Haswell GPU Architecture & Iris Pro

In 2010, Intel’s Clarkdale and Arrandale CPUs dropped the GMA (Graphics Media Accelerator) label from its integrated graphics. From that point on, all Intel graphics would be known as Intel HD graphics. With certain versions of Haswell, Intel once again parts ways with its old brand and introduces a new one, this time the change is much more significant.

Intel attempted to simplify the naming confusion with this slide:

While Sandy and Ivy Bridge featured two different GPU implementations (GT1 and GT2), Haswell adds a third (GT3).

Basically it boils down to this. Haswell GT1 is just called Intel HD Graphics, Haswell GT2 is HD 4200/4400/4600. Haswell GT3 at or below 1.1GHz is called HD 5000. Haswell GT3 capable of hitting 1.3GHz is called Iris 5100, and finally Haswell GT3e (GT3 + embedded DRAM) is called Iris Pro 5200.

The fundamental GPU architecture hasn’t changed much between Ivy Bridge and Haswell. There are some enhancements, but for the most part what we’re looking at here is a dramatic increase in the amount of die area allocated for graphics.

All GPU vendors have some fundamental building block they scale up/down to hit various performance/power/price targets. AMD calls theirs a Compute Unit, NVIDIA’s is known as an SMX, and Intel’s is called a sub-slice.

In Haswell, each graphics sub-slice features 10 EUs. Each EU is a dual-issue SIMD machine with two 4-wide vector ALUs:

Low Level Architecture Comparison
  AMD GCN Intel Gen7 Graphics NVIDIA Kepler
Building Block GCN Compute Unit Sub-Slice Kepler SMX
Shader Building Block 16-wide Vector SIMD 2 x 4-wide Vector SIMD 32-wide Vector SIMD
Smallest Implementation 4 SIMDs 10 SIMDs 6 SIMDs
Smallest Implementation (ALUs) 64 80 192

There are limitations as to what can be co-issued down each EU’s pair of pipes. Intel addressed many of the co-issue limitations last generation with Ivy Bridge, but there are still some that remain.

Architecturally, this makes Intel’s Gen7 graphics core a bit odd compared to AMD’s GCN and NVIDIA’s Kepler, both of which feature much wider SIMD arrays without any co-issue requirements. The smallest sub-slice in Haswell however delivers a competitive number of ALUs to AMD and NVIDIA implementations.

Intel had a decent building block with Ivy Bridge, but it chose not to scale it up as far as it would go. With Haswell that changes. In its highest performing configuration, Haswell implements four sub-slices or 40 EUs. Doing the math reveals a very competent looking part on paper:

Peak Theoretical GPU Performance
  Cores/EUs Peak FP ops per Core/EU Max GPU Frequency Peak GFLOPs
Intel Iris Pro 5100/5200 40 16 1300MHz 832 GFLOPS
Intel HD Graphics 5000 40 16 1100MHz 704 GFLOPS
NVIDIA GeForce GT 650M 384 2 900MHz 691.2 GFLOPS
Intel HD Graphics 4600 20 16 1350MHz 432 GFLOPS
Intel HD Graphics 4000 16 16 1150MHz 294.4 GFLOPS
Intel HD Graphics 3000 12 12 1350MHz 194.4 GFLOPS
Intel HD Graphics 2000 6 12 1350MHz 97.2 GFLOPS
Apple A6X 32 8 300MHz 76.8 GFLOPS

In its highest end configuration, Iris has more raw compute power than a GeForce GT 650M - and even more than a GeForce GT 750M. Now we’re comparing across architectures here so this won’t necessarily translate into a performance advantage in games, but the takeaway is that with HD 5000, Iris 5100 and Iris Pro 5200 Intel is finally walking the walk of a GPU company.

Peak theoretical performance falls off steeply as soon as you start looking at the GT2 and GT1 implementations. With 1/4 - 1/2 of the execution resources as the GT3 graphics implementation, and no corresponding increase in frequency to offset the loss the slower parts are substantially less capable. The good news is that Haswell GT2 (HD 4600) is at least more capable than Ivy Bridge GT2 (HD 4000).

Taking a step back and looking at the rest of the theoretical numbers gives us a more well rounded look at Intel’s graphics architectures :

Peak Theoretical GPU Performance
  Peak Pixel Fill Rate Peak Texel Rate Peak Polygon Rate Peak GFLOPs
Intel Iris Pro 5100/5200 10.4 GPixels/s 20.8 GTexels/s 650 MPolys/s 832 GFLOPS
Intel HD Graphics 5000 8.8 GPixels/s 17.6 GTexels/s 550 MPolys/s 704 GFLOPS
NVIDIA GeForce GT 650M 14.4 GPixels/s 28.8 GTexels/s 900 MPolys/s 691.2 GFLOPS
Intel HD Graphics 4600 5.4 GPixels/s 10.8 GTexels/s 675 MPolys/s 432 GFLOPS
AMD Radeon HD 7660D (Desktop Trinity, A10-5800K) 6.4 GPixels/s 19.2 GTexels/s 800 MPolys/s 614 GFLOPS
AMD Radeon HD 7660G (Mobile Trinity, A10-4600M) 3.97 GPixels/s 11.9 GTexels/s 496 MPolys/s 380 GFLOPS

Intel may have more raw compute, but NVIDIA invested more everywhere else in the pipeline. Triangle, texturing and pixel throughput capabilities are all higher on the 650M than on Iris Pro 5200. Compared to AMD's Trinity however, Intel has a big advantage.

The Prelude Crystalwell: Addressing the Memory Bandwidth Problem
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  • tipoo - Saturday, June 01, 2013 - link

    It still seems to me that this misses where it would benefit most: 13 inch laptops, which currently mostly use dual core processors. GT3e would make something like the Retina MBP 13" much more appealing for instance, but it's paired with processors such that the wattage would be too high. Reply
  • tipoo - Saturday, June 01, 2013 - link

    Oh and I wanted to ask, if the integrated graphics are disabled can the CPU still tap into the eDRAM? Reply
  • Ryan Smith - Saturday, June 01, 2013 - link

    Yes, it's a dedicated cache for both the CPU and the GPU. However it's very unlikely you're going to run into any scenario that uses a Crystalwell-equipped part in such a manner. It's not being sold in socket form, so it will go to OEMs, who in turn would only use it if they didn't include a dGPU. Reply
  • jeffkibuule - Saturday, June 01, 2013 - link

    So pretty much, unless you've got some huge beefy GPU that would absolutely suck up power compared to just using Iris Pro graphics, no one would opt for that SKU? Reply
  • shiznit - Saturday, June 01, 2013 - link

    Right on. A dual core model for the 13" rMPB would have me selling my 2012 immediately. Now I need to decide if I can live with the 15" or even bother. Reply
  • moep - Saturday, June 01, 2013 - link

    If i interpreted the results of this article correctly, I suspect that the 15" MBP is probably going to get a new and even thinner form factor with this refresh. (one chip less, fewer VRM related parts, lower combined TDP)

    A 15" rMBP approaching the weight of a 15" Macbook Air would be very interesting, although a part of me hoped that Apple would wait until Broadwell to ditch the dGPU in the 15".

    Such a step back in GPU performance with the Retina display is surely not going to be very pleasant in 3D applications.
    Reply
  • Galatian - Saturday, June 01, 2013 - link

    I actually hope/suspect, that Apple will go the other road: use a discrete graphic solution on the 15" rMBP until Broadwell comes out, but have a cTDPdown version of the 4850HQ on the 13" rMBP. Maybe they can even get the normal TDP version in there; after all it has the same (good) cooling the 15" rMBP has and I have never heard the fans on mine. I think Apple really designed it with Haswell in mind, so let's see what they'll bring on during the next few weeks. Reply
  • tipoo - Saturday, June 01, 2013 - link

    That's certainly the best case, I really hope they go down that road. The rMBP as a quad with Iris Pro would really make it worth the Pro name. Reply
  • vFunct - Sunday, June 02, 2013 - link

    They'll probably stick with the built in GPU for the 13" model and a discrete GPU for the 15" model, which is what they do right now.

    Apple's top-end MacBook Pro has always had the highest end discrete GPU available.
    Reply
  • Spunjji - Tuesday, June 04, 2013 - link

    I'm guessing you mean "for a given power usage", as there are definitely faster GPUs out there than the 650M. Reply

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