CODEWITHSUNDEEP
cscripting
Matthew Blanchard
Matthew Blanchard

Reputation: 508

How to handle labels in a scripting language I'm writing?

So I've been stewing over this for a long time, thinking about it. Here's a code example first, and then I'll explain it.

:main
    dostuff
    otherlabel

:otherlabel
    dostuff

Alright so in this example, main is where the code starts, and it 'calls' the label 'otherlabel'. This is really just a shortcut for a jump command that changes execution to a different location in memory. My problem though is, how do I handle these labels so that they don't have to be declared before they are called?

At the moment, I'm doing a single step compilation reading straight from the source and outputting the bytecode. I am simply handling labels and adding them to a dictionary when I find them. And then I replace 'otherlabel' with a jump command to the correct location in code. But in this case that code wouldn't compile.

I've thought of a few ways to do this:

First is handling labels before anything else but this requires me to do everything in two steps and I have to deal with the same code twice, this slows down the process and just seems like a mess.

Second is queueing up the label calls until AFTER I've gone through the entire file and compiled everything else and then dealing with them, this seems much cleaner.

I'm writing this in C so I'd rather not implement complex data structures, I'm looking for the most straight forward way to handle this.

Upvotes: 3

Views: 115

Answers (2)

luser droog
luser droog

Reputation: 19494

You could use "backpatching", although it sounds like that's what you've tried already; and it could be consstrued as a complex structure.

When you encounter a call to an undefined label, you emit the jump with a blank address field (probably into a buffer, otherwise this becomes the same as "multipass" if you have to re-read the file to patch it); and you also store a pointer to the blank field in a "patch-up" list in the dictionary. When you encounter the label definition, you fill-in all the blanks in the list, and proceed normally.

Upvotes: 4

orlp
orlp

Reputation: 117681

Use multiple passes. One pass isn't going to suffice for a scripting language, especially when you are getting to the more complex structures.

In a first pass, before compiling, construct your dictionary of labels. In a later pass, when the compiling happens, just use that dictionary.

Upvotes: 5

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