The problem with transforming each Arc function to a C function is the stack space consumed at each call. Sure, gcc does tail call optimization, but not all compilers are gcc.
Chicken fixes this by keeping track of stack and GC'ing the stack when it's full; I'm not sure about Bigloo but I hear it's got a pretty good Scheme function == C function equivalency, so it might be useful to see how they fixed the tail call optimization problem there.
That said, the current execute() function accepts a pc argument, which specifies which Arc function it begins with. It may be possible to pass the pc to go to together with a new stack, but I don't know pthreads.
I think you could trust every decent C compiler to do tail recursion optimization, but if you want full compatibility then the mapping Arc fun -> C fun doesn't work. pthread requires you to pass a pointer to a C function i.e. an adress where to jump. We could wrap every thread within a C function and leave all the other functions as they are currently implemented. The C function would just call execute() with the right parameters. If execute() doesn't use global vars, there will not be race condition problems.
Well, I've started to import bits of execute from globals to locals. However I do have access to a few bits of global variables, specifically the quoted-constants array (those created by 'foo and '(a b c d), etc.); this table is initialized at startup (before the first call to execute). I would suppose this read-only array would be okay to access?
As for wrapping them in C functions: the problem is that the most basic Arc threading function isn't 'thread, it's 'new-thread, which accepts a function as input. 'thread is defined as:
(mac thread body
`(new-thread (fn () ,@body)))
In theory, new-thread could be called with any function:
Sure, it won't happen most of the time, since most people will sensibly use the simpler 'thread, but exploratory, exploratory...
It would be possible to implement if pthreads or whatnot can pass even just a single pointer to the newly-threaded function, but if it can't (why not?) then our alternative is to create a bunch of C functions for each Arc function, which just calls the execute() function with the correct number.
Edit: okay, I did a little eensy-weensy bit of research on pthreads, and it seems that pthreads can pass a pointer to the called C function.
This could work. As for global variables, if they are read-only, then there won't be any problem. What happens if you load in parallel two different modules (with their constants)? Maybe loading a file should be made atomic, or at least a part of it, such as constant values initialization.