Transferring control between code segments, eval, and suchlike things

[Dan Sugalski]

Okay, since this has all come up, here's the scoop from a design perspective.

First, the branch opcodes (branch, bsr, and the conditionals) are all meant for movement within a segment of bytecode. They are *not* supposed to leave a segment. To do so was arguably a bad idea, now it's officially an error. If you need to do so, branch to an op that can transfer across boundaries.

Design Edict #1: Branches, which is any transfer of control that takes an offset, may *not* escape the current bytecode segment.

Next, jumps. Jumps take absolute addresses, so either need fixup at load time (blech), are only valid in dynamically generated code (okay, but limiting), or can only jump to values in registers (that's fine). Jumps aren't a problem in general.

Design Edict #2: Jumps may go anywhere.

Destinations. These are a pain, since if we can go anywhere then the JIT has to do all sorts of nasty and unpleasant things to compensate, and to make every op a valid destination. Yuck.

Design Edict #3: All destinations *must* be marked as such in the bytecode metadata segment. (I am officially nervous about this, as I can see a number of ways to subvert this for evil)

I'm only keeping jumps (and their corresponding jsr) around for nostalgic reasons, and with the vague hope they may be useful. I'm not sure about this.

Design Edict #4: Dan is officially iffy on jumps, but can see them as useful for lower-level statically bound languages such as forth, Scheme, or C.

That leads us to

Design Edict #5: Dan will accommodate semantics for languages outside the core set (perl, python, ruby) only if they don't compromise performance for the core set.

Calling actual routines--subs, methods, functions, whatever--at the high level isn't done with branches or jumps. It is, instead, done with the call series of ops. (call, callmeth, callcc, tailcall, tailcallmeth, tailcallcc (though that one makes my head hurt), invoke) These are specifically for calling code that's potentially in other segments, and to call into them at fixed points. I think these need to be hashed out a bit to make them more JIT-friendly, but they're the primary transfer destination point

Design Edict #6: The first op in a sub is always a valid jump/branch/control transfer destination

Now. Eval. The compile opcode going in is phenomenally cool (thanks, Leo!) but has pointed out some holes in the semantics. I got handwavey and, well, it shows. No cookie for me.

The compreg op should compile the passed code in the language that is indicated and should load that bytecode into the current interpreter. That means that if there are any symbols that get installed because someone's defined a sub then, well, they should get installed into the interpreter's symbol tables.

Compiled code is an interesting thing. In some cases it should return a sub PMC, in some cases it should execute and return a value, and in some cases it should install a bunch of stuff in a symbol table and then return a value. These correspond to:


eval "print 12";

$foo = eval "sub bar{return 1;}";

require foo.pm;

respectively. It's sort of a mixed bag, and unfortunately we can't count on the code doing the compilation to properly handle the semantics of the language being compiled. So...

Design Edict #7: the compreg opcode will execute the compiled code, calling in with parrot's calling conventions. If it should return something, then it had darned well better build it and return it.

Oh, and:

Design Edict #8: compreg is prototyped. It takes a single string and must return a single PMC. The compiler may cheat as need be. (No need to check and see if it returned a string, or an int)

Yes, this does mean that for plain assembly that we want to compile and return a sub ref for we need to do extra in the assembly we pass in. Tough, we can deal. If it was dead-simple it wouldn't be assembly. :)

I think that's it. Let's have at it and see where the edicts need fixing.
--
Dan

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