Developer delves deep inside Intel's Larrabee

— 6:00 AM on April 2, 2009

Michael Abrash of Rad Game Tools (you know, the guys behind Bink Video) has written an interesting and dauntingly thorough technical article about Larrabee, Intel's upcoming x86-based graphics processor.

The piece starts slow with a general overview of why Intel made Larrabee, what Larrabee is, and how Intel retrofitted the x86 ISA for a massively parallel design. Abrash notes that Larrabee is essentially an enhanced x86 CPU. While it lacks the chipset features to "serve as a main CPU running . . . Windows," Larrabee, "running as a GPU device under Windows, can bring up a BSD OS, with the Larrabee graphics pipeline running as just another BSD application."

Abrash then begins an exploration of Larrabee's extensions to the x86 instruction set (dubbed Larrabee New Instructions, or LRBni), Larrabee's vector architecture, and how developers can tap into the hardware to make it fulfill its functions as a GPU. These pages make up the bulk of the article, and they might be a tad difficult to decipher unless you're a programmer versed on subjects like vector registers, vector arithmetic, and vector masks.

Still, the less technically proficient should also find nuggets of more accessible information scattered throughout. At the end, for instance, the article says "any loop written in a traditional programming language can be vectorized, to execute 16 iterations of the loop in parallel on Larrabee vector units"—that's provided the loop meets a couple of criteria beforehand, of course. The piece also includes this quote from Epic Games founder Tim Sweeney:

Larrabee enables GPU-class performance on a fully general x86 CPU; most importantly, it does so in a way that is useful for a broad spectrum of applications and that is easy for developers to use. The key is that Larrabee instructions are "vector-complete."

Abrash concludes that Larrabee's new instructions will pave the way for "major speedups . . . without heroic programming." The author says he plans to write a second piece covering Larrabee's rasterization features, too, so stay tuned. (Thanks to TR regular UberGerbil for the heads up.)

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