If the expression is between the start and end of the block of code and isn't relative (like jmp or call with imm) and references an address that is also within it, or is relative and references an address outside of it, or if the expression is outside of it (in virtual directive) and references an address inside it, then I subtract the start of the code from that operand and add the address of it to my list to add the address it is loaded at to. Except with jmp and call in which case I do the opposite.

How do you detect which one is just a mere constant and which one is a label? Or do you plan to force programmers to add something extra to address references to be able to detect them?

I don't. I just avoid using constants that fall within that range.

I guess I could put the whole thing in a virtual block to move it away from the constant, or use a loop with eq to compare the argument against a list of labels and make sure it matches one of them directly, but I haven't bothered yet. Or maybe the in directive. Or putting an ! in front if I want it relocated and using match.

To compare with a list of labels is easy if you have the list, you can use match. The list has to begin and end with an extra symbol, like '+' (invalid label name anyway), because otherwise the first and last labels would get skipped (i.e no 'a' or 'b' before/after so match would not "match" ):