I took my first glance through Hotspot recently. I’m vaguely curious to see what it would take to port it to PowerPC, not because I have a particular love for that architecture, but because it is important for switching Fedora and RHEL from GCJ to OpenJDK. Also, I’m spending a bit of time researching what I want to work on next, and I think OpenJDK is one of the available options.

Hotspot seems like pretty nice C++ code, overall. The structure is easy to navigate. There are comments pretty much where I expect them.

So far I’ve only looked at simple bits. In particular I tried to read the code that corresponds to code in libgcj with which I am very familiar — the interpreter and the verifier.

The Hotspot verifier is a bit more “C++-y” than mine. Stuff is spread out a bit more and a little more abstract, so I find it somewhat harder to read. But, that’s to be expected. I didn’t try to delve into the deeper things (how do they really handle jsr), since I was only looking at this while waiting for a build today. Some other time.

There seem to be two bytecode interpreters in Hotspot. First, a simple C-ish one, roughly like the libgcj interpreter before I added direct threading. This one is not the default, I suppose it is probably only used when bootstrapping on a new platform or something like that.

The other interpreter is trickier and implements something which is sort of like early Kaffe JITs. The idea is that most bytecodes have an associated function that generates bits of machine code. Before executing a method, a very simple form of JITting is applied where these functions are called to compile code, naively, into a buffer which is then executed.

Well, I hope that is what happens… I’m not trusting myself much this week and the code is fairly convoluted.

I haven’t made much progress on my goal of seeing how hard a port is. The existing ports look pretty small… from 28 KLOC (amd64) to 36 KLOC (sparc).