On Oct 14, 2004, at 12:10 PM, Leopold Toetsch wrote:
Proposal: * we mandate that JIT code uses interpreter-relative addressing - because almost all platforms do it - because some platforms just can't do anything else - and of course to avoid re-JITting for every new thread
FYI, the PPC JIT does already do parrot register addressing relative to the interpreter pointer, which as you said is already in a CPU register. This is actually less instructions than using absolute addressing would require (one rather than three).
We do still re-JIT for each thread on PPC, though we wouldn't have to (just never changed it to not). But, we use this currently, because there is one issue with threads: With a thread, you don't start from the "beginning" of the JITted code segment, but rather you need to start with a specific Parrot function call, somewhere in the middle. But you can't just jump to that instruction, because it would not have the setup code needed when entering the JITted section. So currently, we use a technique whereby the beginning of the JITted section has, right after the setup code, a jump to the correct starting address--in the main thread case, this is just a jump to the next instruction (essentially a noop), but in the thread case, it's a jump to the function which the thread is going to run. So right now the JITted code for a secondary thread differs by one instruction from that for the main thread. We'll need to work out a different mechanism for handling this--probably just a tiny separate JITted section to set things up for a secondary thread, before doing an inter-section jump to the right place.
JEff
