Featherweight Musings

Wednesday, December 16, 2009

VM talks and assembly language

Google are currently developing two (at least only two are public) VMs: Dalvik and V8. Dalvik is a Java VM (but not a "JVM" - it doesn't run Java byte code) targeted at mobile devices (part of Android), V8 is a Javascript interpreter and part of the Chrome browser. There are talks on both:

Dalvik: http://www.youtube.com/watch?v=ptjedOZEXPM

V8: http://www.youtube.com/watch?v=lZnaaUoHPhs

I recommend you watch both, they are both good, the V8 one in particular really got my juices going (man, I'm such a geek).

I think it is interesting to compare the two as the tackle a similar problem in very different ways. Obviously the source languages are different, in particular, V8 does lots of interesting stuff to optimise Javascript out of the dynamic language tar pit. The goals are different: Dalvik's main goal is to minimise space (also, time, but mainly space) and battery life; V8 is a pure speed machine. Dalvik doesn't do any JITing, V8 is (as I understand it) all JITed. Dalvik is optimised by using a new set of byte codes and a more compressed file format and some neat tricks on internal libraries etc. V8 is optimised by using classes and caching call sites. There's tons more on both VMs in the talks, and lots more stuff for an interesting comparison.

All of this machine hackery inspired me to look again at x86 assembly and architecture (and maybe I'll look at ARM next). I used to think x86 assembly was a big scary mess and best shyed away from. I've always output JVM or MIPS assembly in compilers etc. for this reason. Bu actually, x86 is not too bad! There's a few more instructions that do some pretty interesting things, but it really isn't much worse than other assembly languages. And I find it really interesting finding out how the processor works; maybe there's a closet bit-jockey in me after all.