GPU Architecture

Dating back to the original iPhone, Apple has relied on GPU IP from Imagination Technologies. In recent years, the iPhone and iPad lines have pushed the limits of Img’s technology - integrating larger and higher performing GPUs than all other Img partners. Apple definitely attempted to obfuscate its underlying GPU architecture this time around for some reason.

Dating back to a year ago I got a lot of tips saying that Apple would be integrating Imagination Technologies’ PowerVR Series 6 GPU this generation, but I needed more proof.

The first indication that this isn’t simply a Series 5XT part is the listed support for OpenGL ES 3.0. The only GPUs presently shipping with ES 3.0 support are Qualcomm’s Adreno 3xx (which is only integrated into Qualcomm silicon), ARM’s Mali-T6xx series and PowerVR Series 6. NVIDIA’s Tegra 4 GPU doesn’t support ES 3.0, and it’s too early for Logan/mobile Kepler. With Qualcomm out of the running that leaves Mali and PowerVR Series 6.

“All GPUs used in iOS devices use tile-based deferred rendering (TBDR).”

Apple’s developer documentation lists all of its SoCs as supporting Tile Based Deferred Rendering (TBDR). If you ask Imagination, they will tell you that they are the only ones with a true TBDR implementation. However if you look at ARM’s Mali-T6xx documentation, ARM also claims its GPU is a TBDR.

The real hint comes with anti-aliasing support:

The last line in the screenshot above, MAX_SAMPLES = 8. That’s a reference to 8 sample MSAA, a mode that isn’t supported by ARM’s Mali-T6xx hardware - only PowerVR Series 6 (Mali-T6xx supports 4x and 16x AA modes).

There are some other hints here that Apple is talking about PowerVR Series 6 when it references the A7’s GPU:

“The A7 GPU processes all floating-point calculations using a scalar processor, even when those values are declared in a vector. Proper use of write masks and careful definitions of your calculations can improve the performance of your shaders. For more information, see “Perform Vector Calculations Lazily” in OpenGL ES Programming Guide for iOS.

Medium- and low-precision floating-point shader values are computed identically, as 16-bit floating point values. This is a change from the PowerVR SGX hardware, which used 10-bit fixed-point format for low-precision values. If your shaders use low-precision floating point variables and you also support the PowerVR SGX hardware, you must test your shaders on both GPUs.”

As you’ll see below, both of the highlighted statements apply directly to PowerVR Series 6. With Series 6 Imagination moved to a scalar architecture, and in ImgTec’s developer documentation it confirms that the lowest precision mode supported is FP16.

All of this leads me to confirm what I heard would be the case a while ago: Apple’s A7 is the first shipping mobile silicon to integrate ImgTec’s PowerVR Series 6 GPU.

Now let’s talk about hardware.

The A7’s GPU Configuration: PowerVR G6430

Previously known by the codename Rogue, series 6 has been announced in the following configurations:

PowerVR Series 6 "Rogue"
GPU # of Clusters # of FP32 Ops per Cluster Total FP32 Ops Optimization
G6100 1 64 64 Area
G6200 2 64 128 Area
G6230 2 64 128 Performance
G6400 4 64 256 Area
G6430 4 64 256 Performance
G6630 6 64 384 Performance

Based on the delivered performance, as well as some other products coming down the pipeline I believe Apple’s A7 features a variant of the PowerVR G6430 - a 4 cluster Rogue design optimized for performance (vs. area).

Rogue is a significant departure from the Series 5XT architectures that were used in the iPhone 5, iPad mini and iPad 4. The biggest change? A switch to a fully scalar architecture, similar to the present day AMD and NVIDIA GPUs.

Whereas with 5XT designs we talked about multiple cores, the default replication unit in Rogue is a “cluster”. Each core in 5XT replicated all hardware, while each cluster in Rogue only replicates the shader ALUs and texture hardware. Rogue is still a unified architecture, but the front end no longer scales 1:1 with shading hardware. In many ways this approach is a lot more sensible, as it is typically how you build larger GPUs.

In 5XT, each core featured a number of USSE2 pipelines. Each pipeline was capable of a Vec4 multiply+add plus one additional FP operation that could be dual-issued under the right circumstances. Img never detailed the latter so I always counted flops by looking at the number of Vec4 MADs. If you count each MAD as two FP operations, that’s 8 FLOPS per USSE2 pipe. Each USSE2 was a SIMD, so that’s one instruction across all 4 slots and not some combination of instructions. If you had 3 MADs and something else, the USSE2 pipe would act as a Vec3 unit instead. The same goes for 1 or 2 MADs.

With Rogue the USSE2 pipe is gone and replaced by a Unified Shading Cluster (USC). Each USC is a 16-wide scalar SIMD, with each slot capable of up to 4 FP32 ops per clock. Doing the math, a single USC implementation can do a total of 64 FP32 ops per clock - the equivalent of a PowerVR SGX 543MP2. Efficiency obviously goes up with a scalar design, so realizable performance will likely be higher on Rogue than 5XT.

The A7 is a four cluster design, so that four USCs or a total of 256 FP32 ops per clock. At 200MHz that would give the A7 twice the peak theoretical performance of the GPU in the iPhone 5. And from what I’ve heard, the G6430 is clocked much higher than that.

There’s more graphics horsepower under the hood of the iPhone 5s than there is in the iPad 4. While I don’t doubt the iPad 5 will once again widen that gap, keep in mind that the iPhone 5s has less than 1/4 the number of pixels as the iPad 4. If I were a betting man, I’d say that the A7 was designed not only to drive the 5s’ 1136 x 640 display, but also a higher res panel in another device. Perhaps an iPad mini with Retina Display? There’s no solving the memory bandwidth requirements, but the A7 surely has enough compute power to get there. There's also the fact that Apple has prior history of delivering an SoC that wasn’t perfect for the display (e.g. iPad 3).

GPU Performance

As I mentioned earlier, the iPhone 5s is the first Apple device (and consumer device in the world) to ship with a PowerVR Series 6 GPU. The G6430 inside the A7 is a 4 cluster configuration, with each cluster featuring a 16-wide array of SIMD pipelines. Whereas the 5XT generation of hardware used a 4-wide vector architecture (1 pixel per clock, all 4 color components per SIMD), Series 6 moves to a scalar design (think 16 pixels per clock, one color per clock). Each pipeline is capable of two FP32 MADs per clock, for a total of 64 FP32 operations per clock, per cluster. With the A7's 4 cluster GPU, that works out to be the same throughput per clock as the 4th generation iPad.

Imagination claims its new scalar architecture is not only more computationally dense, but also far more efficient. With the transition to scalar GPU architectures in the PC space we generally saw efficiency go up, so I'm inclined to believe Imagination's claims here.

Apple claims up to a 2x increase in GPU performance compared to the iPhone 5, but just looking at the raw numbers in the table above there's far more shading power under the hood of the A7 than only "2x" the A6.

Mobile SoC GPU Comparison
  PowerVR SGX 543 PowerVR SGX 543MP2 PowerVR SGX 543MP3 PowerVR SGX 543MP4 PowerVR SGX 554 PowerVR SGX 554MP2 PowerVR SGX 554MP4 PowerVR G6430
Used In - iPad 2/iPhone 4S iPhone 5 iPad 3 - - iPad 4 iPhone 5s
SIMD Name USSE2 USSE2 USSE2 USSE2 USSE2 USSE2 USSE2 USC
# of SIMDs 4 8 12 16 8 16 32 4
MADs per SIMD 4 4 4 4 4 4 4 32
Total MADs 16 32 48 64 32 64 128 128
GFLOPS @ 300MHz 9.6 GFLOPS 19.2 GFLOPS 28.8 GFLOPS 38.4 GFLOPS 19.2 GFLOPS 38.4 GFLOPS 76.8 GFLOPS 76.8 GFLOPS

GFXBench 2.7

As always, we'll start with GFXBench (formerly GLBenchmark) 2.7 to get a feel for the theoretical performance of the new GPU. GFXBench 2.7 tends to be more computationally bound than most games as it is frequently used by silicon vendors to stress hardware, not by game developers as an actual performance target. Keep that in mind as we get to some of the actual game simulation results.

GLBenchmark 2.7 - Fill Test (Offscreen 1080p)

Twice the fill rate of the iPhone 5, and clearly higher than anything else we've tested. Rogue is off to a good start.

GLBenchmark 2.7 - Triangle Throughput (Offscreen 1080p)

What's this? A performance regression? Remember what I said earlier in the description of Rogue. Whereas 5XT replicated nearly the entire GPU for "multi-core" versions, multi-cluster versions of Rogue only replicate at the shader array. The result? We don't see the same sort of peak triangle setup scaling we did back on multi-core 5XT parts. I don't suppose this will be a big issue in actual games (and likely a better balance between triangle setup/rasterization and shading hardware), but it's worth pointing out.

GLBenchmark 2.7 - Triangle Throughput, Fragment Lit (Offscreen 1080p)

This is the worst case regression we've seen from 5XT to Rogue. Its clear that per chip triangle rates are much higher on Rogue, but with a many core implementation of 5XT there's just no competing. I suspect this change is part of how Img was able to increase the overall density of Rogue vs. 5XT. Now the question is whether or not this regression will actually appear in games? To find out we turn to the two game simulation tests in GFXBench 2.7, starting with the most stressful one: T-Rex HD.

As always, the onscreen tests run at a device's native resolution with v-sync enabled, while the offscreen results happen at 1080p and v-sync disabled.

GLBenchmark 2.7 - T-Rex HD

As expected, the G6430 in the iPhone 5s is more than twice the speed of the part in the iPhone 5. It is also the first device we've tested capable of breaking the 30 fps barrier in T-Rex HD at its native resolution. Given just how ridiculously intense this test is, I think it's safe to say that the iPhone 5s will probably have the longest shelf life from a gaming perspective of any previous iPhone.

GLBenchmark 2.7 - T-Rex HD (Offscreen 1080p)

The offscreen test helps put the G6430's performance in perspective. Here we show the 5s barely falling behind Qualcomm's Adreno 330 (Snapdragon 800). There are obvious thermal differences between the two platforms, but if we look at the G2's performance (another S800/A330 part) we get a better indication of an apples to apples comparison. Looking at the leaked Nexus 5 (also S800/A330) T-Rex HD scores confirms what we're seeing above. In a phone, it looks like the G6430 is a bit quicker than Qualcomm's Adreno 330.

The Egypt HD tests are much lighter and a lot closer to the workload of a lot of games on the store today, although admittedly it is getting a little light.

GLBenchmark 2.7 - Egypt HD

Onscreen we're at Vsync already, something the iPhone 5 wasn't capable of doing. The 5s should have no issues running most games at 30 fps.

GLBenchmark 2.7 - Egypt HD (Offscreen 1080p)

Offscreen, even at 1080p, performance doesn't really change. Qualcomm's Adreno 330 is definitely faster, at least in the MDP/T. In the G2, its performance lags behind the G6430. I really want to measure power on these things.

3DMark

3DMark finally released an iOS version of its benchmark, enabling us to run the 5s through on yet another test. As we've discovered in the past, 3DMark is far more of a CPU test than GFXBench. While CPU load will range from 6 - 25% during GFXBench, we'll see usage greater than 50% on 3DMark - even during the graphics tests. 3DMark is also heavily threaded, with its physics test taking advantage of quad-core CPUs.

With the iOS release of the benchmark comes a new offscreen rendering mode called Unlimited. The benchmark is the same but it renders offscreen at 720p with the display only being updated once every 100 frames to somewhat get around vsync. Because of the new test we don't have a ton of comparison data, so I've included whatever we've got at this point.

3DMark Unlimited - Ice Storm

3DMark ends up being more of a CPU and memory bandwidth test rather than a raw shader performance test like GFXBench and Basemark X. The 5s falls behind the Snapdragon 800/Adreno 330 based G2 in overall performance. To find out how much of that is GPU performance and how much is a lack of four cores, let's look at the subtests.

3DMark Unlimited - Graphics

The graphics test is more GPU bound than CPU bound, and here we see the G6430 based iPhone 5s pull ahead. Note how well the Moto X does because of its very high clocked CPU cores rather than its GPU. Although this is a graphics test, it's still well influenced by CPU performance.

3DMark Unlimited - Graphics Test 1

3DMark Unlimited - Graphics Test 2

3DMark Unlimited - Physics

The physics test hits all four cores in a quad-core chip and explains the G2 pulling ahead in overall performance. Note that I saw no improvement in this largely CPU bound test, leading me to believe that we've hit some sort of a bug with 3DMark and the new Cyclone core. 

3DMark Unlimited - Physics Test

Basemark X

Basemark X is a new addition to our mobile GPU benchmark suite. There are no low level tests here, just some game simulation tests run at both onscreen (device resolution) and offscreen (1080p, no vsync) settings. The scene complexity is far closer to GLBenchmark 2.7 than the new 3DMark Ice Storm benchmark, so frame rates are pretty low.

Unfortunately I ran into a bug with Basemark X under iOS 7 on the iPhone 5/5c/5s that prevented the off screen test from completing, leaving me only with on-screen results at native resolution.

Basemark X - On Screen

Once again we're seeing greater than 2x scaling comparing the iPhone 5s to the 5.

iPhone Generational Performance & iPhone 5s vs. Bay Trail M7 Motion Coprocessor & Touch ID
POST A COMMENT

466 Comments

View All Comments

  • Wilco1 - Thursday, September 19, 2013 - link

    The Geekbench results are indeed skewed by AES encryption. The author claimed AES was the only benchmark where they use hardware acceleration when available. There has been a debate on fixing the weighting or to place hardware accelerated benchmarks in a separate category to avoid skewing the results. So I'm hoping a future version will fix this.

    As for cross-platform benchmarking, Geekbench currently uses the default platform compiler (LLVM on iOS, GCC on Android, VC++ on Windows). So there will be compiler differences that skew results slightly. However this is also what you'd get if you built the same application for iOS and Android.
    Reply
  • smartypnt4 - Thursday, September 19, 2013 - link

    A lot of the other stuff in Geekbench seems to be fairly representative, though. Except a few of the FP ones like the blur and sharpen tests...

    It surely can't be hard to have Geekbench omit those results. I think if they did that, you'd see that the A7 is roughly 50-60% faster than the A6 instead of 100% faster, but I'm not sure there. I'd have to go and do work to figure that out. Which is annoying :-)
    Reply
  • name99 - Wednesday, September 18, 2013 - link

    I'd agree with the tweaks you suggest: (improved memory controller and prefetcher, doubling of L2, larger branch predictor tables).

    There is also scope for a wider CPU. Obviously the most simple-minded widening of a CPU substantially increases power, but there are ways to limit the extra power without compromising performance too much, if you are willing to spend the transistors. I think Apple is not just willing to spend the transistors, but will have them available to spend once they ditch 32-bit compatibility. At that point they can add a fourth decoder, use POWER style blocking of instructions to reduce retirement costs, and add whatever extra pipes make sense.
    The most useful improvement (in my experience) would be to up the L1 from being able to handle one load+store cycle to two loads+ one store per cycle, but I don't know what the power cost of that is --- may be too high.

    On the topic of minor tweaks, do we know what the page size used by iOS is? If they go from 4K to 16K and/or add support for large pages, they could get a 10% of so speed boost just from better TLB coverage.
    (And what's Android's story on this front? Do they stick with standard 4K pages, or do they utilize 16 or 64K pages and/or large pages?)
    Reply
  • extide - Wednesday, September 18, 2013 - link

    Those are some pretty generous numbers you pulled out of your hat there. It's not as easy as just do this and that and bam, you have something to compete with Intel Core series stuff. No. I mean yeah, Apple has done a great job here and I wish someone else was making CPU's like this for the Android phones but oh well. Reply
  • name99 - Wednesday, September 18, 2013 - link

    "Now, I will agree that this does prove that if Apple really wanted to, they could build something to compete with Haswell in terms of raw throughput."

    I agree with your point, but I think we should consider what an astonishing statement this is.
    Two years ago Apple wasn't selling it's own CPU. They burst onto the scene and with their SECOND device they're at an IPC and a performance/watt that equals Intel! Equals THE competitor in this space, the guys who are using the best process on earth.

    If you don't consider that astonishing, you don't understand what has happened here.

    (And once again I'd make my pitch that THIS shows what Intel's fatal flaw is. The problem with x86 is not that it adds area to a design, or that it slows it down --- though it does both. The problem is that it makes design so damn complex that you're constantly lagging; and you're terrified of making large changes because you might screw up.
    Apple, saddled with only the much smaller ARM overhead, has been vastly more nimble than Intel.
    And it's only going to get worse if, as I expect, Apple ditches 32-bit ARM as soon as they can, in two years or so, giving them an even easier design target...)

    What's next for Apple?
    At the circuit level, I expect them to work hard to make their CPU as good at turboing as Intel. (Anand talked about this.)
    At the ISA level, I expect their next major target to be some form of hardware transactional memory --- it just makes life so much easier, and, even though they're at two cores today, they know as well as anyone that the future is more cores. You don't have to do TM the way Intel has done it; the solution IBM used for POWER8 is probably a better fit for ARM. And of course if Apple do this (using their own extensions, because as far as I know ARM doesn't yet even have a TM spec) it's just one more way in which they differentiate their world from the commodity ARM world.
    Reply
  • smartypnt4 - Wednesday, September 18, 2013 - link

    @extide: agreed.

    @name99: It is very astonishing indeed. Then again, a high profile company like Apple has no problem attracting some of the best talent via compensation and prestige.

    They've still got quite a long way to match Haswell, in any case. But the throughput is technically there to rival Intel if they wanted to. I would hope that Haswell contains a much more advanced branch predictor and prefetcher than what Apple has, but you never know. My computer architecture professor always said that everything in computer architecture has already been discovered. The question now is when will it be advantageous to spend the transistors to implement the most complicated designs.

    The next year is going to be very interesting, indeed.
    Reply
  • Bob Todd - Wednesday, September 18, 2013 - link

    How many crows did you stuff down after claiming BT would be slower than A15 and even A12? Remember posting this about integer performance?

    "Silverthorne < A7 < A9 < A9R4 < Silvermont < A12 < Bobcat < A15 < Jaguar"

    Apple's A7 looks great, but you've made so many utterly ridiculous Intel performance bashing posts that it's pretty much impossible to take anything you say seriously.
    Reply
  • Wilco1 - Wednesday, September 18, 2013 - link

    BT has indeed far lower IPC than A15 just like I posted - pretty much all benchmark results confirm that. On Geekbench 3 A15 is 23-25% faster clock for clock on integer and FP.

    The jury is still out on A12 vs BT as we've seen no performance results for A12 so far. So claiming I was wrong is not only premature but also incorrect as the fact is that Bay Trail is slower.
    Reply
  • Wilco1 - Wednesday, September 18, 2013 - link

    Also new version with A7 and A57 now looks like this:

    Silverthorne < A7 < A9 < A9R4 < Silvermont < A12 < Bobcat < A15 < Jaguar < A57 < Apple A7
    Reply
  • Bob Todd - Wednesday, September 18, 2013 - link

    Cherry picking a single benchmark which is notoriously inaccurate at comparisons across platforms/architectures doesn't make you "right", it just makes you look like more of a troll. Bay Trail has better integer performance than Jaguar (at near identical base clocks), so by your own ranking above it *has* to be faster than A12 and A15.

    You show up in every ARM article spouting the same drivel over and over again, yet you were mysteriously absent in the Bay Trail performance preview. Here's the link if you want to try to find a way to spin more FUD.

    http://anandtech.com/show/7314/intel-baytrail-prev...

    Apple's A7 looks great, and IT is still the powerhouse of mobile graphics. The A7 version in the iPad should be a beast. None of that makes most of your comments any less loony.
    Reply

Log in

Don't have an account? Sign up now