Thursday, June 12, 2008

OpenCL / CLang / CL

The most exciting thing that happens under sleep deprivation is a kind of stream of consciousness, where the brain goes into a purely associative mode. I am sleepy now and so I can embark on some journey which I tomorrow morning surely will dismiss as utter nonsense.

So, WWDC is progressing at the moment, some under-the-hood technologies are being introduced into the OS X system that is running on the Macs out there. Two of these sound interesting: "Grand Central Dispatcher" and OpenCL. There has been some speculation that utilizing GPU power (which is 99,9% idle, unless you are a framerate-wanking gamer) could be one of the directions of the future (besides multi-cores). Nvidia and ATI both offer technologies to do this at different levels, but they are incompatible. It is hard to imagine that Apple will provide developers several different and incompatible compilers and tell them to compile their compute intensive kernels with them, bundle them up in some big blob of application and ship it untested (how many developers will buy a bunch of video-cards for testing?).

Apple likes to provide seamless solutions. They have a proven concept in their OpenGL implementation, namely a machine-independent binary format that can be JITted to a wide variety of processors: LLVM. This would make Joe Developer's life pretty easy: compile your computing kernels to LLVM bitcode, and submit a quadruple to a runtime component (bitcode, characteristics, input data, continuation/consumer of results). The characteristics could state that it is integer- or FP-heavy code etc. After deciding which target will run the code, the LLVM JIT could distill the optimal machine code for the target and invoke it on the input data. When done, the continuation would be called, and would consume the results. Add some speculative execution (start the same job in several computing resources), a good deal of caching (no need to recompile bitcode again), heuristics and profile guided recompilation. This could be some yummy sauce.

RoughlyDrafted's blog so far provides the best intro to the new technology, and helped to start my association chain. What comes next is the fun part.

When hearing OpenCL the first time, I thought, what a sucky name. It reminds me of OpenGL, but it is not a graphics language. It reminds me of Common Lisp, but that is not an option. Open Computing Language. Hmmm, all programming languages compute stuff...

There is a little gem, still unpolished, called clang. The light footed successor of gcc in corporate environments. Compiling "curly brace" languages to LLVM! Wasn't OpenCL C-based? Those letters CL appear everywhere. What is the initials of the principal LLVM engineer, again?

1 comment:

podunk said...

I've been experiencing similar associations in my thoughts recently. The most exciting, long-term prospect is in this paper. Imagine a world where fat binaries are replaced with LLVM bc files. In this world, Apple could choose any ambitious and tragically-underutilized CPU architecture in future machines (VLIW, EPIC, Cell, something home-grown...) and the entire operating system could take maximum advantage of it, without requiring 3rd-party software vendors to even recompile. Delicious thought, isn't it?