I got recursive locks working on Friday, which got me back into the framewalker stuff. For HotSpot’s framewalker to see frames as native I need to supply it with something like a program counter can be used to reference into a set of tables that tell it, for example, which stack slots contain pointers for consideration by the garbage collector. It expects this to be in a block of generated code (which won’t really be code at all in Shark), but the core problem is that the “code” you generate goes into a temporary buffer which HotSpot then relocates into the final location so I can’t simply inline pointers from the buffer into Shark’s output. The final location of the “code” can not be determined at compile time, and even if it could it can move at any time as a result of garbage collector activity.
When you invoke a method in zero you start with a methodOop, a pointer to a structure containing (amongst other things) the method’s entry point. The entry point is simply a pointer to the function that you call to execute the method. The address of the final code buffer is also contained within the methodOop, but both the entry point and the code buffer are volatile — they can change at any time — so they need to be read at the same time, in one atomic operation.
What is needed is some way to pass a pointer to the code buffer when calling Shark methods. After a fairly intense thinking session it occurred to me that the entry point is going to be word-aligned, so the bottom two or three bits will always be zero. Code buffer pointers in HotSpot are always word aligned too, so I decided to use the bottom bit as a flag: if the bottom bit is clear then the entry point is a normal pointer-to-a-function entry point, but if it’s set then the “entry point” is really a pointer to the code buffer. The actual entry point can then be read from the code buffer, which in Shark does not contain code but simply whatever data I decide to put in there.
The nice thing about this is that, aside from adding only one or two instructions per method dispatch, it also opens up the possibility of method inlining, something I didn’t think would be possible.
