Programming for Larrabee

The Larrabee programming model is what sets it apart from the competition. While competing GPU architectures have become increasingly programmable over the years, Larrabee starts from a position of being fully programmable. To the developer, it appears as exactly what it is - an arrangement of fully cache coherent x86 microprocessors. The first iteration of Larrabee will hide this fact from the OS through its graphics driver, but future versions of the chip could conceivably populate task manager just like your desktop x86 cores do today.

You have two options for harnessing the power of Larrabee: writing standard DirectX/OpenGL code, or writing directly to the hardware using Larrabee C/C++, which as it turns out is standard C (you can use compilers from MS, Intel, GCC, etc...). In a sense, this is no different than what NVIDIA offers with its GPUs - they will run DirectX/OpenGL code, or they can also run C-code thanks to CUDA. The difference here is that writing directly to Larrabee gives you some additional programming flexibility thanks to the GPU being an array of fully functional x86 GPUs. Programming for x86 architectures is a paradigm that the software community as a whole is used to, there's no learning curve, no new hardware limitations to worry about and no waiting on additional iterations of CUDA to enable new features. You treat Larrabee like you treat your host CPU.

Game developers aren't big on learning new tricks however, especially on an unproven, unreleased hardware platform such as Larrabee. Larrabee must run DirectX/OpenGL code out of the box, and to do this Intel has written its own Larrabee native software renderer to interface between DX/OGL and the Larrabee hardware.

In AMD/NVIDIA GPUs, DirectX/OpenGL instructions map to an internal GPU instruction set at runtime. With Larrabee Intel does this mapping in software, taking DX/OGL instructions, mapping them to its software renderer, and then running its software renderer on the Larrabee hardware.

This intermediate stage should incur a performance penalty, as writing directly to Larrabee is always going to be faster. However Intel has apparently produced a highly optimized software renderer for Larrabee, once that's efficient enough so that any performance penalty introduced by the intermediate stage is made up for by the reduction of memory bandwidth enabled by the software renderer (we'll get to how this is possible in a moment).

Developers can also use a hybrid approach to Larrabee development. Larrabee can run standard DX/OGL code but if there are features developers want to implement that aren't enabled in the current DirectX version, they can simply write those features that they want in Larrabee C/C++.

Without hardware it's difficult to tell exactly how well Larrabee will run DirectX/OpenGL code, but Intel knows it must succeed on running current games very well in order to make this GPU a success.

Cache and Memory Hierarchy: Architected for Low Latency Operation A Tribute to Michael Abrash: The ISA
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  • del - Friday, August 15, 2008 - link

    Don't be a hater. :P Intel has got it goin' on right now. Believe in the POWAH of Larrabee... unless it proves to be a failure upon release.

  • Thatguy97 - Sunday, June 28, 2015 - link

  • atlmann10 - Saturday, August 9, 2008 - link

    Think about this ok AMD originally was a private IBM cpu manufacturer. Then bought out and run as a side unit of INTEL, that was dropped after they were done with them. So in a way the were partners and I'm sure there was some friendliness. As it's always been said keep your friends close but your enemies closer. There have been some things especially in these past two years that struck me kind of odd. Such as AMD's graphics chips running fine on a x38/48 chipset and the physics collaboration things as well as a few other rumors. Then Nvidia starts spouting off about how they could kick INTELS A77 etc. Now AMD has a definite GPU coprocessor in ATI and they wanna break into the market of GPU's etc. They know that there will be graphics competition with Nvidia being there largest competitior because there dedicated to GPU's solely and have a reputation. However now AMD has some chips that compete straight on weakening Nvidia to a point. Then AMD is getting more and more out of there cpu's gpu's and chipsets so INTEl jumps in the CPU GPU market just like AMD. Either way it turns out more are going to go with INTEL cpu's and many other products where AMD is kind of a fringe player. Who would you rather compete against full on 2 major GPU manufacturers or attempt to kind of co-align yourself with there competetitor while the somewhat down. Then throw out a whole new way to do graphics that performs well Nvidia is already loosing market share. So more people try it and the same number of people go with ATI. That leaves a much lower market for Nvidia plus there paying back what some 200 million dollars in bad GPU's right now as well and a few other problems they been having. Now this is not anything I know but knowing INTEL loves to stick it to competitors when there weak think about it.
  • benkantor - Wednesday, August 6, 2008 - link

    if you could fit 10 Larrabees on 143 mm^2, you could fit 40 Larrabees on 286 mm^2, not 20... :P
  • MamiyaOtaru - Saturday, August 9, 2008 - link

    For the love of education. We've already been through this. See the end of page 6 through page 7 in the comments section.

    143mm^2 doesn't mean 143*143. It means 143 square millimeters. 286 square millimeters is twice as many, allowing twice as many cores.">

    The article is right and you are so very wrong.
  • Barack Obama - Wednesday, August 6, 2008 - link

    Derek and Anand deliver again!
  • KGR - Wednesday, August 6, 2008 - link

    I am not a profeesional about software and hardware that is why maybe this question can sound nonsense .
    If larrabee will have a software renderer and programmed by C++ is it possible that it is not depended on windows? I mean if it doesnt need direct X can we run the games on Linux also??
  • npoe1 - Tuesday, August 5, 2008 - link

    I enjoyed reading this so much. I think that this kind of articles is what Anandtech needs; I usually go to Arstechnica to read things like this one.

    Again, thanks!
  • TrEmEnDo - Tuesday, August 5, 2008 - link

    I am definitely impressed with this new development and I expect that this technology will be disruptive down the road, however I feel that somehow they are about to commit another of their megalomaniac mistakes.
    Has anyone stopped for a sec and look where all gaming industry is heading into? Are PCs the future gaming platform? Maybe I am missing something but aren't the big guys already struggling to retain a 'decent' percentage of the multibillion gaming pie (PC gaming alliance anyone...)? I believe that whether us, tech enthusiast, hardcore pc gamers like it or not, it is the console arena where the big guns are going to be playing in a few years from now.
    Guys, we are seeing this happening everyday, we see tittles appearing and disappearing everyday b/c companies don't want to commit the resources to develop games for more than one or two platforms (normally doing a sloppy work BTW). Now that the grandpas of graphic hardware had manage to get DX/D3D derived engines into the last gen consoles (xenos, RSX) and a terribly inertial and rigid developer community avoids and whines about how difficult is to program for the few hardware 'jewels' that we have already in our hands (Cell/RV770/G200) do you think anyone except Intel is in the mood for yet another graphics industry spin?

    I have no doubt that this new development will have its own niche application or someone will definitely find something appropriate for it, but to say that Larrabee CAN do graphics and to say larrabee will kick ass so bad that in 3 years from now we all will be gaming from a Larrabee containing computer are two very different things.
    Congrats to Intel as the fathers of the creature, and congrats to us to see the tech world moving on....but just don't think this will change the world as we know it.
  • hooflung - Tuesday, August 5, 2008 - link

    They are doing something very AMD like and taking it a step further and tossing in a few Power ideals in. I just wonder what the power profile will look like and who will partner up with Intel for it.

    I am sure they will have 4+ of these cores built into integrated chip sets for OEMs and laptops to really boost those areas. And people who buy laptops will see that they can get a desktop with 'bigger larrabee' and play their games faster than their budget/laptop computer.

    So it does make sense. However, it is an empire made on a lot of ifs. It will be fun to watch. Thanks anandtech for the informative article.

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