Sun, 16 Apr 2006

LLVM Thoughts

On Friday I translated my libjit-based JIT to use LLVM. This took a good part of the day; then I spent a chunk of Saturday debugging it.

LLVM has a few drawbacks, as compared to libjit. There's not really any documentation for how to use LLVM as a JIT, so I ended up reading the header files quite a bit; libjit is much better here. LLVM's API is quite a bit bulkier than libjit's, and it is more idiosyncratic. For instance, in LLVM many objects can have a name, and many classes require a name in their constructors; this seems a bit bloaty in a JIT context -- but I didn't measure it. Finally, LLVM is installed strangely; it is mostly static libraries, but with a few random object files thrown in for good measure. This is unfriendly to say the least... also, link times with LLVM are much longer than with libjit, reducing my efficiency.

Some of these I would like to see fixed -- either in LLVM or in whatever ends up, someday, in GCC. Names could perhaps be handled optionally. Other oddities in the interface could be fixed (not that I have a list or anything...). Shared libraries should be made.

All this is gas, though, in a way. LLVM is generally more functional than libjit: it has many more ports, more optimizers, and a friendlier license. It probably represents a better long-term approach.

With a little help on irc from Chris Lattner I got the LLVM JIT running a couple very simple Java programs; with the optimizers enabled it appears that LLVM correctly notices that the empty loops are empty and removes them... so, it is working. There's still a lot of debugging to do ("hello world" still crashes), but this isn't a big deal.

Naturally, exception handling remains a problem. I'm hoping to get Bryce or Andrew to solve that problem :-)

posted at: 18:45 | path: /software | permanent link to this entry