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TODO for SixtyPical

Language

Save values to other-than-the-stack

Allow

save a to temp_a {
    ...
}

Which uses some other storage location instead of the stack. A local non-static would be a good candidate for such. At any rate, the location must not be writeable by anything that is called from within the block. So, probably just restrict this to local non-statics.

Copy byte to/from table

Do we want a copy bytevar, table + x instruction? We don't currently have one. You have to ld a, st a. I think maybe we should have one.

Character literals

For goodness sake, let the programmer say 'A' instead of 65.

Character set mapping

Not all computers think 'A' should be 65. Allow the character set to be mapped. Probably copy what Ophis does.

"Include" directives

Search a searchlist of include paths. And use them to make libraries of routines.

One such library routine might be an interrupt routine type for various architectures. Since "the supervisor" has stored values on the stack, we should be able to trash them with impunity, in such a routine.

Pointers into non-byte tables

Right now you cannot get a pointer into a non-byte (for instance, word or vector) table.

Word and vector tables are stored as two byte tables in memory. This is useful for indexed access, but makes pointer access more difficult.

Laying them out for pointer access would make indexed access more difficult.

Saving non-byte values

Right now you cannot save a word value.

There doesn't seem to be a hugely pressing reason why not.

Analysis

Forbid recursion

What happens if a routine calls itself, directly or indirectly? Many constraints might be violated in this case. We should probably disallow recursion by default. (Which means assembling the callgraph in all cases.)

Analyze memory usage

If you define two variables that occupy the same address, an analysis error ought to be raised. (But there should also be a way to annotate this as intentional. Intentionally making two tables overlap could be valuable. However, the analysis will probably completely miss this fact.)

Optimization

Space optimization of local non-statics

If there are two routines A and B, and A never calls B (even indirectly), and B never calls A (even indirectly), then their non-static locals can be allocated at the same space.

This is not just an impressive trick -- in the presence of local pointers, which use up a word in zero-page, which we consider a precious resource, it allow those zero-page locations to be re-used.

Tail-call optimization

If a block ends in a call can that be converted to end in a goto? Why not? I think it can, if the block is in tail position. The constraints should iron out the same both ways.

As long as the routine has consistent type context every place it exits, that should be fine.

Branch optimization in if

Currently the if generator is not smart enough to avoid generating silly jump instructions. (See the Fallthru tests.) Improve it.

Dead code removal

Once we have a call graph we can omit routines that we're sure aren't called.

This would let us use include-files and standard-libraries nicely: any routines they define, but that you don't use, don't get included.

Analyzing the set of possible routines that a vector can take on would help this immensely.

Implementation

Line numbers in analysis error messages

For analysis errors, there is a line number, but it's the line of the routine after the routine in which the analysis error occurred. Fix this.

Blue-skying

Pointers associated globally with a table(?)

We have point into blocks, but we would also like to sometimes pass a pointer around to different routines, and have them all "know" what table it operates on.

We could associate every pointer variable with a specific table variable, in its declaration. This makes some things simple, and would allow us to know what table a pointer is supposed to point into, even if that pointer was passed into our routine.

One drawback is that it would limit each pointer to be used only on one table. Since a pointer basically represents a zero-page location, and since those are a relatively scarce resource, we would prefer if a single pointer could be used to point into different tables at different times.

These can co-exist with general, non-specific-table-linked pointer variables.

If we have local pointers and space optimization for local non-statics, though, these don't add as much.