I've gone back and forth on this. I initially disallowed this, then allowed
it because I forgot why I disallowed it. The reason to disallow it: if
you return an `addr` to a variable allocated on the stack, the space might
be reused for a different type, which violates type-safety. And once you
can reinterpret bits of one type as another you lose memory-safety as well.
This has some interesting implications for Mu programs; certain kinds of
helper functions become impossible to write. Now I find myself relying a
lot more on scopes (and editor folding support) for abstracting details.
And they won't help manage duplication. We'll see how this goes.
While I'm being draconian about `addr`s on the stack, I'm still abusing
`addr`s on the heap, with the expectation that future checks on reclamation
will protect me. The boon and bane of stack space is that it's constantly
reclaimed.
We haven't run into this limit yet, but everytime I see a 'stream overflow'
error I run into it while going over all the knobs in apps/subx-params.subx,
if I increase Input-size (used by survey.subx) too much.
Before: bytes can't live on the stack, so size(byte) == 1 just for array
elements.
After: bytes mostly can't live on the stack except for function args (which
seem too useful to disallow), so size(byte) == 4 except there's now a new
primitive called element-size for array elements where size(byte) == 1.
Now apps/browse.subx starts working again.
Several bugs fixed in the process, and expectation of further bugs is growing.
I'd somehow started assuming I don't need to have separate cases for rm32
as a register vs mem. That's not right. We might need more reg-reg Primitives.
Most unbelievably, I'd forgotten to pass the output 'out' arg to 'lookup-var'
long before the recent additions of 'err' and 'ed' args. But things continued
to work because an earlier call just happened to leave the arg at just
the right place on the stack. So we only caught all these places when we
had to provide error messages.
Byte-oriented addressing is only supported in a couple of instructions
in SubX. As a result, variables of type 'byte' can't live on the stack,
or in registers 'esi' and 'edi'.
Kartik Agaram