...I'll warn ya when you're 38-42, speaking from experience...you'll need 'cheaters';) ...like our ears, our eyes deteriorate as well ...the elasticity of our lens and ability to change and 'maintain' up close focus just ...goes away! Hence the incredible benefits I've found witg the HiDPI display technology. 3D? Good riddance! It was a joy to see 4k/Ultra HD and even examples of 5/6 & 8K motion display AND capture gear instead of dumbass glasses and crappy off center viewing with limited content.
Apple lead the way and destroyed multi billion dollar monsters in the industry with the iPhone. They then changed consumer display technology's availability. HiDPI and increased resolution is awesome. But to a certain point. Your 1080p 65" LED LCD IPS or AMOLED TV in the living room at 10-12 feet or typical viewing distances is around 100ppi. Quadrupling the resolution (4k) while not exactly linear, will amount to approximately an increase of 100%. To 200ppi. Not four foukd as you may think.
The 'new' ipad(3)/4 -- iPhone 4 -- the rMBPs --- ALL Game Changing home runs. Putting a certain 'joy' back into 'work' again. With SUCH an accurate palate and the ability of OS X'es scaling of the UI (& third party apps the same ...utilizing pixel for pixel when necessary or quadrupling for the GUI simultaneously and without 'glitches' or latency is a marvel in engineering. Coupled with the IrisPro 5200/750m and PCIe SSD at a TB with read and write speeds exceeding a Gb/s, Thunderbolt 2 and its 'one' cable capabilities and abilities to run multiple 4K displays ...shoot man. Seems like yesterday I was hunting the wax pencils and cutting my fingers slicing tape ...now it's immediate and fast as hell in a four pound package at a dozen times the resolution.
Times are good if you're an Apple user. And thats JUST the hardware!
OS X and iOS's march to marriage is incredible. Continuity and Handoff. Air drop between iOS and OS X, as well ...the aggregation and integration between the devices you're using is revolutionary. Period. Only Windows has the power and support vertically and horizontally to compete with what Apple's doing. Vertical & Horizontal backbones.
No one that needs to 'work' is buying a Chromebook
elajt_1 - Friday, October 3, 2014 - linkThis must've been the longest and weirdest Iphone ad I've ever read. ;-)
techconc - Monday, October 6, 2014 - link@shm224 - I now know several people with iPhone 6 and 6+ devices that keep them in their pockets. They all seem to agree that there is no merit to this "bend gate" nonsense. While nobody doubts that these phones can bend under a certain amount of pressure (90 lbs. according to Consumer Reports), from a practical matter, it's a non-issue. Further, I find it rather interesting that phones such as the HTC One which bend under significantly less pressure (70 lbs.) don't receive the same sort of media attention.
As for the 8.0.1 update, yup, Apple screwed that up. Fortunately, for Apple, the update was pulled after about an hour. It's also fortunate that in only affected some phones and only for the over the air update as opposed to the iTunes update. To your point, no, this typically isn't an issue for other phones... then again, neither are regular updates.