Cliff Hacks Things.

Tuesday, February 14, 2006

Reinventing Forth

There's a saying to the effect that any sufficiently complex program or runtime will wind up reimplementing most of Common Lisp. (I mostly hear that from Lispers, so add salt to taste.)

Well, you can imagine how surprised I was to see someone reinventing Forth.

In an indirect threaded interpreter, each op — let's call them words, for the hell of it — is represented by an index into some table holding an address to the code. (Or, in Forth, an address of a word definition, which contains an address of the code to run.)

In a direct-threaded interpreter, each words is represented by the address of the code itself, so the interpreter — which we'll call NEXT — is basically a repeated indirect jump.

At least one Forth saw the obvious next step in performance: replace each reference to a compiled word with a CALL instruction to its address, instead of the address. (This isn't a huge leap for Forth. Direct-threaded Forths usually have a CALL instruction in the word definition's code slot anyway.) On the machine in question (I don't remember which), they used a within-segment jump instruction, and were able to squeeze it the same space that would have been required for the direct-threaded address.

Evidently, this approach has been rediscovered, under the name "context threading."

Context threading does bring a few significant advancements to the table: it generates the calls at runtime from the original bytecode, and inlines basic flow control among all those CALL instructions. This isn't possible in Forth without a lot of trickery, for the same reason it's difficult in Smalltalk: there are no built-in flow control mechanisms. However, high-performance subroutine-threaded Forths will inline short, common words — and you don't get much shorter or more common than IF/ELSE/THEN.

Speaking of which, context threading seems to rely on the original bytecode to resolve flow control operations, where Forth would simply modify the return address.

(Incidentally: yes, this is one of the two tricks I used in the Mongoose interpreter to improve indirect branch prediction. Forth has a lot of good ideas to steal.)

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